Strain-induced Martensitic Phase Transformation Research Articles

Development of two-dimensional temperature field solution method based on the stress–strain response of 316LN stainless steel at cryogenic temperatures

To better understand the tensile fracture behavior and temperature variation during the tension testing of the 316LN at cryogenic temperatures, a two-dimensional temperature field solution method is developed based on two-dimensional strain field measured by the digital image correlation technology combined with the Taylor Quinney coefficient model and the intrinsic model of strain-induced martensitic phase transformation. The method allows for visualization of the strain field and temperature variation of the 316LN austenitic stainless steel at cryogenic temperature. By numerical simulation, the localized temperature elevation of the 316LN is determined to be 13.3 K during the tensile test at 77 K, which is in good agreement with the experimental result of 10.2 K. This model reconstructed the two-dimensional temperature field variation throughout the entire tensile test, which is beneficial for facilitating the analysis of the mutual relation between the mechanical properties and microstructural evolution of the 316LN and other metastable austenitic stainless steels in cryogenic temperatures.

Orientation dependence of the effect of short-range ordering on the plastic deformation of a medium entropy alloy

Multi-principal-element alloys (also known as medium and high entropy alloys) offer a much larger and richer design space than conventional alloys, providing opportunities for discovering new functionalities and their governing physics. Some of these alloys exhibit an outstanding combination of high strength and ductility, linked to the activation of various deformation modes triggered by low-energy stacking faults. However, a pressing question remains: Is the plasticity of medium- and high-entropy alloys governed only by stacking fault energy, or does atomic short-range order (SRO) play a role? To answer this, we investigated how SRO affects the deformation in single-crystalline NiCoCr, with previous contradictory findings. First, we established unique experimental evidence for SRO formation in bulk single crystals using high-energy synchrotron transmission X-Ray Diffraction. By tuning the degree of SRO by aging at high temperatures, twinning density and strain-induced martensitic phase transformation can be significantly increased in the [110] and [111] orientations under tension, increasing the tensile ductility; yet, no increase was observed along the [001] orientation due to lack of TWinning-Induced Plasticity (TWIP) or TRansformation-Induced Plasticity (TRIP), indicating a strong crystallographic orientation dependence. Our first-principles thermodynamic calculations unequivocally show SRO exists and governs the observed microstructural evolution and deformation hardening behavior. Here we find direct proof that SRO triggers a simultaneous TWIP and TRIP in NiCoCr, a rare microstructural evolution path. Our findings establish that the interplay of SRO and plasticity could be exploited to alter deformation modes and yield unprecedented mechanical response in medium- and high-entropy alloys.

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.

Predicting strain-induced martensite in austenitic steels by combining physical modelling and machine learning

Computational materials design has made significant progress lately. However, one underexploited opportunity lies in the combination of physically based modelling and machine learning (ML). In the present work we exploit this combination for modelling of strain-induced martensitic phase transformation (SIMT) in austenitic steels. A fully predictive model for SIMT, responsible for the TRIP effect in many steels, is devised. An experimental dataset correlating SIMT with composition, temperature and strain is collected from the open literature firstly. Secondly, the Olson-Cohen model is applied to make physically based predictions on temperature and strain dependence of SIMT in order to expand the database to the final size of 16,500 entries relating the features and the target. Thirdly, ensemble ML methods are applied to model the data and the final model is validated on a holdout dataset, including also dual-phase alloys. The final model provides accurate predictions of SIMT in a temperature range from −196 to 100 °C and from 0 to 1 in strain. The model can readily be extended to consider further factors such as strain rate and stress state. Moreover, it can be used together with thermodynamic and kinetic calculations, or thermomechanical simulations, for the design of steels and components, respectively.

The strain-induced martensitic phase transformation of Fe–C alloys considering C addition: A molecular dynamics study

Abstract

Role of strain-induced martensitic phase transformation in mechanical response of 304L steel at different strain-rates and temperatures

Abstract The strain-rate and temperature dependencies of the strain-induced martensitic phase transformation (MPT) of AISI 304 L are experimentally investigated. Firstly, the tensile tests of 304 L at room temperature are conducted at the strain-rates ranges from 6.67 × 10−4 s-1 to 80 s-1. The stress-strain curves indicate a significant strain-rate effect of the strain-induced MPT, which is closely relevant to extent of the adiabatic heating. The second batch of tensile tests are conducted at several representative strain-rates and at different temperatures ranging from 0 °C to 100 °C. The martensitic volume fractions at different plastic deformation stages are measured by X-ray diffraction to trace the evolution of martensitic phase in the tests. The test results indicate that the environmental temperature and the strain-rate exert coupling influence on the stress-strain behavior. Variations in strength and ductility of 304 L in the tests are explained with a competition mechanism between strain-induced MPT, thermal softening and strain-rate strengthening. A kinetic model of MPT is modified to take the strain-rate, initial temperature and temperature rise into account.

Strain-induced ε-martensitic transformation and hydrogen embrittlement of SUS304 stainless steel

ABSTRACTWe examined the hydrogen embrittlement mechanism in SUS304 stainless steel by conducting synchrotron radiation diffraction experiments and high-resolution TEM observations. It was found that the strength of SUS304 was lowered by adding hydrogen and that brittle fracture occurred with elongation of approximately 20%. Synchrotron radiation diffraction experiments revealed that the formation of the ε phase having a hexagonal close-packed structure was promoted by hydrogen addition to SUS304. In addition, high-resolution TEM observations revealed the presence of the ε phase with a width of up to approximately 5 nm at the twin boundary in the twin structure due to the γ phase. Furthermore, a large number of stacking faults were formed in the ε phase. These results imply that the strain-induced martensitic phase transformation from the γ phase to the α′ one was suppressed by hydrogen addition to SUS304 and that the ε phase having a large number of stacking faults was stabilised as an intermediate phase during the α′ martensitic phase transformation. The formation of the ε phase having a large number of stacking faults should presumably originate in hydrogen embrittlement.

Stress state-dependent mechanics of additively manufactured 304L stainless steel: Part 1 – characterization and modeling of the effect of stress state and texture on microstructural evolution

The effect of stress state and crystallographic texture on strain-induced martensitic transformation kinetics in 304L stainless steel (SS304L) components made by additive manufacturing were investigated. Mechanical tests under uniaxial tension, uniaxial compression, and pure shear were performed. Experimental results showed that the rate of strain-induced martensitic phase transformation, with respect to plastic strain, was highest under uniaxial compression, followed by uniaxial tension, and lowest under pure shear. The higher rate of phase transformation under uniaxial compression than tension in the additively manufactured SS304L contradicts the trends often, but not always, observed in texture-free conventionally processed austenitic stainless steels. The combined effects of stress state, crystallographic texture, and chemistry were studied, for the first time, to develop a new strain-induced martensitic phase transformation kinetics equation for additively manufactured SS304L that captures the microstructural evolution as a function of plastic strain and these factors.

Effect of chemistry on martensitic phase transformation kinetics and resulting properties of additively manufactured stainless steel

Here we investigate the effect of chemistry, including initial powder chemistry and spatial chemical variations due to vaporization during fabrication, on strain-induced martensitic phase transformation in 304L stainless steel components fabricated by directed energy deposition (DED) additive manufacturing (AM). The austenite stability was altered by mixing pre-alloyed 304L stainless steel powder with Fe powder, which promoted martensitic phase transformation and resulted in an increase in ultimate tensile strength and elongation to failure over pure 304L walls deposited by DED AM. The chemical composition variation with position, due to spatial variations in thermal history, was quantified, showing that austenite stabilizing elements Cr, Mn, and Ni were preferentially vaporized during deposition. We present a martensitic transformation kinetics equation that describes the evolution of martensite volume fraction with respect to plastic strain as a function of chemistry. This allows for the prediction of strain-induced martensite evolution as a function of strain, nominal chemistry, and heterogeneous chemistry due to vaporization within additively manufactured components.

In situ observation of strain and phase transformation in plastically deformed 301 austenitic stainless steel

To inform the design of superior transformation-induced plasticity (TRIP) steels, it is important to understand what happens at the microstructural length scales. In this study, strain-induced martensitic transformation is studied by in situ digital image correlation (DIC) in a scanning electron microscope. Digital image correlation at submicron length scales enables mapping of transformation strains with high confidence. These are correlated with electron backscatter diffraction (EBSD) prior to and post deformation process to get a comprehensive understanding of the strain-induced transformation mechanism. The results are compared with mathematical models for enhanced prediction of strain-induced martensitic phase transformation.

Neutron Bragg Edge Tomography for Phase Mapping

This paper describes non-destructive three-dimensional (3D) mapping of crystallographic phase distributions within the bulk (centimeter range) of samples with micrometer-scale resolution. The technique leverages diffraction contrast due to Bragg scattering and the large penetration power of neutrons through high-atomic-number-element-based structural materials. Our tomographic approach overcomes critical limitations of existing techniques by allowing spatially resolved phase mapping in bulk samples and offers a wide range of potential applications. The technique is demonstrated for (metastable 304L stainless) steel samples that exhibit strain-induced martensitic phase transformation after being subjected to tensile and torsional deformation. The distribution of phase fractions within the 3D reconstructed volumes was verified at select locations using neutron diffraction as well as EBSD, and results agree with transformation kinetics theory.

Radial distribution of martensitic phase transformation in a metastable stainless steel under torsional deformation: A synchrotron X-ray diffraction study

The strain-induced martensitic phase transformation in a metastable 304 L stainless steel under torsional deformation was investigated using synchrotron X-ray diffraction. The measured radial distribution of the martensite phase fraction in a solid cylindrical specimen agrees well with the prediction based on a combination of transformation kinetics and a radial plastic strain distribution equation.

Strain-Induced Martensitic Phase Transformation by Low-Cycle Fatigue in AISI 316L Stainless Steel

The evolution of the strain-induced α′ martensite during the cyclic deformation of austenitic AISI 316L stainless steel (SS) was investigated. The low-cycle fatigue (LCF) test was conducted at various strain amplitudes in air. The amount of α′ martensite was determined for the fatigue-failed specimens as well as for the specimens interrupted after a specific number of cycles. The volume fraction of α′ martensite increased with increasing strain amplitude and number of fatigue cycles. The cyclic hardening behavior was discussed in terms of the dislocation density and strain-induced α′ martensite transformation.

Texture evolution during strain-induced martensitic phase transformation in 304L stainless steel at a cryogenic temperature

The strain-induced martensitic phase transformation during quasi-static uniaxial compression testing of a 304L stainless steel was investigated at 300 and 203 K using time-of-flight neutron diffraction to study the evolution of transformation texture. A number of specimens were precompressed to different strain levels at 300 and 203 K and the texture was investigated. At 203 K, the newly formed martensites are bcc and hcp phases and the texture analysis shows that the martensites are highly textured due to the grain-orientation-dependent phase transformation. The bcc {100} planes are mostly oriented with their plane-normal parallel to the loading direction at the beginning of the phase transformation and this texture is weakened during the subsequent compressive deformation. In the case of fcc to hcp transformation, it is less dependent on the grain orientation, although the fcc grains with {111} plane-normal at an angle close to 40 deg to the loading direction transform easier and the {0001} plane-normal of the newly formed hcp phase tends to rotate toward the loading direction during the texture evolution. The final texture of bcc and hcp martensites is the result of the interaction between deformation texture and transformation texture.

塑性工学 微小硬度を用いた局所的ひずみ誘起マルテンサイト変態特性の測定

By the duplex effect produced by two kinds of phases of austenite and martensite, the transformation-induced plasticity (TRIP) steel is improved in ductility and fracture toughness. The strain-induced martensitic phase transformation could be associated with the strain localization behavior. Accordingly, the measurement of the amount of local transformation is necessary in order to construct a more physical evolution model in the constitutive equation. In this study, a new measurement system using a micro-hardness tester is proposed to obtain a volume fraction map of the martensitic phase expanding in the neighbor of strain localization. Then the system is applied to investigate the inhomogenous transformation behavior around the notch root of SUS 304 stainless steel bar under uniaxial tension.