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Atmospheric plasma sprayed metallic coatings always contain oxides because of the entrainment of ambient air into plasma jet, which significantly limits their performances and applications. This paper presents an approach to deposit highly dense Ni-based alloy coatings with oxide-free particles achieved by introducing boron as deoxidizer element through plasma spraying to generate ultrahigh temperature Ni20Cr4B droplets. During spraying, boron is preferentially oxidized and the formed B2O3 is completely removed due to rapid evaporation. Thus, the in-flight oxidation of metal can be suppressed to achieve oxide-free particles. The oxygen content of plasma sprayed Ni20Cr4B coating is lower than 0.18 wt.% and presents little change over a wide spray distance range from 60 mm to 140 mm. With such a low oxygen content, the Ni20Cr4B coating presents a highly dense microstructure (porosity of 0.21 ± 0.11%) and well-bonded inter-splat interfaces. The hot-corrosion test at 600 °C for 200 h shows that the highly dense Ni20Cr4B coating can effectively block the penetration of the corrosion media and the corrosion only takes place at the coating surface, while severe inner corrosion was observed for the traditional Ni20Cr coating with poor inter-splat bonding.

A 27Cr-4Mo-2Ni super ferritic stainless steel was hot rolled and aged at 800 °C for times ranging from 10 min to 4 h to study the microstructure, precipitation and their effects on mechanical properties by using SEM, EBSD and TEM techniques. Experimental results demonstrate that a great deal of substructure, especially shear bands is obviously observed within elongated ferrite grains after hot rolling. After aging at 800 °C, Laves, chi and sigma phases are identified, and precipitation of these phases is accelerated evidently by the substructure. Laves phase particles are observed formed at dislocations and distributed inside grains with sub-micron scale. Precipitates of chi and sigma phase are found at grain boundaries and along shear bands, as well as around the TiN particles. Recovery during 800 °C aging is impeded by the precipitation of Laves phase at dislocations and chi phase at shear bands. Tensile ductility and micro hardness are significantly affected by the precipitation of intermetallics. The breakage pattern after tensile test transformed from ductile to brittle fracture when the aging time increases from 10 min to 4 h. Keywords: Ferritic steel, Aging treatment, Precipitation, Microstructure, Embrittlement

Thermosetting polymer materials with excellent mechanical properties are highly promising in cryogenic engineering applications. However, the research on the toughening mechanism of thermosetting materials at cryogenic temperatures is always difficult. Herein, one type of high strength hyperbranched epoxy composite was successfully fabricated, and the correlation between microstructure (free volume) and mechanical properties of the HPB/epoxy composites was studied. To disclose the correlations between mechanical properties and microstructures for these composites, positron annihilation lifetime (PAL) measurements were carried out. Thereafter, better impact and compression properties were observed for composites with smaller free volume holes. Meanwhile, the impact and tensile properties of the composites are positively correlated with the free volume holes fraction at 77 K. The correlation between microstructure and mechanical properties provide valuable information for regulating the cryogenic mechanical properties of thermosetting materials in the future.

Inspired by the excellent mechanical performance of bone materials characterized by compact shell and porous core, this study proposes a lightweight, mechanically robust, and thermal-insulating polyarylene ether nitrile (PEN) with a bone-like structure. The supercritical fluid technique was applied to provide PEN with two types of tunable bone-like structures, which makes it simple to manipulated under supercritical treatment conditions. Depending on different bone-like microstructures, their tensile strength can reach up to 75 MPa with the impact strength range of 50–190 KJ/m2. Notably, the impact strength of PEN foam with a saturation time of 4 h is 1.6% higher than that of pure PEN. Furthermore, lightweight PEN foams capable of thermal insulation and their conductivity can reach 0.06 W/m.K without compromising the actual mechanical performance. These lightweight PENs with a remarkable performance in thermal insulation and mechanical robustness show a massive potential in automobile application for effectiveness in energy conservation and reducing air pollution. Based on this experiment, a new method is proposed to prepare biomimetic bone-like materials.

Predicting location specific properties of industrial castings is a critical part of the Integrated Computational Materials Engineering (ICME) framework for casting design and manufacturing. In this paper, a new design methodology is presented to calculate and predict location specific mechanical properties of an aluminum casting based on its location specific microstructure. Cellular Automaton (CA) and Finite Element Analysis (FEA) tools are coupled to model the mechanical response of a cast component from the casting process conditions to its final location specific solidification microstructure. This new CA-FEA methodology can bridge multiple length scales and predict location specific microstructure and mechanical properties of an aluminum wedge casting, which was experimentally validated using X-ray Micro Computed Tomography and mechanical testing.

In this study, the microstructure and corrosion resistance of Mg-6Gd-2Y-χNd-0.2Zr (χ = 0, 0.5, 1 and 1.5 wt%) alloys in extrusion condition are investigated using the correlative microscopic observations, immersion tests and electrochemical measurements. Results show that the addition of Nd can affect the grain size, crystal orientation and secondary phase of the alloys. The corrosion resistance of Mg-6Gd-2Y-χNd-0.2Zr alloys increases with the increase of Nd content from 0% to 0.5 wt%, and then decreases from 0.5 wt% to 1.5 wt%. A minor 0.5 wt% addition of Nd can improve the corrosion resistance of Mg-6Gd-2Y-0.2Zr alloy. The grain size of the four alloys is in a range from 4 μm to 7 μm and the 1.5Nd alloy with the smallest grain has the highest corrosion rate. The highest corrosion resistance of 0.5Nd alloy is attributed to the highest intensity of (0001) basal texture and the less amount of secondary phases. However, the higher fraction of secondary phase is the main reason for the highest corrosion rate of 1.5Nd alloy. Keywords: Magnesium, Intermetallic compounds, Corrosion, Grain size, Crystal orientation

Cold spray is capable to additively manufacture oxide-free metallic parts in open air due to its low processing temperature. However, for metals with relatively high hardness such as Inconel 718 superalloy (IN718), it is still a big challenge to get dense deposits by using low-priced nitrogen gas. In this work, In-situ micro-forging (MF) was introduced to cold spray by mechanically mixing big-sized 410 stainless steel (410SS) particles into the IN718 powder so that the deposited IN718 layer can be hammered and plastically deformed by the MF particles during spraying and results in fully dense deposits. As 50 vol% MF particles were mixed into the IN718 powder, the porosity was decreased from 5.6% to 0.26%. Due to the low impact velocity, 410SS particles were not embedded into the IN718 deposits and the possible contamination was avoided. An oxide scale removal induced increment in deposition efficiency was detected for the first time. Due to the lower porosity and enhanced inter-particle bonding, a great improvement in ultimate strength from 96 to 464 MPa was achieved by the in-situ MF effect. After heat treated at 1200 °C for 6 h, the sample fractured at a high strength of 1089 MPa and revealed a ductile fracture manner. Keywords: IN718 alloy, Additive manufacturing, Cold spray, In-situ micro-forging, Porosity, Mechanical properties

The globularization behavior of the basketweave microstructure in TC17 alloy is quantitatively analyzed during deformation and subsequent heat treatment. Collaborative behavior in the α lamellae and β phase evolution are proposed to affect globularization based on the transmission electron microscopy (TEM) and high-resolution electron backscatter diffraction (EBSD) observations as follows: (i) the interleaved α lamellae restrain the rotation of neighboring α lamellae and promote the formation of low-angle boundaries (LABs) and grooves, and then (ii) the β phase has a positive effect on facilitating the complete separation of α lamellae during deformation. During heat treatment, (iii) the formation of the LABs in the β grain interiors accelerates the appearance of necklace-like α grains, and (iv) the α lamellae promotes the break-up of β phase while the β phase has a positive effect on facilitating the complete separation of α lamellae. In other words, Collaborative behavior in α lamellae and β phase evolution promotes the globularization of TC17 alloy containing a basketweave microstructure. Except for the boundary splitting and termination migration, the mechanism of substructure rotation also participates in the globularization of TC17 alloy containing a basketweave microstructure, which is different to that of the alloys containing a colony microstructure. Keywords: Titanium alloys, Globularization process, Collaborative behavior, Transmission electron microscope, Electron backscatter diffraction

The influence of laser parameters and alloying elements on the dilution rate of FeCoCrNi coating was investigated. It was found that the alloying elements have more significant effect on the dilution rate than the laser cladding parameters. Optical microscopy, scanning electron microscopy, X-ray diffraction analysis, micro-hardness test, wear test, electrochemical corrosion test, and high-temperature oxidation test were conducted to study the influence of dilution rate on the microstructure and properties of the FexCoCrNi coatings. The results have shown that the transfer of Fe element from the substrate could make the mixed entropy of the Fe0CoCrNi coating close to the theoretical mixed entropy of the FeCoCrNi high-entropy alloy. Excess Fe element in the original powder increased the dilution rate, leading to a reduction in the mixed entropy of the coating. However, it did not lead to the generation of new phases in the coating. The atomic radius differences between the elements could lead to large lattice distortion in the FCC crystal lattice. The level of lattice distortion and micro-hardness of the FexCoCrNi coatings increased with increasing Fe element content. Furthermore, the excess Fe elements weakened the self-passivation ability and deteriorated the high-temperature oxidation resistance of the coating. Keywords: Dilution rate, FexCoCrNi, FCC crystal lattice, Lattice dislocation, Self-passivation ability, High-temperature oxidation

The low temperature solid deposition characteristics of cold spraying technology are conducive to the remanufacturing/repair of oxidation sensitive titanium alloy in the atmosphere. Ti6Al4V alloy deposits prepared by cold spray (CS) usually show porous microstructure due to insufficient plastic deformation of Ti6Al4V alloy particles. The interfacial bonding and the microstructural evolution mechanisms of Ti6Al4V particles during cold spray are still unclear. In this study, the Ti6Al4V deposits were prepared by cold spray with nitrogen as process gas at a temperature of 800 °C and a pressure of 4 MPa. The micron CT results show that the porosity is 8.25%. The deformation ultrafine grain zone and un-deformation martensitic laths zone at the bonding interface between Ti6Al4V particles were observed. The EBSD results show that the grain orientation signal was only detected at the edge of the particles deformation area and the 23% of grain sizes detected by EBSD are less than 500 nm, which indicates that the nanocrystallization induced by dynamic recrystallization occurs at the inter-particle boundaries between cold-sprayed Ti6Al4V particles. The TEM and SADPs demonstrate a gradient structure consisting of equiaxed nanograins zone, crushing dendrites transition zone and original martensitic laths zone from the interface to the core of particles.

Microcellular silicone rubber/nanographite foam with the smallest cell diameter and the largest cell density compared to the previous studies was prepared using supercritical carbon dioxide as an environmentally benign foaming agent in this study. Both the cell microstructure and properties of porous composite were investigated in detail. In silicone rubber/nanographite system, the incorporation of nanographite into silicone rubber significantly induced heterogeneous bubble nucleation and enhanced the matrix strength, which resulted in the decreased average cell diameter from 4.97 to 1.12 μm and the increased cell density from 8.87 × 108 cells/cm3 to 1.25 × 1010 cells/cm3. The microcellular rubber foam with decreased cell size, increased cell density and narrow cell distribution was corresponded to the excellent compressive strength and higher hardness, which reduced the content of silica. Furthermore, the thermal decomposition temperature of the obtained foam increased from 380 °C to 413 °C due to the introduction of nanographite. The silicone rubber/nanographite foam with improved performance possessed potential in wide applications. Keywords: Nanographite, Silicone rubber, Microcellular foam, Supercritical carbon dioxide, Thermal and mechanical performance

The ball milling method was used to prepare CeMg12/Ni composite hydrogen storage materials with nanocrystalline and amorphous structure in order to improve the electrochemical hydrogen storage and kinetic properties of alloy materials. The phase composition and crystalline structure were characterized by X-ray diffraction (XRD) and high resolution transmission electronic microscopy (HRTEM). The electrochemical discharging performances were measured though discharge capacity and cycling stability. High rate dischargeability (HRD), electrochemical impedance spectroscopy (EIS) and hydrogen diffusion behavior were utilized for studying the change of electrochemical kinetics. Results indicate that adding Ni significantly strengthens the glass forming ability of alloy samples in the process of ball milling. The maximum discharging capacity of the ball-milled composites monotonously goes upward from 46.7 to 1061.2 mAh/g with an increment in Ni content from 50 to 200 wt%. The increasing Ni content results in an obvious reduction in thermodynamic parameters (including enthalpy change ΔH and entropy change ΔS) of ball-milled composites. Besides, the addition of Ni speeds up the hydrogen diffusion rate inside the alloy. Meantime, the addition of Ni can also diminish the activation energy, which is helpful for ameliorating the electrochemical reaction kinetics on the surface of alloy. Keywords: Cycling stability, Thermodynamic stability, Charge transfer reaction, Hydrogen diffusion coefficient, Activation energy

Ti-6Al-4V matrix composites with 5 vol% TiB and TiC particles were in situ synthesized in this study. Hydrogen was added into the composites directly by melting the composites in gas mixture of hydrogen and argon (Melt Hydrogenation Technology). Microstructure observation results showed that hydrogen addition changed the distribution of ceramic particles to near network structure. The effect of microstructure, deforming temperature, strain rate and hydrogen content on high temperature compressive behavior were investigated. Thermal simulation results of high temperature compression at same strain rate indicated that hydrogen addition caused softening effect at higher deforming temperature and hardening effect at lower temperature, competition of the dominant effect was decided by distribution of ceramic particles and microstructure of matrix. Experimental results of hot compression at same deforming temperature and increasing strain rate indicated that moderate hydrogen content decreased the peak stress and improved the hot workability of composites. Microstructure observation of as compressed composites indicated that hydrogen eliminated defects and encouraged the dynamic recrystallization (DRX) after hot compression, the mechanism of hydrogen induced improvement of DRX, and relationship among hydrogen content, dislocations density and percentage of DRX were discussed. Keywords: Titanium matrix composites, Melt hydrogenation, High temperature deformation, Dynamic recrystallization, Dislocation

Aluminum alloy 5052 sheets (thickness: 1 mm) were joined using a novel method (resistance spot clinching) that combines mechanical clinching and resistance spot welding processes. A pair of molds combine with processing tapes were exerted in this process. The microstructure of the fusion zone under novel process presents more portion of equiaxed dendritic zone compared with that of the resistance spot welded joint, which is due to the different temperature gradient feature during solidification. The mechanical properties of the joints were evaluated via orthogonal trials of tensile shear tests performed on the joints. The results revealed that, even under lower heat input, the load-bearing capacity of the resistance spot clinching joints is superior to that of the traditional resistance spot welding joints. Digital image correlation was used to evaluate the distortion behavior of the weld joints during testing. Compared with the stiffness of traditional joints, the higher stiffness of the resistance spot clinching joint generated a higher tensile stress component in the periphery of the fusion zone during testing (which resulted in a higher peak load). Keywords: Resistance spot clinching, Resistance spot welding, Aluminum alloy, Digital image correlation, Deformation behavior

In this study, the microstructure, mechanical properties and fracture behavior of hastelloy C276-BNi2 brazed joint were investigated. The results show that microstructure and mechanical properties of the joints are greatly affected by the test temperature and inner brazing defects. The brazed joint consists of four regions, i.e. filler zone, fine grain diffusion zone, coarse grain diffusion zone and parent zone. Because of element diffusion in brazing, the diffusion zone becomes harder and the filler zone becomes softer. Compared with base metal, both the yield strength and elongation of brazed joint are relatively small. The filler metal plays a key role in determining the mechanical properties of brazed joint. The specimen fractured in the filler zone with ductile mode by stepped pattern. Because of the existence of brazing defects between grain boundaries, the fracture was accelerated, both the yield strength and ultimate tensile strength of defect-containing joint were decreased compared to the defect-free joint. Keywords: Hastelloy, Brazed joint, Microstructure, Mechanical properties, Fracture

In this study, the influence of solutionizing and aging treatments on the microstructure and mechanical properties of alloy 625 superalloy samples produced by powder bed binder jet printing was investigated. Vacuum melted argon atomized 625 superalloy powder was used to fabricate green parts. Samples vacuum sintered at 1280 °C for 4 h to optimize density to 99.6% were subjected to solution and aging treatments at 1150 °C for 2 h and 745 °C for 20 h and 60 h, respectively, in order to improve microstructure and mechanical properties. Elemental analysis, phase formation and microstructure of the sintered, solution treated and aged samples were investigated by using optical, scanning/transmission electron microscopy with electron dispersive spectroscopy, and X-ray diffraction. The results indicate that the aging treatment resulted in the formation of intermetallic phases such as Ni3Nb and Ni2(Cr,Mo) as well as carbides such as NbC, Cr23C6 in the microstructure. While the microhardness and tensile strength of the aged specimen increased due to these phases, the ductility value decreased. This study shows that alloy 625 produced by binder jet printing subjected to aging treatments has comparable properties to cast alloy 625. Keywords: Additive manufacturing, Inconel 625, Heat treatment, Tensile test, TEM, Precipitation

Pitting corrosion is a common localized corrosion phenomenon, which can lead to cracks and mechanical failure in structural metal materials. On the contrary, pitting corrosion could be a beneficial tool for generating large-area porous structures, which holds a great premise in a number of functional services, such as catalysis, sensing, storage, imprint lithography, and membranes. Herein we presents an electrochemical approach for creating a large-area honeycomb-like porous structure in Zr-based metallic glasses. A pitting process followed by subsurface tunnel etching in NaCl solution elicits to characteristic micrometer scale channels and nanometer size amorphous sidewalls decorated by Cu nanoparticles on the metallic glass substrate. A root-shape growing mechanism of tunnels initiated from pits and penetrating into alloy matrix is postulated. In addition, the effect of alloy composition on the microstructure of honeycomb-like porous metallic glasses is also investigated in detail.

A parametric study was used to investigate the feasibility of dissimilar Al/Cu joining by a thermo-compensated resistance spot welding process. The tensile shear properties of the joints were found to be affected by both the weld's physical properties and metallurgical factors: (1) With increasing heat input, the nugget size and indentation increased, which led to the failure mode of the joint to change from interfacial failure to pull-out mode. (2) There was a critical heat input at which the microstructure changed in the fusion zone. At normal heat input, the fusion zone consisted of a significant amount of α-Al solid solution and a thin transition layer (α-Al/θ-CuAl2 eutectic to θ-CuAl2) at the Al/Cu interface, producing a high nugget's shear strength. At high heat input, the fusion zone mainly consisted of Al–Cu intermetallic compounds (IMCs), producing a low nugget's shear strength. The fractograph revealed cleavage features in the interfacial failure mode and dimples in the pull-out mode, which implied that the crack propagated respectively in the IMC zone and the periphery of the nugget. The Al/Cu thermo-compensated resistance spot weld joints presented a tensile shear peak load comparable to that of Al/Al joints, though their ductility was deteriorated by the IMCs. Keywords: Resistance spot welding, Thermo-compensated method, Aluminum alloys, Copper, Dissimilar metal joining, Tensile shear properties

AISI 201 stainless steel plate was friction stir processed (FSP) at rotation speeds in the range of 500–1000 rpm with traverse speed of 100 mm/min, and rotation speed of 1000 rpm with traverse speed of 50 mm/min using W-Re stirring tool. The microstructural evolution was studied by optical microscopy, scanning electron microscopy, electron backscattered diffraction, electron probe micro analyzer and transmission electron microscopy. The results suggested that the final grain structure in the stir zone (SZ) originated from the competition between continuous recrystallization (CDRX) and discontinuous recrystallization (DDRX). The grain size and the content of δ-ferrite in SZ were closely related to the processing parameters, and a band structure contained 6.5% δ-ferrite formed. Furthermore, the δ-ferrite formed in SZ significantly promoted the occurrence of DDRX and strong (111) [112] annealing texture formed in δ-ferrite. No sigma phase was observed in the SZ, and the density of dislocations and sub-structures was found to be seriously affected by the processing parameters and locations of SZs. The relationship between mechanical properties and structure is elucidated. Keywords: Friction stir processed, AISI201 stainless steel, Grain structure, Recrystallization, δ-Ferrite, Sigma phase, Mechanical properties

Experiments were designed to compare the rollability of magnesium alloy AZ31 sheet in two rolling schedules at the same rolling temperatures: multi-pass electroplastic rolling (ER) and warm rolling (WR). While cracks occur in WR sheet at an accumulative true strain of −0.24, the sheet can be rolled by ER up to an accumulative true strain of −1.09 without edge-cracking. This rollability enhancement in ER sheet was found to be related to the improved homogeneity of plastic deformation and dynamic recrystallization (DRX), which postpones the crack initiation. A ‘twinning - shear banding - DRX’ microstructure evolution model was proposed to explain the effect of pulsed electric current on the homogeneity of deformation and subsequent DRX. The use of ER sheet could pave the way for low-cost applications of lightweight magnesium components. Keywords: Magnesium alloy, Rolling, Shear band, Dynamic recrystallization, Pulsed electric current

Thermal stabilization (TS) and directional solidification (DS) experiments were performed on Ti-43Al-3Si alloy in a Bridgman-type furnace. The effects of thermal stabilization time on the microstructure evolution of mushy zone and initial interface of directional solidification were investigated. The results show that the volume fraction of liquid in mushy zone and the length of mushy zone decrease as the thermal stabilization time increases. The concentrations of Al and Si exhibit a decreasing trend in the complete liquid zone with increasing TS time, while the solute concentrations of Al and Si in complete liquid zone are always higher than those of the original as-cast alloy. A long TS time promotes solute diffusion and provides a solute homogeneous initial interface for subsequent DS process. Isolated α grains and Ti5Si3 particles in mushy zone tend to coarsen with the increase of thermal stabilization time. Long time TS treatment also leads to growth of Ti5Si3 particles which may act as nucleation sites to terminate the continuous growth of dendrite and inheritance of the lamellar orientation. Directionally solidified samples with aligned lamellar structures can be obtained within 30 min TS treatment in Ti-43Al-3Si alloy. Keyword: Microstructure evolution, TiAl alloy, Mushy zone, Thermal stabilization, Directional solidification

A new process to prepare the electroformed nickel without chloride ion activating the anode in the solution is employed in this study, where the sulfur nickel is used as anode. The concentration of nickel sulfamate and boric acid and the pH value were measured during the electroforming process, as well as the anodic passivation curve. The microstructure was observed by means of optical microscope (OM), transmission electron microscope (TEM), and the mechanical property was investigated by tensile experiment. The results show that the concentration of nickel sulfamate and boric acid remains stable with the values of 425 g/L and 35 g/L respectively during the process of electroforming while the pH value keeps stable. The electroforming process could continue without chloride ion as the activator of the anode. The grains of electroformed nickel are very fine and their average size is 5 μm. The elongation of electroformed nickel without annealing treatment reached up to 20%, and the tensile strength was 460 MPa. In this case, electroforming without chloride ion as the activator of the anode is an outstanding method for the electroformed nickel with the excellent mechanical property. Keywords: Electroformed nickel, Without chloride ion, Anodic behavior, High plasticity

High Mn content added to Mg–6Zn–0.3Ca extruded alloy remarkably refined dynamically recrystallized (DRXed) grains, and enhanced tensile and compressive strengths without sacrificing ductility. The large amount of fine Mn precipitates of homogenized alloy that formed effectively restricted new grain growth during the subsequent extrusion and generated ultra-fine DRXed grains. The ultra-fine DRXed grains and large amount of precipitates essentially enhanced the strength and improved the anisotropy of alloy. The Mg–Mn–x ternary systems that potentially formed ultra-fine grains were also discussed. Keywords: Metals and alloys, Microstructure, Mechanical properties, Ultra-fine grains

The effects of alpha grain size on the flow stress, the apparent activation energy for deformation (Q) and the processing maps of Ti–6Al–4V with an equiaxed microstructure are thoroughly investigated using isothermal compression tests, and detailed explanation is given based on the microstructure observation and quantitative analysis. The shapes of flow curves are dependent on the microstructure characteristic of the alloy before deformation and during the deformation process. The flow stress increases with increasing equiaxed alpha phase, but decreases with increasing alpha grain size. The Q-values for dr1 and dr2 are smaller than those for dr3 and dr4, respectively, which is possibly attributed to that Ti–6Al–4V alloy for dr1 and dr2 which exhibits a very strong grain-boundary sliding (GBS) mode besides dominant dislocation glide/climb mechanism. The local efficiency maxima and unstable regions in processing maps change with the alpha grain size, which implies that proper hot-working domains should be modified in different grain size range so as to meet the precision forging process. Keywords: Ti–6Al–4V alloy, Flow stress, Apparent activation energy for deformation, Processing maps, Microstructure

Microstructural evolution of Al2024 alloy, consisting of precipitates, subjected to surface mechanical grinding treatment (SMGT) was investigated to reveal the role of strain and strain rate in structural refinement. Following SMGT, a gradient nanostructure was formed on the coarse-grained substrate, with a significant increase in nanohardness to over 1 GPa at the top surface. At low strains, the presence of particles and increasing strain rate stimulated the rapid transformation of dislocation configurations. However, at larger strains, increasing strain rate effectively stabilized the fine nanoscale structure. Strain gradient from the deformed layers and near the particles significantly promoted the development of microstructure by increasing the boundary misorientations, which was more significant at high strain rates. In particular, at very high rates and large strain gradients, extreme grain sizes may be induced. The presence of minority grains with size below 10 nm in the top 20-μm layers revealed that dislocation processes still operated at sub-nanoscale level. The dislocation-governed ductility was expected to be improved in the nanostructured materials. Keywords: Nanostructured materials, Grain refinement, Strain rate, Strain, Aluminum alloys

Intergranular twinning shear transmission through twin-twin accommodation, which plays an important role on microstructure evolution, is often observed in deformed hexagonal metals. The shear transmitted between two connected twins is m′s, where m′ is the geometrical compatibility factor and s is the magnitude of the original shear. However, the quantitative correlation between the grain boundary misorientation (GBM) and m′ factor for shear transmission is unknown. In the present study, we developed an m′-GBM map to tackle this problem. The map is applied to {11 2 ¯ 2}–{11 2 ¯ 2}, {11 2 ¯ 2}–{11 2 ¯ 4} and {11 2 ¯ 4}–{11 2 ¯ 4} twin pairs in Ti, which have rarely been reported before but are profuse in a cryorolled state. Favorable GBM for the formation of these twin pairs is revealed by the map. Specifically, the map shows that low GBM angles ( 2 ¯ 2}–{11 2 ¯ 2} and {11 2 ¯ 4}–{11 2 ¯ 4} twin pairs. The map also reveals that both the {11 2 ¯ 2}–{11 2 ¯ 2} and {11 2 ¯ 2}–{11 2 ¯ 4} twin pairs tend to occur at grain boundaries with media angles (15°–45°), depending on the location of the corresponding GBM axis in the m′-GBM map. These predictions were then verified by experimental analysis.

An efficient powder metallurgy route including wet dispersion, high energy ball milling (HEBM) and spark plasma sintering (SPS) was proposed to fabricate carbon nanotube reinforced molybdenum‑hafnium‑carbon (CNT-reinforced MHC) composites. To improve the dispersion efficiency of carbon nanotubes (CNTs) in the molybdenum (Mo) matrix, acid treatment was used to de-agglomerate CNTs. Carbon nanotube/hafnium hydride (CNT/HfH2) powder prepared by wet dispersion and the powder subsequently milled with Mo through HEBM to fabricate MHC composite powder. The results show that this powder metallurgy process improved the dispersion uniformly of HfC and grain size. The strengthening mechanism of dispersion strengthening, grain refinement, lattice distortion and purification of free oxygen improved the relative density, microstructures and mechanical properties of the CNT-reinforced MHC composites. As a result, compared to other control groups and Mo based composites, the 30 h ball milling time of CNT-reinforced MHC composite (30 h composite) exhibits simultaneous enhancement in relative density, mechanical properties and improvement in microstructure. This work provides a feasible way to fabricate high-performance Mo matrix composites.

To establish the relationship between mechanical properties and microstructural features, it is necessary to comprehensively describe microstructural characterizations. In present study, an efficient and innovative quantitative method of interleaved degree in lamellar microstructure was developed. Firstly, four types of microstructural variables were defined as key factors that confirmed the microstructural features. Lath length (L) and variation coefficient for distribution of lath angle (CV) were measured to obtain dimension information of laths, and number of lath intersecting points per area (N) and uniformity of lath intersecting points (U) were defined to describe the lath distribution and the lath intersection complexity. Then, the microstructural variables were normalized based on multiple attribute decision-making. Finally, the equation of interleaved degree in lamellar microstructure was established based on entropy weight method. Present quantitative method was applied to analyze the interleaved degree of TC17 alloy containing a basketweave microstructure. It was shown that interleaved degree was higher at a deformation temperature of 930 °C, a height reduction of 40% and a strain rate of 0.1 s−1 and decreased with increasing cooling time. Besides, a model that can theoretically calculate fracture toughness based on tensile properties was utilized to prove the prediction ability of interleaved degree. Keywords: Titanium alloys, Interleaved degree, Quantitative characterization, Lamellar microstructure, Entropy weight method

The present work systematically investigates the effects of starting microstructure and intercritical annealing temperature on the tensile properties and Lüders strain of transformation-induced plasticity steels containing 5 wt.% Mn. It is found that higher intercritical annealing temperature leads to smaller Lüders strain, lower yield strength and higher ultimate tensile strength. The starting cold-rolled microstructure produces much coarser microstructural constituents and larger Lüders strain than the martensitic one. It is concluded that the fraction and size of austenite grains and the amount of carbide formed during intercritical annealing are the most important microstructural factors to determine the Lüders strain rather than the fraction of retained austenite and the grain size of the ferritic phase. Keywords: Lüders strain, Medium Mn TRIP steel, Cottrell atmosphere, Retained austenite, Yield point elongation

In this work, large sized cubic BN (cBN) reinforced nanocomposites are prepared by cold spray deposition of blended powder mixtures of mechanically alloyed 40 vol.% cBN–NiCrAl nanocomposite particles and large sized cBN particles. Deposition behavior of the blended powders, microstructure and mechanical properties including hardness, fracture toughness and two-body dry abrasive wear behavior of the sprayed composites, are investigated. Results show that dense composites with large cBN particle content of 13–20 vol.%, depending on composition of the spray powders, can be obtained. Declining deposition efficiency is detected as increasing large cBN content due to the gradually enhanced sand blasting effect. Increasing content of large cBN in spray powder results in an increment in hardness and a decrement in fracture toughness. Abrasive wear resistance of the 40 vol.% cBN–NiCrAl nanocomposite is doubled by incorporating 20 vol.% of the large cBN particles. Worn surface morphology observation indicates that the improvement is due to the shielding effect of the large protruding cBN particles against SiC abrasive. Material removal mechanism of the sprayed composites during abrasive wear test is also studied. Keywords: Large cBN particle, Nanocomposite matrix, Cold spray, Deposition behavior, Abrasive wear behavior

The impact of electromagnetic stirring (EMS) upon the microstructure and mechanical properties of the Al/Ti resistance spot weld was investigated. The microstructures in the Al/Ti resistance spot weld joint were characterized by comparing them to those in an Al/Al resistance spot weld joint, and the effects of the welding current, welding time, and electrode force upon the tensile shear properties were also studied. The microstructure in the traditional Al/Ti joint consisted of a partially melted zone, a columnar grain zone, and a transition structure. Under the action of EMS, a fine spheroidal grain structure formed in the Al/Ti joint and, compared with the traditional Al/Ti resistance spot weld, the weld created under the EMS effect exhibited a larger bonding diameter, and higher tensile shear force and energy absorption. The large bonding diameter and fine spheroidal grain structure should account for the superior mechanical performance of the Al/Ti EMS joint. Keywords: Resistance spot welding, Al/Ti dissimilar materials, Electromagnetic stirring, Microstructure, Mechanical properties

The present paper reports the microstructure, phase stability and mechanical properties of a new refractory MoNbHfZrTi high-entropy alloy. MoNbHfZrTi alloy consists of a disordered body-centered cubic (BCC) solid solution phase in as-cast and homogenized states. Homogenization treatment and DSC analysis indicate that there is no any phase transition below 1743 K. At room temperature, the compressive yield strength for the alloy in as-cast and as-homogenized states reaches about 1719 MPa and 1575 MPa, respectively and the fracture mechanism is brittle quasi-cleavage fracture. At elevated temperatures, this alloy has compression yield strength of 825 MPa at 1073 K, 728 MPa at 1173 K, 635 MPa at 1273 K, 397 MPa at 1373 K and 187 MPa at 1473 K and some fine grains form at grain boundaries due to partial dynamic recrystallization.

This study aims to investigate durability and microstructure of fluidized bed fly ash and metakaolin based geopolymer exposed to elevated temperatures and acid attack. Geopolymer specimens were prepared by combination of fly ash and metakaolin activated by sodium silicate and sodium hydroxide solutions and were cured in microwave radiation environment plus a heat curing period. Compressive strength and several key durability parameters for geopolymer and ordinary Portland cement (OPC) were assessed and compared. Microstructure formation and development was characterized in terms of morphology and pore structure as well as simulation. The experimental results reveal a dense microstructure of geopolymer compared to OPC. In terms of resistance to the acidic solution and elevated temperatures, geopolymer is superior to OPC as indicated by the relatively lower strength loss and lower mass change. Compressive strength shows a dramatic drop in OPC while geopolymer shows a strength increase after 400 °C. The mass loss curves of geopolymer are similar to OPC, but it shows relatively lower mass loss compared to OPC. The result of saturated water absorption after 28 days curing indicates less water absorption in geopolymer before and after thermal and acid exposure. Durability of geopolymer is demonstrated by monitoring the pore structure.

Thermo-compensated resistance spot welding (RSW) with a Zn interlayer was designed to join AA5052-H12 (Al) aluminum alloy to AZ31B (Mg) magnesium alloy. A stainless steel tape placed between the upper electrode and the Al sheet acts as a heat barrier to prevent heat loss from the spot nugget on the Al side. During welding, more Al was melted to increase the percentage of Al in the liquid nugget. In the resultant Mg/Zn/Al joints produced by thermo-compensated RSW, the formation of Mg–Al brittle intermetallic compounds is restricted. The tensile–shear properties of the weld joint were improved significantly. The failure load of the Mg/Al joints was up to 2199 N, compared with the 833 N and 727 N that is produced by traditional Mg/Al RSW and Mg/Al RSW with Zn interlayer, respectively.

Explosive welding is widely applied to joining Ti with steel in recent years. However, joining the explosion-bonded CP-Ti/Q345 bimetallic sheets has not been thoroughly investigated. In this paper, explosion-bonded CP-Ti/Q345 bimetallic sheets were applied as base metal and intermediate materials were employed to join the bimetallic sheets. Based on the element metallurgical compatibility, Cu/V based filler metals (solid wire and flux-cored wire) were designed as intermediate layers for TIG welding method. To determine the feasibility of these filler metals, microstructures were analyzed by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness tests were used to compare the brittleness of the intermetallics in the weld joints. Tensile strength test was performed to evaluate the strength of the joint. The results showed that the feasibility of the Cu/V based solid wires was poor due to metallurgical incompatibilities. The joint filled with Cu/V solid wires suffered severe cracks. Such defects reduced if a Cu–V flux-cored wire was used. It was attributed to continuously distributed microstructures in weld joint and low heat input with the Cu–V flux-cored wire.

The mechanical behavior and microstructure evolution of directionally solidified twinning-induced plasticity (TWIP) steel were investigated in the present study. It is found that the directionally solidified steel showed improved mechanical properties and different strain hardening behavior along the longitudinal direction of columnar grains compared to traditional steel with equiaxed grains. The ultimate true stress and true strain were as high as 1.13 GPa and 72%, respectively. As the plastic deformation proceeded, a series of complex changes took place in the microstructures including multi-slips, dislocation pile-ups, formation of sub-grains, continuously increased twins and etc. These changes are responsible for the different strain hardening behavior of the present steel. From the microstructure observation, it is demonstrated that the twinning process in the directionally solidified TWIP steel was delayed due to relatively easy slip of dislocations in the columnar grains with very few transverse boundaries.

The effect of Cu with low contents of 10, 12, 15 wt.% on the microstructure and melting point of Al–Si–Cu–Ni alloy has been investigated. Results showed that low-melting-point properties of Al–Si–Cu–Ni alloys with low contents of Cu were attributed to disappearance of Al–Si binary eutectic reaction and introduction of Al–Si–Cu–Ni quaternary reaction. With raising Cu content from 10 to 15 wt.%, the amount of complex eutectic phases formed during low temperature reactions (Al–Cu, Al–Si–Cu and Al–Si–Cu–Ni alloy reactions) increased and the melting temperature of Al–Si–Cu–Ni filler metals declined. Brazing of 6061 aluminum alloy with Al–10Si–15Cu–4Ni (all in wt.%) filler metal of a melting temperature range from 519.3 to 540.2 °C has been carried out successfully at 570 °C. Sound joints can be obtained with Al–10Si–15Cu–4Ni filler metal when brazed at 570 °C for holding time of 60 min or more, and achieved high shear strength up to 144.4 MPa.

A series of Zr-Al-Ni-Cu bulk metallic glasses (BMGs) with network structures were carefully designed and their heterogeneous microstructures were investigated by carefully etching the cross sections of these BMGs. It is found that the heterogeneous microstructures of these BMGs can be uncovered by etching with mixed solution of HF and HNO3. Some of the studied Zr-based BMGs characterize in micrometer network structure. The cell size of the network structure depends on the composition of the BMG and is related with the fractural mode and mechanical property. The found network structure of the Zr-based BMGs benefits for understanding the unique mechanical properties of the Zr-based BMGs. (C) 2014 Elsevier Ltd. All rights reserved.

This paper investigates the effect of electromagnetic stirring (EMS) on the nugget formation, microstructure, and mechanical properties of magnesium alloy resistance spot weld (RSW). The EMS was induced by an external magnetic field (EMF), which produced by a pair of permanent magnets. The results showed that the EMS drove the molten metal in the nugget to flow clockwise and generated a centrifugal movement, which brought high temperature molten metal from the center of the growing nugget to its edge. Compared with the traditional magnesium alloy RSW, the weld under the EMS effect (EMS-RSW) exhibited larger weld diameter, higher tensile shear force and energy absorption, and finer equiaxed crystal and columnar crystal. The EMS-RSW had a higher hardness ratio of fusion zone to pullout failure location, which indicated that the EMS-RSW was more sensitivity to experience pullout failure. Finally, an empirical failure load formula for magnesium alloy resistance spot weld was developed.

The microstructure, texture, thermal conductivity and mechanical properties of the as-extruded Mg–5Zn–1Mn (ZM51) magnesium alloy were investigated on specimens with the extrusion direction (ED), the transverse direction (TD) and the normal to the extrude plane (ND), respectively. The results indicated that the thermal conductivity of ZM51 alloy at room temperature is 125 (W/m K), almost twice as high as other conventional commercial magnesium alloys, such as Mg–Al series and Mg–RE series. The effect of texture on anisotropy of mechanical properties and thermal conductivity has been analyzed. The strong crystallographic texture typical of Mg alloys results in much higher yield strength and tensile strength (UTS) in the extrusion direction, but higher ductility and thermal conductivity in the transverse direction.

Effects of 0–2.1 at.% Si additions on microstructure and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–7.5 at.%Nb–4 at.%Mo–2 at.% Sn (Ti–7.5Nb–4Mo–2Sn), were investigated. The alloys after annealing at 973 K mainly contain β and α″. As the concentration of Si is higher than 1 at.%, Ti5Si3 particles can be found in the alloys, and the number density of the particles increases with the increasing of silicon’s concentration. The addition of Si promotes the strength of the Ti–7.5Nb–4Mo–2Sn due to the Si solid solution strengthening effect and fine Ti5Si3 precipitates. However, as the Si concentration reaches 2.1%, the alloy exhibits a brittle fracture. The 0.5–1.6 at.% Si additions improve the superelasticity of the Ti–7.5Nb–4Mo–2Sn alloy by increasing the critical stress for inducing martensite (σSIM).

In the present study, diffusion bonding of Ti–6Al–4V alloy (TC4) and Oxygen Free Copper (OFC) by using a 0Cr18Ni9 stainless steel interlayer was investigated. The addition of 0Cr18Ni9 stainless steel interlayer improved the bonding quality. The element distribution, microstructure, compositions, mechanical properties and microhardness change of the bonded samples were studied by means of electron probe micro-analyzer (EPMA), scanning electron microscope (SEM), X-ray diffraction system (XRD) and microhardness. The results show that Ni 3 Ti, Fe 2 Ti, TiCr 2 and FeTi formed at the TC4/0Cr18Ni9 interface show better mechanical properties than that of Cu–Ti compounds. Solid solutions are formed at the 0Cr18Ni9/OFC interface. The fracture of the joints had taken place through the TC4 and Ti–Fe compounds. The reaction process at the interface of TC4/0Cr18Ni9/OFC joints consisted of the physical contact stage, the generation of intermetallic compounds and the growth of intermetallic compounds. The tensile strength of joints increased firstly and then decreased with the rising bonding temperature. The sample bonded at 880 °C for 30 min under 5 MPa shows the maximum tensile strength of 140.3 MPa. There is a substantial improvement in the bonding quality, when its tensile strength is compared with 48.32 MPa for directly-bonded TC4/OFC joint.

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.

A double control forming technology of motorcycle wheel was firstly developed to form the complex parts with high mechanical properties. The mechanical properties and microstructure of the AZ91D magnesium alloy motorcycle wheels formed by double control forming and die casting was compared. The results of numerical simulation showed that the velocity magnitude in wheel rib was small than that in other positions. The pressure and temperature decreased from the wheel hub to the overflow launders. Large surface defect concentration indicated the oxides and entrapments existed mainly in the well-designed overflow launders. The experimental results confirmed that the motorcycle wheel’s complex shape can be achieved by the high-speed filling in the injection procedure and the mechanical properties of the parts were greatly improved due to the forging pressure. It is clear from the micrographs that the high pressure caused by forging system results in obvious grain refinement of the primary α-Mg and uniform distribution of the eutectic consisting of the eutectic α-Mg and β-Mg 17 Al 12 . Higher mechanical properties including ultimate tensile strength of 246 MPa, elongation of 9.5% and Vicker-hardness of 87.9 were obtained in the parts formed by double control forming due to the fine and uniform microstructure without defects in comparison to die casting.

The possibility and appropriate processing parameters of diffusion bonding of Mo and Cu using a Ni interlayer was investigated. The microstructure of the bonded joints was studied by SEM, EPMA and XRD, and the main factors affecting diffusion bonding process were analyzed. The results showed that the solid solutions formed in both Mo–Cu and Cu–Ni interfaces during the diffusion bonding process, and no intermetallic compound appeared. The thickness of Ni layer decreased with the bonding temperature raising. The fracture of the joints had taken place in the Mo/Ni interface rather than the Ni/Cu interface. With the increasing of bonding temperature or holding time, the tensile strengths of the joints increased firstly and then decreased. The Mo/Ni/Cu joint bonded at 800 °C for 30 min exhibited a maximum value of tensile strength of 97 MPa.