Kinetics Of Martensitic Transformation Research Articles

ЗАСТОСУВАННЯ МАТЕМАТИЧНОГО МОДЕЛЮВАННЯ ПРИ АНАЛІЗІ ДИЛАТОГРАМ МАРТЕНСИТНОГО ПЕРЕТВОРЕННЯ У СТАЛЯХ

The paper provides practical experience of applying mathematical modeling in the analysis of the kinetics of athermal martensitic transformation in low-alloy structural steel 30HGSA (Fe-0.30С-0.86Si-1.02Mn-0.84Сr, % wt.) using the dilatogram of the real quenching process. The change in the amount of martensite formed in the studied steel as a function of temperature during continuous quenching cooling is shown. The used technique for determining the phase ratio provides in situ correlation with the microstructural mechanisms implemented in the studied material during heat treatment. The obtained results will be used to develop parameters of heat treatment based on the principle of quenching and partitioning (Q&P) to improve the complex of properties of 30HGSA steel due to the creation of a multiphase structure.

Effect of Surface Roughness and Chemical Composition of Metastable Austenitic Stainless Steels on Electrochemical Polishing-Induced Martensitic Transformation

Herein, we analyzed the martensitic transformation kinetics during electrochemical polishing (EP) for stainless steel specimens with varying surface roughness and austenite stability. Martensite fraction measurement demonstrated that specimens with higher surface roughness and lower austenite stability exhibited relatively higher levels of martensitic transformation. To understand these phase transformation characteristics, the amount of charge build-up on the specimen surface during EP was calculated using COMSOL Multiphysics simulations for specimens with different surface roughness. The effect of charge build-up-induced stress was analyzed using previously published first-principles calculations. We found that specimens with higher surface roughness accumulated more charge build-up, resulting in greater stress and a martensitic transformation driving force. Furthermore, the critical energy required for the martensitic transformation was calculated using Thermo-Calc for specimens with different austenite stabilities. We demonstrated that the martensitic transformation kinetics during EP could be explained in terms of austenite stability, similar to the stress-induced martensitic transformation.Graphical

Investigating the role of the austenitizing temperature and cooling rate on the martensitic transformation kinetics in a SAE 9254 spring steel

The effect of austenitizing temperature (ranging from 850 to 1050 °C) and cooling rates on the phase transformations were investigated, particularly when a two cooling steps protocol was adopted. It was possible to observe that the heat treatment parameters play a major role on the martensitic transformation kinetics a possible occurrence of bainitic transformation. The results indicate that a microstructure composed by small austenite grains and the formation of bainite prior the athermal martensitic transformation significantly contribute to increase the martensitic transformation rate. The Koistinen-Marburger model was employed to analyze the non-isothermal kinetics, revealing an increase in the KM and k parameters due to an increase in austenite grain size and the presence of bainite in the microstructure. The results herein demonstrate that the optimization of the heat treatment parameters is crucial to the proper control of the phases that will be present in the microstructure of the alloy, as well as the volume fraction of martensite. Thus, the accurate control of the heat treatment process is a promising approach to enhance the properties of SAE 9254 spring steel, which finds extensive use in the automotive industry.

Revealing the effects of martensitic transformation and dislocation slip in austenite on the micromechanical behaviors of a 9Ni steel using crystal plasticity finite element method

Austenite is an extremely important phase that significantly influence the mechanical properties of (austenite + martensite) duplex steels. Two different deformation mechanisms, i.e., dislocation slip and martensitic transformation, can be activated in the austenite upon plastic deformation. However, these two deformation mechanisms make different contributions to the work hardening and flow stress of the austenite which are hardly separated by experimental methods, making it difficult to clarify the effect of austenite on the micromechanical behavior of (austenite + martensite) duplex steels. In this work, the influence of martensitic transformation and dislocation slip in austenite on the micromechanical behaviors is investigated in a model 9Ni steel consisting of austenite and tempered martensite (TM) using the crystal plasticity finite element method (CPFEM). The austenite and fresh martensite (FM) formed within the austenite grain upon deformation process are regarded as a whole named as FM/A island in the CPFEM. To accurately model the rate of martensitic transformation, the martensitic transformation kinetics law used in the CPFEM is developed by relating the number of possible nucleation sites for fresh martensite to the mechanical driving force originating from the resolved shear stress on each transformation system. The material parameters for the TM were determined by micropillar compression tests. Besides, the method for separating and determining the material parameters accounting for dislocation slip in austenite and martensitic transformation by a combination of neutron diffraction and measurements of stress-strain curves and austenite volume fractions is developed and exemplified. The CPFEM simulation results show that the local concentration of equivalent plastic strain and stress triaxiality in the FM/A island can be enhanced by the dislocation slip in austenite but suppressed by the martensitic transformation. In addition, the martensitic transformation has a remarkable effect on strengthening the local concentration of maximum principal stress in the FM/A island.

Effect of Al doping on the kinetics of reverse martensitic transformation in Ni-Mn-In Heusler alloys

In this work, the effects of Al on the microstructure and kinetics of reverse martensitic transformation in Ni50Mn34In16-xAlx (x = 0, 0.5, 1, and 1.5 (% at)) Heusler alloy was studied. The structural and phase transformation behavior of the samples was evaluated by XRD, FE-SEM, and DSC techniques. In order to evaluate the transformation kinetics parameters in the non-isothermal state upon the heating rates of 5, 10, and 15 K/min, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isoconversional methods were utilized. The results indicated that the reverse martensitic transformation temperatures and activation energy shifted to higher values due to doping the Al content up to 1 at.%. Conversely, the substitution of In by 1.5 at.% Al leads to a decrease in the transformation temperatures and activation energy. Furthermore, due to the lack of dependence on the activation energy and Avrami exponent on the degree of transformation, the reverse martensitic transformation is single-stage for all samples.

Investigations on austenite stability and martensitic transformation kinetics of a medium Mn steel under different strain states

Martensitic transformation has received wide attention due to its significant role in improving mechanical properties of medium Mn steels. In this paper, three typical strain states in sheet metal forming, uniaxial tension, biaxial tension and plane strain states, were selected to investigate the effect of the strain state on martensitic transformation of a medium Mn steel at room temperature. The experimental results show that the austenite stability of the medium Mn steel is seriously strain state dependent. The sample deformed under the plane strain state exhibits the lowest austenite stability, while it progressively increases for biaxial tension and uniaxial tension states. The microstructural evolution and the strain partitioning behavior of the material are characterized. It is indicated that the strains in austenite under different deformation modes are quite different even though the overall deformation conditions are similar, which leads to different transformation rates of γ → α’ transformation. Besides, the ε-martensitic transformation that only generates under the plane strain state further decreases the austenite stability. A strain state dependent α’-martensitic transformation kinetics law is proposed that incorporates the effect of the stress triaxiality and Lode angle parameter, which provides a reasonable prediction of γ → α’ transformation in the medium Mn steel over a wide range of strain states.

A Phase Field Study of the Influence of External Loading on the Dynamics of Martensitic Phase Transformation

An elastoplastic phase field model was employed for simulations to investigate the influence of external loading on the martensitic phase transformation kinetics in steel. The phase field model incorporates external loading and plastic deformation. During the simulation process, the authenticity of the phase field model is ensured by introducing the relevant physical parameters and comparing them with experimental data. During the calculations, loads of various magnitudes and loading conditions were considered. An analysis and discussion were conducted concerning the volume fraction and phase transition temperature during the phase transformation process. The simulation results prominently illustrate the preferential orientation of variants under different loading conditions. This model can be applied to the qualitative phase transition evolution of Fe-Ni alloys, and the crystallographic parameters adhere to the volume expansion effect. It is concluded that uniaxial loading promotes martensitic phase transformation, while triaxial compressive loading inhibits it. From a dynamic perspective, it is demonstrated that external uniaxial loading accelerates the kinetics of martensitic phase transformation, with uniaxial compression being more effective in accelerating the phase transformation process than uniaxial tension. When compared to experimental data, the simulation results provide evidence that under the influence of external loading, the martensitic phase transformation is significantly influenced by the applied load, with the impact of external loading being more significant than that of plastic effects.

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

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

Continuous Heating/Cooling Transformation Kinetics of a Novel CrNiMoV Steel for Large Structural Parts

Continuous heating transformation (CHT) and continuous cooling transformation (CCT) characteristics as well as phase transformation kinetics of a novel Cr–Ni–Mo–V steel for large structural parts are studied using a Gleeble‐3500 thermal–mechanical simulator. CCT curves are obtained. The results show that carbides in initial structure can be completely dissolved during CHT as the heating rates are not greater than 0.2 °C s−1. The final microstructure exhibits obvious sensitivity to cooling rate. It contains martensite with or without bainite. Besides, kinetic models of austenitization are established, and it is confirmed that diffusion is the major mechanism of austenitic transformation in steel during CHT. Meanwhile, bainitic phase transformations kinetic model is developed based on the J–M–A model. The fitted Avrami indexes n combined with transmission electron microscope reveal that the bainite nucleation sites of the steel are mainly located at the prior austenite grain boundaries, and the growth of bainite grains mainly follows a 2D mode. The kinetics of martensitic transformation is better modeled based on the K–M equation. A deep understanding of kinetics and microstructure characteristics during CHT and CCT is of great significance to regulate the final performance of the novel steel under development.

Quantitatively assessing the contributions of temperature-dependent deformation-induced martensitic transformation to uniform elongation and work hardening of TRIP-assisted duplex stainless steel via crystal plasticity

The martensitic transformation produced by transformation-induced plasticity (TRIP) assisted duplex stainless steels (TDSSs) can significantly enhance plasticity and strength of the material under deformation. At different deformation temperatures, the stability of austenite varies, which further affects the varied kinetics of deformation-induced martensitic transformation (DIMT). In the work, The mechanical response during tensile deformation of TDSS in a temperature range of 20 °C–60 °C was investigated. Especially, the quantitative determination of strength- and elongation-contributions of DIMT was characterized by employing crystal plasticity finite element method (CPFEM) considering thermal softening of temperature effect. The higher the temperature is, the weaker the DIMT and resultant TRIP effect are, and hence the contributions of TRIP to work hardening and uniform elongation are correspondingly reduced. The local strain increment is limited after martensitic transformation due to the significant deformation resistance of martensite, and the strain concentration phenomenon has slowed down at martensitic transformation positions. As a result, the maximum value of strain drops, leading to a decrease in the coefficient of variation (CV) of the strain distribution. Providing improvement of the homogeneity of strain, the material exhibits substantial enhanced plasticity characteristics. The global stress increment is lower than the ideal linear law calculated based on stress mixing that the global stress increment is proportional to the volume fraction of martensite. The stress increment caused by martensitic transformation has a certain degree of postponed effect and transformation rate dependence.

Annealing parameters effect on microstructure evolution, tensile properties and deformation behaviors of direct-cold-rolled UNS S32101 duplex stainless steel with heterogeneous layered structure

Microstructure evolution, strain-induced martensite transformation (SIMT) kinetics, tensile properties, deformation behaviors of UNS S32101 duplex stainless steel (DSS) with heterogeneous layered structure (HLS) were investigated. HLS composed of multiscale grains (spanning coarse, fine, and ultrafine grains) was prepared by direct cold rolling in combination with short-time annealing, being dominated by coarse-grained ferrite (CGed α) and fine-grained austenite (FGed γ). A quantitative SIMT kinetics model was established to predict the α′-martensite fraction at various strain/annealing parameters, indicating that increased average grain size (AGS) for γ not only contributed to the SIM formation but also promoted the monotonic increase of SIMT rate until annealing for 10 min. Relatively high stacking fault energy (SFE, 35.89∼39.34 mJ/m2) favored mechanical twinning as the dominant deformation mode of γ accompanied by SIMT and dislocation glide. And α deformation was mainly coordinated by wavy slip. Both SFE and Olson-Cohen parameters were strongly correlated with the γ AGS, which could reasonably interpret the dependence of SIMT on the AGS. The A and B values increased progressively with grain coarsening along with the rapid decline in SFE, facilitating the martensite formation. Further increasing the AGS beyond the peak region severely suppressed SIMT probably due to the low probability of martensite embryo generation at deformation twins (DTs) intersections, coinciding with the sharp decrease A value.

Crossover from mechanical stabilization of martensite to spanning avalanche-type reverse transformation of deformed Cu-Al-Ni shape memory crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.

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.

High Ductile Medium Mn Lightweight Alloy: The Role of Intensive Quenching and Deep Cryogenic Treatment

Medium Mn lightweight steels with a relatively higher Mn content of 9–12 wt% have been actively developed recently to meet the demands of crashworthiness and lightweight vehicles. In this study, a combined intensive quenching (IQ) and deep cryogenic treatment (DCT) was first proposed to achieve the microstructural homogeneity as well as the final strength–ductility synergy of medium Mn lightweight steels with Mn segregation bands, together with a comparison with the conventional intercritical annealing. The proposed IQ and DCT process induced the formation of finer large fractioned plate-like martensite in the austenite matrix and thereby contributed to finer and uniform austenite grains after subsequent intercritical annealing. The martensitic transformation rate (dVγ/dε) and transformation kinetics (k value) were used to evaluate the mechanical stability of retained austenite, showing that the D700&750 sample exhibited a similar dVγ/dε value and extremely low k value when compared to the conventional IA650–850 samples, implying that the former had the higher mechanical stability of austenite. The higher mechanical stability of austenite enabled the TRIP effect to occur in a larger strain range, leading to continuous strain hardening behavior. Thus, the highest yield strength (728 MPa) and the largest total elongation of 61.6% were achieved in the D700&750 sample, where the ductility was more than three times higher than that of the conventional IA samples. The grain size and morphologies of retained austenite were believed to be the main factors influencing the strain-hardening behavior of this type of ultrafine lamellar and equiaxed ferrite and austenite duplex structure.

Improvement of tensile properties by controlling the microstructure and crystallographic data in commercial pearlitic carbon-silicon steel via quenching and partitioning (Q&P) process

In the current research, a complex microstructure and crystallographic data were developed through quenching and partitioning (Q&P) process to improve tensile properties of commercial pearlitic carbon-silicon steel. Two-stage Q&P process, including full austenitization, quenching at 220 °C, followed by two different partitioning temperatures, was applied to the as-received specimen to generate a complex microstructure composed of tempered martensite, bainite, ultrafine carbides/martensite-austenite/retained austenite particles. Microstructure and crystallographic data were investigated by scanning electron microscopy, electron backscattered diffraction (EBSD), and X-ray diffraction techniques. Then, hardness and tensile properties were evaluated to confirm the improvement of mechanical properties. Dilatation-temperature curves exhibited the kinetics of martensitic and bainitic transformation during quenching and isothermal partitioning stages. The presence of nano-carbide particles inside athermal martensite was confirmed by electron microscopy due to the pre-formed martensite carbon depletion during the partitioning stage coupled with bainitic transformation. The formation of preferential atomic-compact <111> direction in BCC (martensite/bainite) plates characterized by EBSD, could enhance ductility by providing adequate slip systems. Point-to-point misorientation analyses demonstrated a slight dominance of low angle boundaries proportion in bainitic dominance structure in Q&P-220-375 specimen, which could be used in phase characterization. Results revealed that the development of nanoscale carbide dispersed in refined bainite/martensite matrix boosted the yield and ultimate tensile strength by over 100% and 110% compared to the initial pearlitic microstructure. However, ductility reduced to half value in Q&P-220-325 and Q&P-220-375 specimens.

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.

Developing NiAl-strengthened HSLA steels by controlling nanoscale precipitation and high-angle boundaries

In this work, an attempt was made to develop NiAl precipitation-strengthened HSLA steels by introducing Al to the alloy composition of conventional HSLA steels. A novel HSLA steel with a superior combination of yield strength (1234 MPa) and total elongation (13.3%) was achieved. The precipitation behaviors of nanoparticles (including NiAl and carbides) and the collateral effects of Al on the microstructural evolution in this steel were studied. The results show high densities of Ni(Al, Mn) and (Mo, V)C precipitate in this alloy, but low precipitation kinetics. The high Al/Ni atom ratio in the alloy composition and high dislocation density promote the precipitation of Ni(Al, Mn). Additionally, the Ni(Al, Mn) nanoparticles serve as heterogeneous nucleation sites for the (Mo, V)C carbide, significantly increasing the nucleation rates of the carbide. Except for precipitation, increasing Al also increases the proportion of high-angle grain boundaries by altering martensitic transformation kinetics, resulting in a refinement of the equivalent grain size. This microstructure evolution also optimizes the matrix's dislocation distribution, allowing the steel to retain good ductility after precipitation strengthening. Finally, theoretical models are used to quantitively evaluate the contributions of the above changes induced by Al to yield strength.

Formation of core-shell austenite as promoted by alloying solutes in hot-rolled medium Mn steel

Recently, there has been a new trend to design chemically heterogeneous austenite by adjusting the various thermal treatment processes in medium Mn steel. In this work, we found that the single or synergistic addition of ferrite stabilizers (e.g., Al) accelerated the heterogeneous distribution of Mn and the formation of a core-shell austenitic structure compared with the addition of an austenite stabilizer (e.g., Cu) under the conventional quenched-annealing process. In addition, the Mn-enriched austenite shell played a significant role in suppressing the martensitic transformation of the Mn-depleted austenite core upon cooling. The sharp Mn gradient distribution corresponded to the heterogeneous kinetics of the deformation-induced martensitic transformation, resulting in the gradual transformation-induced plasticity (TRIP) effect.

Load partitioning behavior in a metastable austenitic stainless steel subjected to pre-deformation below and above Md temperature: Experimental and modeling studies

A physics-based model coupling the evolution of dislocation density in the austenite and strain-induced martensite phases with composite flow strength and kinetics of martensite transformation was developed to describe ambient temperature uniaxial tensile deformation behavior of mill annealed and pre-deformed 304 SS. The applicability of the model was evaluated by fitting the experimental stress-strain data and validated with the experimentally estimated dislocation density in the austenite phase of the steel. The modeling results revealed that the initial microstructure of the steel significantly influences the dislocation accumulation and annihilation rates as well as the load partitioning between austenite and martensite phases. Despite a lower initial dislocation density than the pre-deformed conditions, the mill annealed condition revealed a higher capacity (approximately two orders of magnitude) for dislocation accumulation in the austenite phase. The initial volume fraction of strain-induced martensite phase and the initial dislocation density of pre-deformed 304 SS are governed by the degree of pre-deformation and pre-deformation temperature, either below or above Md temperature (where Md is defined as the temperature above which plastic deformation does not result in austenite to martensite transformation). It is observed that storage, as well as annihilation rates in the austenite phase, are higher for the steel having a higher initial volume fraction of martensite and dislocation density. Though the pre-deformation altered the evolution of martensite volume fraction, the steel exhibited a minor variation in dislocation accumulation capability in the martensite phase for different conditions.