Strain-induced Martensitic Transformation Research Articles

High-Speed Erichsen Testing of Grain-Refined 301LN Austenitic Stainless Steel Processed by Double-Reversion Annealing

Austenitic Cr–Ni stainless-type 301LN steel was subjected to a double-reversion annealing (DRA) treatment to develop bulk grain-refined microstructures. The tensile properties and formability of the DRA structures were determined by high-speed tensile and Erichsen cupping tests at a strain rate of 1.5 s−1 (50 mm s−1) and compared with those of coarse-grained steel. Detailed microstructural features of the DRA structures were characterized using the electron backscatter diffraction technique and X-ray diffraction analysis. The DRA structures achieved by annealing for 1 second at 800 °C and 900 °C exhibited a superior combination of yield (~ 950 and 770 MPa, respectively) and tensile (~ 1050 and 950 MPa, respectively) strengths and ductility (~ 35 and 40 pct, respectively, as well as reasonable Erichsen index values under high-speed biaxial strain. Due to adiabatic heating, the DRA structures had higher austenite stability during high-speed stretch forming, i.e., were less prone to strain-induced martensitic transformation. The finite-element method (FEM) was used to conduct coupled field thermomechanical analyses of the high-speed deformation processes for the coarse-grained and DRA structures. Comparison of the FEM analyses with the experimental results revealed a considerable influence (~ 20 pct) of martensitic transformation on the adiabatic temperature rise. The balance of the yield strength and Erichsen index value of the developed nanograined microstructure is comparable to that of coarse-grained commercial steel.

Compressive deformation behavior and energy absorption characteristic of additively manufactured sheet CoCrMo triply periodic minimal surface lattices

CoCrMo sheet triply periodic minimal surface (sheet-TPMS) lattices with two different topologies (Neovius, IWP) were fabricated by laser power bed fusion (LPBF). Compressive behavior and energy absorption characteristic for each topology and direction were investigated. The unique pore channel structures of the fabricated Neovius and IWP lattices were well formed. The microstructural observation revealed that fine cellular substructures existed inside the sheets, and most of their constituent phases were confirmed to be the FCC phase. The results of the compression tests demonstrated that Neovius lattice had higher yield strength and first peak strength than IWP lattice, regardless of the compressive direction. As for the stress–strain curves of the lattices, an abrupt stress drop was found in neither of the two topologies. A comparison of the energy absorption characteristics confirmed that Neovius lattice had higher energy absorption efficiency and ideality than IWP lattice. The deformed structures showed that Neovius lattice displayed a gradual collapse mode, whereas a layer-by-layer compaction mode was indicated in IWP lattice. After the deformation, Neovius showed an evenly distributed and high fraction of the HCP phase caused by strain-induced martensitic transformation. The topology of lattice and strain-induced martensitic transformation effects on the compressive mechanical responses of the LPBF-built TPMS lattices were also discussed.

Mechanical behaviour and micromechanical modelling of medium-Mn steel microstructure evolution

Intense research efforts are focused on the development of advanced medium-manganese steels intended for lighweight automotive components and other applications, where high strength and good formability are crucial. The mechanical and technological properties of such steels depend on the strain-induced martensitic transformation (SIMT) of metastable retained austenite (RA), which is affected by several factors including a deformation temperature. The response of the retained austenite to strain-induced martensitic transformation at various temperatures (-60 °C-200 °C) was studied in the advanced medium-Mn 0.17C-3.3Mn-1.7Al-0.22Si steel via tensile testing. It was found that the efficiency of martensitic transformation decreases with increasing the deformation temperature in a range from -60 °C to 100 °C. At 140 °C and 200 °C, the strain hardening ability and strain-induced martensitic transformation were additionally affected by thermally activated processes. The obtained experimental data was applied into a thermodynamic model in order to predict the temperature-dependant stability of retained austenite. Furthermore, the micromechanical modelling using the finite element method was utilized to perform an analysis at the microscale level of the SIMT of the RA during plastic deformation. The experimental and modelling approaches were found to be complementary methods to comprehensively evaluate the mechanical response of investigated steel as well as microstructure evolution of structural constituents at progressive strain levels.

Solid-state depositions of multilayer SS304 by friction stir metal deposition

The present work aims to analyse the microstructure and mechanical properties of solid-state depositions of multilayer SS304 by friction stir metal deposition. The microstructure analysis suggests continuous dynamic recrystallization as the main mechanism for grain refinement in the deposited region. The grains at the interface of deposited layers are significantly refined due to severe plastic deformation. Further, the reduction of twin boundary and martensite fractions is noticed in the bulk of the layers. However, strain-induced martensitic transformation is noticed at the interface of the successively deposited regions, due to relatively high stresses owing to lower initial temperatures. The average tensile strength and fracture strain of deposited SS304 is 692 ± 14 MPa and 87.3± 16%, respectively.

Temperature-Stimulated Morphological Features of Advanced High-Strength Medium-Mn TRIP Steel.

Advanced High-Strength Steels (AHSSs) are one of the most rapidly developing group of Fe-based metallic materials. Their excellent combination of high strength, ductility and formability is due to their complex microstructure and strain-induced martensitic transformation of metastable retained austenite (RA), which favors extra ductility of the sheet steels. A deformation temperature is one of the most important factors affecting the phase transformation behavior in these Fe–C–Mn–Al–Si systems. Therefore, the present study aimed at understanding the temperature-dependent phase transformations and structural phenomena in an advanced medium-Mn–Al-alloyed steel. The 3Mn steel was thermomechanically processed and subjected to tensile testing in a temperature range from 20°C to 200°C. The different extent of the strain-induced martensitic transformation and some softening phenomena of bainitic ferrite matrix were revealed using transmission electron microscopy and electron backscatter diffraction techniques. It was found that the thermal stability of RA is strongly dependent on the deformation temperature. Moreover, the dynamic recovery and carbide precipitation play a key role when the deformation temperature is increased to 140°C and higher temperatures.

A novel crystal plasticity model incorporating transformation induced plasticity for a wide range of strain rates and temperatures

Transformation induced plasticity (TRIP) is an effect common to several classes of advanced high strength steels (AHSS) with promising automotive applications. This effect is characterized by the transformation of retained austenite (RA) to martensite, resulting in increased hardening, increased fracture resistance, and improved formability. Accurate thermo-mechanical modeling over a range of strain-rates and temperatures is critical to fully utilize the improved performance of AHSS exhibiting the TRIP effect. In this work, a novel thermodynamically consistent rate-dependent crystal plasticity formulation is developed, which incorporates strain-rate and temperature dependent strain-induced martensitic transformation. Thermodynamic arguments are used to derive plastic slip and transformation driving forces accounting for various physical mechanisms (e.g. applied stress, temperature, crystal orientation, stored dislocation energy), as well as a constitutive law governing temperature evolution. RA and transformed martensite mechanical thermo-elasto-viscoplastic behavior is explicitly modeled, and a modified Taylor homogenization law is proposed to determine strain partitioning while accounting for transformation. The model is then calibrated and validated for a QP3Mn alloy over a large range of temperatures (-10°C–70°C) and strain-rates (5 × 10-4s−1–200s−1). The evolution of the Taylor–Quinney coefficient and the orientation dependence of transformation are found to match well with trends in literature. The fully calibrated model is compared to a model recalibrated without strain-rate dependent transformation, demonstrating that capturing strain-rate dependent transformation may be necessary even for materials where no direct experimental strain-rate dependence. Plane strain and equibiaxial tension simulations are conducted using the calibrated model, showing that increasing triaxiality results in increased transformation. An extension to the calibrated model is proposed for materials which do not match the predicted trend.

A Computational Simulation on Size-dependent Plastic Behavior in Polycrystalline Austenitic Steel based on the Notion of Microforce

Steel containing the metastable austenitic phase shows excellent mechanical properties such as strength, ductility and toughness. The transformation-induced plasticity (TRIP) in the steel can optimize these mechanical properties. Since strain-induced martensitic transformation (SIMT) causes TRIP, deep understanding of the continuous development of SIMT is very important to control the mechanical properties by TRIP. In addition, a dislocation attributes to the strain hardening so it is very important to clearly explain its complexity. It is necessary to construct a model on the dislocation deeply related to size effects. In this study, a hardening model and yield conditions for monocrystalline austenitic steel are established based on the notion of microforce. By incorporating the finite element crystal plasticity method, the two-dimensional unit cells of polycrystalline austenitic steel are obtained by Voronoi tessellation, and the deformation behavior and microstructure are simulated computationally.

Modeling of the Kinetics of Strain‐Induced Martensite Transformation and the Transformation‐Induced Plasticity Effect in a Lean‐Alloyed Metastable Austenitic Stainless Steel

This article investigates the influence of temperature and strain on second‐phase transformation strengthening and the resulting mechanical properties in a lean AISI 301LN austenitic stainless steel within a temperature range of −60 to 180 °C. The volume fraction of martensite evolved is determined using nondestructive magnetic Ferritescope measurements that are adjusted by using a calibration factor of 1.7, which is established using the saturation magnetization measurements, X‐ray, and neutron diffraction measurements. The kinetics of strain‐induced martensite transformation (SIMT) as a function of strain and temperature is accurately described by a set of modified constitutive Boltzmann sigmoidal equations at temperatures below 75 °C. For this steel, the Md (30/50) temperature is determined as 61 °C. The absolute Md temperature is established as ≈109 °C, and no athermal transformation to martensite is observed upon cooling to −270 °C using cryogenic neutron diffraction facilities. Extended JMAK analysis of the transformation is used to shed light on the mechanism of martensitic transformation. It is found that the transformation‐induced plasticity (TRIP) effect due to SIMT is at a maximum at 75 °C, which is the maximum elongation temperature (MET) and calculations are performed regarding alloy development which will reduce the MET to room temperature.

Wear Behaviors of Stainless Steel and Lubrication Effect on Transitions in Lubrication Regimes in Sliding Contact

The wear behavior of AISI304 stainless steel was investigated under dry, water-, and oil-lubricated conditions. A block-on-disk wear test was conducted in this work, since the test conditions could be controlled easily. For oil-lubricated contact, a significant amount of thin and elongated cutting chip-like debris was observed. This is attributed to the high lubricating effect of oil. Strain-induced martensitic (SIM) transformation was observed for all AISI304 blocks after the wear test, while AISI304 consisted of a single γ-austenite phase prior to the wear test. The Stribeck curve and the corresponding lubrication regimes were also considered to explain the wear behaviors and lubrication effect of AISI304. In comparison to the dry or water-lubricated conditions, which fall in the boundary lubrication regime at a low rotation speed, it is considered that the high viscosity of the oil-based lubricant causes the lubrication condition to enter the mixed lubrication regime early at a lower speed, thus reducing the specific wear rate over the 100–300 rpm range.

Molecular dynamics simulation of vacancy and void effects on strain-induced martensitic transformations in Fe-50 at.% Ni model concentrated solid solution alloy

Strain-induced martensitic transformations can improve the strength and ductility of some face centered cubic (FCC) metals and alloys. Irradiation-induced defects such as vacancies, dislocation loops, and voids activate martensitic transformations over a wider range of mechanical loading conditions than in pristine material. However, the mechanisms underlying irradiation-enabled martensite transformations remain unclear. In this work, we use molecular dynamics simulations to study the effect of vacancies and voids on strain-induced martensitic transformations in a model concentrated solid solution alloy Fe-50 at.% Ni. It has been found that single vacancies have no resolvable effect on the transformation because they reduce the stacking fault energy by a relatively insignificant margin and do so only if the vacancy is located on the stacking fault plane. Voids, however, activate the martensite transformation through shear strain accumulation around the void due to dislocation pileup. The larger the void, the more pronounced this effect.

Studies of Bainitic Steel for Rail Applications Based on Carbide-Free, Low-Alloy Steel

This paper describes the development and characterisation of bainitic steel for rail applications based on carbide-free, low-alloy steel. The results show that after rolling and subsequently cooling, the designed carbide-free bainitic steel exhibits better mechanical performance than standard pearlitic steel. This is because of its fine, carbide-free bainitic microstructure, which consists of bainitic ferrite and retained austenite laths. Microstructural and mechanical property analysis was carried out using scanning and transmission electron microscopy, X-ray diffraction, hardness measurements, tensile and low-cycle fatigue tests. The obtained results demonstrate that during low cyclic deformation, a partial transformation of the retained austenite into deformed martensite α′ takes place, and strain-induced martensitic transformation occurs. The initial strengthening of the material during low-cycle fatigue was caused by the transformation of austenite into martensite and the increase in the dislocation density of the steel. In addition, an optimal amount of retained austenite in the form of thin layers and islands (dimensions not exceeding 1 µm) made it possible to obtain a high yield while maintaining the high plasticity of the steel. These microstructural features also contributed to the high crack resistance of the tested carbide-free bainitic steel.

Flow and fracture of austenitic stainless steels at cryogenic temperatures

In this paper, we have characterized the microstructural evolution and the plastic flow and fracture behaviours of AISI 304L and AISI 316LN stainless steel grades at liquid nitrogen temperature (77K) and at liquid helium temperature (4K). Uninterrupted tensile experiments, where the sample is continuously deformed under quasi-static loading conditions until fracture, have been carried out with a Single-Section Sample to obtain the stress–strain characteristics of the two grades. Interrupted tensile experiments, in which the sample is unloaded before fracture, have been performed with a novel Double-Section Sample to later characterize the strain-induced martensitic transformation at different levels of deformation. The content of martensite has been determined post-mortem, using magnetic induction, electron backscatter diffraction and quantitative light optical micrography. The results obtained with the three methods show quantitative agreement, and reveal that the martensitic transformation in AISI 304L occurs faster and to a greater extent than in AISI 316LN both at 77K and at 4K. To the authors’ knowledge, in this paper we provide the first experimental results for the evolution of the content of strain-induced martensite in AISI 304L and AISI 316LN samples tested at liquid helium temperature. In addition, the experimental data for the evolution of the martensite volume fraction with the strain have been used to identify the temperature-dependent parameters of the martensitic transformation kinetic model proposed by Olson and Cohen (1975). Moreover, Mode I fracture tests with fatigue-precracked Compact Samples have been carried out to determine the fracture properties of the two investigated materials using the “resistance curve procedure” (ASTM-E1820-20a). The crack-growth resistance curves have been obtained with four different methods here referred to as ASTM Compliance Method, W–N Compliance Method, Modified W–N Compliance Method and ASTM Normalization Method, which is an original methodological contribution of this paper. While the four approaches yield similar results for the fracture toughness, only the W–N Compliance Method and the Modified W–N Compliance Method, the latter being proposed in this paper, fulfil all the requirements of the standard ASTM-E1820-20a so that the calculated fracture toughness can be accepted as a material property. The comparison of results for both materials and testing temperatures shows that AISI 316LN displays higher fracture toughness than AISI 304L. Moreover, post-mortem microstructural analysis of the Compact Samples near the fracture surface has revealed that the content of martensite is greater in AISI 304L than in AISI 316LN. Furthermore, for AISI 304L more martensite is formed in the sample tested at 77K because the plastic deformation near the crack is more than at 4K.

Deformation kinetics of a TRIP steel determined by in situ high-energy synchrotron X-ray diffraction

The microstructure design and the development of predictive approaches exploiting the transformation-induced plasticity (TRIP) effect require a keen understanding of the kinetics governing the strain-induced martensitic transformation. In this work, in situ high-energy synchrotron X-ray diffraction is applied to track the deformation kinetics of a commercial AISI 301LN metastable austenitic stainless steel in real-time. The kinetics obtained, providing the behaviour of the bulk material during room temperature tension up to a true strain of 0.3, unambiguously reveals the transformation sequence of ε and α′ martensite which is discussed with respect to the evolution of texture and slip. These results are enhanced with microstructure analysis including electron backscattered diffraction and transmission Kikuchi diffraction. The insights provided shed light on the role of ε during α′ transformation in metastable austenitic stainless steels and show that the latter is triggered by the general activation of slip.

Crystallographic orientation dependence of Charpy impact behaviours in stainless steel 316L fabricated by laser powder bed fusion

Despite the extensive research on the mechanical properties of stainless steel 316L (SS316L) produced by laser powder bed fusion (LPBF), only a handful of studies have investigated its impact performance even though it is critical for structural applications experiencing rapid loading. In this study, the crystallographic-orientation-dependent Charpy impact behaviours of LPBF-SS316L were systematically investigated. Samples with their longitudinal directions (LDs) along the 〈100〉, 〈110〉 and 〈111〉 crystallographic directions were tested at both room temperature (RT) and liquid nitrogen temperature (LNT). At RT, the 〈100〉 and 〈111〉 samples exhibited similar impact toughness, while that of the 〈110〉 samples was the lowest. At LNT, the absorbed energy of the 〈100〉 and 〈111〉 samples experienced varying decreases. However, the impact toughness of the 〈110〉 samples did not suffer from the cryogenic temperature, exhibiting the highest absorbed energy value at LNT. The percentage of the shear area on the fracture surface was well correlated with the impact toughness. EBSD analysis showed that all the samples fractured at RT contained deformation twins. The lower impact toughness of the 〈110〉 samples was attributed to the slow strain hardening recovery due to their moderate twinning-induced plasticity (TWIP) effect under tension. At LNT, deformation twinning still prevailed in the 〈100〉 samples, while the 〈110〉 and 〈111〉 samples favoured strain-induced martensite, featuring dispersed ε-martensite and a considerable amount of α'-martensite, which led to strong transformation-induced plasticity (TRIP) effect. In particular, the 〈110〉 samples were characterized by a more widespread strain-induced martensitic transformation, resulting in their highest impact toughness at LNT among the three orientations. This study provides valuable insights into the design of crystallographic texture of LPBF-SS316L towards better impact properties for future structural applications. • Impact behaviours along the 〈100〉, 〈110〉 and 〈111〉 directions of LPBF-SS316L. • Great impact toughness at room temperature and liquid nitrogen temperature (LNT). • No deterioration in impact toughness of the 〈110〉 samples at LNT. • ε-martensite in LPBF-SS316L after deformation. • Correlations between impact toughness and deformed microstructure.

Low-cycle fatigue life and plasticity mechanisms of a Fe−15Mn−10Cr−8Ni−4Si seismic damping alloy under cyclic loading at various temperatures

A recently developed Fe−15Mn−10Cr−8Ni−4Si (FMS) seismic damping alloy has a superior fatigue life at room temperature due to reversible dislocation motion associated with the γ → ε martensitic transformation. The effect of temperature on the low-cycle fatigue life (Nf) and the associated plasticity mechanisms of the FMS alloy were evaluated between 253 and 393 K. The longest Nf of 18,029 cycles was obtained at 333 K (at the total strain amplitude of 1%). The Nf was dependent on the plasticity mechanisms, which were subdivided into three temperature regions with respect to the upper temperature limits for stress-assisted and strain-induced martensitic transformation (Msσ and Md, respectively). At temperatures below Msσ, the Nf exceeded 5,000 cycles, where ε-martensite was dominant and a long period stacking ordered structure was formed. The longest Nfvalues of over 10,000 cycles were obtained in the dual γ/ε-phase formed between Msσ and Md, while Nf decreased rapidly as the deformation temperature increased beyond Md. The effect of temperature on the Nf of the FMS alloy was comparable to that of Fe–28Mn–6Si–5Cr shape memory alloy, and to the chemical composition dependence of Nf of Fe–30Mn–(6 – x)Si–xAl (x = 0-6) transformation- and twinning-induced plasticity (TRIP/TWIP) steels. A new set of thermodynamic parameters was established to calculate the Gibbs free energy difference between the phases (ΔGγ→ε) and the stacking fault energy of austenite (ΓSFE). The superior Nf was associated with the cyclic strain-induced martensitic transformation when ΔGγ→ε was between –50 and 100 Jmol−1, while reversible martensitic transformation relied on a ΔGγ→ε of ∽0 Jmol−1.

A new fcc-bcc orientation relationship observed in the strain-induced martensitic transformation of an austenitic stainless steel

Orientation relationship (OR) of the face-centered cubic austenite (γ-fcc) and body-centered cubic structured martensite (α′-bcc) is a critical parameter affecting the microstructure-properties of metallic materials. So far, the ORs of Kurdjumov-Sachs (K-S), Pitsch and Nishiyama-Wassermann (N-W) are the most reported relationships involved in the martensitic transformation, where the K-S and Pitsch are preferred as the α′-bcc formed via the intermediated phase of hexagonal closed packed (hcp) ε-plates, while the N-W only appeares as the α′-bcc direct nucleated from the γ-fcc. Here, we observed a α′-bcc, formed at the intersection of parallel ε1- and parallel ε2-plates with a new OR in an austenitic steel SS304 after shot peening. The new OR is distinctive among all of the known relationships in fcc-bcc systems, which was formed by the relative rotation of −12.8° from the Pitsch one, and expressed as the (002)fcc 12.8° deviated from (101-)bcc, [11-0]fcc//[11-1]bcc.

A Crystal Plasticity Simulation on Strain-Induced Martensitic Transformation in Crystalline TRIP Steel by Coupling with Cellular Automata

In transformation-induced plasticity (TRIP) steel, the strain-induced martensitic transformation (SIMT) has a close relationship with the shear band formation. At a small length scale such as that of a crystal, the explicit analysis of the shear band structure with the formed microstructure is quite important for an adequate understanding of the SIMT. Here, a study on the microstructures formed by SIMT, related to shear band formation in both single and polycrystal TRIP steels, is presented. The constitutive equation for single crystal TRIP steel considering the transformation strain on each variant system is derived based on a rate-dependent crystal plasticity theory. To express the martensitic transformation, the cellular automata approach, including a transformation criterion acting as a local rule, is introduced. Numerical simulation is conducted with patterning processes of the martensitic phase at an infinite medium under the plane strain tension. It is found that the similar distributions of the plastic strain and the martensitic phase are dependent on the initial crystal orientation and appear as the shear band structures. In addition, the sizes of embryo and cell strongly influence the shear band formation and the martensitic volume fraction of crystal TRIP steel.

Mechanical and thermal stability of retained austenite in plastically deformed bainite-based TRIP-aided medium-Mn steels

Advanced medium-Mn sheet steels show an opportunity for the development of cost-effective and light-weight automotive parts with improved safety and optimized environmental performance. These steels utilize the strain-induced martensitic transformation of metastable retained austenite to improve the strength–ductility balance. The improvement of mechanical performance is related to the tailored thermal and mechanical stabilities of retained austenite. The mechanical stability of retained austenite was estimated in static tensile tests over a wide temperature range from 20 °C to 200 °C. The thermal stability of retained austenite during heating at elevated temperatures was assessed by means of dilatometry. The phase composition and microstructure evolution were investigated by means of scanning electron microscopy, electron backscatter diffraction, X-ray diffraction and transmission electron microscopy techniques. It was shown that the retained austenite stability shows a pronounced temperature dependence and is also stimulated by the manganese addition in a 3–5% range.

Development of Low-Yield Stress Co\u2013Cr\u2013W\u2013Ni Alloy by Adding 6 Mass Pct Mn for Balloon-Expandable Stents

This is the first report presenting the development of a Co–Cr–W–Ni–Mn alloy by adding 6 mass pct Mn to ASTM F90 Co–20Cr–15W–10Ni (CCWN, mass pct) alloy for use as balloon-expandable stents with an excellent balance of mechanical properties and corrosion resistance. The effects of Mn addition on the microstructures as well as the mechanical and corrosion properties were investigated after hot forging, solution treatment, swaging, and static recrystallization. The Mn-added alloy with a grain size of ~ 20 µm (recrystallization condition: 1523 K, 150 seconds) exhibited an ultimate tensile strength of 1131 MPa, 0.2 pct proof stress of 535 MPa, and plastic elongation of 66 pct. Additionally, it exhibited higher ductility and lower yield stress while maintaining high strength compared to the ASTM F90 CCWN alloy. The formation of intersecting stacking faults was suppressed by increasing the stacking fault energy (SFE) with Mn addition, resulting in a lower yield stress. The low-yield stress is effective in suppressing stent recoil. In addition, strain-induced martensitic transformation during plastic deformation was suppressed by increasing the SFE, thereby improving the ductility. The Mn-added alloys also exhibited good corrosion resistance, similar to the ASTM F90 CCWN alloy. Mn-added Co–Cr–W–Ni alloys are suitable for use as balloon-expandable stents.

Improvement of Mechanical Properties of Nanocrystalline FeCrC Alloy via Strain-Induced Martensitic Transformation

Improvement of Mechanical Properties of Nanocrystalline FeCrC Alloy via Strain-Induced Martensitic Transformation