Evolution Of Martensite Research Articles

Effect of the cooling behavior on phase transformation and mechanical property of RAFM steel

The phase transition behavior of RAFM steel during Hot Isostatic Pressing (HIP) diffusion bonding process would extensively impact the service performance of blanket component in the future fusion reactor. In this paper, the influence of cooling behavior on phase transformation and mechanical property of RAFM steel was studied using a combination of Electron Backscatter Diffraction (EBSD) and a thermal dilatometer. The results show that as the cooling rate decreases after austenitizing, the phase category transitions from a single-phase of martensite to the dual-phase of martensite and ferrite, along with an increase in the rate of ferrite, minor martensite packets and high-angle grain boundaries (HAGB). As the cooling rate decreases, the initial temperature for martensite transformation significantly increases, and the transformation driving force slowdown. Moreover, specimens processed with HIP cooling show lower strength, higher elongation, and a greater standard deviation of hardness compared to air cooling. These differences can be attributed to the formation of ferrite and the diffusion of carbon atoms within martensite. The research findings reveal the phase transition and microstructural evolution of martensite under varying cooling conditions, providing a reference for the controlling the cooling process after HIP bonding of blanket steel components in the future fusion reactor.

Role of grain size in the evolution of stress-induced martensite and the tensile deformation behavior of Ti-7333 alloy

The evolution of martensite and the tensile deformation behavior of Ti-7333 alloy with different β grain size was studied. It is found that the β grain size (45–260 μm) significantly affects the triggering stress of stress-induced α″ martensite (SIMα″), and then affects the distribution and morphology of α″ martensite. At the initial stage of deformation, the larger β grain can inhibit the stress-induced secondary α″ martensitic transformation and the formation of α″ twinning. While under the same strain, the stress-induced martensite and martensite twinning can be activated simultaneously in the alloy with the smaller β grain, including {111}α″ type Ⅰ twinning, {130}<310>α″ compound twinning, <211>α″ type Ⅱ twinning, {021}<512>α″ type Ⅱ twinning, which can effectively accommodate localized strains and improve the work hardening rate of Ti-7333 alloy. When the β grain size varies in the range of 45–84 μm, there is a higher work hardening plateau in the deformation stage II of Ti-7333 alloy. The yield strength and β grain size are in accordance with the Hall-Petch relation. As the grain size increases to 128 even to 260 μm, the work hardening rate continues to decrease and the yield strength inversely increases. It is demonstrated that the triggering stress for SIMα″ exhibits U-shaped fluctuations as β grain size increases, and the inflection point of Ti-7333 alloy is between 84 μm and 260 μm.

Physics-informed deep operator network for predicting martensite evolution in superelastic shape memory alloys through cyclic tensile tests

Superelastic shape memory alloy (SMA) wires and rods possess unique deformation and energy dissipation capabilities. For the assessment of their stress response, commonly cyclic tensile tests are conducted. An important but subtle parameter in this procedure is the martensite evolution. In scenarios where conducting thermal experiments is impractical, inverse modeling from cyclic tests serves as a viable alternative. However, employing constitutive models in this process presents distinct challenges, such as parameter identification and calibration, or numerical stability issues. To address these challenges, this paper proposes a data-driven method based on a physics-informed deep operator network (DeepONet) to estimate the martensite evolution. Constraint with a stress equation, the network requires only strain–stress data for training and provides stress responses in addition to the martensite evolution. From the training data, the network learns to consider the effects included in the response. The DeepONet can be coupled with experiments to provide online estimates from noisy sensor-based strain inputs, while remaining numerically stable. Moreover, this approach avoids the need for separate parameter identification or calibration. This paper details this method and evaluates its performance through experiments conducted on superelastic SMA wires. Furthermore, as an alternative approach, training using a constitutive model is provided.

Microstructure of Stress‐Induced Martensite in Micron‐Sized Zirconia Single Crystal Spheres

Martensitic phase transformation of zirconia (ZrO2), when realized from the superelastic effect, has immense potential in energy applications, such as micro‐actuation and high‐energy dissipation/damping. In this study, martensitic transformation in ZrO2 is studied using a newly developed microscale phase field model, which is scale‐independent and contains an averaged gradient term. The model is implemented using the FEniCS package in Python. With a thorough explanation of the model construction, the mechanics of phase transformation are detailed, and the material behaviors under different stress conditions are discussed. Mesh sensitivity, multivariant evolution, and the effect of particle size are analyzed to understand the martensite evolution in superelastic ZrO2. This work provides new insight into the martensitic phase transformation behaviors of micron‐sized spherical ZrO2 particles.

Investigating quench crack susceptibility in disc springs: a microstructural analysis of martensitic evolution and its implications

Investigating quench crack susceptibility in disc springs: a microstructural analysis of martensitic evolution and its implications

Hydrogen promoted Ti3Al precipitation during Ti-6Al-4V martensite tempering

Temporary hydrogen alloying (THA) treatments are extensively used to refine coarse Ti-6Al-4V microstructures. The present study focuses on a THA treatment which promotes martensite decomposition; it highlights the effect of hydrogen on tempered martensite evolution by comparing martensite decomposition in hydrogenated and control samples. Microstructure evolution was characterised using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). Bulk XRD measurements indicate that quenching hydrogenated Ti-6Al-4V from the beta region causes a partial beta to hexagonal (α’) plus orthorhombic (α’’) martensite transformation. The EBSD, TKD and HRTEM analyses show that hydrogen saturated plate martensite decomposes to form a nanobanded structure which consists of nanosized, periodical Al-rich, and V-rich HCP regions that stabilise D019 Ti3Al within 3 h of tempering. In comparison, typical martensite decomposition is observed in the control experiment. Quenching non-hydrogenated Ti-6Al-4V from the beta region causes a full beta to hexagonal (α’) martensite transformation. Elemental partitioning occurs, but not to the extent that D019 Ti3Al is promoted and compositional banding is not observed. Formation of the nanobanded structure suggests that hydrogen altered the transformation pathway in hydrogenated martensite because of the presence of α’’possibly promoting spinodal decomposition.

Martensite evolution in additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures

Based on classical nucleation and growth theory, the phase evolution of martensite α′ phase of additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures (600–960 °C) was systematically investigated. The results show that: Owing to insufficient diffusion of alloy elements during laser deposition, numerous β-stabilizing elements are retained in α′ phase and the microstructure is made up of 95.4% α′ phase and 4.6% β phase. According to TEM, EBSD, SEM/EDX results, the α′ phase can be divided into α′ phase containing more β-stabilizing elements (α′β), α′ phase containing more Al (α′Al). Under annealing condition, martensite decomposition tends to occur at the α′/α' interfaces and the matrix of α′β phase. In the matrix of α′β phase, fine adjacent α′ phase precipitates at β phase because a lot of β-stabilizing elements are consumed by precipitated β phase, thus improving the precipitation of adjacent α′ phase. The decomposition of α′ phase is controlled by diffusion of alloy elements. As the temperature increases, the diffusion of alloy elements and degree of α′ martensitic decomposition become more sufficient. When the Al content in the α′ phase reaches that in equilibrium α phase, the lattice parameters of α′ phase increase from a = 2.97 Å, c = 4.72 Å to a = 3.04 Å, c = 4.77 Å and meanwhile the α′ martensitic decomposition is finished.

Achieving ultra-high flow stress and hardness in a medium Mn stainless steel by the formation of strain-induced martensite and G-phase precipitates

This work demonstrates how the deformation of a medium Mn austenitic stainless steel and its subsequent thermal treatment results in the steel having an ultra-high flow stress, and with a microstructure containing strain-induced martensite (α′) and G-phase precipitates. A 14 wt.%Cr-8 wt.%Mn-1 wt.%Ni austenitic stainless steel was uniaxially compressed at room temperature and quasi-static strain rates to determine the evolution of strain-induced martensite with strain. Uniaxial compression resulted in a sigmoidal stress-strain curve (enhanced work-hardening) with the flow stress reaching ∼2.2 GPa, and the α′ fraction saturating at 0.8 true strain. Atom-probe tomography showed that aging at 400 °C for 72 h formed 2–3 nm sized G-phase precipitates containing Cu-rich clusters and having a nominal composition of Ni8Si2Mn20Cu17Cr8Fe44. Such a microstructure resulted in a steel with an ultra-high tensile strength and a compressive flow stress of around 2 GPa. Though the tensile ductility was low, there was sufficient flowability in compression.

Microstructure evolution and enhanced mechanical properties of as-rolled TiB/(TA15-Si) composite via heat treatment

To improve the mechanical properties of the as-rolled composite under large rolling deformation, the effect of solid solution and aging on the microstructural characteristics and mechanical properties of the as-rolled TiB/(TA15-Si) composite was detailedly investigated. The solid solution temperatures of 1050 °C in the β-phase field and different aging temperatures of 550–750 °C were chosen to regulate the evolution of matrix and silicides. After solid solution treatment, the matrix transformed from elongated and kinkled lamella into acicular martensite microstructure. And the silicides completely redissolved into matrix and a uniform diffusion of Si atom was presented. Meanwhile, the aging temperature can obviously affect the evolution of martensite and silicides. As aging temperature increased, the martensite gradually decomposed into acicular α+β lamella (at 650–700 °C) and abnormal growth of α phase appeared (at 750 °C), which dominated the decreasing trend of microhardness and high-temperature strength. Besides, the silicides gradually precipitated along the prior β phase and TiB whiskers (at 600 °C) as well as α/β phase boundaries (at 650 °C). Eventually, 600 °C and 700 °C strengths of the as-rolled composite can be obviously improved by 39.7 % and 84.6 % after solid solution and aging processes. And the composite aged at 600 °C can obtain the highest 600 °C strength of 992 MPa, which is mainly attributed to martensite, precipitation of silicides and weak α2 phase.

Characterization of microstructural evolution with pre-strain in BG801 bearing steel: Grain, carbides, retained austenite and martensite

The method of multiple cryogenic and tempering treatments significantly limits the microstructure regulation of BG801 bearing steel. In this work, the pre-strain was introduced in the vacuum-melted BG801 steel combing with heat treatment (HT) to adjust the microstructures. The samples were subjected to uniaxial tensile at room temperature with different strains (engineering strain of 0, 0.15, 0.175, 0.2), then heating to 1323 K and water-cooling. The results showed that higher pre-strain was beneficial to obtain finer prior austenite grain (PAG) and martensite grains, and a low fraction of retained austenite (RA) in BG801 steel. With increasing pre-strain, the PAG was elongated along the RD direction (rolling direction) without a change in size. The recrystallized grains with a size of about 5–6 μm nucleated inside the PAG during HT. Meanwhile, the high-density dislocations were introduced in the tensile samples by pre-strain, promoting the growth of micron-M6C by accelerating the diffusion of alloy elements during HT. And the evolution of RA and martensite indicated that the pre-strain accelerates transformation from austenite to martensite by transferring elements from the matrix to M6C and introducing shear bands into the matrix to promote the nucleation of martensite.

Determination of Martensite Formation Rate by Magnetic In Situ Measurement in CrMnNi–N Steel

The plastic deformation and martensite evolution of austenitic CrMnNi–N stainless steel with 8.6 wt% Mn and 4 wt% Ni is investigated in a temperature range of −40 to 100 °C. Martensite evolution is determined by in situ magnetic measurements during tensile test. The triggering stress and strain for martensite formation decrease with decreasing temperature. Ex situ volumetric magnetic measurements are used to determine the strain‐induced α′‐martensite volume fractions. The characterization of the microstructure is carried out with scanning electron microscope. Using electron backscatter diffraction, strain‐induced α′‐martensite is detected within deformation bands in the austenite. The strain hardening curve at −40 °C shows a typical progression for metastable austenitic stainless steels with pronounced strain‐induced α′‐martensite formation and can be divided into four hardening stages. At this temperature, the studied steel achieves the highest strain hardening rate. The amount of martensite increases with decreasing test temperature and reaches a maximum volume fraction of 76 vol% at −40 °C. The highest ductility of 83% is achieved at 40 °C, accompanied by a tensile strength of 732 MPa. The in situ magnetic measurement confirms that the inflection point in the strain hardening curve coincides with the maximum martensite formation rate.

MORPHOLOGICAL DETAILS OF STRAIN-INDUCED MARTENSITE IN COLD-ROLLED Cr – Ni STEEL

"The investigation presents the evolution of strain-induced martensite during cold rolling of AISI 304 stainless steel at 10% and 50% degree of deformation. The morphologies and characteristics of ’ martensite have been studied using optical-image analysis, scanning and transmission electron microscope. The influence of strain-induced martensite on mechanical properties and hardness of AISI 304 steel have been reported. Plastic deformation during the cold rolling of AISI 304 steel induces a martensitic transformation (γ→α’) in the whole range of applied deformation degree. The equiaxial grains of austenite containing annealing twins were observed within the microstructure of steel at delivery state. During the deformation process the shape of austenite grains was changed into long slim bands parallel to the rolling direction. Many shear bands and micro-twins were observed in the microstructure at large deformations degrees. The martensite, which was formed as a result of the transformation (γ→α’) induced by deformation, has a distinct influence on properties of the investigated steel grade. It was found that the effect of steel hardening is a result of a significant increase of the volume fraction of the ’-martensite in the structure. "

Polarization contrast neutron imaging of magnetic crystallographic phases

Non-destructive characterization methods to observe phase transformations and thus, gain insights to transformation mechanisms in representative volumes are key for the development of advanced materials and manufacturing. Conventional methods are constrained to the surface and small sizes, thus, access to the bulk often implies tedious destructive approaches and hinders in-situ observations of phase evolution. In this work, we introduce a non-destructive technique that overcomes key limitations prevailing today for mapping the spatial distribution of magnetic phases in bulk materials. The use of polarized neutrons, being sensitive to sub-percent fractions of ferromagnetic phases and able to penetrate centimeter sized samples, enables micrometer-scale spatial and second-scale time resolutions. We demonstrate ex-situ and in-situ quantitative mapping of magnetic phases, in particular the evolution of martensite induced by uniaxial- and biaxial deformation in metastable 304 steel. The quantitative results obtained during in-situ deformation testing prove polarization contrast neutron imaging to be particularly effective in detecting small fractions of martensite. Especially during early stages of deformation where neutron diffraction, which in contrast does not provide full field spatial resolution, fails. The short exposure times and high sensitivity renders the method well suited for rapid 3D tomographic mapping and/or operando investigations of phase distributions.

Influence of Structural Porosity and Martensite Evolution on Mechanical Characteristics of Nitinol via In-Silico Finite Element Approach.

Nitinol (NiTi) alloys are gaining extensive attention due to their excellent mechanical, superelasticity, and biocompatibility properties. It is difficult to model the complex mechanical behavior of NiTi alloys due to the solid-state diffusionless phase transformations, and the differing elasticity and plasticity presenting from these two phases. In this work, an Auricchio finite element (FE) model was used to model the mechanical behavior of superelastic NiTi and was validated with experimental data from literature. A Representative Volume Element (RVE) was used to simulate the NiTi microstructure, and a microscale study was performed to understand how the evolution of martensite phase from austenite affects the response of the material upon loading. Laser Powder Bed Fusion (L-PBF) is an effective way to build complex NiTi components. Porosity being one of the major defects in Laser Powder Bed Fusion (L-PBF) processes, the model was used to correlate the macroscale effect of porosity (1.4–83.4%) with structural stiffness, dissipated energy during phase transformations, and damping properties. The results collectively summarize the effectiveness of the Auricchio model and show that this model can aid engineers to plan NiTi processing and operational parameters, for example for heat pump, medical implant, actuator, and shock absorption applications.

Influence of Plastic Strain Control on Martensite Evolution and Fatigue Life of Metastable Austenitic Stainless Steel

Metastable austenitic stainless steel was investigated during fatigue tests under strain control with either constant total or constant plastic strain amplitude. Two different material conditions with coarse-grained and ultrafine-grained microstructure were in focus. The influence of plastic strain control of the fatigue test on both the martensitic phase transformation as well as on the fatigue lives is discussed. In addition, an approach for calculating the Coffin–Manson–Basquin parameters to estimate fatigue lives based on strain-controlled tests at constant total strain amplitudes is proposed for materials undergoing a strong secondary hardening due to martensitic phase transformation.

A large deformation constitutive model for plastic strain-induced phase transformation of stainless steels at cryogenic temperatures

In this paper, a new constitutive model for plastic behavior of the metastable austenitic stainless steels at cryogenic temperatures is presented. The constitutive model is a phenomenological hyperelastic-based large deformation model developed in the framework of continuum damage mechanics considering the dissipative phenomena of strain-induced phase transformation and damage growth during plastic deformation. To solve the coupled nonlinear set of equations, incremental equations and the related implicit integration method have been developed. Numerical analysis is performed by implementing the constitutive model in a UMAT subroutine in ABAQUS/STANDARD. In addition, experiments have been conducted to identify the material model parameters for AISI304 steel at the 77K temperature. In addition to calibration, the constitutive model was validated via a tensile test of a notched sample. Also, the model has been compared with the experimental data at 4.2 K available in the literature. The results show that the model is well able to predict the hardening behavior, martensite evolution, and damage growth during the plastic deformation until the fracture with acceptable accuracy.

Effect of Ausforming Strain on the Microstructural Evolution of Lath Martensite in Low Carbon Steel

Effect of Ausforming Strain on the Microstructural Evolution of Lath Martensite in Low Carbon Steel

Influence of CDFW Process Parameters on Microstructure and Mechanical Properties of U75V Rail Steel Welded Joint

In the present paper, the continuous-drive friction welding (CDFW) technology has been successfully applied to join the U75V rail steel. The base metal (BM) of U75V rail steel is lamellar pearlite, and the weld zone could be clearly divided into three subzones (i.e., heat affected zone, thermo-mechanical affected zone (TMAZ), and central weld zone (CWZ)). Electron back-scattered diffraction examinations revealed the martensitic evolution in TMAZ and CWZ, suggesting that the experienced high temperature, severe plastic deformation, and fast cooling rate induce the microstructure transition during the CDFW process. The hard and brittle martensite structure explains the raised microhardness profiles and the reduced impact absorption energy of the as-welded joints. The CDFW process parameters govern the joint properties via influencing the welding heat input and plastic deformation by spindle speed and friction pressure at the friction stage, and the plastic deformation layer (flash) extrusion by upsetting pressure at the upsetting stage. More favorable results could be obtained at small set values of spindle speed (1800 rpm) and friction pressure (75 MPa) with less heat input and plastic deformation, and a large set value of upsetting pressure (175 MPa) with more flash extrusion, whose tensile strength reached 94.3% of that of the BM.

In-situ strain induced martensitic transformation measurement and consequences for the modeling of medium Mn stainless steels mechanical behavior

This work aims at studying the control of medium manganese austenitic stainless steels’ mechanical behavior and austenite to martensite transformation via chemical composition. Six grades of austenitic stainless steel were cast with chemical compositions variations small enough able to keep relatively constant the stacking fault energy (SFE) value, but high enough to change the martensitic start temperature Ms. These experiments allow the effect of the martensite transformation on the mechanical behavior in tension to be compared while the austenitic mechanical behavior is almost unchanged. Tensile tests were performed at room temperature and at a low strain rate (10−4 s−1). In-situ magnetic measurement is implemented to quantify the martensite volume fraction evolution with strain. Strain heterogeneities are detected by digital image correlation (DIC) analysis. For all grades, stress–strain curves exhibit Portevin–le-Châtelier (PLC) phenomenon related to localized martensite transformation within the deformation bands. For the two most unstable alloys, a Lüders plateau is detected at the yield stress. The martensite evolution is modeled using the Olson–Cohen approach, which confirms slower kinetics for the most stable grades. Moreover, it is shown that the martensite fraction evolves linearly with stress after a stress threshold function of the austenite stability represented by Ms. The martensite volume fraction vs stress slope is constant whatever the composition. This result leads to the development of a coupled metallurgical/mechanical model which depends on a single parameter Ms related to the chemical composition.

Understanding Shape Memory Alloy Torsional Actuators: From the Conceptual to the Preliminary Design

Shape memory alloy actuators have been studied for more than thirty years. Many experimental tests have been performed, and several patents have been registered. However, designing such devices is still a challenging task. On the one hand, models are not yet able to provide the accuracy required to replace a substantial portion of the experimental tests; on the other hand, it seems that a gap exists in the literature between the main ideas behind SMA torsional actuators and their actual implementation. This work is a systematic effort to fill this gap, helping researchers and designers in developing SMA torsional actuators with a particular focus on aeronautical applications. This paper reports all the steps toward the preliminary design of such devices, using a state-of-the-art, commercially available FEM software. Moreover, the SMA rods’ behaviour under mechanical and thermal loading is thoroughly examined, looking at monitoring stress, temperature, torque and martensite evolution simultaneously, and thus providing a holistic vision of the macroscopic phenomena involved during phase transformations. Simple aerodynamic load predictions are also performed, using Xfoil for three classes of aircraft (medium size UAV, Four-Seat Aircraft and Regional Transport Aircraft).