Deformation-induced Martensitic Transformation Research Articles

Abnormal work hardening in a TRIP-assisted metastable β titanium alloy under high strain rate loading

In this study, the dynamic tensile deformation behavior and microstructural evolution of a metastable β titanium alloy (Ti–10 V–2Fe–3Al, Ti-1023) with a single β phase structure were investigated at ambient temperature and high strain rates ranging from 1000 s−1 to 4000 s−1 on a split Hopkinson tension bar (SHTB) system. The dynamic tensile properties and work hardening ability exhibited a significant strain rate dependence. The calculated strain rate sensitivity (0.026–0.090) showed a strong strain and strain rate hardening ability. Microstructural analyses indicated that the dynamic deformation mechanisms were dislocation slip and deformation-induced α" martensite at all strain rates, while deformation-induced ω phase was activated beyond a strain rate of 2000 s−1. Deformation-induced phase transformations during dynamic tensile deformation led to a significant transformation-induced plasticity (TRIP) effect and a double-yielding phenomenon. As strain rate increased, deformation-induced martensitic transformation and the corresponding TRIP effect were suppressed, while work hardening rate first increased and then rapidly decreased. This abnormal TRIP effect on work hardening behavior was mainly due to the deformation-induced ω phase, promoting the dislocation plasticity in the β matrix and contributing to extra work hardening ability.

Significant effects of grain size on mechanical response characteristics and deformation mechanisms of metastable austenitic stainless steel

Grain size of a 304 austenitic stainless steel was tuned via deformation-induced martensite transformation and austenite reversion annealing, and the significant effects of grain size on mechanical properties, strain hardening behavior, deformation mechanisms were studied. The results showed that grain refinement strengthening significantly increased the yield strength from 235 MPa (~20 μm) to 1030 MPa (~200 nm), meanwhile, the interaction of Lüders deformation and deformation-induced martensite transformation realized the stable propagation of Lüders bands, leading to a considerable total elongation (above 30%). It was inferred that martensitic transformation was enhanced with a decrease in grain size. The underlying reasons were attributed to the formation of carbide precipitation (M 23 C 6 ) and the change of martensite nucleation sites depending on grain size. In coarse grains, α'-martensite formed at shear bands with or without the aid of ɛ-martensite, whereas α'-martensite mainly nucleated at grain boundaries and deformation twins in ultrafine austenite grains. Besides TRIP effect and dislocation glide, TWIP effect also became one of the dominate deformation mechanism with the decrease in austenite grain size. • Nano/ultrafine-grained (NG/UFG) 304 stainless steels with high strength and good tensile plasticity are achieved. • Deformation-induced martensite transformation is enhanced as austenite grain sizes decrease. • The enhanced strain-hardening caused by martensitic transformation supports the propagation of Lüders bands. • The dominant strain hardening mechanisms change from TRIP effect in CG steels to TRIP and TWIP effects in NG/UFG steels.

Characterization of deformation-induced martensite with various AGSs upon Charpy impact loading and correlation with transformation mechanisms

Deformation-induced martensitic transformation (DIMT) during quasi-static loading has drawn much attention in recent years since it is considered as one of the key strengthening mechanisms in advanced high-strength steels (AHSS). However, systematic investigations on martensitic transformation at high strain-rates are scarce due to difficulties in experimental designs. In this contribution, interrupted Charpy tests were used to study the transformation sequence of DIMT under high strain-rates (~10 2 –10 3 s −1 ). Austenitic 321 stainless steel samples with increasing austenite grain sizes (AGSs) were designed to demonstrate the change in transformation sequence of α'-martensite and its morphology. Quasi-in-situ electron backscattered diffraction (EBSD) and selected transmission electron microscopy (TEM) experiments have revealed that, as the AGSs increases, the nucleation sites of α'-martensite change from austenite grain boundaries in ultra-fine-grained (UFG) samples to additional twin boundaries in fine-grained (FG) samples, and finally to ε-martensite in coarse-grained (CG) samples. Due to the spatial constraints imposed by different crystallographic boundaries, the morphology of transformed α'-martensite shifts from blocky in UFG samples to film-like in CG samples, with FG samples having a mixed morphology. It is found that the changed transformation sequence can be understood by considering the combined effect of stacking fault energy and twinning nucleation stress of austenite. • Quasi-in-situ observation of DIMT under dynamic loading conditions. • TRIP (ε + α'), TWIP+TRIP (α'), and TRIP (α') occurred with decreasing austenite grain sizes. • The morphology of DIM is controlled by intermediate deformation products. • The absence of twins and ε-martensite resulted in blocky α'-martensite in UFG steels. • High SFE and twinning nucleation stress suppressed the formation of ε-martensite and twins.

Effect of deformation-induced martensitic transformation on nonuniform deformation of metastable austenitic steel

A digital image correlation technique combined with in situ neutron diffraction was used for the first time to investigate the Portevin–Le–Chatelier (PLC) behavior in an Fe–24Ni–0.3C steel. Propagating type A + B PLC bands observed in the early stage of plastic deformation changed to the non-propagating type C bands due to the occurrence of deformation-induced martensitic transformation.

On the enhanced hardening ability and plasticity mechanisms in a novel Mn-added CoCrNi medium entropy alloy during high-pressure torsion

Microstructure and texture evolution during high-pressure torsion (HPT) of a novel Mn-added CoCrNi medium entropy alloy (Co33Ni33Cr19Mn15) is investigated for the first time. The alloy exhibited a rapid rise in hardness at relatively low shear strains (γ≤20). It is attributed to an extensive dislocation activity to achieve saturation in dislocation density of ~1016 m−2, combined TWIP and TRIP effects and microstructural refinement. At higher shear strain, hardness increased at much reduced rates owing to saturation of dislocation density, twin fault probability and the TRIP effect, besides continued grain refinement for severe nano-structuring led to subsequent strengthening. The FCC phase showed remarkable stability except a small degree of initial deformation-induced HCP martensitic transformation in an early stage of HPT. The ideal shear texture components were observed at low shear strain, and these continued to evolve up to 5 turns of HPT processing. For similar HPT processing conditions, the studied alloy showed superior hardness (~650 Hv) compared to a wide spectrum of FCC materials, which is ascribed to a combination of the strengthening mechanisms of Taylor hardening, the TRIP and TWIP effects and Hall-Petch strengthening resulting from the nano-structured grains having an average size of ~35 nm.

Effect of Microstructure on the Onset Strain and Rate per Strain of Deformation-Induced Martensite Transformation in Q&P Steel by Modeling.

The effect of microstructure on the onset strain and rate of deformation-induced martensitic transformation (DIMT) in Q&P steel is studied by a mean-field micromechanics model, in which the residual austenite (RA) and primary martensite (M) phases are treated as elastoplastic particles embedded into the ferrite (F) matrix. The results show that when the volume fraction of the RA increases with a constant fraction of the M, the onset strain of DIMT increases and transformation rate decreases, in contrast to the case of the RA fraction effect with a fixed F fraction. Increasing the volume fraction of the M postpones the DIMT, regardless of the corresponding change from the RA or F fraction, which is similar to the effect of the RA fraction with the constant M but to a higher degree. Conversely, when increasing the fraction of the matrix F, the onset strain of DIMT increases and the rate decreases, and the effect is greater when the corresponding fraction change comes from the M rather than from the RA. Moreover, when the aspect ratio of the RA increases, the onset strain of DIMT decreases with a gradual increase in transformation rate, in agreement with the experimental observation that the equiaxial austenite is more stable in Q&P steels. However, the aspect ratio effect of the M is opposite to that of the RA, indicating that the lath-shaped primary martensite could protect the austenite from DIMT.

Double-humped strain hardening in a metastable ferrous medium-entropy alloy by cryogenic pre-straining and subsequent heat treatment

Significant benefits of either heterogeneous microstructures or deformation-induced martensitic transformation (DIMT) in metallic materials stem from their superior strain hardening and tensile properties. Herein, we present an unprecedented strain hardening behavior at 77 K in a ferrous medium-entropy alloy comprising a metastable face-centered cubic (FCC) bimodal microstructure with different grain sizes. Pre-straining yields fine body-centered cubic (BCC) martensites, and subsequent heat treatment causes reverse martensitic transformation from BCC to FCC, affording the bimodal FCC microstructure. Furthermore, the yield strength is enhanced due to the presence of submicron FCC grains and geometrically necessary dislocations (GNDs) that are generated during the pre-straining. Profuse GNDs in reversely transformed FCC promote DIMT. Moreover, when true strain exceeds 0.2, widespread DIMT in the interior of the remaining FCC drastically increases the strain hardening rate and consequently delays necking. The outstanding tensile properties derived from this thermomechanical process are because of the DIMT and hetero-deformation-induced strengthening.

Effect of Deformation-Induced Martensitic Transformations in a SMAW Butt Joint of the 304 Stainless Steel on its Strain Field Distributions

Effect of Deformation-Induced Martensitic Transformations in a SMAW Butt Joint of the 304 Stainless Steel on its Strain Field Distributions

Decoupling the Impacts of Strain Rate and Temperature on TRIP in a Q&P Steel

The individual effects of strain rate and temperature on the strain hardening rate of a quenched and partitioned steel have been examined. During quasistatic tests, resistive heating was used to simulate the deformation-induced heating that occurs during high-strain-rate deformation, while the deformation-induced martensitic transformation was tracked by a combination of x-ray and electron backscatter diffraction. Unique work hardening rates under various thermal–mechanical conditions are discussed, based on the balance between the concurrent dislocation slip and transformation-induced plasticity deformation mechanisms. The diffraction and strain hardening data suggest that the imposed strain rate and temperature exhibited dissonant influences on the martensitic phase transformation. Increasing the strain rate appeared to enhance the martensitic transformation, while increasing the temperature suppressed the martensitic transformation.

Two-Step Deformation-Induced Martensitic Transformation in Additively Manufactured High-Si Stainless Steel

Two-Step Deformation-Induced Martensitic Transformation in Additively Manufactured High-Si Stainless Steel

Phase-field and dislocation-based crystal plasticity analysis on lath martensitic structures formation in polycrystalline metastable austenitic steel

Although strength of TRIP steels can be compatible with ductility owing to deformation-induced martensitic transformation, there is currently no model describing simultaneously the deformation and microstructure formation processes of TRIP steel. In this report, we introduce the basis vectors of martensitic transformation derived in the previous report into the crystal plasticity model and develop a new material model based on a dislocation-based crystal plasticity model coupled with a multi-phase-field one. Also, we consider plastic deformation of austenite phase for more realistic analyze. After that, we use this model to reproduce the formation process of martensitic lath-block structure in a metastable austenitic polycrystal. As a result, it is shown that the microstructure of lath martensite is numerically formed in a polycrystalline micro-specimen using 6 variants in a CP group.

Effects of deformation-induced martensitic transformation on cryogenic fracture toughness for metastable Si8V2Fe45Cr10Mn5Co30 high-entropy alloy

In this study, fracture toughness was evaluated at room and cryogenic temperatures for a metastable Si8V2Fe45Cr10Mn5Co30 (at.%) alloy, and roles of deformation-induced martensitic transformation (DIMT) on fracture behavior were investigated with respect to types of transformation mechanism and martensite morphology. The as-annealed alloy consisted mainly of face-centered-cubic (FCC) phase with athermal hexagonal-close-packed (HCP) martensite. The DIMT from FCC to body-centered-cubic (BCC) via intermediate HCP resulted in an excellent combination of strength and ductility by high strain-hardening effect. Considerably high fracture toughness and ductile-dimpled fracture mode appeared at room temperature, but the fracture toughness significantly deteriorated at cryogenic temperature. The BCC martensite exhibited a thick-plate morphology as a resultant product of stress-induced transformation from low-stability FCC, and the plates had an alternating variant characterized by a ladder martensite, based on a 60°-BCC-twin relationship with {112} twin planes. This plate-type BCC martensite had the fewer phase boundaries and geometrically necessary dislocations than the lath-type BCC martensite. In addition, cleavage {001}BCC-planes of two martensite blocks having the alternating variants were slightly deviated, thereby leading to the much lower toughness along with the brittle fracture mode of lamellar cleavage facets at cryogenic temperature. Thus, the present work suggests that the proper control of FCC stability is required to enhance the resistance to low-temperature embrittlement for securing cryogenic applications.

Enhancement of Uniform Elongation by Temperature Change during Tensile Deformation in a 0.2C TRIP Steel

It is important to control the deformation-induced martensitic transformation (DIMT) up to the latter part of the deformation to improve the uniform elongation (U.El) through the TRIP effect. In the present study, tensile tests with decreasing deformation temperatures were conducted to achieve continuous DIMT up to the latter part of the deformation. As a result, the U.El was improved by approximately 1.5 times compared with that in the tensile test conducted at 296 K. The enhancement of the U.El in the temperature change test was discussed with the use of neutron diffraction experiments. In the continuous DIMT behavior, a maximum transformation rate of about 0.4 was obtained at a true strain (ε) of 0.2, which was larger than that in the tensile test at 296 K. The tensile deformation behavior of ferrite (α), austenite (γ), and deformation-induced martensite (α′) phases were investigated from the viewpoint of the fraction weighted phase stress. The tensile test with a decreasing deformation temperature caused the increase of the fraction weighted phase stress of α and that of α′, which was affected by the DIMT behavior, resulting in the increase in the work hardening, and also controlled the ductility of α and α′, resulting in the enhancement of the U.El. Especially, the α phase contributed to maintaining high strength instead of α′ at a larger ε. Therefore, not only the DIMT behavior but also the deformation behavior of γ, α, and α′ are important in order to improve U.El due to the TRIP effect.

Relationship between the Tensile Properties and Damping Capacity of Fe-22%Mn-12%Cr-4%Co-3%Ni-2%Si Alloys by Fatigue Stress.

The relationship between the tensile properties and damping capacity of fatigue-damaged Fe-22%Mn-12%Cr-4%Co-3%Ni-2%Si alloy under various magnitudes of fatigue stress was investigated. Analytical results show that α′- and ε-martensite were formed due to fatigue stress. The formed α′- and ε-martensite followed a specific orientation and surface relief and intersected with each other. TEM observation and pattern analysis reveal that both α′- and ε-martensites formed on the austenite. As a result of X-ray diffraction, with an increase in fatigue stress, the volume fractions of α′- and ε-martensite were increased, and the increasing rate of the volume fraction of α′-martensite was higher than that of the ε-martensite. As the fatigue stress increased, the tensile strength and damping capacity increased, but the elongation decreased. Besides, as the strength increased and the elongation decreased, the damping capacity decreased. This result is inconsistent with the general tendency for metals but similar to that of alloys undergoing deformation-induced martensite transformation.

Excellent cryogenic strength-ductility synergy in duplex stainless steel with heterogeneous lamella structure

Ferrite and austenite duplex stainless steel with heterogeneous lamella structure was fabricated by severe cold-rolling and short-time annealing. The processed steel exhibited superior cryogenic mechanical properties with a tensile strength of ∼1650 MPa and an ultimate tensile strain of ∼30%. The excellent combination of cryogenic strength and ductility was attributed to fine grain size, nanotwins, back-stress hardening, and deformation-induced martensitic transformation.

Surface hardening of high- and medium-entropy alloys by mechanical attrition at room and cryogenic temperatures

Surface hardening by mechanical attrition at room and cryogenic temperatures has been studied on CrMnFeCoNi high- and CrCoNi medium-entropy alloys. The hardness gradient is produced by severe plastic deformation through dislocation slip and mechanical twinning, finally leading to an ultrafine or even nanocrystalline structure. The hardness of samples after surface attrition at cryogenic temperature is slightly less than that of samples deformed at room temperature. A small amount of deformation-induced martensitic transformation is only observed in a narrow surface layer of the medium-entropy alloy severely deformed under cryogenic conditions. Reasons for these observations are discussed.

Role of martensitic transformation sequences on deformation-induced martensitic transformation at high strain rates: A quasi in-situ study

For the mechanical behavior of steels with metastable austenite, it was widely accepted that high strain rates could weaken deformation-induced martensitic transformation (DIMT) and reduce work-hardening behavior. This limits the application of these steels for energy-absorption applications, e.g., impact conditions. However, the mechanism of strain-rate dependent DIMT is still not well understood and it greatly limited the further alloy design. Thus, with the aim of revealing the intrinsic relations between strain rate and martensitic transformation, quasi-in-situ EBSD (Electron Backscatter Diffraction) tests were performed under different strain rates in this study. The influence of the martensitic transformation sequences on the DIMT volume fraction was investigated. With increasing strain rate, the γ→ε→α’ transformation was inhibited, and the γ→α’ transformation sequences dominated. The martensitic transformation sequences firstly affect the nucleation behavior. The α′-martensite nucleation sites changed from ε or twin to twin only, and thus, the α′-martensite nucleation barrier increased. In addition to nucleation, the variant selection phenomenon caused by the absence of ε-martensite under high strain rates increased the elastic strain energy, which inhibited the growth of α′-martensite. By affecting both nucleation and growth, high strain rates suppress the γ→ε→α’ transformation, and inhibit DIMT kinetics. Thus, by tailoring the martensitic transformation sequences, two methods are proposed to enhance DIMT behavior via suitable control of the stacking fault energy. This work provides a comprehensive explanation for the strain-rate dependence of DIMT, which can be used to tailor the mechanical properties of steels with metastable austenite.

Delayed deformation-induced martensite transformation and enhanced cryogenic tensile properties in laser additive manufactured 316L austenitic stainless steel

Abstract The cellular dislocation structure is an important microstructure aspect in fusion-based laser metal additive manufactured (MAM) parts. Its role in increasing strength and ductility at room temperature (298 K) is a known phenomenon. In this work, we have shown that the cellular dislocation structure not only improves the strength but also dramatically increases the mechanical stability of the austenite and retards the deformation-induced martensite transformation (DIMT) of austenite at cryogenic temperature (77 K). The delayed and slow rate of DIMT led to an enhanced strength-ductility synergy in the direct energy deposited and selective laser melted 316 L stainless steel samples at 77 K compared to the wrought processed samples. The current study establishes the capability of MAM technologies in controlling the DIMT behavior and realizing materials with high strength and ductility combination.

On low-temperature strength and tensile ductility of bulk metallic glass composites containing stable or shape memory β-Ti crystals

Bulk metallic glass composites (BMGCs) with in-situ formed crystals can display tensile ductility at room temperature, overcoming brittleness of monolithic bulk metallic glasses (BMGs). However, the ductility and toughness of BMGCs severely deteriorate at low temperatures, which has been a longstanding bottleneck hindering their low-temperature applications. Here, the low-temperature tensile properties of two Ti-based BMGCs containing stable β-Ti crystals with dislocation-mediated plasticity (Fe1) or shape memory β-Ti crystals (Fe0) were investigated. With temperature decreasing from 298 to 77 K, the strength of Fe1 increases but its tensile ductility decreases. In comparison, the shape memory Fe0 BMGC exhibits a simultaneous enhancement of strength and tensile ductility as temperature decreases from 298 to 143 K. The superior low-temperature properties of Fe0 are attributed to its combined structure of a glassy matrix and shape memory crystals. As temperature decreasing, the glassy matrix possesses an increased strength which softens as shear bands form, and the shape memory crystals are more inclined to deformation-induced martensitic transformation. Therefore, the combination of characteristic properties of metallic glasses and shape memory crystals in shape memory BMGCs can successfully overcome the low-temperature brittleness. These findings are believed to facilitate the development of BMGCs with superior low-temperature properties and promote their applications at low temperatures.

Effect of Al content on the mechanical properties and toughening mechanism of Zr-Co-Al alloys prepared by rapid solidification

Zr50-x/2Co50-x/2Alx (x = 0, 4, 7) alloys fabricated by water-cooled copper mold casting yields excellent mechanical properties like high strength and large plasticity. Al can reduce the stacking fault energy and simultaneously improve strength and yield strength of the alloys. With increasing Al content, the effect of grain refinement strengthening, solid solution strengthening and second phase precipitation strengthening are enhanced to a certain extent. The good plasticity of these alloys primarily originates from the dislocation entanglement at the end of dislocation pileup and the transformation-induced plasticity associated with B2-to-B33 deformation-induced martensitic transformation. Zr46.5Co46.5Al7 alloy shows the highest ultimate compressive strength of 1787 MPa, and the largest ultimate failure strain of more than 25%.