Martensitic Transformation Process Research Articles

Effect of heat treatment on the microstructure and superelasticity of NiTi alloy via selective laser melting

In this study, a Ni50.4Ti49.6 shape memory alloy (SMA) was fabricated by selective laser melting (SLM), and it is found that heat treatments were effective to change the phase transformation behavior, microstructure, and superelasticity under compression for SLM Ni50.4Ti49.6. Detailed results based on EBSD demonstrated that, for SLM Ni50.4Ti49.6 during solid solution and aging (450 °C) treatment, the ultrafine cellular and columnar grains (∼16.5 μm) in as-fabricated Ni50.4Ti49.6 became equiaxed grains (∼70.2 μm), and the frequency of low-angle grain boundaries (26.7%→13.5%) and kernel average misorientation (0.37 → 0.21) decreased. The SEM and TEM analyses show that Ti-rich Ti2Ni/Ti4Ni2Ox and Ni-rich Ni4Ti3 precipitate phases were formed in the matrix simultaneously. After solid solution treatment, the size and distribution of Ti2Ni/Ti4Ni2Ox almost unchanged with aging temperature increasing from 350 °C to 550 °C, however, the size of Ni4Ti3 in aging-treated NiTi samples increased with aging temperature obviously, and it is interesting to find that the distribution of Ni4Ti3 became uneven and dislocation networks were formed in the matrix when the aging treatment reached up to 550 °C, which originated from the internal stress generated by supersaturated Ni atoms. Furthermore, the results of superelasticity for aging-treated NiTi demonstrated that the characteristics of Ni4Ti3 precipitate phase were powerful to influence the process of superelasticity decay/fatigue instead of determining the ultimately recovery strain after 10 compressive cycles. The aging-treated NiTi with evenly-distributed, high-density, and relatively-small Ni4Ti3 precipitate phase shows minimum recovery strain at 1st cycle compression and best superelasticity stability among three aging-treated NiTi, which was attributed to strong obstruction effect of these Ni4Ti3 precipitate for the occurrence of stress-induced martensitic transformation process and formation of dislocations. In contrast, the aging-treated NiTi with unevenly-distributed and low-density Ni4Ti3 precipitate shows maximum recovery strain at 1st cycle compression and worst superelasticity stability. These results in this work can bring some meaningful insights into tailoring the microstructure and superelasticity of SLM NiTi SMAs.

Phase-field simulation and high-temperature confocal laser scanning microscope experimental study of Martensite transformation during the disk laser quenching process

Laser quenching has significant advantages over conventional heat treatment. The main microstructure of C30E4 steel after quenching at a specific temperature is Martensite. Accurately obtaining the microscopic characteristics of the Martensite transformation process is the key to obtaining the best material properties, which is significant to material design. In this paper, the C30E4 steel was disk-laser-quenched, the metallographic and high-temperature confocal laser scanning microscope experiment was carried out. The microstructure of the fully phase-change hardened zone, heat-affected transition zone and matrix zone after quenching was obtained. The C30E4 martensitic transformation process in-situ was obtained. The mathematical physics model of Martensite transformation from metastable to steady state was established by phase field method. The finite-element method was used to solve the model, and the quenching Martensite transformation law was obtained. The Martensite formation process in austenite grain boundaries and microstructure defects was discussed. The experimental and the numerical simulation were compared to get consistent results, which verify the validity of the calculation method. This study lays a significant foundation for effectively revealing the Martensite formation and transformation during the disk laser quenching process, and provides a theoretical basis for quantifying the influence of Martensite transformation process on material properties.

Effect of Zr on the microstructure and properties of high-strength Ti-Mo microalloyed steel after quenching and tempering

Investigation on the effects of Zr and quenching and tempering processes on the microstructure and properties of high-strength Ti-Mo microalloyed steel using SEM, TEM, mechanical property testing, and First-principles calculations. The results show that within the tempering temperature range of 550–650 °C, the microstructures of Ti-Mo microalloyed steel and Ti-Zr-Mo microalloyed steel consist of ultra-low carbon martensite and carbides. Specifically, the strength of Ti-Mo microalloyed steel initially increases and then decreases with the increase in tempering temperature, while the strength of Ti-Zr-Mo microalloyed steel increases with the increase in tempering temperature. During tempering, the addition of Zr element affects the diffusion of C element, delaying the martensitic tempering transformation process, and slowing down the nucleation, growth, and coarsening processes of alloy carbides. First-principles calculation results indicate that ZrC preferentially forms during tempering, resulting in a certain sequential gradient of precipitated microalloy carbides, thereby enhancing the tempering stability of microalloyed steel. Compared to Ti-Mo steel, Ti-Zr-Mo steel exhibits narrower lath martensite width and finer dispersed precipitation phases after tempering at 600 °C and 650 °C.

Effect of phase transformation sequence on the low cycle fatigue properties of bainitic/martensitic multiphase steels

The three microstructures of bainite-fresh martensite, pre-martensite-bainite and carbide-free bainite were obtained by modulating the phase transformation sequence by the heat treatment process in the medium-carbon multiphase steel. The effects of martensitic and bainitic phase transformation sequences on the low cycle deformation behavior of multiphase steels were investigated. The results showed that the sample of the fresh martensite transformation process (295 °C–15 min) exhibited the best fatigue life among all processes under strain control. The samples of the preformed martensite transformation process (250 °C–295 °C) and the carbide-free bainite transformation process (295 °C–120 min) exhibited similar fatigue life. The sample of 295 °C–15 min possesses good resistance to plastic destabilization, resulting in higher fatigue life. The results of the EBSD crack propagation path analysis demonstrate that high carbon M/A islands block crack propagation in the sample of 295 °C–15 min, and high angular grain boundaries skew the crack propagation path. Fracture scans observed that secondary cracks in the sample of 250 °C–295 °C extend inward with aggregation of voids, which are more likely to form crack sources.

Whole martensitic transformation process in Fe–Mn–Si–Cr shape memory alloy by improved characterization of volume resistivity

Since Fe–Mn–Si–Cr shape memory alloy (Fe-SMA) has been discovered, an improvement of the shape memory effect (SME) is challenged. Numerous attempts have been made thus far to improve SME. To effectively control such performance, it becomes imperative to comprehend the real-time behavior of the stress-induced martensitic transformation (SIMT), which is strongly related to SME, during deformation. In the past, the measurement of volume resistivity has been proposed and applied to capture the forward SIMT behavior during the tensile loading process at various strain rate. However, it is quite hard to find investigations of the reverse martensitic transformation after the loading, which is more essential for understanding SME. Therefore, a comprehensive evaluation of the whole SIMT process including the loading process at various strain rate is strongly required. In this study, an improved measurement of volume resistivity is established by introducing both the four-probe and active-dummy techniques with a development of a diameter measurement at high temperature. Then, the volume resistivity of Fe-SMA is measured during the whole deformation including loading under tension at various strain rate, unloading, and heating processes. Besides, the microstructures of Fe-SMA after unloading and heating processes are observed, respectively. Finally, the real-time whole martensitic transformation in Fe-SMA is first investigated with the novel optimization method. The negative strain rate of change in volume resistivity during whole processes can be found. More amount of austenite can be obtained at lower strain rate, resulting in a larger SME.

Effect of tempering temperature and subzero treatment on microstructures, retained austenite, and hardness of AISI D2 tool steel

The presence of retained austenite in the hardening process of tool steel often causes the lower hardness compared to the hardness requirements and poor dimensional stability in the tool steel. The purpose of the present research is to determine the relationships between the tempering process with and without cryogenic treatment to the hardness and retained austenite amount of as-hardened D2 tool steel. The austenitizing temperature was 1020 °C, the tempering temperatures have variations of 180 °C, 280 °C, 380 °C, 480 °C, and 580 °C, and the subzero treatment has a temperature of −172 °C, followed by tempering at 180 °C, 380 °C, and 580 °C. This study aims to determine the appropriate treatment to obtain a minimum retained austenite percentage to prevent and mitigate the failure of AISI D2 tool steel in the industrial application process. An optical microscope with image processing software (Image-J analysis), as well as Brinell and Vickers hardness testing, is the characterization method used in this work. In general, plate martensite, bainite, retained austenite, and primary and secondary carbides are the phases contained in the microstructure. Tempering temperatures have the effect of increasing the secondary carbide precipitation and decreasing the retained austenite content (γr 3,671%–2,769%). However, the cryogenic treatment can provide a more efficient martensitic phase transformation process and minimal retained austenite content (γr 2,257%–1,199%). The increase in tempering temperature causes a decrease in hardness at a temperature of 180 °C–380 °C. On the other hand, the secondary hardening and phase transformation phenomena cause an increase in the hardness of the as-tempered sample at a temperature of 480 °C, before the sample reexperiences a significant decrease in hardness at a temperature of 580 °C due to diffusion that decreases the carbon content.

Atomistic simulations of martensitic transformation processes for metastable FeMnCoCr high-entropy alloy

Atomistic simulations of martensitic transformation processes for metastable FeMnCoCr high-entropy alloy

Effect of Hot Deformation Parameters on Heat-Treated Microstructures and Mechanical Properties of 300M Steel.

The high strength of 300M steel originates from the heat treatment process after forging, but how hot deformation affects the heat-treated microstructure and mechanical properties is unclear. In this study, compression tests under different hot deformation parameters and post-deformation heat treatment experiments were carried out, and the martensite transformation process was investigated using in situ observation. The results show that the grain size of the specimen deformed at low temperature and high strain rate is smaller, and annealing twins will be formed. Both austenite grain boundaries and twin boundaries hinder the growth of martensite blocks, reducing the size of martensite units after heat treatment and thus resulting in higher yield strength. Besides, the mathematical models were established to describe the relationship between hot deformation parameters and grain size after deformation, martensite packet size and martensite block width, respectively, after heat treatment. The relationship between yield strength and hot deformation parameters was also analyzed. According to the results and models, the hot deformation parameters would be optimized more reasonably to improve the final mechanical properties of 300M steel forgings.

Релейний захист від електричної дуги та аварійного перегрівання елементів електроенергетичного розподільчого пристрою

Relay protection against electric arc and emergency overheating of elements of a high-voltage switchgear, triggered with a high degree of reliability, was created by NUFT scientists when using a material with a shape memory (ESM) effect. Each metal and alloy has its own crystal lattice, the architecture and dimensions of which are predetermined. But in many me¬tals with changes in temperature, pressure, the lattice does not remain unchanged. In some moment its restructuring takes place. Such a change in the type of lattice, a polymorphic transformation, can be carried out in two ways. According to this scheme, the lattice is rearranged if the mobility of the atoms is diffusion, high enough to ensure their movement to new places. This is possible when a polymorphic transformation occurs at high temperature. This method was discovered during hardening. As a result of quenching, a phase is created with a new crystal lattice — martensite. The reverse transition of martensite to austenite (high-temperature phase) cannot pass through the "explosive" mechanism. It is necessary to significantly heat the alloy so that austenite crystals begin to emerge and grow in the depths of martensite. Moreover, their shape, as a rule, does not renew (atoms do not fall into place). If the lattice is so complex that there is no choice, there is only one way to reverse the restructuring — movement to ascending positions. Only in this case, the martensitic transformation provides the crystal with an ascending memory. Moreover, their shape, as a rule, does not renew (atoms do not fall into place). But the memory of a single crystal is not the memory of the whole volume. The alloy, as a rule, has a polycrystalline structure, consists of many individual crystals (grains), differing from one another in the orientation of the crystal lattices. Changing the shape of the entire sample will occur only if it is created a certain order in the placement of the crystals. It is ideally to orient all the crystals in one direction. During cooling, the atoms leave their old places and occupy new ones, they will move in the direction of action of an external force. Thus, the process of martensitic transformation causes atoms to move, and the external load sets the direction of movement.

Role of planar faults in martensite formation in nano-polycrystalline iron by molecular dynamics simulation

The martensitic transformation in pure Fe and its alloys has been studied over many decades. Several theoretical models have been proposed to describe the atomic motion that leads to the fcc-to-bcc martensitic transformation. However, such models do not account for the effect of pre-existing planar defects such as twin boundaries and stacking faults, present in the high-temperature austenite phase prior to the transformation process. This work systematically studies the role of nano-spaced planar faults with different inter-spacing on the martensitic transformation using molecular dynamics simulations. Research shows that the investigated planar defects affect the nucleation and growth mechanisms during martensite formation, the morphology of the resulting microstructure, the specific atomic path leading to the phase transformation, and the martensite start temperatures. Martensite variants were identified by the analysis of the atomic shears and slip systems during the transformation process. A crystallographic analysis is done to explain the existence of different shear mechanisms of martensite transformation at different locations in the fcc austenite. The present investigation provides fundamental insights into the martensitic transformation process in presence of pre-existing planar defects and can be applied to other material systems, e.g., Fe alloys.

Phase transformation mediated anomalous plasticity of titanium under severe loading conditions

Titanium (Ti) with ω phase is always considered brittle and non-deformable. Therefore, little is known of the role of ω phase in the anomalously high plasticity of titanium alloys undergoing severe plastic deformation. The deformation mechanisms of shocked Ti samples in uniaxial compression and severe shear deformation was investigated by using molecular dynamics (MD) simulations. Results show that anomalous plasticity of Ti is accommodated by a forward and reverse α-ω martensitic transformation and subsequent twin-twin interactions, rather than conventional dislocation activity. The dislocation-free deformation mechanism in ω-Ti is due to an energy barrier that favors ω→α martensitic transformation compared to dislocation-based slip. Moreover, the α↔ω martensitic transformation process is accompanied by grain/domain refinement, which can be explained by the symmetry change during the phase transformation. This work advances the understanding of the interplay between phase transformation and plastic deformation in titanium under extreme loading conditions.

Martensitic Transition and Superelasticity of Ordered Heat Treatment Ni-Mn-Ga-Fe Microwires

The preparation of Ni-Mn-Ga and Ni-Mn-Ga-Fe master alloy ingots and microwires was completed by high vacuum electric furnace melt melting furnace and melt drawing liquid forming equipment, and the lattice dislocations and defects formed inside the microwires during the preparation process were corrected by stepwise ordered heat treatment. The micro-structure and phase structure were characterized using a SEM field emission scanning electron microscopy and an XRD diffractometer combined with an EDS energy spectrum analyzer; the martensitic phase transformation process of the microwires was analyzed using a DSC differential scanning calorimeter; and the superelasticity of the microwires was tested by a Q800 dynamic mechanical analyzer. The results indicate that Fe doping can refine the grain, transform the phase structure from parent phase to single 7M martensite, reduce the number of martensitic variants, and increase the mobility of the twin grain boundary interface. The MT phase transition temperature (MS) is substantially increased in the martensite transition (MT) process by the increase of the number of free electrons in its lattice. During the superelasticity (SE) test, both microwires displayed superior recover-ability of SE curves, and the Fe doping curves showed similar characteristics of “linear superelasticity”, showing higher critical stress values and complete SE in the experiment. The critical stress satisfies the Clausius-Clapeyron equation and exhibits higher temperature sensitivity than Ni-Mn-Ga microwires.

Giant elastocaloric effect induced by lower stress in Ni-Mn-In-Fe ferromagnetic shape memory alloys

The influence of the substitution of Fe on the mechanical properties and elastocaloric effect (eCE) in Ni50Mn36−xIn14Fex (x = 0.5, 1, 1.5, 2, 4, 6) ferromagnetic shape memory alloys (FMSMAs) have been investigated. The results show that mechanical properties are improved by iron doping. The fracture stress and strain of arc melting Ni50Mn34In15Fe2 alloy are 411 MPa and 10.1 %, respectively. A large adiabatic temperature change (ΔTad) of up to 9.8 K was obtained via the reverse process of compressive stress-induced martensitic transformation with a lower driving stress of 175 MPa compared to the undoped alloy (256 MPa). These results show that this approach is a practical solution to the challenge of obtaining shape memory alloys with a large elastocaloric effect.

The nucleation mechanism of martensite and its interaction with dislocation dipoles in dual-phase high-entropy alloys

The recent progresses in experiments have proven that the recently synthesized dual-phase high-entropy alloys (DP-HEAs) exhibit ultrahigh ductility without sacrificing the high strengthen. However, the original atomic mechanisms of the martensitic transformation process and its interaction with dislocations remain mysterious. In this work, the nucleation mechanisms of martensite are thoroughly investigated with various arrangement of dislocation dipoles via molecular dynamics (MD) simulations. Both stress-induced and thermal-induced martensitic transformation are reproduced in our simulations, which obeys the Burgers orientation relationship. Under the thermal fluctuation, the nucleation of martensite is controlled by the localized stress levels and strain energies. The thermal-induced martensite is found to nucleate near the dislocation dipoles once the strain energy reaches the required value. Upon loading, the fast-moving stress-induced martensite bends and wraps the dislocation dipoles in propagation path. These results provide a fundamental understanding of the martensitic nucleation in DP-HEA and its strengthening and toughening mechanisms, contributing benefits to design new alloys.

In Situ Observation of Thermoelastic Martensitic Transformation of Cu-Al-Mn Cryogenic Shape Memory Alloy with Compressive Stress.

The thermoelastic martensitic transformation and its reverse transformation of the Cu-Al-Mn cryogenic shape memory alloy, both with and without compressive stress, has been dynamically in situ observed. During the process of thermoelastic martensitic transformation, martensite nucleates and gradually grow up as they cool, and shrink to disappearance as they heat. The order of martensite disappearance is just opposite to that of their formation. Observations of the self-accommodation of martensite variants, which were carried out by using a low temperature metallographic in situ observation apparatus, showed that the variants could interact with each other. The results of in situ synchrotron radiation X-ray and metallographic observation also suggested there were some residual austenites, even if the temperature was below Mf, which means the martensitic transformation could not be 100% accomplished. The external compressive stress would promote the preferential formation of martensite with some orientation, and also hinder the formation of martensite with other nonequivalent directions. The possible mechanism of the martensitic reverse transformation is discussed.

An integral transformation model for the combined calculation of key martensitic transformation temperatures and martensite fraction

Understanding the process of martensitic transformation and accurately predicting the fraction of retained austenite and martensite after athermal martensitic transformation are of great significance for optimizing the mechanical properties of various advanced high strength steels. This paper proposes an integral transformation model for the description of both the martensitic transformation temperature and its volume fraction considering both chemical driving force and non-chemical free energy. The proposed model was validated in both incomplete and complete martensitic transformation systems with different austenite grain sizes. Compared with the conventional Koistinen and Marburger model, the integral transformation model can predict the Ms and Mf temperatures as well as the transformation fraction in an integrated manner and depict the relationship between martensite fraction and temperature as a more realistic S-shaped curve. This can be attributed to the fact that the integral transformation model is developed from a generic energy viewpoint and takes into account not only the chemical driving force but also the significant non-chemical free energy contribution.

Martensitic Transformation and Magnetic-Field-Induced Strain in High-Entropy Magnetic Memory Alloy Ni20Mn20Ga20Gd20Co20 by Hot-Magnetic Drawing.

The wires with chemical composition Ni20Mn20Ga20Gd20Co20 were prepared by hot-magnetic drawing and the microstructure evolution characteristics, martensitic transformation and MFIS process were investigated in detail, respectively. The results showed that a multiphase structure with γ phase and martensite was observed in samples when the magnetic field was 0 T to 0.2 T during the hot-magnetic drawing process. With the magnetic field increased to 0.5 T, due to the atomic diffusion by severe thermoplastic deformation and high external magnetic field, a single-phase structure with L10 type twin martensite was found in the sample. Moreover, an obvious increasing trend in martensitic transformation temperature in the sample was found by the enhancement of the magnetic field during the hot-magnetic drawing process. The highest phase transition temperature rose to about 600 °C when the magnetic field reached 0.5 T. Finally, the property of SME and MFIS in the sample can be enhanced by the magnetic field increasing during the hot-magnetic drawing process, excellent performance of SME was obtained at low total strain, and MFIS was achieved at 4.47% at a magnetic field of 8007 Oe in the sample in the 0.5 T magnetic field during the hot-magnetic drawing process.

Elastic interaction energy analysis during twin plane formation in the martensitic transformation process of α"-martensite in Ti-Nb-Al

To understand the twin selection mechanism that occurs in the initial stage of the martensitic transformation of shape memory alloys, the stress state during twin plane formation was numerically analyzed by micromechanics. The self-accommodation microstructure of α''-martensite of the Ti-23Nb-3Al (at%) shape memory alloy, in which the lattice parameter was adjusted to eliminate the formation of internal twinning due to lattice-invariant deformation, was adopted for analysis. The two lattice correspondence variants (CVi and CVj) formed a twin plane. CVi was assumed to be spheroid, and the elastic strain energy during twin plane formation was evaluated by analyzing the elastic interaction energy when CVj formed around CVi. There are three types of twins ({011}α" compound twin, {111}α" type I twin, and <211>α" type II twin) in the self-accommodation microstructure of α"-martensite. Regardless of the CVi geometry (i.e., aspect ratio (1 to 0.01) and minor axis orientation), the {111}α" type I twin exhibited minimum elastic interaction energy, indicating that minimum elastic strain energy was generated during this twin plane formation. This result is consistent with the experimental results, which showed the selective formation of {111}α" type I twins in the initial stage of martensitic transformation.

Interactions of nanoprecipitates, metastable austenite, and Lüders banding in an ultra-low carbon medium Mn steel with abnormal strength dependence of hydrogen embrittlement

A hierarchical microstructure, containing an ultrafine-grained matrix with mixed lamellar and equiaxed morphology, metastable austenite, and intermetallic nanoprecipitate, was designed to improve the hydrogen embrittlement (HE) resistance of ultra-low carbon medium Mn steels. Compared with the warm-rolled (WR) state, the warm-rolled-tempering (WRT) sample shows a simultaneous enhancement of yield strength (YS) and tensile ductility, but lower total elongation loss (∼3%) after hydrogen charging, thus exhibiting an abnormal strength dependence of HE. The microstructural analysis with numerical calculation proves that an additional tempering not only introduces a heterogeneous distribution of Ni(Al, Mn) precipitations, but also results in a reverse γ→α transformation with Mn partitioning, thus contributing to a self-stabilization of austenite with refined grain size and enriched element stabilizer. On one hand, the introduction of intermetallic Ni(Al, Mn) precipitations could be beneficial for the intrinsic hydrogen capture and storage, although its limited number density only contributes little to impeding hydrogen diffusion. Moreover, the introduction of nanoprecipitate can improve the strain compatibility and suppress the strain concentration in ferrite phases, which is consistent with a lower kernel average misorientation (KAM) value of the WRT sample after deformation. Meanwhile, the increased austenite stability leads to a shorter yield point elongation (YPE) and an effective transformation-induced plasticity (TRIP) effect at large strains, and the transformation product of nano-lamellar α’-martensite from self-stabilized austenite contributes little to the hydrogen-accelerated strain localization in weak α/γ interfaces of the WRT sample. In contrast, the low HE resistance of the WR sample is mainly attributed to the enhanced martensitic transformation process during the propagation of Lüders banding, which leads to a strong localized stress concentration in α/α’ interfaces, and the premature failure due to hydrogen-enhanced decohesion effect (HEDE). The interactions of nanoprecipitate, metastable austenite, and Lüders banding provide new insight into the design of high-strength steel with excellent HE resistance.

The cyclic strain evolution and the fatigue prediction in non-proportional multiaxial loadings of NiTi SMAs

NiTi shape memory alloy is a kind of smart material which present unique properties of superelasticity, shape memory and excellent biocompatibility. The applications of such alloys are generally the connectors and fasteners, vibration-dampers, and endovascular stents, etc.. The cyclic deformation and fatigue failure of NiTi SMAs are key issues that need to be investigated since they usually endure complex cyclic loadings during the using process. In this paper, the evolutions of the peak/valley strains in non-proportional multiaxial fatigue loadings of NiTi SMAs are considered, and the fatigue failure mechanism is investigated. The results show that the martensite transformation and martensite re-orientation process greatly influence the transformation ratcheting and fatigue life of NiTi SMAs, and a life-prediction model that considers these deformation mechanisms particular for NiTi SMAs is also proposed to give reasonable predictions.