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In this study, a high-strength maraging stainless steel (CX stainless steel) was prepared using selective laser melting (SLM) technology. Solution aging was employed to enhance the microstructure and mechanical properties of the printed part. The heterogeneous microstructures of CX steel were studied, which mainly consisted of martensitic laths with high-density dislocations and numerous cellular substructures. Precipitation of NiAl with an average particle size of 12 nm and the occurrence of alumina inclusions with an average size of about 70 nm were observed. A high ultimate tensile strength of 1647 MPa, micro-hardness of 520 HV0.2 and a good elongation of 8.9% was achieved for CX stainless steel. Moreover, the strengthening mechanism and crack propagation law are analyzed from the perspective of microdefects, such as fine grains, high density dislocations, cellular structure, nano-precipitates and inclusions. [ABSTRACT FROM AUTHOR]

The strength of ultra-low carbon maraging stainless steels can be significantly enhanced by precipitating nanoscale intermetallic secondary phases. Retained or reversed austenite in the steel can improve its toughness, which is key to achieving an ideal combination of strength and toughness. Ti and Al are often used as cost-effective strengthening elements in maraging stainless steels but the synergistic toughening and strengthening mechanisms of Ti and Al have not been studied. To investigate the synergistic toughening and strengthening mechanisms of Ti and Al in Co-free maraging stainless steels, this paper focuses on the microstructure and mechanical properties of three alloys: Fe-12Cr-11Ni-1.7Al-0.5Ti (Steel A), Fe-12Cr-11Ni-0.5Ti (Steel B), and Fe-12Cr-11Ni-1.7Al (Steel C). The impact of Ti and Al on the microstructure and mechanical properties was investigated using X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), and thermodynamic simulations. The relationship between microstructure, strength, and toughness is also discussed. The results indicated that Steel A, containing both Al and Ti, exhibited the highest strength level after solution treatment at 900 °C, with an ultimate tensile strength reaching 1571 MPa after aging at 540 °C. This is attributed to the simultaneous precipitation of spherical β-NiAl and rod-shaped η-Ni3Ti phases. Steel B, with only Ti, formed a significant amount of Ni-rich reversed austenite during aging, reducing its ultimate tensile strength to 1096 MPa. Steel C, with only Al, showed a high strength–toughness combination, which was achieved by forming dispersive nano-sized intermetallic precipitates of β-NiAl in the martensitic matrix with a slight amount of austenite. It is highlighted that Al has superior toughening and strengthening effects compared to Ti in the alloy system. [ABSTRACT FROM AUTHOR]

Periodic electric current pulses, i.e. electropulses, were applied on selective laser melting prepared 15-5 precipitation hardening stainless steel for direct aging. Traditional aging treatment typically takes hours to achieve peak hardening and could induce reverted austenite. Herein, electropulsing treatment on as-built sample can dramatically reduce the processing time to 6 min. Moreover, the nucleation of reverted austenite triggered by the segregation of Ni atoms on the interfaces between Cu-rich precipitates and martensite was not observed. These two unique phenomena were rationalized to the unsynchronized enhancement of diffusivity of Cu and Ni atoms under electropulses due to their different electron configurations.

Periodic electric current pulses, i.e. electropulses, were applied on selective laser melting prepared 15-5 precipitation hardening stainless steel for direct aging. Traditional aging treatment typically takes hours to achieve peak hardening and could induce reverted austenite. Herein, electropulsing treatment on as-built sample can dramatically reduce the processing time to 6 min. Moreover, the nucleation of reverted austenite triggered by the segregation of Ni atoms on the interfaces between Cu-rich precipitates and martensite was not observed. These two unique phenomena were rationalized to the unsynchronized enhancement of diffusivity of Cu and Ni atoms under electropulses due to their different electron configurations. Electropulsing treatment selectively boosted the diffusivity of Cu atoms in additive manufactured maraging stainless steel and dramatically reduced the direct aging time from hours to minutes without inducing reverted austenite. [ABSTRACT FROM AUTHOR]

The evolution of the microstructural and mechanical properties of maraging stainless steel specimens with or without sub-zero treatment was examined in the present study. Through various microstructural analysis techniques, the primary microstructural characterization after various heat treatments can be summarized as follows: First, the amount of reversed austenite increased monotonically as the aging temperature increased. Second, the lattice strain of the martensite matrix was increased by sub-zero treatment but later relieved by the aging process due to the recovery effect of the martensite matrix. Furthermore, the fraction of high-angle grain boundaries (HAGBs) grew when the aging treatment was executed at higher temperature, which was ascribed to the merging of laths and blocks. Third, numerous η-Ni3Ti particles having a hexagonal closed-packed (HCP) crystal structure and adopting a Burgers orientation relationship with martensite matrix precipitated with homogeneous distribution in the steels. In addition, the mechanical properties of the tested maraging stainless steel were also consistent with the microstructural features of the specimens. For instance, in comparison with specimens without sub-zero treatment, specimens with sub-zero treatment had higher ultimate tensile strength (UTS) and yield strength (YS). These differences can be primarily ascribed to lattice strain, i.e., dislocations, introduced in the matrix upon sub-zero treatment. That lattice strain led to precipitation strengthening in the steel due to the greater number of nucleation sites facilitating the precipitation of η-Ni3Ti particles. [ABSTRACT FROM AUTHOR]

The effect of cobalt on the precipitation hardening behavior of the maraging stainless steels was studied. The hardness test indicated that cobalt addition could advance peak aging time and generate a higher peak hardness. The statistical results of nano-size precipitates distribution demonstrated that cobalt increased the density of precipitates, thus resulting in a stronger precipitation hardening contribution. Based on the first-principles calculations and atom probe tomography (APT) results, a possible mechanism has been proposed to reveal the underlying mechanism: cobalt promotes the nucleation of precipitates by decreasing the diffusion activation energy of Ni and Ti.

Hydrogen embrittlement (HE) behavior strongly threatens the safety of metallic structural materials. In this study, the HE resistance of Custom 465 maraging stainless steel is dramatically enhanced by long-term aging process. The influence of long-term aging on the hydrogen-assisted cracking behavior of the maraging stainless steel was thus unveiled using slow strain rate tensile test (SSRT), x-ray diffraction (XRD), and electron back-scattered diffraction (EBSD). The results showed that, compared with the peak-aged steel, the long-term aged (over-aged) ones possessed higher volume fraction of reversed austenite. A certain amount of austenite observed in the crack tips and microvoids of long-term aged steel acted as dual effective traps of both cracks and diffusible hydrogen atoms. Besides, the ripening of intermetallic η-Ni3Ti precipitates in long-term aged steel relaxed the stress concentration during SSRT and reduced the density of deleterious reversible hydrogen traps, which synergistically improved the cracking resistance of the matrix. The crystallographic orientation analysis of the hydrogen-induced cracks demonstrated that, in the long-term aged steel, higher fraction of low-angle grain boundaries (LAGBs) with lower kernel average misorientation (KAM) value and fraction of Σ3 coincidence site lattice (CSL) grain boundary favored the retardation of hydrogen-assisted crack propagation. [ABSTRACT FROM AUTHOR]

Slow strain rate tensile tests under hydrogen cathodic charging are conducted on 17-4 PH steel with two surface conditions: mirror polished and ball-burnished. In both cases, significant subcritical cracking initiating at the surface is observed leading to considerable reduction in elongation to fracture. However, ball-burnished specimens show better elongation and much less secondary cracking than the polished ones. Ball-burnishing introduces high compressive residual stresses in the specimen sub-surface. However, EBSD showed a very limited impact of ball-burnishing on the microstructure, so little effect on hydrogen trapping is expected. The beneficial effect of ball-burnishing on the resistance of the hydrogen assisted cracking is mainly explained by the high compressive longitudinal stress at the specimen surface, which makes crack initiation more difficult and hence delays specimen failure. In addition, it is estimated that the amount of hydrogen introduced at the specimen surface is decreased by approximately 30% due to the high compressive hydrostatic stress. [Display omitted] • Ball-burnished specimens are mechanically tested in hydrogen-rich environment. • Ball-burnishing results in high compressive residual stress and limited roughness. • A better resistance to HE is observed when specimens are ball-burnished. • The introduction of CRS at the surface delays the specimen failure. [ABSTRACT FROM AUTHOR]

The effect of aging on transformation behavior of reverted austenite and impact toughness in Co-free maraging stainless steel were investigated via thermodynamic calculation, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Excluding the film-shaped austenite growing along the phase interface, other shapes of reverted austenite are evolved from the growth and aggregation of acicular austenite in the range of 300-600 °C. Under N-W orientation relationship, {111}γ grows inside the martensite lath along <100>α, and austenite merges in <100>α and <110>α simultaneously. Under K-S orientation relationship, the growth direction of {111}γ is 60° or parallel to <112>α. From 300 to 500 °C, Ni prefers to diffuse into η-Ni3Ti and matrix. The precipitation of Ni3Ti hinders the formation of reverted austenite and significantly deteriorates the toughness. Above 500 °C, due to the coarsening of Ni3Ti and the recovery of matrix, the resistance to the formation of austenite is obviously weakened, and then austenite plays a leading role in the improvement of toughness. When the aging temperature reaches 600 °C, the dissolution of Ni3Ti promotes the formation of austenite and η-Ni3Ti changes to γ'-Ni3Ti. The interaction between Ni3Ti and reverted austenite essentially depends on the diffusion behavior of Ni. [ABSTRACT FROM AUTHOR]

A high strength maraging stainless steel (MSS) was developed utilizing multiple nanoprecipitates and reverted austenite. After aging treatment, this steel shows remarkable mechanical properties with a yield strength of ∼1750 MPa, a tensile strength of ∼1910 MPa, and a total elongation of ∼10.5 %. The precipitation and strengthening mechanisms of multiple nanoprecipitates were clarified. The precipitation sequence upon aging can be identified as: Ni-rich cluster→ Ni-rich cluster + Mo-rich cluster→ Ni 3 Ti + Mo-rich phase→ Ni 3 Ti + Mo-rich phase + α′-Cr. Ni-rich cluster and Mo-rich cluster is the precursor of Ni 3 Ti and Mo-rich phase, respectively. The Ni-rich clusters, Mo-rich clusters, Ni 3 Ti and α′-Cr, contribute to the precipitation strengthening mainly by shearing mechanisms, among them the ordered strengthening plays the most significant role. Upon aging, Ni 3 Ti is wrapped by Mo-rich phase, restraining their growth. In contrast, the Mo-rich phase can be significantly coarsened, leading to a shift in the precipitation strengthening from dislocation shearing to Orowan looping mechanism. The reverted austenite improves the ductility of the MSS via a transformation induced plasticity (TRIP) effect. [ABSTRACT FROM AUTHOR]

After solid solution at 850 °C, the Custom 455 was aged at 420 °C for different times. The hardness changes were measured using a Vickers hardness tester, and the microstructure and composition of the second phase were studied using atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM). The APT results indicate that in the early stage of aging, Cu atoms first segregate and attract Ti to form CuTi clusters; As the aging time increases, Ti atoms in CuTi clusters attract Ni and form CuTiNi clusters, resulting in a rapid increase in sample hardness; Subsequently, CuNiTi clusters grew and gradually separated into Cu-rich and NiTi-rich particles; As the aging time further increases, some NiTi-rich particles grow into rod-shaped Ni3Ti precipitates, while others have Si atoms and grow into spherical G phase particles. The orientation relationship between Cu-rich phase, Ni3Ti phase, and G phase is: [100]M//[0–11]Cu//[-12–10]Ni3Ti//[100]G, (011)M//(111)Cu//(0004)Ni3Ti//(022)G. The formation of the Cu-rich, Ni3Ti, and G phase resulted in a hardness peak of 538 HV after 16 h of aging. Subsequently, the decrease in hardness caused by the coarsening of the Cu-rich and Ni3Ti particles was offset by the newly formed G phase, resulting in a slow decrease in hardness. [ABSTRACT FROM AUTHOR]

In this research, we studied the influence of process parameters on the quality of selective laser melting of 18Ni300 maraging steel. The effects of laser power and scanning speed on the relative density and hardness of 18Ni300 were studied by single-factor experiment and the orthogonal experimental method. The relative optimal process parameters of 18Ni300 were obtained when the layer thickness was 0.03 mm, and the hatch space was 0.1 mm. The microstructures and mechanical properties of the samples formed under different process parameters were characterized. The results showed that the optimal hardness and relative density of the sample were 44.7 HRC and 99.98% when the laser power was 230 W and the scanning speed was 1100 mm/s, respectively; the microstructure of the material was uniform and dense, exhibiting few pores. Some columnar crystals appeared along the boundary of the molten pool due to vertical epitaxial growth. The orientation of fine grains at the boundary of the molten pool was random, and some coarse columnar crystals in the molten pool exhibited a certain orientational preference along the <001> orientation. In the case of optimal process parameters, the SLM-formed 18Ni300 was composed of 99.5% martensite and 0.5% retained austenite; the indentation hardness was distributed in the range of 3.2–5 GPa. The indentation modulus was between 142.8–223.4 GPa, exhibiting stronger fluctuations than the indentation hardness. The sample's mechanical properties showed obvious anisotropy, while the tensile fracture characteristics exhibited necking. The tensile fracture morphology was ductile, and large equiaxed dimples and holes could be observed in the fiber area, accompanied by tearing characteristics. [ABSTRACT FROM AUTHOR]

This work focuses on the structure and properties of a laser powder bed-fused (LPBF) precipitation hardening stainless steel and its chemically analogous wrought counterpart, both subjected to an identical combination of solution and aging treatments with the objective of maximizing the material hardness. It was observed that both the LPBF and wrought alloy follows similar evolution of their phase composition, microstructure, and mechanical properties throughout the different stages of the technological workflow. After a solution treatment at 850 ◦C for 0.5 h and an aging at 525 ◦C for 2 h, both alloys achieve their highest hardness of ~50 HRC. Notwithstanding this similarity, the LPBF alloy offers a finer microstructure and a lower amount of retained austenite than its wrought counterpart. This microstructure provides comparable strength characteristics to both the LPBF and wrought alloys, while offering a significantly higher ductility to the former as compared to the latter. The elongation at break of the LPBF alloy is, however, strongly build orientation-dependent, with a measured anisotropy of 23%. This anisotropy is caused by the presence of processing-induced pores (average pore size ~23 µm) in the LPBF alloy preferably distributed in planes oriented perpendicular to the build direction. [ABSTRACT FROM AUTHOR]

The mechanical properties and microstructure of tested steel strengthened via Al at different solution and aging temperatures were studied by mechanical property test, optical microscope and transmission electron microscope. The results show that the tensile strength and yield strength of tested steel reach up to 1876MPa and 1762MPa respectively, with the higher strength and better match of toughness. Fine lath martensite with high-density dislocations is formed after solution and the precipitation of dispersive NiAl phase from matrix makes the strength greatly improved during the aging. The strength of tested steel decreases rapidly after reaching peak due to the coarsening of particles, namely, overaging phenomenon occurs. The optimum comprehensive mechanical properties of tested steel can be obtained after solution treatment at 820℃, cold treatment at -70℃ and aging treatment at 540℃.

[Display omitted] • The effects of a novel functionally-graded material on the compressive performances of LPBF Corrax maraging stainless steel lattices were investigated. • The FGM-V lattice exhibited higher SEA than the FCCZ and cubic lattices due to the effective alleviation of strain localization. • The specific energy absorptions of LPBF Corrax lattices were comparable to those of LPBF Ti-based lattices in the literature. The effects of a functionally-graded material (FGM) designed with two types of unit cells, cubic and face-centered cubic with Z-axis struts (FCCZ), on the compressive performances and fracture mechanisms of powder bed fusion-laser beam\metals (PBF-LB\M) Corrax maraging stainless steel lattices were investigated. FGM lattices with gradient directions parallel and vertical to the compressive loading were respectively designated as FGM-P and FGM-V lattices. The results showed that the FCCZ lattice exhibited higher compressive properties than the cubic lattice did. The fracture modes of FCCZ and cubic lattices were respectively ∼45° shear fracture and layer-by-layer fracture. The FGM-V lattice exhibited higher specific energy absorption at 50 % strain than the FCCZ and cubic lattices did by 7.6 % and 19.4 %, respectively. This phenomenon can be attributed to the effective alleviation of strain localization in the FGM-V lattice postponing the fracture. Furthermore, the specific energy absorptions of PBF-LB\M Corrax lattices were comparable to those of PBF-LB\M Ti-based alloy lattices in the literature. Thus, the PBF-LB\M Corrax lattice appears to be a potential low-cost material for high-performance and lightweight structural applications. This lattice design extends the flexibility of additive manufactured lattices and provides a specific guideline for further improving their mechanical performances. [ABSTRACT FROM AUTHOR]

PH13-8Mo is a precipitation hardened martensitic stainless steel, known for its high strength but also for its high sensitivity to hydrogen embrittlement. The aging condition of the alloy is known to play a key role in the mechanical behavior of the as-hydrogen-charged state. Yet the exact effects of residual trapped hydrogen on this behavior were not explored. In this study a novel simple methodology utilizing the small punch test (SPT) and thermal desorption analysis (TDA) is developed. It is used to investigate the effects of hydrogen trapping at different aging conditions on the mechanical properties and fracture behavior of PH13-8Mo steel. The effects of hydrogen were tested at the fully-aged (550 °C for 4 h) and over-aged (600 °C for 4 h) conditions. Samples were cathodically hydrogen charged and were tested in both the as-charged condition and also after holding at ambient condition for specific delay times. SPT load-displacement behavior and fractographic scanning electron microscopy (SEM) examinations were performed. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses were used as complementary characterization tools. It is shown that the over-aged condition (related specifically to the reverted austenite content), in contrast to the general notion, exhibits an embrittling effect on the mechanical behavior due to residual trapped hydrogen. The significance of these findings and their relation to the microstructure are discussed. • The effect of trapped hydrogen is demonstrated for PH13-8Mo. • By combining TDA and SPT hydrogen trapping effects were quantified. • The combined microstructure and hydrogen sensitivity is described in details. [ABSTRACT FROM AUTHOR]

The influence of austenitizing temperature on the microstructure and mechanical properties of an Fe-Cr-Ni-Mo-Ti maraging stainless steel was investigated. The grain size, Laves phase, and retained austenite in steels given different solution treatments were observed using optical microscopy, scanning electron microscopy, and x-ray diffraction. Relationships with mechanical properties were measured by tensile testing. The grain growth rate was relatively slow at temperatures of 800-1000 °C then rapidly increased at higher temperatures. Low-temperature austenitization augmented the retention of austenite, the fraction of which decreased with an increase in austenitizing temperature. The Laves phases, which precipitated in austenite during austenitization at 800-900 °C, were preserved after quenching. The solution treatment had a significant effect on the final tensile properties of the steel after aging, due to the presence of retained austenite and Laves phase reducing its strength. [ABSTRACT FROM AUTHOR]

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Super-high-strength maraging stainless steel (MSS) is of great interest for the next generation aircraft. The ultrasonic fatigue behavior was studied for a 1.9 GPa grade ultra-high-strength MSS with a composition of 13Co-13Cr-3.5Mo-4.5Ni-0.5Ti (at. %). The ultra-high cycle fatigue limit was 560 MPa. Cracking initiated on the surface or at non-inclusion interior sites. A crack initiation and propagation model was proposed through a discussion of the fracture mechanism: (i) the surface defect-induced failure was caused by machining marks with a critical size of 0.6 μm; and (ii) the interior matrix induced failure was caused by a defect larger than 3.3 μm. The fatigue number (N f) of the surface induced failures ranged from 6 × 104 to 1.4 × 105, while that of the interior induced failures was much longer, ranging from 8.6 × 106 to 3.2 × 108. [ABSTRACT FROM AUTHOR]

Abstract Hierarchical microstructure engineering is an efficient design path for ultra-high strength steels. An excellent example of this is maraging stainless steel, which achieves its high-performance by combining the hierarchic martensitic microstructure and nano-sized precipitates. Relating this complex microstructure with mechanical properties, e.g. strength, is not trivial. In the present work, we therefore explore the relationship between the hierarchic microstructure, evolving with the tempering of a Cu-containing maraging stainless steel 15–5 PH, and its strength. Comprehensive microstructure characterization, including the quantification of dislocation density, effective grain size, precipitates and retained austenite fraction is performed after quenching and tempering at 500 °C. The microstructure data is subsequently used as input for assessing the evolution of individual strength contributions and thus the increase in strength of tempered martensite contributed by Cu precipitation strengthening is evaluated. It is found that the Cu precipitation and dislocation annihilation are two major factors controlling the evolution of the yield strength of the tempered martensite. The Cu precipitation strengthening is also modelled using our previous Langer-Schwartz-Kampmann-Wagner model based predictions of the Cu precipitation, and modelled precipitation strengthening is compared with the evaluated Cu precipitation strengthening from the experiments. The work exemplifies the promising approach of combining physically based precipitation modelling and precipitation-strengthening modelling for alloy design and optimization. However, more work is needed to develop a generic predictive framework. [ABSTRACT FROM AUTHOR]

Predicting the mechanical properties of additively manufactured parts is often a tedious process, requiring the integration of multiple stand-alone and expensive simulations. Furthermore, as properties are highly location-dependent due to repeated heating and cooling cycles, the properties prediction models must be run for multiple locations before the part-level performance can be analyzed for certification, compounding the computational expense. This work has proposed a rapid prediction framework that replaces the physics-based mechanistic models with Gaussian process metamodels, a type of machine learning model for statistical inference with limited data. The metamodels can predict the varying properties within an entire part in a fraction of the time while providing uncertainty quantification. The framework was demonstrated with the prediction of the tensile yield strength of Ferrium® PH48S maraging stainless steel fabricated by additive manufacturing. Impressive agreement was found between the metamodels and the mechanistic models, and the computation was dramatically decreased from hours of physics-based simulations to less than a second with metamodels. This method can be extended to predict various materials properties in different alloy systems whose processstructure- property-performance interrelationships are linked by mechanistic models. It is powerful for rapidly identifying the spatial properties of a part with compositional and processing parameter variations, and can support part certification by providing a fast interface between materials models and part-level thermal and performance simulations. [ABSTRACT FROM AUTHOR]

An anisotropic microstructure and mechanical properties are big challenges in the development of various laser powder bed fusion (LPBF) alloys. This study investigated the roles of heat treatment and building direction (BD) on the microstructure and anisotropic mechanical properties of LPBF Corrax maraging stainless steel. The effects of solution treatment (ST) and integrated solution-aging treatment (SAT) were clarified. The results show that the grain size of martensite, amount of austenite, and features of grain boundaries were slightly varied with the building direction due to the thermal history. In the as-built state, the weak <111> α′ ||BD, <1 1 ‾ 0> α′ ||X, and <001> α′ ||BD textures could be found. After the SAT process, the <1 1 ‾ 0> α′ ||X texture was slightly intensified due to the coarsening of large columnar grains. However, the texture of the SAT sample was still weak. Furthermore, the building direction and heat treatment did not lead to obvious anisotropic tensile properties or change the ductile fracture mode. The weak texture and pores in LPBF Corrax did not dominate the tensile properties. Irrespective of sample state, the horizontally-built samples exhibited comparable strengths and slightly higher elongation than the vertically-built ones did. In the as-built condition, this phenomenon can be mainly attributed to the transformation-induced plasticity effect. In the ST and SAT conditions, smaller grain sizes of martensite and higher high-angle grain boundary ratios in the horizontally-built samples provided more resistance to crack propagation. LPBF Corrax maraging stainless steel exhibited superior tensile performances and low anisotropic tensile properties, which are very beneficial to the stability of the material during service. [ABSTRACT FROM AUTHOR]

A new maraging stainless steel with superior strength-toughness-corrosion synergy has been developed based on an innovative concept of alloy design. The high strength-toughness combination is achieved by forming dispersive nano-sized intermetallic compounds in the soft lath martensitic matrix with a slight amount of residual austenite. The good corrosion resistance is guaranteed by exactly controlling the Co content based on understanding the synergistic effect between Co and Cr. The fine structure characteristics of two dominant strengthening precipitations including Ni3Ti and Mo-rich phases were finely characterized associated with transmission electron microscope (TEM) and atom probe tomography (APT) analyses. The relationship among microstructure, strength and toughness is discussed. The precipitation mechanism of different precipitates in the new maraging stainless steel is revealed based on the APT analysis. [ABSTRACT FROM AUTHOR]

Metastable austenite plays an important role in improving the cryogenic impact toughness of maraging steels by strain-induced martensitic transformations. However, a significant volume fraction of austenite decreases the yield strength of the material. In this study, to overcome the strength-toughness issue in maraging steels, austenite with a compositional core-shell structure and specific volume fraction are designed in Fe–Cr–Ni–Co–Mo maraging stainless steel. The core-shell compositional structure comprises retained austenite with bulk content in the core region and high Ni content reverted austenite layers in the shell region. During tensile testing, a step transformation of austenite with a continuous lattice is observed; herein, the shell regions are preserved. In contrast, the core regions transform to martensite. Close to the impact fracture, the thin austenite shell layers are retained from the martensitic transformation, which further contribute to the impact toughness. Under the combined influence of austenite and nanoprecipitates (Laves phase), the investigated alloy reaches a yield strength >1200 MPa at room temperature with good cryogenic impact toughness (77 K, >90 J), thereby outperforming conventional maraging steels and several high-entropy alloys. The current study demonstrates that the chemical heterogeneity within metastable austenite may create unique mechanical properties in structural materials. • A novel Fe–Cr–Ni–Co–Mo maraging stainless steel was designed with dual phase microstructure. • The steel reaches a yield strength >1200 MPa with good cryogenic impact toughness (77 K, >90 J). • Austenite with a compositional core-shell structure was designed in the steel. • A step transformation of austenite with a continuous lattice at deformation and impact test were characterized. • Austenite reversion process was simulated by DICTRA. [ABSTRACT FROM AUTHOR]

A new ultra-high strength maraging stainless steel with composition of 13Cr–13Co–4.5Ni–3.5Mo–0.5Ti (at.%) has been developed. It was demonstrated that the ultimate tensile strength of the steel could reach 1.9 GPa with reasonable ductility. This breakthrough was achieved by a combined strengthening of three different species of precipitates. The evolution of precipitates with respect of size, morphology and chemical composition during aging at 500 °C was characterized using atom probe tomography (APT) and transmission electron microscopy (TEM). The precipitates were identified to be η-phase Ni 3 (Ti, Al) phase, Mo-rich R′ phase and Cr-rich α′ phase, developing out of the precursor clusters, Ni–Ti–Al-rich cluster, Mo-rich cluster and Cr-rich cluster, separately. The segregation of Mo and Cr atoms at the precipitate/matrix interfaces was detected and is considered to impede the coarsening of η-phase. Based on the characterizations, the precipitation process of these phases and their effect on mechanical properties were analyzed. [ABSTRACT FROM AUTHOR]

In this study, FeCrNiCoMo maraging stainless steel was fabricated by laser melting deposition. The initial nonequilibrium microstructure exhibited cellular segregation of Cr and Mo with a 10–20 μm cell size. We investigated the microstructural evolution, particularly during aging with different time(1 h, 3 h, 5 h), for three post heat treatments: direct aging (500 °C) and low-temperature solution treatment (750 °C) and aging without and with additional prior homogenization (1000 °C). Microsegregation profoundly affected the austenite reversion behavior. The segregation at the cell walls promoted the nucleation and growth of reverted austenite, which eventually linked to form a net-like structure after prolonged aging. The net-like austenite in the direct-aged samples exhibited low thermal stability and result in brittle fracture upon impact at the temperature of liquid nitrogen (−196 °C). Net-like austenite and abundant lath austenite in the cell interior were found in the low-temperature solution-treated samples, which exhibited well-matched strength, toughness, and annealing temperature. Prior homogenization eliminated the microsegregation-induced net-like austenite. The measured mechanical properties indicate that the segregation-induced net-like austenite failed to benefit both the strength and toughness of the studied steel. [ABSTRACT FROM AUTHOR]

A maraging stainless steel was studied by in-situ XRD to monitor the austenite reversion process. The partitioning of Ni aided austenite reversion in this steel was identified from its negative effect on the lattice parameter of the reverse austenite. This chemical enrichment partially retained the reverse austenite against re-transformation to fresh martensite upon cooling to room temperature. The precipitation hardening effect of Cu and martensite tempering controlled the mechanical properties during low temperature aging (i.e., 550–600 °C), despite a significant austenite reversion at 600 °C. On the other hand, the reverse austenite formation by the austenite reversion process during heating and its subsequent transformation to fresh martensite upon cooling dominated the microstructure for higher aging temperatures (i.e., 650–700 °C), and resulted in a serrated prior austenite grain boundary structure. The change in strengthening mechanism from precipitation hardening to martensite strengthening caused a notable change in the tensile flow behavior (especially the yield ratio) with an increase in the aging temperature. As a result, the mechanical strength for the 700 °C aging condition approached the as-quenched value. This suggests that the aging heat treatment at higher temperature resulted in the re-appearance of martensite strengthening due to fresh martensite formation assisted by the austenite reversion process. • In-situ XRD set-up was employed to examine the reverse austenite formation by austenite reversion process. • Reverse austenite formation caused a serrated prior austenite grain structure at 650 °C and 700 °C. • Cu-precipitation dominated the mechanical properties at 550-600 °C, despite a significant austenite reversion at 600 °C. • Strengthening mechanism changed from precipitation hardening to martensite strengthening due to austenite reversion process. [ABSTRACT FROM AUTHOR]

Aging is an important heat treatment process for maraging stainless steel. During aging, the interaction between intermetallic compounds and dislocations has a significant impact on the mechanical properties of the material. In this article, the effect of aging temperature on the precipitation behavior and mechanical properties of Fe–Cr–Ni maraging stainless steel was studied by means of a three-dimensional atom probe (3DAP) and high-resolution transmission electron microscopy (HRTEM). The results show that coherent Ni 3 (Ti, Al) precipitates by heterogeneous nucleation from Ni/Ti/Al coclusters formed at defects (mainly dislocations) in the range of 350–450 °C, which leads to a rapid increase in strength and a sharp decrease in toughness. At 450–500 °C, the interface between Ni 3 (Ti, Al) and the matrix gradually changes from coherent to semicoherent, and an increase in the equivalent precipitate radius is accompanied by a decrease in their density. The tensile strength of the material reaches its peak value, and the toughness is improved. Within the range of 500–600 °C, Ni 3 (Ti, Al) continues to grow, aggregate and coarsen, and the amount of reverted austenite increases significantly. Both effects lead to a prominent decrease in the tensile strength and a substantial increase in toughness. The Ni 3 (Ti, Al) grows axially along the <111> dislocations through tube diffusion. The radial growth is parallel to [110] α and [1 1 ‾ 2] α , and the process is affected by the climbing and slipping of misfit edge dislocations. The strength increment of precipitates with different sizes formed at the peak aging temperature is calculated according to the critical transition radius, and the superposed results of the cutting and bypassing mechanisms during the yielding stage are found to be consistent with the experimental data, indicating that the two mechanisms work simultaneously. The findings provide a more accurate method for predicting the yield strength of materials. [ABSTRACT FROM AUTHOR]

The microstructure and high cycle fatigue (HCF) property of maraging stainless steels with Ti addition have been systematically studied. The two steels showed similar hardening responses in the aging process, which should be attributed to their identical microstructure characters, typical lath martensite immerged with dispersive precipitates Ni 3 Ti and R phase. It is proposed that the HCF crack initiation mechanism depends on the value of normalized applied stress (NAS). At high NAS level (slip band control region), the yield strength dominated by nano-size precipitates is the main factor controlling the HCF lifetime. At low NAS level (inclusion control region), the distance between TiN inclusion and the specimen surface is the main factor controlling the HCF lifetime. TiN inclusion could sharply increase the fatigue crack growth rate. It is found that TiN inclusion acted as the nucleation site of microcrack at front of main crack tip, thus deteriorated the crack propagation resistance of the maraging stainless steel. [ABSTRACT FROM AUTHOR]

In this work, the effect of built direction on the small-scale mechanical properties and microstructure of a novel maraging stainless steel (SS-CX) manufactured through the selective laser melting (SLM) process was studied. Advanced electron microscopy and nanoindentation techniques were utilized to evaluate retained austenite fraction and micro-mechanical properties, respectively. Different thermal histories caused by the change of the built direction resulted in microstructures with different volume fractions of retained austenite and grain morphology. Furthermore, the slower cooling rates in the vertically built sample was found to result in the formation of large elongated grains and lower hardness values during the nanoindentation experiments. • The building direction changes the microstructure of selective laser melting product. • Changing the building direction resulted in different volume fractions of retained austenite. • The thermal history of the product depends on the printing orientation. • The laser scan path changes the static and dynamic recovery processes. [ABSTRACT FROM AUTHOR]