Deformation-induced Martensitic Transformation Research Articles

Atomic-scale investigation of the mechanisms of deformation-induced martensitic transformation at ultra-cryogenic temperatures

Liquefied natural gas storage and transportation as well as space propulsion systems have sparked interest in the martensitic transformation and behaviours of 316L stainless steels (SS) under ultra-cryogenic deformation. In this study, high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations were used to investigate the atomic arrangements and crystalline defects of deformation-induced γ-austenite → ε-martensite → α′-martensite and γ → α′ martensitic transformations in 316L SS at 15 and 173 K. The γ → ε transformation involves the glide of Shockley partial dislocations on (111)γ planes without a change in atomic spacing. The formation of an α′ inclusion in a single ε-band is achieved by a continuous lattice distortion, accompanied by the formation of a transition zone of α′ and the expansion of the average atomic spacings due to dislocation shuffling. As α′ grows further into γ, the orientation relationship (OR) of the α′ changes by lattice bending. This process follows the Bogers-Burgers-Olson-Cohen model despite it not occurring on intersecting shear bands. Stacking faults and twins can also serve as nucleation sites for α′ at 173 K. We also found that direct transformation of γ → α′ occurs by the glide of 6aγ[112¯]/12 dislocations on every (111)γ planes with misfit dislocations. Overall, this study provides, for the first time, insights into the atomic-scale mechanisms of various two-step and one-step martensitic transformations induced by cryogenic deformation and corresponding local strain, enhancing our understanding of the role of martensitic transformation under ultra-cryogenic-temperature deformation in controlling the properties.

Synergistic effect of fatigue and hydrogen-induced damage under asymmetric loading of TRIP steel

This study investigates the synergistic assessment of cyclic plasticity damage and hydrogen-induced cracking of transformation-induced plasticity (TRIP) steel subjected to asymmetric strain-controlled fatigue loading. The presence of hydrogen in TRIP steel results in cyclic softening throughout its life, suppressing the hardening behavior induced by martensitic transformation. The mild hardening observed in hydrogen-free specimens is attributed to the predominance of hardening induced by martensitic transformation over cyclic softening. The increase in mean strain leads to higher mean stress relaxation, suppressing the compressive stress developed during the deformation-induced martensitic transformation phenomenon. Geometrically necessary dislocation (GND) density map and fractographic images substantiate the hydrogen-enhanced decohesion failure mechanism, decreasing fatigue resistance for hydrogen-induced TRIP steel. The failure mechanisms of the fatigue damage due to the ingress of hydrogen atoms in the material are proposed for further insights.

Tailoring deformation-induced martensitic transformation through cellular engineering in laser powder bed fusion processed 316L stainless steel

The microstructural effect of 316L stainless steel (SS) fabricated by laser powder bed fusion on deformation-induced martensitic transformation (DIMT) was investigated. Both opposing effects, i.e., disturbing/promoting DIMT were observed. While the cellular structure and low-angle grain boundaries reduce the DIMT rate by disturbing shear band formation, the high dislocation density can activate direct BCC martensite nucleation and accelerate the DIMT. This observation suggests that controlling the two competing effects is the key to cellular engineering using additive manufacturing.

Cryogenic Tensile Behavior of Ferrous Medium-entropy Alloy Additively Manufactured by Laser Powder Bed Fusion

The emergence of ferrous-medium entropy alloys (FeMEAs) with excellent tensile properties represents a potential direction for designing alloys based on metastable engineering. In this study, an FeMEA is successfully fabricated using laser powder bed fusion (LPBF), a metal additive manufacturing technology. Tensile tests are conducted on the LPBF-processed FeMEA at room temperature and cryogenic temperatures (77 K). At 77 K, the LPBF-processed FeMEA exhibits high yield strength and excellent ultimate tensile strength through active deformation-induced martensitic transformation. Furthermore, due to the low stability of the face-centered cubic (FCC) phase of the LPBF-processed FeMEA based on nano-scale solute heterogeneity, stress-induced martensitic transformation occurs, accompanied by the appearance of a yield point phenomenon during cryogenic tensile deformation. This study elucidates the origin of the yield point phenomenon and deformation behavior of the FeMEA at 77 K.

Effects of pressurized water reactor environment and cyclic loading parameters on the low cycle fatigue behavior of 304L stainless steel

Austenitic stainless steels used in light water reactor coolant environments are susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. This study investigates the effects of a high-temperature pressurized water reactor (PWR) water environment and cyclic loading parameters on the low cycle fatigue behavior of austenitic 304L stainless steel. Prolonged exposure to a PWR environment and cyclic loading conditions such as a lower strain rate or a higher fraction of slow strain rate enhances the initiation and accelerates the crack growth rate of fatigue cracks, resulting in decreased fatigue life. The deformation-induced α'-martensite is observed in proximity to fatigue crack tips primarily in specimens tested in simulated PWR primary water, while cellular dislocation structures are more frequently observed near crack tips in specimens tested in high-temperature air. The deformation-induced martensitic transformation from γ-austenite to α'-martensite, occurring via the precursor ε-martensite phase, contributes to the accelerated fatigue crack growth rate in a PWR environment with hydrogen.

Texture Development in Austenite during Tensile Deformation of 5%Mn Steel Accompanied by Deformation-Induced Martensite Transformation

Texture Development in Austenite during Tensile Deformation of 5%Mn Steel Accompanied by Deformation-Induced Martensite Transformation

Uncovering the generic and alloy-specific governing parameters of deformation-induced martensitic transformation in austenitic steel

In this work, a hybrid modeling approach, combining machine learning (ML) and computational thermodynamics, has been applied to predict deformation-induced martensitic transformation (DIMT) and explore the generic and alloy-specific parameters governing DIMT in austenitic steels. The DIMT model was established based on the ensemble ML algorithms and a comprehensive set of physical variables. The developed model is highly generalizable as validated on unseen alloys. The generic governing parameters of DIMT are in good agreement with previous studies in the literature. However, the evaluated alloy-specific governing parameters reveal large differences between grades, e.g., 204 series of austenitic stainless steels has a quite balanced correlation between strain, stress, temperature, and DIMT, while the 301 series has much stronger correlation between stress and DIMT. The findings in the current study emphasize the importance that a general DIMT model for steels should include both stress and strain, as well as other governing parameters, since DIMT can be both stress-assisted and strain-induced transformation, and often the effect of applied mechanical driving force and the formation of new nucleation sites interact.Graphical abstract

Deformation-induced Martensitic Transformation at Tensile and Compressive Deformations of Bainitic Steels with Different Carbon Contents

Deformation-induced Martensitic Transformation at Tensile and Compressive Deformations of Bainitic Steels with Different Carbon Contents

Atomic simulation study of the factors affecting nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron

Molecular dynamics simulations were performed to investigate the dominant factors affecting the nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron, and the effectiveness of microstructure designs incorporating the obtained dominant factors was verified. First, the local nucleation stress required for grain-boundary nucleation was lower than that for in-grain nucleation, indicating that the grain boundary is an effective nucleation site for martensitic transformation. Next, we investigated the dominant factors affecting the in-grain nucleation and the grain-boundary nucleation, and found that the dominant factor of the in-grain nucleation is the magnitude of the loading-direction component of the transformation strain induced by lattice deformation (Bain distortion), while the dominant factors of grain-boundary nucleation are the grain-boundary misorientation angle and grain-boundary free volume. Finally, the possibility of controlling the nucleation timing by designing the microstructures based on the obtained nucleation dominant factors was demonstrated by comparing two polycrystalline models with the same texture but different distributions of grain-boundary misorientation angles.

Mechanical properties of lamellar-structured 18Ni300 maraging steel manufactured via directed energy deposition

The combination of repetitive melting–solidification cycles, directional laser scanning, and microstructure design using additive manufacturing (AM) is effective for enhancing the mechanical properties of metallic alloys. Because the ultrafine metastable phases in as-built metallic alloys are beneficial to their mechanical properties, there is a design window to enhance the strength–ductility combination by controlling the metastable phase distribution with AM. In this study, ultrafine lamellar-structured maraging steel consisting of martensite and austenite was manufactured using directed energy deposition. The stripe laser scan strategy induced linear (Mo, Ti, Ni)-rich segregations that formed lamellar-structured austenite by reducing the martensitic transformation starting temperature in the high-Ni-content maraging steel. The deformation-induced martensitic transformation and back-stress hardening near the austenite–martensite interfacial region enhanced the plasticity capability of the AM-processed maraging steel and achieved a better strength–ductility combination compared with conventional and recently developed AM-processed maraging steels. This result indicates that artificial microstructural design via AM is important for enhancing the performance of metallic alloys, which can also be achieved with conventional metallic alloys by distributing the metastable phase with processing variable control.

Unexpected deformation-induced martensitic phase transformations in Ni–Cr and Ni–Cr–Fe alloys

Deformation of Ni–Cr and Ni–Cr–Fe alloys has historically been understood to occur through dislocation slip, while deformation-induced martensitic transformations are believed to be suppressed by the high stacking fault energy (SFE). The present study challenges these longstanding beliefs with experimental and theoretical evidence of deformation-induced martensitic transformations in Alloy 625 and Ni–20Cr, respectively. Systematic investigations are conducted along the three principal grain orientations; experiments employ nanoindentation coupled with post mortem transmission electron microscopy (TEM) analysis, while molecular dynamics (MD) simulations are conducted in uniaxial tension. Experimental results reveal γ→ε and γ→ε→α′ martensitic transformations in Alloy 625 consistent with the Bogers-Burgers-Olson-Cohen (BBOC) intersecting shear mode. The orientation relationship is characterized as a distorted Shoji-Nishiyama OR between FCC/HCP and Kurdjumov-Sachs OR between FCC/BCC. By contrast, Alloy 690 does not exhibit martensitic transformations despite having a lower SFE than Alloy 625, due to Mo solute strengthening in Alloy 625. MD simulations reveal consistent transformation mechanisms across grain orientations, but orientation-dependent differences in Schmid factors and critical resolved shear stress affect the strain evolution and extent of the transformation. This study concludes that SFE alone may not comprehensively dictate whether deformation-induced martensitic transformations will occur; other factors such as free energy and strain rate also influence transformability.

EFFECT OF COLD ROLLING PROCESS ON TEXTURE EVOLUTION IN AUSTENITIC STEEL SHEETS

The investigation presents the evolution of texture and microstructure of cold-rolled austenitic AISI 301 stainless steel sheets. The austenite steel was deformed by cold-rolling to 30%, 50% and 80% of rolling reduction. It was found, that during cold plastic deformation, phase transformation of austenite into martensite takes place. The transformation progress was confirmed by the microscopic investigation, as well as by X-ray diffraction methods. Texture analysis was performed on the basis of the orientation distribution functions (ODFs) calculated from the experimental pole figures. The texture of the deformed austenite is dominated by fiber orientations α =110║KN, mainly Goss orientation {110}001 and alloy type orientation {110}112. On the other hand, plastic deformation-induced martensitic transformation of the type (γ') significantly affects the development of martensite texture ' in the cold rolling process. After rolling with a 30% of deformation degree in the texture of  martensite, the strongest orientation is {001}110, while after maximum deformation (about 80%), the {111}112 and {112}110 type orientations dominate. It was found that in AISI 301 steel structure after deformation range from 30% to 80% proportion of α’ martensite phase changes from about 26.7% to about 67.8%.

Grain-level effects on in-situ deformation-induced phase transformations in a complex-phase steel using 3DXRD and EBSD

A novel complex-phase steel alloy is conceived with a deliberately unstable austenite, γ, phase that enables the deformation-induced martensitic transformations (DIMT) to be explored at low levels of plastic strain. The DIMT was thus explored, in-situ and non-destructively, using both far-field Three-Dimensional X-ray Diffraction (3DXRD) and Electron Back-Scatter Diffraction (EBSD). Substantial α′ martensite formation was observed under 10% applied strain with EBSD, and many ɛ grain formation events were captured with 3DXRD, indicative of the indirect transformation of martensite via the reaction γ→ɛ→α′. Using ɛ grain formation as a direct measurement of γ grain stability, the influence of several microstructural properties, such as grain size, orientation and neighbourhood configuration, on γ stability have been identified. Larger γ grains were found to be less stable than smaller grains. Any γ grains oriented with {100} parallel to the loading direction preferentially transformed with lower stresses. Parent ɛ-forming γ grains possessed a neighbourhood with increased ferritic/martensitic volume fraction. This finding shows, unambiguously, that the nearby presence of α and α′ promotes ɛ formation in neighbouring grains. The minimum strain work criterion model for ɛ variant prediction was also evaluated, which worked well for most grains. However, ɛ-forming grains with a lower stress were less well predicted by the model, indicating crystal-level behaviour must be considered for accurate ɛ formation. The findings from this work are considered key for the future design of alloys where the deformation response can be controlled by tailoring microstructure and local or macroscopic crystal orientations.

Achieving prominent high-temperature mechanical properties in a dual-phase high-entropy alloy: A synergy of deformation-induced twinning and martensite transformation

Dual-phase high-entropy alloys are renowned for their favorable balance of strength and ductility at ambient conditions. However, their mechanical properties, especially the associated deformation mechanisms at elevated temperatures, remain less explored. In this study, the AlCoCrFeTi0.5Ni2.5 HEA consisting of FCC (i.e., L12) and BCC (i.e., B2 + A2) phases was subjected to compressive test at elevated temperatures ranging from 500 to 800 °C. The results demonstrate that this HEA exhibits excellent high-temperature mechanical properties up to 700 °C, superior to most refractory HEAs and Inconel 718 superalloy. Notably, its yield strength surpasses 1000 MPa, with pronounced strain-hardening evident at 600 °C. Upon reaching 700 °C, despite strain-softening occurring at the late deformation stage, the yield strength remains above 900 MPa. Microstructural analysis of the sample deformed at 600 °C indicates increased stacking faults and deformation twins in the FCC phase. Additionally, the BCC phase displays a high density of dislocation entanglements and nanoscale martensite lathes. It is revealed that the high-temperature twins in the FCC phase are triggered by the pre-existing local chemical ordered domains, while the martensitic laths in the BCC phase are activated via stress-induced phase transformation driven by stress concentration at the B2/A2 interface. The synergistic effect of multiple deformation mechanisms operating in both the FCC and BCC phases significantly enhances the strain-hardening capability, contributing to the remarkable mechanical properties at elevated temperatures. These findings provide valuable insights for developing dual-phase HEAs with outstanding mechanical properties over an extensive range of temperatures.

Mechanical property of the shape memorable Ti–Zr–Nb–Sn alloy manufactured by in-situ alloying in directed energy deposition

Ni-free β-type Ti alloys have been developed to manufacture high-strength, low-elastic-modulus, shape-memorable medical, and non-toxic components. Because the machinability of these alloys is generally poor, processing via additive manufacturing is an important step in the development of order-made medical devices. Although many studies evaluated the mechanical properties of additively manufactured Ni-free β-type Ti alloys, investigation of their heterogeneous microstructure resulting from the rapid melting-solidification cycling has not been discussed yet. In this study, the role of a heterogeneous microstructure on the performance of an in-situ alloyed Ti–Zr–Nb–Sn alloy was investigated. The metastable and nanosized ω phase, which is initiated in the non-equilibrium environment, enhances the yield strength without severe ductility degradation. Additionally, deformation-induced martensitic transformation occurs near the unmelted particle/matrix interface, and this phase transformation not only contributes to transformation-induced plasticity but also provides a superelasticity to the present alloy. Although the remained partially melted particle induces a localized corrosion in the matrix, the present results show that the heterogeneous microstructure of the in-situ alloyed Ti–Zr–Nb–Sn alloy exhibits outstanding mechanical properties and superelasticity, which will be suitable to fabricate biomedical parts via additive manufacturing.

Дюрометричний аналіз зміцнення приповерхневого шару ADI при терті під впливом TRIP-ефекту

Features of strengthening of the near-surface layer of ADI during friction due to strain-induced martensitic transformation were analyzed by duromeric methods. Indentation under continuous loading (Meyer hardness, HM) and Vickers microhardness Hμ were used. Pop–ins are observed on the ADI continuous load curves, which indicate martensitic transformations during indentation. The effect usually exists at a load of ~0,1 H and an depth of ~1,5 μk. The average microhardness of the initial sample is Hμ ≈ 4,89 GPa. After wear, the average value increases to Hμ ≈ 6,92 GPa. Statistical analysis of the microhardness distribution of the sample after wear revealed that a third of the indentations have abnormally high hardness, which is characteristic of deformation-induced martensite. Probably, these indents are obtained from regions of the structure where deformation-induced martensitic transformation took place. Increasing the indentation load practically does not affect the determination of the microhardness of the initial sample, but reduces the hardness of the sample after wear. This indicates the gradient nature of deformation and phase-structural rearrangements in the near-surface layer during wear. As the friction temperature increases, there is a decrease in microhardness in the wear zone. This is explained by the departure from the temperature range of the martensitic transformation, due to which the TRIP effect is weakened. The maximum degradation of microhardness is observed between room temperature and 50 oC. Keywords: ADI materials, durometric studies, TRIP-effect, wear.

Grain size independence of cryogenic strain recovery behavior in high-Zr β-Ti alloy

A significant increase in the recovery strain of a high-Zr β-Ti alloy from 2.25 % to 5.5 % when decreasing the deformation temperature from 300 K to 77 K is reported in this study. It is found that the super-elasticity of this alloy is independent of the β-grain size at 77 K. The results reveal that a coarse-grained specimen exhibited approximately the same super-elasticity as its ultra-fine grain counterpart at 77 K. The relative easiness of deformation-induced martensitic transformation and dislocation slip was substantially changed at 77 K, with a strong suppression of dislocation slip, which overshadowed the effect of grain refinement on the super-elasticity.

Phenomenological Modeling of Deformation-Induced Martensite Transformation Kinetics in Austenitic Stainless Steels

Phenomenological Modeling of Deformation-Induced Martensite Transformation Kinetics in Austenitic Stainless Steels

Quantitatively assessing the contributions of temperature-dependent deformation-induced martensitic transformation to uniform elongation and work hardening of TRIP-assisted duplex stainless steel via crystal plasticity

The martensitic transformation produced by transformation-induced plasticity (TRIP) assisted duplex stainless steels (TDSSs) can significantly enhance plasticity and strength of the material under deformation. At different deformation temperatures, the stability of austenite varies, which further affects the varied kinetics of deformation-induced martensitic transformation (DIMT). In the work, The mechanical response during tensile deformation of TDSS in a temperature range of 20 °C–60 °C was investigated. Especially, the quantitative determination of strength- and elongation-contributions of DIMT was characterized by employing crystal plasticity finite element method (CPFEM) considering thermal softening of temperature effect. The higher the temperature is, the weaker the DIMT and resultant TRIP effect are, and hence the contributions of TRIP to work hardening and uniform elongation are correspondingly reduced. The local strain increment is limited after martensitic transformation due to the significant deformation resistance of martensite, and the strain concentration phenomenon has slowed down at martensitic transformation positions. As a result, the maximum value of strain drops, leading to a decrease in the coefficient of variation (CV) of the strain distribution. Providing improvement of the homogeneity of strain, the material exhibits substantial enhanced plasticity characteristics. The global stress increment is lower than the ideal linear law calculated based on stress mixing that the global stress increment is proportional to the volume fraction of martensite. The stress increment caused by martensitic transformation has a certain degree of postponed effect and transformation rate dependence.

Unveiling the transformation pathways of hierarchical γ90 – εtwin – α’ triple phase structure formation at ε-ε martensite intersection

Deformation-induced martensitic transformation (γ-austenite → ε-martensite/α’-martensite) in austenitic steels has garnered significant interest owing to its transformation-induced plasticity effect. To elucidate the orientation-dependent intricate γ/ε/α’ phase microstructure at deformation-induced ε-ε intersection, a single crystal of 316 austenitic stainless steel was compressed along the [001]γ axis at a cryogenic temperature (173 K). Electron backscattered diffraction analysis was employed to reveal the deformed microstructure on the (110)γ surface. A hierarchical triple phase structure was discovered at ε–ε intersection, where the γ rotated 90° from the matrix (γ90), {101¯2} ε-twin, and α’ phases coexist. Depending on the operative shear angle with a common intersection axis, either 90° (Type I) or 30° (Type II), three distinct atomic rearrangements of the intersection volume were observed: γ90 was present at Type I intersection, and α’-phase was developed at Type II intersection, respectively. {101¯2} ε-twin also occurred at Type I intersection, serving as an accommodation mechanism alongside the intersection γ90. EBSD results confirm the Shoji–Nishiyama (SN), Kurdjumov–Sachs (KS), and Burgers (B) orientation relationships within the complex γ – ε – α’ triple phase structures at ε–ε intersection. Transformation paths for three intersection products were visualized by the unified tetrahedron model, considering T/2 or T/3 γ-twinning shear as an intermediate state. Moreover, a novel scheme is proposed to index the α’-martensite crystallographic variants at ε-ε intersections by establishing a correlation between the Bain distortion and double shear process.