Martensitic Transformation Start Research Articles

Enhancing elastocaloric effect of NiTi alloy by concentration-gradient engineering

By formulating the martensite transformation start temperature (Ms) of NiTi shape memory alloys (SMAs) as a function of Ni concentration, a Ni-concentration dependent phase field model is newly proposed in this work. Then, based on correspondent phase field simulations, the elastocaloric effects (eCEs) of the NiTi SMA samples with different Ni-concentration gradients and various aspect ratios are investigated and discussed. The simulated results show that the martensite transformation start stress of NiTi SMAs in the sample with a Ni-concentration gradient along the tensile direction progressively increases during the tensile loading, leading to a step-by-step martensite transformation; hence the suppression of microscopic instability results in a small stress-strain hysteresis loop in the process of tensile-unloading and a tremendous improvement in the coefficient of performance of material (COPmat) if addressing the eCE of the sample. When the aspect ratio of the sample is relatively small (e.g., 1:1), the microscopic instability can be suppressed due to the strong boundary constraint, indicating that reducing the aspect ratio is an efficient way to obtain a small stress-strain hysteresis loop and improve the COPmat of NiTi SMA samples. Moreover, for the sample with a Ni-concentration gradient, when its aspect ratio is relatively large, its eCE is mainly determined by the Ni-concentration gradient; while, when the aspect ratio becomes relatively small, the eCE of the sample is strongly dependent on both the Ni-concentration gradient and the aspect ratio. It implies that the NiTi SMA sample with both a small Ni-concentration gradient and a small aspect ratio and that with both a large Ni-concentration gradient and a large aspect ratio demonstrate the optimal eCE. In other words, a pioneering approach, i.e., concentration-gradient engineering, can be put forward to enhance the eCE of NiTi SMAs based on the detailed phase field simulations. The new findings in this work provide available guidance for improving the eCE of NiTi SMAs and open a novel way for developing advanced SMA-based elastocaloric materials.

Microstructure, martensitic transformation and shape memory effect of novel Cu–Al-Ga based shape memory single crystals

In this study, the shape memory effect of novel iron-alloyed Cu–Al-Ga shape memory single crystals were reported for the first time. The microstructure, crystal structure, martensitic transformation and shape memory effect of Cu–10Al-6Ga (master alloy), Cu–10Al-6Ga-3Fe, Cu–12Al-4Ga-3Fe and Cu–14Al-4Ga-3Fe (wt.%) were investigated. All studied alloys consist of dominant martensite and fine bcc nanoparticles with richer Fe content and poorer Cu content except master alloy. The martensitic transformation start temperatures are 241.3 °C, 251.3 °C and 71.9 °C for Cu–10Al-6Ga-3Fe, Cu–12Al-4Ga-3Fe and for Cu–14Al-4Ga-3Fe alloys, respectively. For master alloy, the martensitic transformation disappears resulted from the precipitation of a large amount of α-Cu during heating. In addition, the shape memory effects clearly increase as a result of increase of the pre-strains. The Cu–14Al-4Ga-3Fe single crystal obtains a maximum shape memory effect of 6.8% with 100% strain recovery when 10% pre-strain is applied.

Development of an alternating heat engine, actuated by shape memory alloys

The paper introduces a new type of heat engine actuated by shape memory alloy (SMA) commercial wires. The originality of the design consists in using two SMA active elements that are martensitic at room temperature (RT) and compensate each other in such a way that each element becomes, on a turn, austenitic while the other is martensitic and vice-versa. The SMA wires were manufactured from Ti50Ni45Cu5 SMAs undergoing thermoelastic martensitic transformation characterized, among other things, by superior stiffness of austenitic state as compared to martensitic state. This feature has enabled thermoelastic SMAs to develop work generating shape memory effect (SME), while undergoing reverse martensitic transformation, during heating. Based on this principle, solid state heat engines have been built even since the discovery of SMAs. The paper reviews the most prominent types of SMA driven solid state engines, before introducing a new type of engine. In the present heat engine constructive solution, solar energy is used to directly heat up the active element, which has a straight austenitic “hot shape” and was bent into a curved shape at RT, while being in martensitic state. During heating, the element becomes fully austenitic, partially recovers its straight shape, by decreasing its curvature radius, and bents the bias martensitic element which is softer. The latter will be brought with its most deformed area into the focus of an optical lens, thus being instantly heated up. It develops, in its turn, work generating SME. Based on this principle, a linear-alternative heat engine has been patented. Its functioning principle was tested for different numbers of heating–cooling cycles. By means of an original testing setup, using hot air heating and sprayed water cooling, the variations of displacement with temperature were recorded and the critical temperature for then start of reverse martensitic transformation (As) was determined.

Improved elastocaloric effect of NiTi shape memory alloys via microstructure engineering: A phase field simulation

By newly introducing a grain boundary energy, a grain size (GS) dependent and thermo-mechanically coupled phase field model was proposed to investigate the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs), and discuss the effects of GS, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the eCE through simulations. The simulated results show that with decreasing the GS, the gradual reduction of the degree of martensite transformation (MT) and the variation of transformation mode in the nano-polycrystalline NiTi SMA system lead to the decreasing absorption of transformation latent heat, and the gradual weakness and even disappearance of local instability during MT decreases the hysteresis, resulting in the increase of the coefficient of performance (COPmat); the deformation dependence of COPmat and the grain orientation dependence of transformation instability lead to a dependence of COPmat on texture, which can be enhanced with decreasing the GS. It is further found that the microstructure engineering (from the perspectives of GS, texture, gradient-nanograin and bimodal grain structures) can significantly improve the COPmat, and the introduction of geometric gradient can effectively reduce the MT start stress. Several microstructure and geometric schemes with considerable adiabatic temperature change (ΔTad) and high COPmat as well as relatively low MT start stress are reported, which can provide an important theoretical guidance for improving the eCE of NiTi SMAs and exploiting excellent SMA-based elastocaloric materials. The proposed phase field model and the characterization method for texture can be used as a reference to simulate the eCE and anisotropic MT of other SMAs.

Austenite Decomposition of a Lean Medium Mn Steel Suitable for Quenching and Partitioning Process: Comparison of CCT and DCCT Diagram and Their Microstructural Changes.

The present work deals with the dilatometric study of a hot-rolled 0.2C3Mn1.5Si lean medium Mn steel, mainly suitable for the quenching and partitioning (Q&P) heat treatment in both hot-rolled or cold-rolled condition, subjected to a variation of austenitization temperature. These investigations were performed in a temperature range of 800–1200 °C. In this context, the martensite transformation start temperature (Ms) was determined as a function of austenitization temperature and in turn obtained prior austenite grain size (PAGS). The results show rise in prior austenite grain size due to increasing austenitization temperature, resulting in elevated Ms temperatures. Measured dilatation curves were confronted with the metallographic analysis by means of scanning electron microscopy (SEM). The present paper also focuses on the construction of a continuous cooling transformation (CCT) and deformation continuous cooling transformation (DCCT) diagram of the investigated lean medium Mn steel in a range of cooling rates from 100 to 0.01 °C/s and their subsequent comparison. By comparing these two diagrams, we observed an overall shift of the DCCT diagram to shorter times compared to the CCT diagram, which represents an earlier formation of phase transformations with respect to the individual cooling rates. Moreover, the determination of individual phase fractions in the CCT and DCCT mode revealed that the growth stage of ferrite and bainite is decelerated by deformation, especially for intermediate cooling rates. Microstructural changes corresponding to cooling were also observed using SEM to provide more detailed investigation of the structure and present phases identification as a function of cooling rate. Moreover, the volume fractions obtained from the saturation magnetization method (SMM) are compared with data from X-ray diffraction (XRD) measurements. The discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements. In that regard, the volume fraction of retained austenite increased with a decrease of cooling rate as a result of larger volume fraction of ferrite and bainite. The hardness of the samples subjected to the deformation was slightly higher compared to non-deformed samples. The reason for this was an evident grain refinement after deformation.

Investigations of Deformation Behavior and Microstructure of Al Tailored Ti–Mo High Temperature Shape Memory Alloys during Isothermal Holding at 393 K

To utilize β-Ti based high temperature (HT) shape memory alloys (SMAs), a high Al concentration of 14 mol% was designed for sufficient suppressing the undesired ω-phase. HTSMA exhibits shape memory effect (SME) above 373 K, and thus the operating temperature is over 373 K. However, the SME and the mechanical properties of most β-Ti SMAs deteriorate after holding at elevated temperatures due to the ω-embrittlement. The Ti-4.5Mo-14Al alloy (mol%) and the Ti-6Mo-7Al alloy as a comparison, both of which possess the identical reverse martensitic transformation start temperature of 407 K, were isothermally held at 393 K for up to 360 ks, and deformation behaviors and microstructures were investigated. It was found that after the isothermal holding, the deformation behavior of the Ti-6Mo-7Al alloy altered significantly; on the other hand, that of the Ti-4.5Mo-14Al alloy remained almost intact. Transmission electron microscopy observations revealed that the isothermal ω-phase (ωiso) was successfully suppressed in the Ti-4.5Mo-14Al alloy, while the ωiso phase grew in Ti-6Mo-7Al alloy. Moreover, the isothermal α″-phase coexisted in the Ti-4.5Mo-14Al alloy. It is concluded that a high Al concentration is a crucial prerequisite in the practical β-Ti HTSMAs. The presented design could be a useful guideline for developing Ti SMAs with comparable Mo- and Al-equivalents.

Phase transition kinetics and microstructures of A508-3 steel under continuous cooling process

The phase transition kinetics of A508-3 steel during continuous cooling were studied by high temperature transition instrument which was based on thermodilatometry in this paper. The microstructures during continuous heating and cooling were also observed. The transition points were determined to be austenite formation temperatures, A c1 = 757 °C, A c3 = 823 °C, and martensitic transformation start and final temperatures, Ms = 382 °C, Mf = 222 °C at the cooling rate of 5 °C/s. The curves of Austenite transition fraction and time during continuous heating show a typical S shape. And the Austenite will show a maximum transition rate value at the middle temperature. The volume fraction of new phases translated from Austenite as functions of time and temperature during the cooling process can be obtained using the lever law. The pearlite transitions can occur in high temperature region at cooling rate smaller than 0.1 °C/s. And Bainite can be gotten at a lower temperature region. The S type curves can be used to describe the relationships between the transition volume fraction and time. Increasing the cooling rate enhances the volume fraction of Martensite at the cooling rate ≥5 °C/s. And only Martensite transition can be observed at the cooling rate > 10 °C/s. And Ms and Mf increase with increasing cooling rate. The Martensite transition kinetics equation with changing temperature were obtained. Finally, the continuous cooling transformation curves of A508-3 steel was constructed, which can be used to predict the microstructure and design the heat treatment technologies.

Microstructural evolution in a thin wall of 2Cr13 martensitic stainless steel during wire arc additive manufacturing

A thin wall of well-formed dense 2Cr13 martensitic stainless steel was successfully fabricated by wire arc additive manufacturing (WAAM). Heterogeneous distributions of the hardness and microstructure were found in the wall consisting of 20 layers along the building direction. The hardness profile of the thin wall mainly depended on the degree of tempering of the martensite. Based on the simulated and tested thermal history, the inter-pass temperature was lower than the martensitic transformation start (M s ) temperature in the first 10 layers. The thermal effect of the subsequent passes partially tempered the quenched martensite formed in the previous pass during cooling. By contrast, the inter-pass temperature was greater than the M s temperature in the last 10 layers. The martensitic transformation mainly occurred after the final pass, resulting in a relatively homogeneous microstructure. This study suggests that the inter-pass temperature is a dominant factor in controlling the microstructural evolution during the WAAM of 2Cr13 martensitic stainless steel. • A thin wall of martensitic stainless steel was fabricated by wire arc additive manufacturing. • Hardness and microstructure were heterogeneously distributed along the building direction. • Hardness profile mainly depended on the degree of tempering of martensite. • Inter-pass temperature was a dominant factor to control the microstructural evolution.

Effects of annealing temperature on microstructures and shape memory effect of Ti-19Zr-11Nb-2Ta alloy sheets

The effects of annealing temperature on the microstructures, mechanical properties and shape memory effect of Ti-19Zr-11Nb-2Ta alloy sheets have been investigated. The X-ray diffraction results show that the as-received Ti-19Zr-11Nb-2Ta alloy and those annealed between 873 K and 1073 K are composed of single α″-martensite. The recrystallization temperature of the alloy is between 823 K and 873 K indicated by the optical morphologies, and the thickness of martensite variants increases with the elevating of annealing temperature as detected by transmission electron microscopy (TEM) observations. The reverse martensitic transformation start temperature (AS) slightly increases after annealing above 873 K. The as-received alloy exhibits elongation of only 4.3%, and the plasticity is remarkably improved after annealing above 873 K. The alloy annealed at 873 K exhibits the largest elongation of 21.2% and a maximum shape memory strain of 2.7% among all the samples corresponding to an ultimate tensile strength of 607 MPa.

Novel Ti–20zr–49.7pd High Temperature Shape Memory Alloy with Facilitated Detwinning and Precipitation Strengthening

It is desirable to develop high temperature shape memory alloys with a martensitic transformation start temperature (Ms) above 100°C and a recoverable strain of about 4‐6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We have previously demonstrated the facilitation of detwinning for Ti–Zr–Ni–Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti–(15‐20)Zr–49.7Pd (at.%) and surrounding Ni‐containing compositions are candidates. In this study, aging temperature was optimized for a Ti–20Zr–49.7Pd alloy, and an excellent shape memory effect with Ms above 130°C, low reorientation stress around 200 MPa, high plastic deformation stress around 1800 MPa, and large recovery strain of 4.5% was achieved. On the other hand, short‐range ordering of solute atoms occurs just above the reverse transformation temperature and decreases Ms.

Large recoverable strain with suitable transition temperature in TiNb-based multicomponent shape memory alloys: First-principles calculations

TiNb-based shape memory alloys (SMAs) have great potentials in biomaterials. However, high transition temperature or small recoverable strain limit their application. Using first-principles method, we systematically study the recoverable strain and transition temperature of TiNb-based binary, ternary, and high-entropy alloys (HEAs), and aim to lower the transition temperature and improve the recoverable strain at the same time. We find that the employed approach describes accurately the lattice strain by comparing with the available experimental results. It is well known that there is a positive correlation between lattice strain and recoverable strain in SMAs. Thus, we have evaluated the magnitude of recoverable strain of SMAs by calculating the lattice strain. Meanwhile, we correlate the available measured martensitic transformation start temperature (Ms) with the calculated energy difference between β and α’’ phases in Ti-Nb binary alloys. According to this relation, we evaluate the Ms in other TiNb-based alloys. We find that Zr is a good alloying element that can decrease considerably the Ms and keep the lattice (recoverable) strain almost unchanged simultaneously. Finally, an Al-containing Ti24Nb25Zr24Sc24Al3 HEA has been designed to have simultaneously large recoverable strain and low transition temperature.

Machine learning assisted empirical formula augmentation

Tuning the martensite transformation temperature through composition design has become an important way to broaden the applicable temperature range of shape memory alloys (SMAs). The empirical formula based on traditional statistics is a key reference for composition design. Due to the lack of experimental data, a large deviation may exist among the prediction results from the empirical formulas obtained by different data sources. In present work, we proposed an augmentation strategy of empirical formula based on a machine learning method to build the relationship between martensite transformation start temperature (Ms) and compositions in Cu-Al-based SMA system. A series of ML models were established by physical and chemical features and a Gaussian radial basis kernel function support vector machine (SVR.rbf) model was screened out based on mathematical and domain knowledge criteria. An augmented empirical formula of Ms as the function of compositions was fitted based on the abundant augmented dataset combined experimental data with predicted data by the SVR.rbf model. Compared with previous empirical formula fitted by small experimental dataset, the accuracy and robustness of the augmented empirical formula was significantly improved without additional experimental cost. This strategy offers a recipe to build empirical formula based on a small experimental dataset.

Partitioning of the High Strength Steel Contained Ti Microalloy Element at the Temperatures Above and Below Martensitic Transformation Start Temperature (Ms)

Partitioning of the High Strength Steel Contained Ti Microalloy Element at the Temperatures Above and Below Martensitic Transformation Start Temperature (Ms)

Transformation behavior and superelasticity of TiZrHfNiCoCu multi-component high-temperature shape memory alloys

Multi-component (TiZrHf)1Ni1.50Co0.60Cu0.90, (TiZrHf)1Ni1.48Co0.58Cu0.88, (TiZrHf)1Ni1.45Co0.57Cu0.86, and (TiZrHf)1Ni1.40Co0.54Cu0.82 high-temperature shape memory alloys (HTSMAs) were prepared by arc-melting and then microstructures, transformation temperatures, phase constituents and superelasticity were investigated by scanning electron microscope observation, differential scanning calorimetery, X-ray diffraction and dynamic mechanical analysis in tensile mode, respectively. The microstructure of solution treated TiZrHfNiCoCu HTSMAs specimens consisted of (TiZrHf)(NiCoCu)-type matrix and (TiZrHf)2(NiCoCu)-type second phase. The area fraction of the second phase increased from 1.7% to 17.2% with the increased in (TiZrHf) content from 50 at% to 52 at%. Martensitic transformation start temperature of the solution treated specimens increased from 53.5 °C to 188.7 °C with the increase in (TiZrHf) content from 50 at% to 52 at%. TiZrHfNiCoCu HTSMAs showed clear superelasticity in the solution treated state. The superelastic recovery strain of solution treated TiZrHfNiCoCu HTSMAs was in the range of 4.2–4.6% depending on the alloy composition.

Shape Memory Effect and Superelasticity of [001]-Oriented FeNiCoAlNb Single Crystals Aged under and without Stress

The two-step ageing of Fe-28Ni-17Co-11.5Al-2.5Nb (at.%) single crystals under and without stress, leads to the precipitation of the γ′- and β-phase particles. Research has shown that γ–α′ thermoelastic martensitic transformation (MT), with shape memory effect (SME) and superelasticity (SE), develops in the [001]-oriented crystals under tension. SE was observed within the range from the temperature of the start of MT upon cooling Ms, to the temperature of the end of the reverse MT upon heating Af, and at temperatures from Af to 323–373 K. It was found that at γ–α′ MT in the [001]-oriented crystals, with γ′- and β-phase particles, a high level of elastic energy, ΔGel, is generated, which significantly exceeds the energy dissipation, ΔGdis. As a result, the temperature of the start of the reverse MT, while heating As, became lower than the temperature Ms. The development of γ–α′ MT under stress occurs with high values of the transformation hardening coefficient, Θ = dσ/dε from 2 to 8 GPa and low values of mechanical Δσ and thermal ΔTh hysteresis. The reasons for an increase in ΔGel during the development of γ–α′ MT under stress are discussed.

Atomic scale modeling of the coherent strain field surrounding Ni4Ti3 precipitate and its effects on thermally-induced martensitic transformation in a NiTi alloy

Precipitation of Ni4Ti3 in NiTi alloy profoundly affects material properties, while the coherency strain fields and their effects on the thermally-induced martensitic transformation are not known in detail, especially for the B19’ variants and morphologies. Therefore, molecular dynamics simulations, as well as an Eshelby solution and phase-field microelasticity theory were applied to investigate the strain fields caused by the Ni4Ti3 precipitates with different aspect ratios. The maximum strain (along the central axis of the precipitate) in the matrix and its relative position are formulated as function relationships. Ms (martensitic transformation start temperature) and Af (austenitic transformation finish temperature) during martensitic transformation were determined and analyzed. Two previously published self-accommodation B19’ structures, the triangular and “herring-bone’’ morphologies are investigated in detail. An intermediate state between the triangular and “herring-bone’’ morphologies, termed mixed self-accommodation, is observed for the first time and proved to be the most unstable structure, providing a potential design route for low hysteresis microactuators. The formation processes of these self-accommodation morphologies are analyzed and discussed. Our simulations are the first time to reveal a variety of twinned B19’ morphologies selected by precipitates and their effects on thermally-induced martensitic transformation at the atomic scale.

Martensite Transformation Start Temperature During Quench and Austempering in Fe-8Ni-0.2C Alloys

It is important for the improvement of mechanical properties of quenching and partitioning (QP) steels to control the martensite transformation behavior and the stability of untransformed austenite. In this study, we evaluated the martensite transformation restart temperature (Mr) via a dilatometer during the reheat austempering pattern. This heat pattern corresponding to the typical heat treatment for QP steel involves quenching to TQ, a temperature between the martensite transformation start temperature (Ms) and the finish temperature, and austempering above Ms. In Fe-8Ni-0.2C alloy, Mr equals TQ, regardless of the progress of bainite transformation during austempering at 673 K (400 °C). In Fe-8Ni-1Si-0.2C alloy, Mr decreases parabolically from TQ with the progress of bainite transformation, and this behavior corresponds to the assumed carbon concentration by bainite transformation. These results indicate that the self-stabilization of the entire untransformed austenite during the first quench from Ms to TQ was preserved, regardless the bainite transformation. Moreover, the stabilization by the carbon concentration in untransformed austenite was added to this self-stabilization.

H-phase precipitation and its effects on martensitic transformation in NiTi-Hf high-temperature shape memory alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Effect of Cooling Rate below Ms Temperature on Hydrogen Embrittlement of TRIP-Aided Martensitic Steels

Low alloy TRIP steel is expected to be applied to automobile bodies because of its high strength, high ductility, and excellent impact properties and press formability. It has been reported that the low alloy TRIP steel of hydrogen embrittlement resistance is improved by utilizing the hydrogen storage characteristics of highly stable retained austenite. Therefore, for the purpose of increasing the volume fraction of retained austenite, it was produced at various cooling rates below the martensite transformation start temperature. As a result, the volume fraction of retained austenite increased, and then the effect of hydrogen embrittlement decreased. The matrix phase and retained austenite is refined with decrees of the cooling rate. It is considered that the size and surface area of the retained austenite also affected the improvement of hydrogen embrittlement resistance.

A study on the cold workability and shape memory effect of NiTiHf-Nb eutectic high-temperature shape memory alloy

In this work, we report a (Ni50.1Ti34.9Hf15)85Nb15 alloy that shows excellent cold workability and good high-temperature shape memory effect. The as-cast sample shows eutectic microstructure consisting of β-Nb and the NiTiHf(Nb) matrix. The homogenized (Ni50.1Ti34.9Hf15)85Nb15 alloy shows high compressive fracture strain of 40% and excellent cold-rolling reduction ratio of 80%. It is suggested that the formation of (semi-) coherent β-Nb is the main reason on improving the ductility of NiTiHf alloy. After solution treatment, the material shows a high martensitic transformation start temperature of 146 °C, and a maximum shape memory effect strain of 2.3%. This work demonstrates that (Ni50.1Ti34.9Hf15)85Nb15 alloy with good cold workability and high-temperature shape memory effect may provide new opportunities for applications.