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The demand for steel components with tailored properties is constantly growing. To obtain a specific variation of microstructures and mechanical properties along the component it must undergo a controllable cooling. One way to control the cooling rates along the component is by using different simultaneous water jet impingements on a hot austenitized surface. This can be done by a newly developed test rig for water Impinging Jet Quenching Technique (IJQT). This work discusses the effect of IJQT on mechanical properties and fracture behavior of 15 mm steel plates containing 0.27 and 0.38 mass-% carbon. The samples were cooled in a specifically designed setup of the technique to obtain simultaneous water and air cooling resulting in diverse microstructures. The mechanical property gradients of both steels were analyzed through hardness measurements and tensile tests. The fracture surfaces and the near fracture regions were observed using scanning electron microscope and light optical microscope respectively. The results from tensile tests showed that the larger part of the sample with higher carbon content was fully hardened, however smoothly transitioning to a more ductile region. The sample with lower carbon content combined various degrees of hardening and transitioned from higher to lower ultimate tensile strength values. Fracture behavior of higher carbon steel was predominantly brittle transitioning to a ductile, while the lower carbon steel had a small region showing brittle fracture transitioning to a larger region of predominant ductile fracture behavior.

The effect of aging time and temperature variants on the Ni-Ti Shape Memory Alloy (SMA) has been investigated in this study. After the aging process, microstructural characterization was carried out using Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) data. Using an innovative technique, Austenite finish (Af) temperature was determined. Aging effects on the hardness of the material were studied using Vickers Microhardness Test. SEM results revealed uniform distribution of α-Ni and β-Ti phases, appearance and disappearance of martensite plates and presence of precipitates. XRD analysis of the samples indicated that etching resulted in formation of oxides. The Af temperature was observed to increase with increase in aging temperature. In the Vickers Microhardness Test, it was seen that the mid-range time variant samples showed lower HV values.

The fundamental understanding of the effects of rare earth (RE) on the phase transformation of Fe-based alloys is essential in the development of advanced RE-containing high strength steels. In this work, we investigated the effects of La and Ce on the martensitic phase transformation of Fe in terms of the driving force and minimum energy path (MEP) using the first-principles calculations. The calculations of formation energy show that the RE substitution increases the stabilization of fcc-Fe and decreases that of bcc-Fe, thus decrease the driving force of phase transformation. Meanwhile, the analysis based on the generalized solid-state nudged elastic band (G-SSNEB) method indicates that in the RE-containing systems, an additional energy barrier appears when the phase transformation proceeds along the Bain transition path to c/a ratio of 1.14 in the FM state, which is associated with a sudden decrease in magnetization due to the hybridization between the orbitals of the RE atoms and those of the nearest neighbor Fe atoms. The results of potential-energy surfaces (PES) confirm that magnetic ordering and lattice volume change simultaneously with the Bain transition, and in comparison with pure Fe, the variation between the onset and the end structural volumes of the phase transformation is slight for the RE-containing Fe systems.

The frequent replacement of worn rails on tracks brings an immense economic burden on the railroad industry, and also causes significant interruptions to railroad operation. Restoration of worn rails via laser powder deposition (LPD) can considerably reduce the associated maintenance costs. This study was focused on the use of LPD to repair the worn profile of a standard U.S. rail. The microstructure of the 304L stainless steel deposits with a minimum hardness of 85 HRB was composed of austenite, δ-ferrite, and sigma. Micropores were dispersed throughout the deposit, and microcracks were found at the rail-deposition interface. The pearlitic rail substrate showed a moderate hardness of 94 HRB. The fine-grain, pearlitic-ferritic heat affected zone had the maximum hardness of 96 HRB, which was still below the minimum required hardness of 97 HRB for a typical rail. To increase the hardness to or above 97 HRB and to mitigate the microstructural defects, the as-repaired rail went through a heat treatment process. The average hardness of the as-treated rail was increased significantly, i.e., to 103 HRB. Besides, the porous and coarse-grain deposition materials were transformed into an impermeable and fine-grain microstructure. However, heat treatment intensified the microcracks at the rail-deposition interface and also led to the formation of martensite and augmentation of the micropores in the parent rail. Isolation of the base rail during heat treatment and preheating were suggested as solutions for the problematic results. The LPD process ultimately was found to be a promising technique for repairing rails.

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

In the present study, the interface of martensite twins in Ti-Ni-Hf shape memory alloy composite was studied by high resolution transmission electron microscopy in terms of atomic structure. The results reveal that two varieties of interface structure for {111} type I twins are observed, which is determined by the interior substructure (001) compounds twin. The formation of {111} type I twins can be attributed to the interaction between the misfit dislocations on (001) plane and ( 1 ¯ 11) twin boundary. In addition, the corresponding characteristic parameters of misfit dislocation were calculated in theory. The theoretical value of disregistry factor and average spacing for the misfit dislocations are well in good agreement with the experimental results.

Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength, ductility, and toughness in high-strength steels. In this work, the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography, transmission electron microscopy, and mechanical tests. Our results indicate that Cu accelerates the austenite reversion kinetics in two manners: first, Cu, as an austenite stabilizer, increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation, and second, Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation. In addition, the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite. During tensile deformation, the metastable reverted austenite transforms to martensite, which substantially improves the ductility and toughness through a transformation-induced plasticity (TRIP) effect. The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of ∼1.3 GPa, an elongation of ∼15%, and an impact toughness of ∼58 J.

Ultrahigh-strength quenching and partitioning (Q&P) steels have attracted strong interests in the auto manufactory, while the comprehensive understanding in the microstructure and mechanical behavior of their welded joints is highly needed to enrich their applications. In the present work, it is designed to make an insight into these imperative conundrums. Equal strength Q&P 1180 steel joints to parent metal were successfully fabricated via friction stir welding (FSW) technique under different parameters. Apparent hardening and softening were observed in stir zone (SZ) and heat-affected zone (HAZ) respectively, whose microstructures strongly depended on the peak temperature and cooling rate during welding. The formation of fresh martensite was the main mechanism for the SZ hardening, while the decomposition of metastable phases played key roles in the microhardness drop of the HAZ. A heat source zone-isothermal phase transition layer model was proposed to clarify the impregnability of the joint strength under parameter variation. The dual-phase structure, nano-carbide particles, tempered initial martensite, and ultrafine-grained ferrite synergistically improved the strain hardening ability of the HAZ, which eventually resulted in the equal strength FSW joints.

Epitaxial Ni–Mn–Ga thin films have promising application potential in micro-electro-mechanical sensing and actuation systems. To date, large abrupt magnetization changes have been observed in some epitaxial Ni–Mn–Ga thin films, but their origin - either from magnetically induced martensite variant reorientation (MIR) or magnetic domain evolution - has been discussed controversially. In the present work, we investigated the evolutions of the magnetic domain and microstructure of a typical epitaxial Ni–Mn–Ga thin film through wide-field magneto-optical Kerr-microscopy. It is demonstrated that the abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the MIR.

Laser engineered net shaping (LENS) is an advanced additive manufacturing technology combining rapid prototyping and synchronization technology. Its multi-powder feeder delivery system enables multi-materials building in single deposition, which is appropriate for additive manufacturing of functionally graded materials (FGMs). In this study, martensitic-stainless steel (MSS)/austenitic stainless steel (ASS) FGMs with composition transitioning from 100% MSS incrementally graded to 100% ASS by 25% composition gradients are fabricated by LENS. The Vickers hardness of MSS/ASS FGMs ranges from 358 to 170 HV. The decrease of hardness is found to relate to the grain-growth region with the increase of austenite. The obtained specimens of MSS/ASS FGMs show a tensile strength of 669 MPa and an elongation of 19%. In addition, the fracture location of MSS/ASS FGMs in tensile test is in the region of 100% ASS, which is dominated by austenite structure. Finally, the Scheil–Gulliver model is introduced to validate the phase formation of MSS/ASS FGMs’ failure region. Experimental and modeling results indicate that precipitation of α-ferrite in the austenite structure leads to reduced ductility of MSS/ASS FGMs.

Laser solid forming (LSF) technology can be used to rapidly manufacture and repair high-strength steel parts with superior performance, but the value of the heat input during operation is difficult to quantify, which has a substantial impact on the microstructure and mechanical properties of the parts. A promising method to improve the forming efficiency and quality of LSFed parts is to accurately control the heat input and explore its relationship with the microstructure and mechanical properties. To remove the interference of other variables from the experiment, the dimensionless heat input Q* was introduced. The Q* values were designed in advance to calculate the experimental parameters used to perform the LSF experiment. The microstructure was observed at different regions of the sample, and its mechanical properties were analyzed. From the results, the following conclusions were drawn. The Q* value was directly related to the cooling rate and heat accumulation in the top structure, leading to the formation of different microstructures; it also modified the original structure at the bottom, affecting the subsequent thermal cycle and indirectly changing the tempered martensite morphology. The heat input also affected the mechanical properties of the sample. The hardness of the stable zone decreased with increasing Q* value, and the lowest value was 190 HV. Similarly, the tensile strength and yield strength of the LSFed samples decreased considerably with increasing Q* value, and the lowest values were 735 and 604 MPa, respectively. Only the elongation and reduction in the area increased after a slight decrease. The Q* value had a significant effect on heat treatment. When Q* = 2.9, the increase in tensile strength and yield strength after heat treatment was the largest (29% and 44%, respectively).

The excellent shape memory and mechanical properties of TiNi shape memory alloys (SMAs) fabricated using selective laser melting (SLM) are highly desirable for a wide range of critical applications. In this study, we examined the simultaneous enhancement of mechanical and shape memory properties using heat-treatment homogenization of Ti2Ni precipitates in a Ti50.6Ni49.4 SMA fabricated using SLM. Specifically, because of the complete solution treatment, nanoscale spherical Ti2Ni precipitates were homogeneously dispersed throughout the grain interior. Interestingly, the resultant SMA exhibited an ultrahigh tensile strength of 880 ± 13 MPa, a large elongation of 22.4 ± 0.4%, and an excellent shape memory effect, with a recovery rate of > 98% and ultrahigh recoverable strain of 5.32% after ten loading–unloading cycles. These simultaneously enhanced properties are considerably superior than those of most previously reported TiNi SMAs fabricated using additive manufacturing. Fundamentally, the enhancement in tensile strength is ascribed to precipitation strengthening and work hardening, and the large plasticity is mainly attributed to the homogeneous nanoscale globular Ti2Ni precipitates, which effectively impeded the rapid propagation of microcracks. Furthermore, the enhanced shape memory properties are derived from the suppression of dislocation movement and formation of retained stabilized martensite by the presence of high-density dislocations, nanoscale Ti2Ni precipitates, and abundant interfaces. The obtained results provide insight into the enhancement of the two types of properties in TiNi SMAs and will accelerate the wider application of SMAs.

The increase in demand for the joining of dissimilar materials for various manufacturing industries. Modern technology attracts dissimilar materials to utilize for multiple applications to reduce the cost and weight. However, the joining of dissimilar materials is challenging to attain quality joints. There are several joining methods applied for the fabrication of dissimilar materials based on the applications. However, the conventional welding methods unable to produce successful joints. Tungsten Inert Gas welding is one of the candidate process to weld dissimilar materials. In this study, TIG welding was applied to join the high temperature materials of Inconel 718 to high-strength steel. Two kinds of filler materials were used to join the dissimilar materials and optimize the welding parameters. The metallurgical and mechanical properties of the joints were also analyzed and compared between the two filler materials. The results showed that welding current has a significant influence on the properties of the dissimilar welded joints. Microstructural analysis was revealed the fusion boundary consisting of the martensite phase on high strength steel fusion boundary. Moreover, there are a small number of precipitates in the Inconel 718 fusion line. EDS line scan analysis identified that the formation of γ′ and γ′′, NbC in the weld metal. The tensile strength of the welded joints is higher for the Inconel-based filler than the stainless steel-based filler material.

17–4 PH stainless steel is of martensite structure well known for its combination of corrosion resistance in multiple environments while retaining high strength properties, including hardness. There exists a lack of study in optimization of machining parameters to control its cycle time and tool wear of the WC cutting tool used in machining difficult-to-cut material for improved mass production capabilities. The correlation analysis allowed it to be determined that the speed and cutting force have a negative interconnection whereas the feed and depth of cut have a positive interconnection with the cutting force for both cryogenic treated and conventional WC tool in varying degrees. Thus, setting the stage for predictive maintenance algorithms and possible improvement in throughput time.

Silicon is an essential alloying element in quenching and partitioning (Q&P) steels, because it is known to suppress carbide precipitation during partitioning step and promote carbon partitioning to stabilize austenite. When 2 wt% Si was added to 10Mn-2Al-0.2C steel, the size and fraction of the carbides formed during partitioning became smaller than in the Si-free counterpart. Moreover, the suppression of carbide formation promoted C partitioning into austenite as expected. However, austenite stability was always lower with Si under the equivalent partitioning condition because Si effectively decreased the stacking fault energy of austenite. As partitioning progressed, the both yield and tensile strengths of the Si-added steel exceeded that of the Si-free steel with the similar ductility level. This was because Si was an effective solid solution strengthener, and the austenite in the Si-added steel exhibited the appropriate stability to gradually transform into martensite throughout the deformation. The resulting strengthening effect compensated for the softening caused by martensite recovery. Consequently, strain hardening rate decreased continuously throughout deformation, which resulted in high tensile strength and ductility.

The effect of precipitate type and distribution to mitigate the hydrogen embrittlement of high strength martensitic steels containing different levels of C, Nb, and V was studied. Even low additions of Nb and V contributed to an increase of the material resistance. The steel with a more homogeneous distribution of carbides, higher Nb, and lower V showed increased hydrogen solubility and presented a lower level of loss of ductility in tensile tests with hydrogen pre-charging. The presence of segregation of precipitates was responsible for a rise in the amount of diffusible hydrogen, thus increasing the hydrogen embrittlement susceptibility.

The present study investigated hydrogen-related fractures at different deformation temperatures ranging from −100 °C to 100 °C in low-carbon martensitic steel. The sensitivity to hydrogen embrittlement increased as the temperature decreased from 100 °C to 0 °C, while it decreased as the temperature decreased further below 0 °C. We characterized the fracture surface types from the morphological and crystallographic aspects and found that the fraction of hydrogen-embrittled surfaces exhibited a similar temperature dependence on the sensitivity to hydrogen embrittlement. The qualitative discussion suggested that the degree of hydrogen accumulation exhibits a peak value in the medium temperature range, which has the same tendency as the sensitivity to hydrogen embrittlement confirmed experimentally. Thus, we proposed that the effect of deformation conditions on the sensitivity to hydrogen embrittlement could be explained on the basis of the hydrogen accumulation behavior.

Ultrafast heating (UFH) at the rates of 10–300 °C/s was employed as a new strategy to anneal a cold-rolled 7wt% Mn steel, followed by the immediate cooling. Severely deformed strain-induced martensite and lightly-deformed thermal martensite, both had been already enriched with C and Mn before, transformed to fine and coarse austenite grains during the UFH, leading to the bimodal size distribution. Compared with the long intercritical annealing (IA) process, the UFH processes produced larger fraction of RA grains (up to 37%) with a high density of dislocation, leading to the significant increase in yield strength by 270 MPa and the product of strength and elongation up to 55 GPa% due to the enormous work hardening capacity. Such a significant strengthening is first attributed to high density dislocations preserved after UFH and then to the microstructural refinement and the precipitation strengthening; whilst the sustainable work hardening is attributed to the successive TRIP effect during deformation, resulting from the large fraction of RA instantly formed with the bimodal size distribution during UFH. Moreover, the results on the microstructural characterization, thermodynamics calculation on the reverse transformation temperature and the kinetic simulations on the reverse transformation all suggest that the austenitization during UFH is displacive and involves the diffusion and partition of C. Therefore, we propose that it is a bainite-like transformation.

High entropy alloy (HEA) with shape memory effect (SME) has been the subject of great interest for the past few decades. However, with the increased demands for new materials for high thermal applications, the research activities on the multi elemental high entropy shape memory alloys (HESMA) have been increased by many folds recently. The nano crystalline HEA powder with shape memory effect developed in this study, HfZrTiFeMnSi, was produced by mechanical alloying (MA) for the first time. In this method equiatomic ratio of Hf, Zr, Ti, Fe, Mn, and Si were mixed together and milled by MA process for 100 h. The powder formed was of amorphous in nature. Elemental mapping of the powder from SEM-EDS revealed homogeneity of the alloying elements confirming successful fabrication of HfZrTiFeMnSi HEA powder. The DSC studies from ambient to 500 °C of the annealed alloy powders showed reversible austenitic to martensitic (A↔M) transformations. The A↔M transformation hysteresis seemed to vary with the milling time and annealing temperature. The enthalpy values, ΔH, for the transformation were calculated from the DSC plots using tangent method for peak area calculation. Regardless of the annealing temperature, the thermal analysis revealed that the ΔH, equilibrium temperature (T0), and crystallization temperature values decreased with the increasing milling time.

The underwater laser cladding layers of Ti-6Al-4V titanium alloy were prepared using underwater wire-feeding laser cladding (UWLC) technology under different heat inputs and gas flow rates. At optimum process parameters, the uniform underwater cladding layer without oxidation was obtained, and the cracks inside layer were restrained successfully. The deposition morphology, grain characteristics, microstructure and properties of the cladding layer were analyzed, and the mechanism of crack elimination was investigated. With the increase of heat input, the grain size and thickness of the α lath increased, while the amount of α' decreased, and the oxidation phenomenon had also improved. The sizes of columnar β-crystal grains and spherical β-crystal grains both increased. The microstructure of the different heat input layers was composed of lamellar α phase and acicular α′ phase with different sizes. Besides, the reduction of the cooling rate inhibited the unidirectional growth of acicular martensite, so that the martensite arrangement was close to a disordered state, and the deformation of the cladding layer was more uniform, which effectively prevented the cracks and melt expansion. The corrosion resistance of the coating was improved, and the microhardness was reduced.

The purpose of this research project is to generate an algorithm that allows predicting roughness, an important factor for measuring the surface finish of machined parts and thus establishing adequate machining parameters that have an impact on manufacturing cost time. The research carried out uses analysis of experiments based on the ANOVA and Taguchi methodology, the data were generated from tests carried out in a machine with computerized numerical control on specimens of AISI P20 martensitic steel material, the machining process was carried out on a convex surface with the application of finishing strategies arranged in a CAM software, there were 27 tests, with ZIGZAG, ZIGZAG Concentric and ZIGZAG RADIAL strategies, with cutting speed between 70 and 110 m/min and a feed per tooth that varies from 0.05 to 0.15 [mm/tooth]. With the analysis of the data, the following equation Ra = 2.1720–0.2829A + 0.6417C is generated, where A is the strategy and C the advance, the validation of the algorithm was carried out by contrasting the theoretical value of 1.335 [µm] and the value measured in the specimen of 1.311 with an error of 3.4% between the 2 values. By having the algorithm validated, it is possible to establish a desired roughness and verify machining times and their impact on the final cost of the product.

The roles of pre-formed martensite (PM) in below-Ms bainite formation, microstructure, crystallography, strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction, transmission electron microscopy, micro digital image correlation technique and mechanical tests. It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms, indicative of its acceleration effect at the early stage of transformation. This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite, with initial bainite tending to form around the PM. However, the finishing time for below-Ms bainite transformation, especially at even lower temperatures, is retarded, owing to the dividing effect of PM on parent austenite grains, the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite. PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite. The specimens with PM show relatively uniform strain partitioning among various phases, contrasting with the specimens without PM, for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite (M/A) blocks or between adjacent M/A blocks. The impact absorption energies of the specimens with PM, when austempered at 30–60 °C below Ms, are more than twice higher than those of the specimens without PM, at no expense of tensile properties.

A two-fold approach is considered to study hydrogen (H) diffusion characteristics in martensitic steels. Initially, a multi-trap stress coupled H diffusion finite element model was developed to investigate the role of various trap states on effective H trapping during a four point bend test. The calculations show that high angle boundaries are more influential in controlling H diffusion in presence of low initial (bulk) H concentrations, while dislocations can have more pronounced impact, when the bulk H concentration is higher. A microstructural model comprising of prior austenite grains and packets was further developed. The study highlights the importance of packet boundaries (PBs) moderating H diffusion in martensite microstructure. The presence of retained austenite content affecting H diffusion paths was also studied. Overall, this parametric study presents complementary techniques in numerical modeling, as well as implications on the role of various microstructural entities affecting H diffusion.

The development of ultrastrong maraging stainless steels (MSSs) is always in high demand. However, traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic (BCC) lath martensitic matrix. Here, we design a novel ultrahigh strength MSS (Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B, wt.%) using a cluster formula approach. A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-NiAl nanoprecipitates (3−5 nm) in BCC martensitic matrix was successfully obtained. This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa, but also a decent uniform elongation of 4.2%−5.1%, which is almost triple of the value observed in existing MSSs. We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit high-performance MSSs.

The in-situ nanoscale TiC particles reinforced Ti6Al4V composites was prepared by selective laser melting (SLM) from the mixture of Ti6Al4V alloy powders and graphene powders, the microstructure and interface bonding of the composites were studied. The composites were mainly composed of α, α' martensite and α+β structures. More TiC were formed in the overlapping regions than that in the molten pool, the solidification rate in overlapping regions was slow, which was beneficial to TiC nucleation and growth. The TiC and Ti matrix displayed following orientation relationships: axis [0 0 1]β-Ti∥[-1 -1 1]TiC∥[0 -1 1 0 ]α-Ti, lattice plane (2 2 2)TiC //(0 0 0 2)α-Ti, lattice plane (1 1 1)TiC//(0 0 0 2)α-Ti, lattice plane (1 1 1)TiC//(1 0 -1 0)α-Ti. The TiC and Ti matrix have a small degree (6.2%) of mismatch between (1 1 1)TiC and (0 0 0 2)α-Ti, and the semi-coherent interface was occurred. TiC preferentially nucleated and grew up along its (1 1 1) plane. The situ synthesized TiC was added into the Ti6Al4V alloy, refined the grains, and increased the microhardness of the alloy. The microhardness of the composites was 13.6% higher than that of Ti6Al4V alloy. The TiC effectively pinned the dislocations and grain boundaries were responsible for the enhanced mechanical properties of the composites.

Acoustic emission (AE) of 316L stainless steel with of low Ni content shows, under tension, simultaneously three avalanche processes. One avalanche process relates to the movement of dislocations, the others to martensitic transformations and detwinning/twinning. Detwinning/twinning occurs predominantly at the early stage of the plastic deformation while martensitic transformations only become observable after large plastic deformation. All processes coincide during deformation with variable effect on AE. An excellent fingerprint for the detection of the coincidence between the several mechanisms is the observation of multivalued E ~ A2 correlations. AE signals from moving dislocations decay more slowly (~ 7×10−3s) and show long avalanche durations. In contrast, AE signals during martensitic transformations and detwinning/twinning decay rapidly (

This work highlights a new process for direct manufacturing of ductile iron that is based on the innovative combined technology of quenching and partitioning, and low-temperature-transformation of nano bainite as used for high strength steels. In this process, a commercial unalloyed ductile iron is austenitized at 890 °C for 20 min, then rapidly quenched to 180 °C for 5 s, and finally partitioned at 220 °C for 240 min. A maximum tensile strength of more than 1600 MPa, a hardness of 55 HRC at an elongation in excess of 5%, and the number of repeat tensile fatigue failure of 2.5 × 104 cycles at a stress amplitude of 600 MPa, are achieved. This is comparable to those of a high strength carbon alloy steel. These properties are due to the synergistic strengthening and toughening effects of a multiphase structure comprising tempered martensite, bainitic ferrite and retained austenite in the matrix, and a spherical graphite morphology. This work provides a guidance on how to produce unique steel-like ductile iron with a high level of strength and acceptable toughness.

Copper/steel is a typical bimetal functional material, combining the excellent electrical and thermal conductivity of copper alloy and the high strength and hardness of stainless steel. There has been recent interest in manufacturing copper/steel bimetal by directed energy deposition (DED) due to its layer-by-layer method. However, cracks tend to form on the copper/steel interface because of the great difference in thermal expansion coefficient and crystal structure between copper and steel. In this work, interfacial characteristics and mechanical properties of the copper/steel bimetal were studied from one layer to multilayers. The laser power has a great influence on the Cu element distribution of the molten pool, affecting the crack formation dramatically on the solidification stage. Cracks tend to form along columnar grain boundaries because of the Cu-rich liquid films and spherical particles in the cracks. Crack-free and good metallurgical bonding copper/steel interface is formed at a scanning velocity of 800 mm/min and the laser power of 3000 W. The ultimate tensile strength (UTS) and the break elongation (EL) of the vertically combined crack-free copper/steel bimetal are 238.2 ± 4.4 MPa and 20.6 ± 0.7%, respectively. The fracture occurs on the copper side instead of the copper/steel interface, indicating that the bonding strength is higher than that of the Cu-Cr alloy. The UTS of the horizontally combined crack-free copper/steel bimetal is 746.7 ± 22.6 MPa, which is 200% higher than that of the Cu-Cr alloy substrate. The microhardness is 398.6 ± 5.4 HV at the steel side and is 235.3 ± 64.1 HV at the interface, which is 400% higher than that of the Cu-Cr alloy substrate. This paper advances the understanding of the interfacial characteristics of heterogeneous materials and provides guidance and reference for the fabrication of multi-material components by DED.

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450-460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

Welding processes were accomplished by means of a 600 W Continuous Wave (CW) fiber laser instrument in bead on plate manner with 60, 80 and 100 J mm−1 heat inputs. δ-TRIP Steel samples were similar in chemical composition and only contained different Nb (40Nb: 0.040 & 72Nb: 0.072 wt.% Niobium) amounts. Elevation of heat input resulted in changes in microstructure and hardness properties, which are desirable for welders to guarantee safe joints in engineering structures. An increase in the heat input can lead to reduction of the austenite in weld metals of both steel sheets. The reduction of the austenite could be attributed to the increase in δ-ferrite volume fraction which was induced by availability of sufficient time for segregation of aluminum and formation of more dendrite structured δ-ferrite at lower cooling rates. Increase in δ-ferrite content results in decreasing austenite content required for transformation of austenite to martensite. The direct consequence of a decrease in the martensite volume fraction is reduction of hardness. It is shown that an increase in the heat input from 60 to 100 J mm−1 results in reduction of hardness difference between weld and base metals by 40% and 37% for 40Nb and 72Nb steels respectively.

In this experiment, we systematically investigated the microstructure evolution, mechanical properties, and deformation behaviors as functions of annealing temperature using cold-rolled Fe-0.05C–6Mn–1Al-1.5Si steel. Almost all annealed specimens are composed of the equiaxed and granular α-ferrite (α) grains due to recrystallization, reversed austenite (γ) grains because of reversed transformation from deformed martensite, and fibrous δ-ferrite (δ) grains only undergoing recovery during intercritical annealing except for 760 °C. It is interesting that several annealing twins are formed within γ grains. The outstanding combination of strength and ductility is obtained for annealing at 740 °C. The tensile strength and total elongation are 980 MPa and 32.1%, which are attributed to the ultrafine grains and optimal TRIP effect associated with rational γ content and stability. A phenomenon that yield point elongation (YPE) gradually shortens with increased γ content and decreased γ stability is observed. What's more, the mechanism that active α′ formation can promote the improvement of work hardening rate and further curtail YPE is verified using deep-cryogenic treatment. Based on SEM observation, microscopic strain of δ grains appears nearly consistent with matrix (α and γ grains) during tensile process, although δ phase presents as fibrous morphology with coarse size.

The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

A dual phase structure consisting of ferrite and martensite has been achieved in Ti-bearing steels microalloyed with Mo or Cu, respectively, which were treated under different austenitizing and hot deformation conditions to investigate the effects of second microalloying elements and thermomechanical processes on the microstructural evolution and mechanical properties. The results showed the largest microhardness values, 343 HV0.1 for Ti–Mo steel and 307 HV0.1 for Ti–Cu steel, in microalloyed steels austenitized at 1200 °C with 30% deformation strain. Furthermore, critical patent analysis revealed that higher tensile strength is achieved in a Ti–Mo steel having a larger Ti/Mo atomic ratio, but no such relationship between tensile strength and Ti/Cu atomic ratio applies to Ti–Cu steel. This finding can be ascribed to the different metallurgical effects of Mo and Cu in the Ti-bearing steels. For example, the addition of Mo in a Ti-bearing steel leads to tiny carbides due to the decrease in coarsening via the synergistic effect of Ti and Mo. On the other hand, the separate precipitation of TiC and Cu in the Ti-bearing steels is consistent with the results of patent analysis. That is, the tensile strength of these steels is not associated with the Ti/Cu atomic ratio.

The traditional welding processes are challenging to obtain an excellent jointed structure with a thickness of over 80 mm. Meanwhile, cyclic heating for large-scale manufacturing components worsens microstructure and mechanical properties after welding. Laser Melting Deposition (LMD) technology is put forward to break through the limit of large-scale jointing components widely used for manufacturing aerospace products. Therefore, the LMD experiment with coaxial powder feeding for joining a pair of X-groove Ti-6Al-4V thick plates (80 mm) with different thermal behaviors (including laser scanning speeds and cooling rates) is carried out in this paper. The morphology evolution of its microstructure at the equiaxed grain zone (EQZ) and the deposition area under different thermal behaviors are systematically investigated. Meanwhile, the relationship between microstructure evolution mechanism and tensile properties is further explored. The volume fraction and size of acicular α/α' phases at the EQZ or the deposition area keep a stable rising when the laser scanning speed increase from 10mm/s to 20mm/s. The α-Ti cluster phases are usually precipitate around the β grain boundary with a low scanning speed and cooling rate. By contrast, the orthogonal acicular martensitic α/α' phase can precipitate inside the β grain, forming a Basketweave structure with a high scanning speed and cooling rate. The Basketweave structure forming in β grains can coordinate the deformation and reduce the stacking effect of dislocations, which is beneficial for improving ductility. However, the α-Ti cluster phases precipitating around the β grain boundary usually cause local stress concentration at these locations, forming microcracks at this location and deteriorating the tensile properties.

This study aims to find out the effect of welding heat inputs on microstructure and mechanical properties relationship of naval high strength steel. The physical weld thermal cycles with four different heat inputs between 10 to 200 kJ/cm were experimented using the thermomechanical simulator Gleeble®3800. The weld coarse-grained heat-affected zone (CGHAZ) microstructure was characterized in detail by using optical microscopy, field emission scanning electron microscope (FESEM), and transmission scanning electron microscope(TEM) facilities and mechanical properties such as impact toughness, hardness and tensile strength were obtained using Charpy impact, microhardness, and tensile testing facilities. It was often noticed that the impact toughness decreased with increasing heat inputs attributed to the grain and bainite lath coarsening and the formation of new phases with different morphologies in CGHAZ regions. At lower heat input, the microstructure consisted of a fully martensite structure, while at higher heat inputs consisted of granular bainite and bainitic ferrite; however, in medium-range heat inputs, it consisted of bainite and martensite. The different phases were confirmed through TEM images. The mechanical properties were found to be decreased with an increase in heat input. It was due to the formation of coarse bainite lath, granular bainite, and hard M/A constituents. The weld structure properties were established in detail.

In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

This paper aims at the phase identification and quantification in transformation induced plasticity duplex stainless steel (TDSS) base and weld metal containing ferrite, austenite, and martensite. Light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis were employed to analyze phases. Samples were either mechanically or electrolytically polished to study the effect of the preparation technique. Mechanical polishing produced up to 26% strain-induced martensite. Electrolytic polishing with 150 g citric acid, 300 g distilled water, 600 mL H3PO4, and 450 mL H2SO4 resulted in martensite free surfaces, providing high-quality samples for EBSD analysis. Martensite identification was challenging both with LOM, due to the similar etching response of ferrite and martensite, and with EBSD, due to the similar lattice structures of ferrite and martensite. An optimized Beraha color etching procedure was developed that etched martensite distinctively. A novel step-by-step EBSD methodology was also introduced considering grain size and orientation, which successfully identified and quantified martensite as well as ferrite and austenite in the studied TDSS. Although here applied to a TDSS, the presented EBSD methodology is general and can, in combination with knowledge of the metallurgy of the specific material and with suitable adaption, be applied to a multitude of multiphase materials. It is also general in the sense that it can be used for base material and weld metals as well as additive manufactured materials. This study received support from the EU-project H2020-MSCA-RISE-2018 Number 823786, i-Weld, and the Swedish Agency for Economic and Regional Growth through the European Union–European Development Fund

Compared with the gas metal arc welding (GMAW), the cold metal transfer technology can significantly reduce the welding heat input owing to obtain the output current of almost zero in the droplet detaching, which has a greater advantage in additive manufacturing. In this work, the cold metal transfer was employed to study the law of appearance and mechanical behavior of additively manufacturing of Ti–6Al–4V alloy. The results showed that the continuous and uniform weld appearance could be obtained when the current of 120 A–130 A was used. In the top layer, middle layer and bottom layer of thin-walled components, the same microstructure was obtained with the different parameters, which were mainly consisted of the columnar original β grains and acicular martensite α’. The max-hardness was obtained in the bottom zone (BZ) of thin-walled component. With the increase of welding current, the hardness values in BZ were decreased. With the increases of current, the tensile strength was reduced, which caused by the grain growth.

Scanning electron beam micro-polishing is a new technique to reduce surface roughness using molten metal to fill pits under gravity's action spontaneously. In this study, through the establishment of the electron beam heat source model and heat conduction model, it is easy to obtain the mathematical and physical model of heat balance for continuous scanning electron beam process under micro-melting polishing and to discuss the influence of regularity of electron beam process parameters on the surface morphology and microstructure. The results showed that the surface roughness of 45 steel was reduced after scanning electron beam processing. The lowest value was 0.34 μm with a reduction of 83%. It was found that the melt zone was mostly martensite with fine grains by further measuring and analyzing the microstructure and hardness of the polished sample. The maximum hardness was 870 HV in the subsurface layer.

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants. Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep, which is known as premature failure. However, the microstructural origin of the premature failure remains unclear. Here in this study, we have carefully investigated the microstructural transformations and their influences on creep rupture behavior, showing that the evolution of martensite and M23C6 carbides as well as Laves phase are responsible for the premature failure. By using multi-step TTP-LMP method, we confirmed a three-stage creep rupture behavior under different stress regions. Further quantitative analysis showed that the coarsening of M23C6 carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region, while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region.

TiN/Ni alloying of 38CrMoAl steel surface was carried out by continuous scanning electron beam. The effect of electron beam process parameters on the surface microstructures and mechanical properties of 38CrMoAl steel was studied. The results shows that the sample surface was divided into three regions: alloying layer, heat affected zone and substrate. Alloying layer made up with equiaxed crystals and short columnar crystals, microstructure of the heat affected zone was acicular and lath martensite, microstructure of substrate was tempered sorbate. The microhardness of the sample section first increased and then decreased, at a depth of 400 μm, the microhardness reached a maximum of 817.9 HV. When the beam current reached 12 mA and the moving speed reached 5.5 mm/s, the surface microhardness of the sample reached the highest 1209HV and the abrasion loss reached the lowest 4.6 mg.

Super ductile thermo-mechanically treated (SD TMT) reinforcing steel bars are commonly used these days for RC constructions given their excellent thermal and seismic resistant properties. The superior performance of SD TMT rebars is due to their distinct cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite, which impart better combinations of strength and ductility than conventional classes of steel. This paper presents an investigation carried out to study the influence of corrosion-induced damage to cross-sectional phase distribution (CSPD) on mechanical properties of SD TMT bars in comparison with the performance of conventional mild steel and ordinary TMT steel bars exposed to an aggressive environment. The influence of corrosion-induced damage to cross-sectional phase distribution (CSPD) on the stress–strain response of SD TMT bars has been investigated through tensile tests in a displacement controlled UTM. The study involved tensile tests carried out on SD TMT bars corroded to 25 different levels ranging from (0–45%) on five different rebar diameters (8mmO, 10mmO, 12mmO, 16mmO and 20mmO). The results suggest better characteristic performance of SD TMT steel bars over conventional hot-rolled rebars in mechanical behaviour's critical aspects. A correlation predicting the reduction of mechanical properties of corroded TMT bars' residual capacity concerning corrosion percentage has been suggested.

In this work, the hydrogen embrittlement (HE) characteristics of 1400 MPa bolt steels with three different vanadium contents of 0, 0.17 wt% and 0.34 wt% were evaluated. The characteristics of the microstructure and dislocation density of the experimental steels were analyzed, and their effects on HE were also discussed. The results showed that with increasing V content, the HE resistance of the experimental steels was improved, and the experimental steel with the highest V content possessed the best HE resistance. The V-precipitates of steels with V contents of 0.17 wt% and 0.34 wt% were reversible hydrogen traps, and the inhibitory effect of V-precipitates on hydrogen-dislocation interactions improved HE resistance. In addition, a lower dislocation density and finer martensitic structure were also beneficial for hindering hydrogen-induced cracking (HIC).

This paper develops a deep learning scheme of phase recognition for steel materials. A convolutional neural network classifier is established, such that the martensite phase, which has a substantial impact on the mechanical properties of steels, can be recognized from microstructure images and its volumetric fraction can also be estimated from multi-phase microconstituents. The testing results on an ultrahigh carbon steel dataset proved that the developed scheme has a rational phase recognition accuracy. The estimated martensite fraction can be used as an essential feature to predict the mechanical properties of materials in additive manufacturing.

The high-manganese steels are important structural materials, owing to their excellent toughness at low temperatures. However, the microstructural causes for their unusual properties have not adequately been understood thus far. Here, we report a reversal relationship between impact toughness and grain size in a high-manganese steel and its unrevealed microscopic mechanisms, which result in an excellent low-temperature toughness of the steel. Our investigations show that with increasing grain size the impact toughness of the steel can be improved drastically, especially at low-temperatures. Advanced electron microscopy characterization reveals that the enhanced impact toughness of the coarse-grained steel is attributed to the twinning induced plasticity and transformation induced plasticity effects, which produce large quantities of deformation twins, ehcp-martensite and α′bcc-martensite. Inversely, in the fine-grained steels, the formation of deformation twins and martensite is significantly inhibited, leading to the decrease of impact toughness. Microstructural characterizations also indicate that ehcp-martensite becomes more stable than α′bcc-martensite with decreasing temperature, resulting in characteristic microstructures in the coarse-grained samples after impact deformation at liquid nitrogen temperature. In the coarse-grained samples under impact deformation at -80 °C, ehcp-martensite transformation, α′bcc-martensite transformation and deformation twinning all occur simultaneously, which greatly improves the toughness of the steel.

This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.

In this paper, Al/Ti joints are produced by resistance element welding (REW). A hole is made in the Al sheet and a welding element (Ti6Al4V rivet) is inserted into the hole followed by resistance welding to create a metallurgical bond between the Ti rivet and Ti sheet. The mechanical properties, fracture mode, microstructure, and hardness distribution of the joints are analyzed. The REW joints show higher tensile-shear load and energy absorption than present riveted joints. The maximum peak load and energy absorption of the REW joints improved with the increasing of rivet diameter. The failure mode for Al/Ti REW joints transits from interfacial to pullout mode, and then to fully pullout mode with increasing welding current. The nugget microstructure mainly consists of acicular α′ phase martensite. A discontinuous intermetallic compound layer is formed at the Ti/Al interface, and it consists of TiAl2, TiAl, or a mixture thereof.

Super ductile TMT reinforcing steel bars are commonly used these days for RC constructions, given their excellent thermal properties. SD TMT rebars are superior due to their distinct cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite. This paper investigates the influence of the superimposition of corrosion-induced damage by the exposure to elevated temperatures on properties of reinforcing steel simulating corroded structural members exposed to an accidental fire. The experimental program involved exposing bare bars to accelerated corrosion to achieve desired corrosion levels in terms of mass loss using the impressed current accelerated corrosion technique to a precision level of ± 1%. Subsequently, the identical specimens were heated to 250 °C, 550 °C, 800 °C, and 950 °C and allowed to achieve a steady-state at that temperature before being cooled down to ambient temperatures in the furnace itself and tested in tension to failure. A detailed analysis of the effect of various combinations of corrosion-induced mass loss and elevated temperatures on ductility characteristics of reinforcing steel using various prevalent methods in the available literature is presented. The results indicate a notable reduction in reinforcing steel's critical mechanical properties due to the superimposition of the two phenomena.