Deformation-induced Martensite Research Articles

An Ultra-Low Modulus of Ductile TiZrHfTa Biomedical High-Entropy Alloys through Deformation Induced Martensitic Transformation/Twinning/Amorphization.

Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is simultaneously achieving an ultra-low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi-component body-centered cubic (BCC) biomedical high-entropy alloy (Bio-HEA), which is composed of non-toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio-HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress-induced martensite transformation with extremely low trigger stress. The transformation-induced plasticity and work hardening capacity are achieved via the stress-induced martensite transformation. Additionally, the hierarchical stress-induced martensite twin structure and crystalline-to-amorphous phase transformation provide robust toughening mechanisms in the Bio-HEA. The cytotoxicity test confirms that this Bio-HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra-low Young's modulus, excellent biocompatibility, and decent ductility.

Probing the impact of grain size distribution on the deformation behavior in fine-grained austenitic stainless steel: A critical analysis of unimodal structure versus bimodal structure

This study investigated the effect of grain size distribution on the deformation behavior in fine-grained (FG) austenitic stainless steels (γ-SSs). Commercial Cr–Mn–N γ-SS was selected as the coarse-grained (CG) specimen, which was cold rolled and then annealed at 1000 °C for 60 s and 700 °C for 3600 s to obtain FG specimens with similar grain sizes but different grain size distributions. The CG specimens annealed at 1000 °C for 60 s and 700 °C for 3600 s were referred to as unimodal-FG and bimodal-FG specimens, respectively. The tensile properties revealed that the yield strength was enhanced from 383 MPa to 685 MPa when the grain size was refined from CG (10.7 μm) to FG (∼1.8 μm). Moreover, the bimodal-FG specimen exhibited a higher total elongation of 48% than 36.5% for the unimodal-FG specimen. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to identify the microstructural characteristics during tension. The results indicated that the main deformation mechanisms of γ-SSs specimens during tension were dislocation generation and accumulation at the initial stage, deformation twinning at the intermediate stage, and deformation-induced martensite (DIM) at the final stage. Compared with the unimodal-CG counterpart, grain refinement in the unimodal-FG specimens restricted the formation of DIM due to the increased austenite stability. A newly discovered phenomenon was that the bimodal-FG specimens promoted the formation of geometrically necessary dislocation (GND) and DIM compared to the unimodal-FG, which was due to the uneven deformation in grains located on hetero-boundaries (HBs). Moreover, the bimodal-FG specimen was preferentially formed DIM on HBs, especially in the smaller grains side rather than inside larger grains due to the higher stress concentration in smaller grains.

Characteristics of negative creep aging and its microstructure-oriented tensile behavior

UNS S17400 steel was creep-aged under a constant stress of 300 MPa at 480 °C and 600 °C for 1, 3, and 5 h, and its microstructural phase evolution was investigated. During the initial creep aging at 480 °C for 15 min, negative creep strain occurred because of the stress-assisted precipitation, which resulted in Cu-rich precipitates impeding the movement of dislocations. By contrast, the sample creep-aged at 600 °C had a typical creep-strain curve comprising a primary creep stage and secondary steady-state stage. The creep stress accelerated the growth of Cu-rich precipitates and austenite γ phase reversion, which were clearly observed in samples creep-aged at 600 °C. The reverted γ phase formed due to the stress-assisted aging at 480 °C for 3 h, as well as at 600 °C due to the elevated temperature aging. The samples with the longer creep aging times decreased in tensile yield strength but increased in strain hardening rate. Strain hardening after strain softening was observed in the sample creep-aged at 600 °C for 5 h, which had the highest strain hardening rate. Strain hardening was attributed to the dislocations becoming entangled with nanoscale Cu-rich precipitates, the formation of microtwins, and the presence of deformation-induced martensite.

Optimizing crack initiation energy in austenitic steel via controlled martensitic transformation

Although the seemingly negative effect of deformation-induced martensite (DIM) volume fraction on the impact toughness of austenitic steels has been well documented, it relies mostly on analyzing crack propagation without explicitly considering the crack initiation process which, however, plays a crucial role in these ductile alloys. The dependence of crack initiation energy (Ei) on martensitic transformation mechanisms is still ambiguous, inhibiting the precise design of damage-tolerant and ductile alloys. Here, we explore the temperature-dependent crack initiation energy of a SUS321 stainless steel at various temperatures (25, –50, and –196 °C). Contrary to the crack propagation energy (Ep), the Ei has a weak correlation with the volume fraction of α′-martensite but a strong correlation with the martensitic transformation rate. Also contrary to the traditional viewpoint of Ep considering ε-martensite as a detrimental phase, a high volume fraction of ε-martensite turns out to be beneficial to the increase of Ei, thereby enhancing impact toughness. As such, an optimal value (15 mJ/m2) for the stacking fault energy (SFE), which dictates the γ→ε→α′ transformation sequence, is given as a new design guideline for enhancing the Ei and consequently the impact toughness of ductile steels. The generality of this guideline is further validated in multiple austenitic steels with different compositions and grain sizes.

Atomic-scale understanding of twin intersection rotation and ε-martensite transformation in a high Mn twinning-induced plasticity steel

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibiting deviation angles of ∼0-15 o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions.

Formation of nano martensite grain with specific angle grain boundary during the drawing process of 316L stainless steel

The evolution of the deformation-induced martensite in the drawn 316L stainless steel and its effect on mechanical performance are examined. Both the yield strength and ultimate tensile strength of the 316L wire reach over 2.3 GPa at a drawing strain of 3.44. The deformation production of the wire during the drawing process is dislocation, twin and finally, deformation-induced α′-martensite. The neighboring misorientation between α′-martensite shows a preferential distribution around 60° due to variant selection limited by a double K–S orientation relationship. This preferential misorientation distribution around ∼60° is stable during the drawing process, and the α′-martensite has been refined to possess an average boundary spacing of ∼50 nm at a strain of 3.44.

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

Effect of high current pulsed electron beam on surface microstructure and properties of cold-rolled austenitic stainless steel

High current pulsed electron beam (HCPEB) was used to modify the surface microstructure and performance of AISI 304 stainless steel with different cold-rolling deformation. The phase, microstructure, microhardness, and corrosion performance were characterized and analyzed after cold-rolling and HCPEB treatment. The results show that after cold-rolling, deformation-induced martensites (DMs) are generated in the deformation twin lamellae or at the intersections of multiple twin lamellae, resulting in a grid-like morphology, smaller size, and a certain orientation relationship with the parent austenite. The content of DMs increases with increasing cold-rolling deformation. After HCPEB treatment, many craters with different sizes and similar densities are produced on the sample surface, and the DMs are transformed into austenite again in the surface layer. The difference is that the grain size of the regenerated austenite after HCPEB is much smaller than that of the initial austenite (before cold-rolling). Such high-density equiaxed ultrafine grains (UFGs) of austenite formed after HCPEB present a twin feature, which is the inheritance of the orientation relationship between DMs and austenite formed during rolling. HCPEB treatment can effectively reshape the phase and microstructure of the surface layer of the cold-rolled deformed austenitic steel but reduce the surface hardness and increase pitting corrosion tendency.

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.

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.

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

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

Dynamic Strain Aging Induced by Synergistic Effects of Deformation-Induced Martensite and Deformation Twins in Fe–Cr–Ni Metastable Austenitic Stainless Steel

Dynamic Strain Aging Induced by Synergistic Effects of Deformation-Induced Martensite and Deformation Twins in Fe–Cr–Ni Metastable Austenitic Stainless Steel

Effect of cooling rate on the final microstructure and tensile property in an Fe–Mn–Si–C-based multiphase TRIP steel

The effect of cooling rate on the formation of the final microstructure and the tensile property was studied in Fe–Mn–Si–C-based multiphase steel. Three different cooling rates of 5 °C/s, 10 °C/s, and 30 °C/s (CR5, CR10, and CR30) were applied from 800 °C to 400 °C and then the specimens held at 400 °C for 30 min. EPMA, EBSD, and TEM analyses clarified the constituent phases' volume fractions and the mechanical stability of retained γ. Appling a faster cooling led to a remarkable increase of tensile properties without a loss of elongation. This increase was accompanied by a higher fraction of bainite as the cooling rate increased All the specimens showed a similar fraction and mechanical stability of γ. In-situ tensile EBSD experiment revealed the accelerated deformation-induced martensite transformation (DIMT) in the faster cooling rate condition. The increased flow stress from a larger bainite fraction brought faster DIMT kinetics and improves tensile properties. This study provides a detailed understanding of the influence of cooling rate on microstructural evolution and tensile properties.

Strain-Induced Surface Roughening of Thin Sheets and Its Effects on Metal Forming and Component Properties

Straining of sheet metal leads to surface roughness changes. In this study, foils of AISI 201 and AISI 304 stainless steel were strained in uniaxial tension to impose roughening of their surfaces. Thereafter, the corrosion resistance, electrical resistivity, magnetic field density, and lubricated friction of the resulting surfaces were evaluated. The effect of strain-rate on the surface roughening, and thereby on the friction against tools, corrosion resistance, and occurrence of deformation-induced martensite was investigated. The AISI 304 material showed higher roughening than AISI 201 at low strain-rate. Lubricated friction is clearly affected by the changes to the surface of the strained foils that occur. When simulating a micro-forming process, the effect of strain-induced changes should be included where possible to maintain a high fidelity of the simulation. Strain-rate, in the range tested in this work, had only a minor effect on corrosion properties; however, the martensite fraction was reduced for material elongated at higher strain-rates.

The significant role of bimodal lamellar heterostructure for Lüders deformation and TRIP effect in 18Cr–8Ni austenitic stainless steel

A series of 18Cr–8Ni ASSs with homogeneous or heterogeneous structures were prepared to reveal the unique deformation mechanism of hetero-deformation-induced (HDI) hardening in lamellar heterostructure. By adjusting the annealing temperature, the nano-grained (NG) and coarse-grained (CG) unimodal homogeneous structures were obtained by annealing at 600 °C and 1000 °C, respectively, while the nano-grained/ultrafine-grained (NG/UFG) and ultrafine-grained/fine-grained (UFG/FG) bimodal heterogeneous structures were achieved by annealing at 700 °C and 800 °C. Compared to the conventional bimodal homogeneous structure (UFG/FG), the hetero-zone boundaries (HBs) of bimodal lamellar heterostructure (NG/UFG) can not only provide extra HDI hardening effect, but also coordinate the inhomogeneous deformation for Lüders deformation. The HDI hardening and Lüders deformation greatly facilitate the transformation induced plasticity (TRIP) effect to achieve. This is due to the lower austenite stability in soft region (coarser grain), leading to the formation of deformation-induced martensite (DIM) under lower stress, on the other hand, the Lüders deformation achieved a dynamic balance between both sides of hetero-boundary, avoiding the stress concentration and promoting the TRIP effect. Consequently, the DIM content of the NG/UFG specimen is considerably higher than that of the UFG/FG specimen (78 vol% to 27 vol%).

Simultaneously enhancing the strength and strain hardenability by Si substitution in metastable high-entropy alloys

We have demonstrated the potential to simultaneously enhance the strength and strain hardenability of metastable dual-phase high-entropy alloys (HEAs) by substituting the constituent transition metal element with Si. This leads to the development of a face-centered cubic single-phase Fe50Mn30Cr10Si10 HEA with excellent strength-ductility synergy, low density and low alloying costs. The substantial solid-solution hardening effect achieved through Si-alloying leads to a higher yield stress for this HEA compared to its dual-phase counterpart when their grain sizes are comparable. In addition, the Si-alloying imparts a low stacking fault energy (∼12.8 mJ/m2) to this HEA, facilitating the progressive formation of hexagonal close-packed martensite plates during plastic deformation. Furthermore, the Si-alloying increases the volume fraction of the deformation-induced martensite plates (up to 75.5%) and renders their thickness ultra-thin (<20 nm), thereby leading to remarkable strain hardenability.

Leakage failure analysis of a 60 m3 LNG storage tank

This work aimed to analyze a leak that occurred in a 60 m3 LNG storage tank made of 304 stainless steel. Optical microscopy (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and mechanical property and hardness tests were conducted to characterize the failed storage tank. The results show that the crack originates near the circumferential weld of the lower head and propagates perpendicular to the weld. There are dense corrosion pits near the crack paths, and high contents of Cl and S in the crack and pits. Besides this, deformation-induced martensite transformation occurs during the cold spinning process of the head, and the microstructure undergoes embrittlement. In service, the lower head subjected to greater internal pressure, plus the structural stress caused by martensite transformation and the welding residual stress, combined with the action of corrosive medium, stress corrosion cracking (SCC) occurs near the circumferential weld of the lower head. To prevent SCC on storage tank, it is suggested to utilize 304 stainless steel with a low content of δ-ferrite, and apply solution treatment after spin molding of the head of tank.

Can Martensitic Phase Transformation Measured by Magnetic Methods be an Indicator of Fatigue Damage in Austenitic Steel at Elevated Temperature and Thermo-Mechanical Loading?

The phenomenon of phase transformation from paramagnetic γ austenite to ferromagnetic α' martensite in metastable austenitic stainless steels under fatigue loading is well investigated, especially at ambient temperature. Generally, it can be said that at ambient temperature, the amount of martensite is proportional to the accumulated plastic strain and, therefore, can be an early indicator of fatigue damage. The problem arises at elevated temperatures when the thermal energy can severely or even completely inhibit the phase transformation. In this case, the amount of α'-martensite can no longer be a good indicator of fatigue damage. The question is, where is the limit the deformation-induced martensite can still be used as a fatigue damage indicator? To answer this question, several fatigue tests have been performed on AISI 347 (X6CrNiNb18-10), a metastable austenitic stainless steel often used in the piping of European nuclear powerplants. Tests included isothermal fatigue tests as well as thermo-mechanical fatigue (TMF) loading in the range of 25 to 320 °C. Deformation-induced martensite was measured in situ by means of a uniaxial magnetic balance, from which the rate of phase transformation was obtained. Fatigue specimens were then cut to investigate the distribution of martensite in the gauge length by means of magnetic force imaging. The results showed that isothermal fatigue loading results in a steady decrease in phase transformation rate with temperature. Initial martensite nucleation was observed to be different at elevated temperatures and occurred only at the surface, where sufficient shear deformation was present. TMF tests showed higher than isothermal rates of phase transformation. Peak temperatures above 300 °C were nevertheless enough to severely reduce the formation of α' martensite, even when cooled back to ambient temperature in the other half of the TMF cycle.

Tailoring the austenite characteristics via dual nanoparticles to synergistically optimize the strength-ductility in cold rolled medium Mn steel

In this work, we proposed a novel Cu/θ dual nanoparticles strategy to tailor the austenite characteristics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties. We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process. Both experiments and numerical simulations verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix. In addition, quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder the α/γ interface migration and the recrystallized grain growth, thereby further refining the final microstructure. With contributions from the effects of dual nanoprecipitates on the austenite reversion, the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation. Therefore, the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33% to 45%. This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.

Quantitatively evaluating respective contribution of austenite and deformation-induced martensite to flow stress, plastic strain, and strain hardening rate in tensile deformed TRIP steel

Transformation-induced plasticity (TRIP)-assisted steels exhibit an excellent combination of strength and ductility due to enhanced strain hardening rate associated with deformation-induced martensitic transformation (DIMT). Quantitative evaluation on the role of DIMT in strain hardening behavior of TRIP-assisted steels and alloys can provide guidance for designing advanced materials with strength and ductility synergy, which is, however, difficult since the phase composition keeps changing and both stress and plastic strain are dynamically partitioned among constituent phases during deformation. In the present study, tensile deformation with in situ neutron diffraction measurement was performed on an Fe-24Ni-0.3C (wt.%) TRIP-assisted austenitic steel. The analysis method based on stress partitioning and phase fractions measured by neutron diffraction was proposed, by which the tensile flow stress and the strain hardening rate of the specimen were resolved into factors associated with each phase, i.e., the austenite matrix, deformation-induced martensite, and the transformation rate of DIMT after differentiation, and then the role of each factor in the global strain hardening behavior was discussed. In addition, the plastic strain partitioning between austenite and martensite was indirectly estimated using the dislocation density measured by diffraction profile analysis, which constructed the full picture of stress and strain partitioning between austenite and martensite in the material. The results suggested that both the transformation rate and the phase stress borne by the deformation-induced martensite played important roles in the global tensile properties of the material. The proposed decomposition analysis method could be widely applied to investigating mechanical behavior of multi-phase alloys exhibiting the TRIP phenomenon.