Strain-induced Martensitic Transformation Research Articles

Experimental investigation on the spalling failure of a railway turnout made from Hadfield steel

In a rail track structure, a turnout is used as a transition between straight and branching rails and therefore withstands complex dynamic rolling and impact loading. This paper reports a comprehensive failure investigation of a railway off-track turnout made from Hadfield austenitic cast steel. The worn turnout was examined using optical microscopy, scanning electron microscopy, micro-hardness, and quantitative X-ray diffraction analyses. A ball-on-disc sliding wear test was also applied to evaluate the impact of work-hardening on the wear property. The turnout exhibited surface embrittlement, severe cracking, delamination, and spalling characteristics. Severe plastic deformation of the worn turnout was observed with the formation of densely packed mechanical twins, which facilitated crack propagation. The rail top developed a gradient hardness (HV0.1) profile from the rail top of 8.9 GPa, the cracked subsurface of 5.9 GPa to the bulk steel of 2 GPa. The rail top was also found to have accumulated an extremely high residual compressive stress of 500–650 MPa. Strain induced martensite transformation was repeatedly evidenced by X-ray diffraction peak of the ferritic (110) plane although the highly strained and nanocrystallised ferrite was only detected on the severely deformed rail top. Comparative sliding wear tests showed highly deteriorated wear resistance of the strain-hardened rail top as compared to the bulk steel.

An experimental and numerical study on strain controlled fatigue response of TRIP steel under influence of hydrogen

The present work aims to study the influence of hydrogen charging on the transformation induced plasticity steel under strain-controlled low cycle fatigue (LCF). Hydrogen charging had detrimental effect on strain-controlled LCF lives. Cyclic hardening in un-charged specimen due to strain-induced martensitic transformation (SIMT) dominated over cyclic softening. In contrast, generation of large fraction of low angle grain boundaries in hydrogen charged specimen leads to cyclic softening and supresses cyclic hardening due to SIMT. Additionally, Cyclic plasticity modelling using Ohno-Wang kinematic hardening model is validated by simulating cyclic stress–strain hysteresis loop and cyclic softening curve of uncharged and hydrogen charged specimens.

Phenomenological Model of Cavitation Erosion of Nitrogen ION Implanted Hiped Stellite 6

Abstract Stellites are a group of Co-Cr-C-W/Mo-containing alloys showing outstanding behavior under cavitation erosion (CE) operational conditions. The process of ion implantation can improve the CE resistance of metal alloys. This work presents the elaborated original phenomenological model of CE of nitrogen ion implanted HIP-consolidated (Hot Isostatically Pressed) cobalt alloy grade Stellite 6. The ultrasonic vibratory test rig was used for CE testing. The nitrogen ion implantation with 120 keV and fluence of 5 × 1016 N+/cm−2 improves HIPed Stellite 6 cavitation erosion resistance two times. Ion-implanted HIPed Stellite 6 has more than ten times higher CE resistance than the reference AISI 304 stainless steel sample. Comparative analysis of AFM, SEM and XRD results done at different test intervals reveals the kinetic of CE process. The model includes the surface roughness development and clarifies the meaning of cobalt-based matrix phase transformations under the nitrogen ion implantation and cavitation loads. Ion implantation modifies the cavitation erosion mechanisms of HIPed Stellite 6. The CE of unimplanted alloy starts on material loss initiated at the carbides/matrix interfaces. Deterioration starts with cobalt matrix plastic deformation, weakening the carbides restraint in the metallic matrix. Then, the cobalt-based matrix and further hard carbides are removed. Finally, a deformed cobalt matrix undergoes cracking, accelerating material removal and formation of pits and craters’ growth. The nitrogen ion implantation facilitates ɛ (hcp—hexagonal close-packed)) → γ (fcc—face-centered cubic) phase transformation, which further is reversed due to cavitation loads, i.e., CE induces the γ → ɛ martensitic phase transformation of the cobalt-based matrix. This phenomenon successfully limits carbide removal by consuming the cavitation loads for martensitic transformation at the initial stages of erosion. The CE incubation stage for ion implanted HIPed Stellite 6 lasts longer than for unimplanted due to the higher initial content of γ phase. Moreover, this phase slows the erosion rate by restraining carbides in cobalt-based matrix, facilitating strain-induced martensitic transformation and preventing the surface from severe material loss.

Strain-rate dependent deformation behavior of additively manufactured stainless steel with different fractions of δ-ferrite

The effect of strain rate on the tensile deformation behavior of additively manufactured CrNi stainless steel with different fractions of δ-ferrite (14 % in the as-built material and 6 % after the post-built solid-solution treatment) was revealed with the focus on the deformation stage of macroscopic strain-induced γ → α' martensitic transformation. Tension tests were performed with the initial strain rates of 5 ×10−4, 5 ×10−3, and 5 ×10−2 s−1 at two different temperatures, at which slow (300 K) and fast (183 K) kinetics of martensitic transformation were realized. Post-built solid-solution treatment provided a partial dissolution of the δ-ferrite in the additively manufactured steel specimens and assisted higher strain hardening at the stage associated with γ → α' martensitic transformation. Independently on δ-ferrite fraction and test temperature, an increase in the strain rate weakly influences the stress and strain, at which the stage of macroscopic strain-induced transformation starts, but visibly decreases the strain hardening rate at this deformation stage. The latter testifies the slower kinetics of strain-induced martensitic transformation at higher strain rates.

Properties of medium-manganese steel processed by laser powder bed fusion: The effect of microstructure in as-built and intercritically annealed state on energy absorption during tensile and impact tests

The presented research was aimed at applying laser powder bed fusion (LPBF) technology and TRIP (Transformation Induced Plasticity) steel with medium Mn content to produce thin-walled parts with enhanced crashworthiness. It was expected that the simultaneous use of the excellent combination of strength and ductility of the new type of medium-Mn steel and the advantages of LPBF technology would allow the development of an alternative to the conventional method of manufacturing the energy absorbing components. The energy absorption capability of LPBF-processed Fe-0.15C-7.5Mn-1.8Al steel was evaluated based on its deformation behavior under monotonic and impact loading. Tensile and Charpy V-notch specimens were fabricated by LPBF with layer thicknesses of 30 and 60 μm, using processing windows set established according to the criteria of low porosity (<0.3%) and good surface quality. Specimens were tested in as-built state and after intercritical annealing at a constant temperature (670 °C), but for different holding times and cooling rates to obtain a large fraction (30–50 vol%) of retained austenite capable of strain-induced martensitic transformation. Based on multiscale microstructure examinations (OM, SEM, EBSD, XRD, EDS) the relationships between the morphology of LPBF-specific microstructures before and after intercritical annealing, as well as between their behavior in tensile and impact test, were analyzed and discussed. For both layer thicknesses, the best combination of strength, elongation and impact toughness was obtained after annealing at 670 °C for 6 h followed by argon cooling. LPBF technology has been shown to be suitable for the fabrication of thin-walled energy-absorbing components from medium-manganese TRIP steel with the potential to reduce fabrication time by doubling the layer thickness compared to standard.

In Situ Observation of Retained Austenite Transformation in Low-Carbon Micro-Alloyed Q&amp;P Steels

Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&amp;P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&amp;P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&amp;P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&amp;P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

Gradient Microstructure and Texture Formation in a Metastable Austenitic Stainless Steel during Cold Rotary Swaging

The paper aimed to study the evolution of the microstructure and texture gradient of a 321-type metastable austenitic stainless steel during cold rotary swaging. Cold rotary swaging was carried out with a reduction of up to 90% at ambient temperature. Pronounced gradients of the α'-martensite volume fraction, the axial texture of austenite (⟨111⟩ and ⟨001⟩) and α'-martensite (⟨101⟩), and non-uniform microhardness distribution along the rod diameter were obtained after a reduction of 80-90%. According to the finite element analysis, moderate tensile stresses were attained in the center, whereas high compressive stresses operated at the edge. Due to water cooling of the rod surface and heating of the rod center during processing, a temperature gradient was also derived. Features of strain-induced martensitic transformation, microstructure and texture evolution, and non-uniform hardening during cold rotary swaging were discussed.

Plasticity Improvement in a Co-Rich Co40Fe25Cr20Ni15 High-Entropy Alloy via Al Alloying.

The mechanical properties of high-entropy alloys (HEAs) can be regulated by altering the stacking fault energy (SFE) through compositional modulation. The Co-rich HEAs, exhibiting deformation twinning and even strain-induced martensitic transformation at room temperature, suffer from insufficient ductility at high strength. In this work, we developed Co-rich (Co40Fe25Cr20Ni15)100-xAlx (x = 0 and 5 at.%) HEAs and investigated their tensile behaviors at room temperature. The addition of Al resulted in a massive improvement in the strength-ductility product, even at similar grain sizes, and also altered the fracture mode from quasi-cleavage to ductile dimple fracture. Interestingly, both alloys were deformed by mechanical twinning, which was also verified by molecular dynamics (MD) simulations. The MD simulations revealed the SFE increased upon Al addition; however, the slip energy barrier was reduced, which favored the mobility of dislocations and twinning propensity to prolong strain hardening. The present findings provide further insights into the regulation of mechanical properties of HEAs by Al-alloying.

Temperature-Dependent Deformation Behavior of “γ-austenite/δ-ferrite” Composite Obtained through Electron Beam Additive Manufacturing with Austenitic Stainless-Steel Wire

Temperature dependence of tensile deformation behavior and mechanical properties (yield strength, ultimate tensile strength, and an elongation-to-failure) of the dual-phase (γ-austenite/δ-ferrite) specimens, obtained through electron-beam additive manufacturing, has been explored for the first time in a wide temperature range T = (77–300) K. The dual-phase structures with a dendritic morphology of δ-ferrite (γ + 14%δ) and with a coarse globular δ-phase (γ + 6%δ) are typical of the as-built specimens and those subjected to a post-production solid–solution treatment, respectively. In material with lower δ-ferrite content, the lower values of the yield strength in the whole temperature range and the higher elongation of the specimens at T &gt; 250 K have been revealed. Tensile strength and stages of plastic flow of the materials do not depend on the δ-ferrite fraction and its morphology, but the characteristics of strain-induced γ→α′ and γ→ε→α′ martensitic transformations and strain-hardening values are different for two types of the specimens. A new approach has been applied for the analysis of deformation behavior of additively fabricated Cr-Ni steels. Mechanical properties and plastic deformation of the dual-phase (γ + δ) steels produced through electron beam additive manufacturing have been described from the point of view of composite materials. Both types of the δ-ferrite inclusions, dendritic lamellae and globular coarse particles, change the stress distribution in the bulk of the materials during tensile testing, assist the defect accumulation and partially suppress strain-induced martensitic transformation.

Microscale Strain Localizations and Strain-Induced Martensitic Phase Transformation in Austenitic Steel 301LN at Different Strain Rates

Microscopic strain and strain-induced phase transformation during plastic deformation in metastable austenitic steel were investigated at different strain rates. Quasi in-situ tension tests were performed sequentially with well-defined elongation intervals at room temperature at strain rates of 10−3 s−1 and 10−1 s−1. The tests were monitored by high-resolution optical imaging with a microscopic lens at a resolution of 0.23 µm/pixel. The macroscopic temperature was also measured with an infrared (IR) camera. The microstructure-level strain localizations were observed on the surface of the etched specimens by means of microscale digital image correlation (µDIC). Additionally, the microstructure was characterized by electron backscatter diffraction (EBSD) at the same location before and after deformation. The results of the study indicated that microscopic strain localizations favored the formation of α′-martensite particles. At the lower strain rate, high local strain concentrations formed at several locations in the microstructure, correlating with the areas where the formation of large martensite islands occurred. Martensite particles of various sizes formed nearby each other at the lower strain rate, whereas at the higher strain rate, martensite islands remained small and isolated. Although the macroscopic increase in temperature at both the studied strain rates was very low, at the higher strain rate, local heating on the microscopic scale could take place at the newly nucleated martensite embryos. This inhibited the further growth of the martensite particles, and local strain distribution also remained more homogeneous than at the lower strain rate.

A systematic study on microstructure evolution, mechanical stability and the micro-mechanical response of tensile deformed medium manganese steel through interrupted tensile test

In the current work, the microstructure evolution, mechanical stability of austenite and the micro-mechanical response of tensile deformed Fe-0.18C-4Al-7Mn (Wt%) steel are investigated. Interrupted tensile tests on the annealed samples (750 °C, 1 h) were performed at a tensile elongation of 5, 10, 20, 30, 40 and 60% to understand the deformation behavior and transformation-induced plasticity (TRIP) effect. Strain-induced martensitic transformation (SIMT) of reverted austenite is explored using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) analysis. The coefficient of mechanical stability of austenite (k-parameter) is quantified through the interrupted tensile tests using XRD analysis. Instrumented nanoindentation test was conducted on individual phases of the interrupted tensile-tested samples to determine their hardness and modulus values. The scanning electron microscopy (SEM) analysis reveals the gradual decrease in width and increase in length of austenite-martensite islands with an increase in the percentage of elongation. Evolution of dislocation lines, dislocation tangles and dislocation accumulations occur at ferrite grain boundaries and martensite lath boundaries during tensile deformation. The volume fraction of austenite decreases, and the transformation of austenite to martensite and dislocation density of austenite increase with the increase in the tensile elongation. Austenite is found to be mechanically more stable at 30% of tensile elongation due to the low austenite transformation ratio. Nanoindentation results show that the hardness increases with an increasing percentage of deformation. Multiple pop-ins are observed in load (P)–indentation depth (h) curves due to dislocation interaction at the interphase boundary.

Cyclic deformation response and strain-induced martensitic transformation of β single phase Ti-35421 alloy

Compared with bimodal microstructure, the low-cycle fatigue (LCF) behavior and cyclic deformation mechanism of a near β Ti-3Al-5Mo-4Cr-2Zr-1Fe (Ti-35421) alloy with single β were systematically investigated. Lower strength, but higher ductility and longer LCF life were found for Single β. The nanoscale α'' martensite was detected in Single β by selected area electron diffraction pattern. Besides, slips and strain induced martensitic transformation played the major deformation mechanisms in Single β. Moreover, the existence of α'' contributed to the limit of microcracks nucleation in Single β, which resulted in a better fatigue life.

A novel Fe–Cr–Ni–Co–Mo maraging stainless steel with enhanced strength and cryogenic toughness: Role of austenite with core-shell structures

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.

Improvement of mechanical properties of Co–Cr–W–Ni alloy tube suitable for balloon-expandable stent applications through heat treatment

This study focuses on the changes in the microstructure and mechanical properties of the Co–20Cr–15W–10Ni (CCWN, mass%) alloy tube, which, with heat treatment, is used as a balloon-expandable stent. To the best of our knowledge, this is the first report on the improvement of mechanical properties by microstructure control in CCWN alloy tubes. In addition, optimized heat treatment conditions are proposed for the development of next-generation small-diameter stents. The cold-worked CCWN alloy tube for stents was statically recrystallized by heat treatment at temperatures of 1373–1473 K for holding times of 60–900 s. The recrystallized alloy tubes exhibited a microstructure with a γ(fcc)-phase matrix and an average grain size of 5.8–34.1 μm. The strength and plastic elongation increased with decreasing grain size. In the coarse-grained structure, a large plate-like ε-phase formed along the twin boundary and stacking fault. The ε-phase grew to the grain boundaries, such that large cracks could form, resulting in early fracture. In the fine-grained structure, a small ε-phase formed by strain-induced martensitic transformation was uniformly dispersed, and the formation of large cracks was suppressed, leading to high ductility. The effect of low-temperature heat treatment (LTHT) at 873 K after recrystallization depended on the grain size of the alloy tube. In the alloy tube with a grain size of >15 μm, ε(hcp)-phase formation during plastic deformation was suppressed through the decrease in stacking fault density during LTHT, which resulted in improved ductility. In contrast, in the alloy tube with a grain size of <15 μm, the amount of precipitates increased during LTHT, resulting in reduced ductility.

Plasticity enhancement mechanism: Effect of the annealing temperature on strain-induced segmented martensitic transformations and Portevin–Le Chatelier bands in 7Mn steel after deep cryogenic treatment

This study investigated the microstructural evolution and strain localization of warm deformed Fe-0.15C-7Mn steel after deep cryogenic treatment (DCT) and intercritical annealing (IA) at 600, 630, and 660 °C. After DCT and IA, the steel contained nanoscale austenite grains with inhomogeneous thermal stability and grain size. Lüders bands appeared in all the samples, whereas the Portevin–Le Chatelier (PLC) effect only occurred in the samples annealed at 630 and 660 °C. The occurrence and initiation position of the PLC bands depended on the IA temperature, which affected the final grain size, retained austenite (RA) stability, and interactions between the solute atoms/C-Mn pairs and mobile dislocations. This indicates that segmented martensitic transformations occurred in the samples annealed at 600 and 630 °C, and the transformation sequence γ → ε → α’ occurred in the samples annealed at 630 and 660 °C. The segmented martensitic transformation depended on the IA temperature, which directly affected the amplitude of fluctuations in the Mn concentration in the RA grains and resulted in the heterogeneous thermal stability of the RA. The excellent uniaxial tensile properties of the samples were ascribed to the transformation of RA with heterogeneous thermal stability to segmented martensite during deformation. The segmented martensite acted as a protective shell that shielded the encapsulated austenite in the core. • Microstructural evolution and strain localization of a medium Mn steel were studied. • The annealing temperature affects the PLC bands and microstructural evolution. • Relationship between heterogeneous stability of RA and martensitic transformations. • Mechanisms underlying the observed physical properties were investigated. • Segmented martensite transformation behavior occurs and plays a protective role in the encapsulated austenite.

An in situ synchrotron X-ray study of reverse austenitic transformation in a metastable FeMnCo alloy

This study concerns reverse austenitic transformation of plastic strain-induced hexagonal close-packed martensite. With the aid of in situ synchrotron X-ray diffractometry, the kinetic features of the transformation and the defect content evolution in a metastable (Fe60Mn40)85Co15 alloy are quantitatively examined using 5, 20, and 100 °C/min heating rates. It is found that the reverse austenitic transformation can be activated below 200 °C and completes within a short time scale. Through a Kissinger-style kinetic analysis, the activation energy of the reverse austenitic transformation is determined as 171.38 kJ/mol, confirming its displacive nature. Although exponential attenuation is observed in both stacking fault probability and dislocation density upon the initiation of the transformation, the resulting microstructure (single-phase face-centered cubic structure) remains highly defected, exhibiting high Vickers hardness, but still preserving somewhat strain hardenability. Atomistic mechanisms for the reverse austenitic transformation are further conceived according to the crystallographic theory of martensitic transformation.Graphical abstract

Ductilization of high carbon, high silicon carbide-free nanostructured bainitic steel

In the present study, the influence of the volume fraction of blocky austenite on the microstructure and mechanical properties of novel bainitic steel was investigated. The rolled homogenized sheets were austenitized at 950 °C and austempered at 250 °C for 18h (B15VA-1) and 50 h(B15VA-2). X-ray diffraction study of B15VA-1 & B15VA-2 confirmed the presence of carbide-free nanostructured bainitic ferrite (BF) and retained austenite (RA). Optical, scanning and transmission electron microscopic studies delineate the morphology of retained austenite to filmy and blocky forms. On the tensile deformation, majority of blocky austenite in B15VA-1 transformed to strain-induced martensite of ε type. Hardness and yield strength (YS) were increased with an increasing amount of bainitic ferrite, however, ultimate tensile strength (UTS) was found to be a strong function of the extent of strain-induced martensitic (SIM) transformation. As a result of the high amount of SIM, UTS of B15VA-1 increased to 1889 MPa. Ductility is mainly controlled by the transformation-induced plasticity of blocky austenite. Uniform plastic elongation and total plastic elongation were found to be 25% and 28%, respectively. The work hardening behavior can be described by the Swift model. Deformation under tensile load takes place in three stages which are homogeneous deformation, strain-induced martensitic transformation and multiphase deformation-led interaction of dislocations and their recovery. The alloy with nanostructured bainitic ferrite fails by mixed mode of ductile-brittle fracture.

Enhancing the strength–ductility synergy of medium-Mn steel by introducing multiple gradient structures

In this study, novel multiple gradient structures with various distributions of austenite/martensite phase, grain sizes, and dislocation densities were fabricated in medium-Mn steel by using torsion treatment, and the corresponding tensile properties and deformation mechanisms were investigated. The results showed that the yield strength and the total elongation of the multiple-gradient-structured samples increased by 27% and 25%, respectively, compared with their homogeneous counterparts. A distinct phase transformation behavior was observed in the present gradient medium-Mn steel. That is, the strain-induced martensitic transformation was promoted in the center during the entire tensile deformation process; however, it was suppressed at the surface during the initial deformation stage and then significantly triggered during the remaining deformation process. Moreover, active strain partitioning at the macroscale and microscale occurred during plastic deformation, which led to a higher hetero-deformation induced (HDI) stress in the multiple-gradient samples. A combination of a strong and persistent transformation-induced plasticity effect, active HDI strengthening and HDI hardening, and dislocation strengthening contributed to the superior strength–ductility synergy of the gradient-structured medium-Mn steel.

Alternative Approach for the Intercritical Annealing of (Cr, Mo, V)-Alloyed TRIP-Assisted Steel before Austempering

TRIP-assisted C-Si-Mn steels are usually subjected to austempering with a preliminary intercritical annealing that is targeted at the multi-phase structure with 40–60 vol.% of proeutectoid ferrite. The kinetics and the mechanism of phase-structural transformations can be impacted due to the additional alloying of TRIP-assisted steel by the strong carbide forming elements, thus necessitating an alternative approach for the selection of intercritical annealing parameters. This issue is analyzed in the present work, which investigates the effect of the temperature of intercritical annealing on the “Structure/Properties” correlations in 0.2 wt.% C-Si-Mn-Nb steel additionally alloyed by 0.55 wt.% Cr, 0.20% Mo, and 0.11 wt.% V. The annealing temperature ranged from 770 °C to 950 °C, and austempering was performed at 350 °C for 20 min. It was observed that the addition of the (Cr, Mo, and V) complex significantly improved the steel hardenability. However, the annealing of steel at 770 °C (to gain 50 vol.% of proeutectoid ferrite) resulted in the precipitation of coarse cementite lamellas during bainite transformation, thus lowering the amount of retained austenite (RA) and decreasing the strength and ductility of the steel. At higher annealing temperatures, carbide-free bainite was formed, which presented a 2.5–3.5 times increase in the RA volume fraction and a 1.5 times increase in the RA carbon content. The optimal combination of the mechanical properties (UTS of 1040 MPa, TEL of 23%, V-notch impact toughness of 95 J/cm2, PSE of 23.9 GPa·%) referred to annealing at a temperature close to the Ac3 point, resulting in a structure with 5 vol.% ferrite and 9 vol.% RA (the residue was carbide-free bainite). This structure presented an extended manifestation of the TRIP effect with an enhanced strain hardening rate due to strain-induced martensite transformation. The impact of the alloying elements on the carbon activity in austenite served as the basis for the analysis of structure formation.

Adjustable room temperature deformation behavior of Mg–Sc alloy: From superelasticity to slip deformation via TRIP effect

This study investigated the transformation-induced plasticity (TRIP) effect in Mg–Sc alloys with β single-phase (body-centered cubic) structure. A Mg-19.8 at% Sc alloy showed fracture strain of ∼53 % accompanied by necking propagation with increasing the tensile strain at room temperature. An X-ray diffraction analysis and transmission electron microscopy observations confirmed the existence of a hexagonal close-packed phase beside the β matrix phase during and after the tensile test; this indicates a strain-induced martensitic transformation, i.e., the TRIP effect induced superior ductility. Moreover, the study revealed a threshold Sc content of ∼19.5 at% Sc at which the deformation behavior at room temperature changes from superelasticity to TRIP. The TRIP Mg–Sc alloys exhibited fracture strain from 45 % to 66 % and ultimate tensile strength (UTS) above 220 MPa to overcome the tradeoff between the fracture strain and UTS of conventional Mg alloys.