Martensitic Transformation Temperature Research Articles

Processing Effects on the Martensitic Transformation and Related Properties in the Ni55Fe18Nd2Ga25 Ferromagnetic Shape Memory Alloy.

The influence of processing on the martensitic transformation and related magnetic properties of the Ni55Fe18Nd2Ga25 ferromagnetic shape memory alloy, as bulk and ribbons prepared by the melt spinning method and subjected to different thermal treatments, is investigated. Structural, calorimetric, and magnetic characterizations are performed. Thermal treatment at 1173 K induces a decrease in both the Curie and the martensitic transformation temperatures, while a treatment at 673 K produces the structural ordering of the ribbons, hence an increase in TC. A maximum value of the magnetic entropy variation of −5.41 J/kgK was recorded at 310 K for the as quenched ribbons. The evaluation of the magnetoresistive effect shows a remarkable value of −13.5% at 275 K on the bulk sample, which is much higher than in the ribbons.

Influence of Austenitisation Time and Temperature on Grain Size and Martensite Start of 51CrV4 Spring Steel

51CrV4 spring steel is a martensitic steel grade that is heat treated by quenching and tempering. Therefore, austenitisation is an important step that influences steel properties. The main goal of austenitisation is to obtain a single-phase austenite structure that will transform into martensite. We studied the influence of austenitisation parameters on grain growth and martensite transformation temperatures. The samples were quenched from different austenitisation temperatures (800–1040 °C) and were held for 5, 10 and 30 min. The martensite start transformation temperatures (MS) were determined from dilatometric curves, and the hardness was measured using the Vickers method. The microstructure of the samples and the size of the prior austenite grains were characterised using optical microscopy. The increase in the size of the prior austenite crystal grains increases the MS temperature. However, this trend is visible up to 960 °C, where the results start to deviate. High temperatures, 960 °C and above, cause both grain growth and increased carbide dissolution along with chemical homogenization of the steel. The added influence of strong solute diffusion caused a big deviation in the results. The stability of carbides during austenitisation were evaluated with scanning electron microscopy (SEM) and thermodynamic calculations of equilibrium phases using the Thermo-Calc program. MC-type vanadium carbides are stable up to 956 °C under equilibrium conditions, but the SEM results show that they were present in the microstructure even after annealing at 1040 °C. This means that crystal growth is slowed down, which is positive, and that the austenite contains less carbon, so the hardness is lower.

Effect of Fe addition on the microstructure, transformation behaviour and superelasticity of NiTi alloys fabricated by laser powder bed fusion

ABSTRACT NiTi-Fe ternary shape memory alloys were produced by laser powder bed fusion (L-PBF) using the pre-alloyed NiTi and elemental Fe powders. A large process window exists to fabricate dense NiTiFe0.3 and NiTiFe0.5 alloys, while severe cracks occur in the NiTiFe2 alloy. The cracking susceptibility is studied through thermodynamic analysis. Fe is mainly dissolved in the NiTi matrix, which is mainly due to the low Fe addition and rapid cooling associated with L-PBF process. The martensite transformation temperatures (MTTs) decrease monotonously with the increase of scanning speed, the decrease of laser power, or the increase of Fe addition, providing a feasible approach to tailoring the MTTs. Due to the decrease of the MTTs, the NiTiFe0.3 and NiTiFe0.5 alloys show good superelasticity, e.g. the NiTiFe0.3 alloy shows a superelastic recoverable strain of 5.2% (pre-strain of 6%). This work suggests that L-PBF is a potential method to in-situ synthesise ternary NiTiFe alloys.

Microstructure, phase transformation and mechanical properties of NiMnGa particles/Cu composites fabricated by SPS

Microstructure, phase transformation and mechanical properties of NiMnGa particles/Cu composites prepared by spark plasma sintering method were investigated by SEM, EDS, XRD, susceptibility measurements and mechanical tests. The NiMnGa particles were found to react with Cu matrix and the composites exhibited a similar crystal structure to the Cu matrix. The martensitic transformation and Curie transition of the composites were weakened due to the composition change of NiMnGa particles caused by reactions. With increasing NiMnGa particles content, the martensitic transformation and Curie transition of the composites were enhanced to some extent. However, the martensitic transformation temperature and Curie transition temperature were decreased by ~50 K as compared to those of the original NiMnGa particles. The compressive strength of the composites increased with the increase of NiMnGa particles content, whereas the compressive strain was decreased gradually.

Effect of Hf on the microstructure and martensitic transformation behavior in Ti–Pd–Hf alloy

We investigated the microstructure and the crystallography of martensite variants in Ti–Pd–Hf alloy using electron microscopy. The martensitic transformation temperature decreased with increasing amount of Hf substituted into the Ti50−xPd50Hfx alloy. No indication of a martensitic transformation was observed in the as-quenched Ti25Pd50Hf25 and Ti10Pd50Hf40 alloys, even when the samples were cooled to 150 K. The Ti40Pd50Hf10, Ti35Pd50Hf15, and Ti30Pd50Hf20 alloys were composed of a B19 orthorhombic phase having plate-like habit-plane variants consisting of both major and minor correspondence variants several hundred nanometers in width and several tens of micrometers in length at room temperature. Lattice-invariant shear of these alloys was a {111}B19 Type I twin. A solution-treated sample of the Ti25Pd50Hf25 alloy consisted of a B2 cubic matrix with local atomic displacement and lattice distortions, and rounded H-phase particles several tens of nanometers in diameter were precipitated inside the B2 matrix upon aging of the specimen at 823 K for 10.8 ks. The as-quenched Ti10Pd50Hf40 alloy contained rounded H-phase particles several tens of nanometers in diameter embedded in a B2 matrix at room temperature. The martensitic transformation temperature in the Ti50−xPd50Hfx alloy was drastically decreased by both the fully coherent nanosized H-phase particles and the short-range order originating from clustering of solute atoms.

Shape memory behavior of sputter-deposited Ni46.2Ti51.1Co2.7 (at.%) thin films

In order to analyze the relationship between the shape memory behavior of Ti-rich NiTiCo thin films and the microstructures obtained after different annealing treatments, a batch of Ni 46.2 Ti 51.1 Co 2.7 (at.%) thin films were produced by magnetron co-sputtering and annealed at 773 K, 873 K and 973 K for 60 min. The microstructure of the thin films was studied by X-ray diffraction and transmission electron microscopy. Electrical resistivity measurements were performed to determine martensitic transformation temperatures. Additionally, a home-made device was specially designed for studying the mechanical behavior associated with the martensitic transformations of the thin films at different constant external stresses. The sample annealed at 773 K presented Guinier-Preston (GP) zones within the NiTiCo grains, while the samples annealed at 873 K and 973 K showed Ti 2 Ni precipitates inside the grains with semi-coherent interface with the matrix. The appearance of GP zones can explain why the sample annealed at 773 K reaches higher values of recoverable strain than the other samples, with negligible irreversible strain. The maximum recoverable strain of 5.7 % for this specimen represents a noticeable mechanical behavior, similar or even better than the one observed in the binary alloy with similar Ti concentration. The maximum recoverable strains were analyzed in terms of the Sachs bound, which represents a theoretical upper limit. • Ti-rich NiTiCo thin films show an excellent mechanical behavior. • The addition of Co offers a controlled lowering of martensitic transformation temperatures. • Specimens annealed at 773 K present GP zones within the NiTiCo grains. • Specimens annealed at 773 K reach a recoverable strain of 5.7 % with negligible irreversible strain. • The recoverable strain of 5.7 % represents a promising response, in comparison with the Sachs bound.

Machine-learning model for prediction of martensitic transformation temperature in NiMnSn-based ferromagnetic shape memory alloys

Martensitic transformation temperature (TM) of NiMnSn-based ferromagnetic shape memory alloys (FSMAs) is crucial to identifying the operating range of an application. From a materials design point of view, an efficient method that can predict the TM accurately should be strongly pursued, to meet various applications with different operating temperatures. In this paper, we demonstrate that machine learning (ML) can rapidly and accurately predict the TM in NiMnSn-based FSMAs. We evaluate the performance of four machine learning models, including Random Forest Regressor (RFR), Support Vector Regression (SVR), Linear Regression (LR), and XGBRegressor (XGBR) model. Three important features of Numa, Arc, and avg Ven are selected as the optimal feature combination for building the model. Moreover, to ensure the best generalization ability of the model, multiple methods of cross-validation (Leave-One-Out Cross-Validation, 3-fold Cross-Validation, and 5-fold Cross-Validation) are used. Finally, the XGBR model exhibits the best performance for predicting the TM (R2 = 0.903 and RMSE = 5.4, R5f2 = 0.869 and R3f2 = 0.838). The results of small deviation and variance proven that the XGBR model, proposed in this work, is suitable to be used to predict the TM of unknown NiMnSn-based FSMAs. This work is expected to promote the targeted design of FSMAs.

Comparisons of performances in Ti-V-Al composites with single and multiple in-situ reinforcements

In the present study, various kinds of in-situ reinforcements were introduced into Ti-V-Al shape memory alloys to enhance strength. The results revealed that the addition of TiB2 and B4C ceramic particles can form TiB whiskers and a mixture of TiB whiskers and TiC particles, respectively. Meanwhile, both TiB2 and B4C ceramic particles can refine grain size of Ti-V-Al shape memory alloys. The addition of TiB2 and B4C ceramic particles caused an increase in martensitic transformation temperature. The grain refinement and introduction of in-situ TiB whisker and TiC particles resulted in an obvious improvement in strength without sacrificing the strain recovery characteristics. For comparison, the addition of B4C ceramic particles can obtain high-performance Ti-V-Al shape memory alloys.

Tailoring the microstructure, martensitic transformation temperature and mechanical properties of 4D printed NiTi alloys

This study explores the control of the microstructure and properties of NiTi alloy through the adjustment of 4D printing process parameters. The results indicate that increasing the scanning speed from 450 mm/s to 750 mm/s leads to a change in the phase composition from near full martensitic to a two-phase structure of austenite and martensite. The increase in scanning speed can also reduce the evaporation of Ni element and lower the phase transformation temperature. [Formula: see text] temperature and [Formula: see text] temperature decrease from 93°C and 63°C at 450 mm/s to 37°C and 12°C at 750 mm/s, respectively. Moreover, the scanning speed can alter the texture characteristics and mechanical behavior of NiTi alloy. The hardness value and critical stress increase with the increase in scanning speed, while the strain distribution during tensile deformation becomes uneven at high scanning speed due to the unevenly distributed dual-phase microstructure. These findings provide insights for the optimization of 4D printing parameters to tailor the properties of NiTi alloy for various applications.

Microstructure, martensitic transformation and excellent shape memory effect of Ga-alloyed Cu–Al–Mn–Fe shape-memory single crystal

A Cu–10Al–6Mn–3Fe–4Ga (wt.%) single crystal was simply prepared, and its microstructure, reversible martensitic transformation, mechanical and SME properties were investigated. The results show that this single crystal consists of dominant 2H martensite retained L2 1 parent and fine Fe-rich nanoparticles. Its reversible martensitic transformation temperatures are respectively 313 K ( M s ), 299 K ( M f ), 339 K ( A s ), and 354 K ( A f ), which are suitable for practical applications. A large stress platform with a strain of approximately 8% was associated with the reorientation of existing martensite and stabilization of stress-induced martensite. Furthermore, full shape recovery characteristics are obtained all the time while changing the pre-strains from 6% to 11%. The maximum shape memory effect is up to 9.2% with a 100% shape recovery rate.

Effect of energy density on the superelastic property of Ni-rich NiTi alloy fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) has proved to be a promising process for tuning the microstructure and mechanical response in NiTi-shaped memory alloys using a varied energy density input by modifying the process parameters such as laser power or scanning speed. The microstructure, precipitation, chemical composition, and their relationship with austenite–martensite transformation temperatures (TTs) and superelastic properties have been extensively investigated in Ni-rich NiTi alloys processed at varying volume energy densities (52–110 J/mm3) through LPBF. A specific amount of Ni3Ti precipitation was observed and primarily located along the boundary and at the center of the melt pool, and nanoscale spherical NiTi2/Ni2Ti4OX precipitation was homogeneously dispersed in the B2 matrix and B19’ martensite, but no Ni4Ti3 was observed as frequently reported in the literature. Decreasing the volume energy density (VED) led to lower TTs and a higher recovery strain ratio. However, there is an exception for the sample built with the highest laser power of 250 W that possesses the largest Ni/Ti atomic ratio and therefore the highest superelastic property despite the mediated VED (∼87 J/mm3) employed. We concluded that the variation in TTs and superelastic properties among the samples is caused by the varying amounts of Ni, which causes competition among alloying elements in evaporation, condensation, and precipitation during LPBF.

Grain-size effects on the temperature-dependent elastocaloric cooling performance of polycrystalline NiTi alloy

Grain-size (GS) effects on the temperature-dependent elastocaloric cooling performance of NiTi with the average GS of 11, 22, 30, 45 and 70 nm are investigated over a temperature range from -50 ℃ to 80 ℃. It is found that GS refinement is conducive to improving the thermal stability of superelasticity and the associated elastocaloric effect, while the trade-off between cooling temperature drop ΔT and effective working temperature span Tspan, more or less, is inevitable regardless of GS. The large ΔT of the 70 nm-GS specimen, which is characterized by sharp first-order martensitic transformation and strong temperature dependence of the transformation stress dσtr/dT (= 5.7 MPa/℃), is restricted to a narrow Tspan (= 19 ℃). Tspan can be widened by five times via reducing GS to 11 nm but at the expense of a significant sacrifice in ΔT, as the combined result of high strength and small dσtr/dT (= 0.9 MPa/℃). Among the five microstructures, the 30 nm-GS one achieves a favorable compromise between ΔT and Tspan owing to the mild transformation nature together with robust mechanical properties. Consequently, its ΔT can reach 50% – 440% of that of the 70 nm-GS counterpart and the resultant cooling efficiency can be enhanced by a factor of half to six. The work demonstrates that GS engineering is a feasible approach for reconciling various elastocaloric cooling metrics of the NiTi refrigerant.

Influence of Nb addition and process parameters on the microstructure and phase transformation behavior of NiTiNb ternary shape memory alloys fabricated by laser powder bed fusion

NiTiNbx (x=1,3,5) alloys were fabricated by laser powder bed fusion (L-PBF) of powder mixtures containing pre-alloyed NiTi and elemental Nb powders. Upon Nb addition, the dependence of the martensite transformation temperatures (MTTs) on the L-PBF process parameters was significantly altered. In case of the NiTiNb1 alloy, the MTTs increased as the laser energy density was increased, whereas it decreased in case of the NiTiNb5 alloy. It is suggested that the net effect of Nb addition is to lower the MTTs, while Ni evaporation has the opposite effect. The latter contribution is predominant in alloys with low Nb addition (1 at.%), while the Nb-related effects become the major influencing factor in the high-Nb alloys (5 at.%). In the case of the NiTiNb3 alloys, the factors that promote and suppress MTTs are almost completely compensated, and the MTT is rather stable despite of the large variation of L-PBF process parameters.

Large elastocaloric effect in as-cast Ni-Mn-Sn-Fe ferromagnetic shape memory alloys

The Ni-Mn-Sn ferromagnetic shape memory alloys exhibit excellent both magnetic field-induced magnetocaloric effect and uniaxial stress-induced elastocaloric effect. However, the intrinsic brittleness and narrow refrigeration temperature regions hinder their practical applications in elastocaloric and magnetocaloric as well other fields. In this work, the as-cast Ni44-xFexMn46Sn10 (x = 1, 2, 3) alloys are prepared directly by cooling solidification. The as-cast alloys improve the mechanical properties and broaden the martensitic transformation temperature region due to the presence of dendrites. The dendrites prevent cracking formation and propagation along the grain boundaries, and the addition of Fe-atoms enriched in the interdendritic region further optimizes the mechanical performance. In addition, the as-cast Ni41Fe3Mn46Sn10 alloy exhibits a large adiabatic temperature variation up to −10.3 K at a moderate stress of 350 MPa. Accordingly, direct cooling solidification of as-cast alloys could be a novel and feasible method for studying elastocaloric cooling technology.

Correlations between processing technology, microstructure and properties of a Ni-rich Ti–Ni shape memory alloy

A Ti-50.8Ni (at.%)/Ti-55.9Ni (wt.%) VIM processed ingot was rotary swaged and rolled in parallel, obtaining bars with a total Area Reduction above 90% (from forged/rolled raw state one). Throughout the successive stages, the microstructural evolution, thermal and mechanical properties were compared. Deformation bands, grain morphology, precipitates and oxidation were evaluated (by OM and SEM/EDS). An essentially (∼100%) austenitic phase was detected at room temperature (via XRD/Rietveld method), while the Martensitic Transformation temperatures occurred at negative temperatures (via DSC). The rolled bar got through the process more regularly, with homogeneous and refined structure. Typical defects such as superficial microcracks and creases were relevant in the last stages of the two-dies rotary swaging with inductive heating. The work hardening level (via Hardness Test) was similar in both metal forming processes, being 5–7% more pronounced at the edge area of the bars due to redundant work.

Damping performance and martensitic transformation of rapidly solidified Fe-17%Mn alloy

The rapid solidification of Fe-17%Mn alloy was performed to investigate the influence of cooling rate on its damping performance and martensitic transformation mechanism. A proper heat treatment was also carried out to clarify its coupled effects with rapid solidification. The stacking fault probability and martensitic transformation temperature were determined to demonstrate their relationship with the cooling rate and the heat treatment process. With the increase of cooling rate, the volume fraction of ε-martensite increased and the stacking fault probability of ε-martensite was enhanced. The formation of ε-martensite phase was remarkable for the increase of damping capacity and microhardness. It was found that rapid solidification was beneficial for the formation of ε-martensite and the improvement of damping capacity. This effect can be facilitated by the incorporation of the heat treatment process.

Effect of aluminium on microstructure and shape memory effect in Cu-Al-Ag-Mn shape memory alloys

ABSTRACT The objective of the present investigation is to synthesise and characterise the Cu-Al-Ag shape memory alloys with minor Mn addition. Synthesis of Cu-Al-Ag-Mn SMA was done by induction furnace and gravity die casting, with constant Ag/Mn ratio (5% Ag and 2% Mn), the aluminium composition is varied from 8% to 14%. The as-cast sample will be further subjected to thermo-mechanical treatment to produce a sheet which exhibits shape memory effect. This study is to determine the influence of Al content on shape memory characteristics, microstructure, and phase transformation temperatures. The phase fraction of β’1 increased from 56% to 67% along with subsequent reduction of γ1’ phase fraction from 23% to 17%, which is essential for improvement of shape memory properties with increase of aluminium constituent. X-ray diffraction revealed that the crystalline size decreased 79 to 32 nm and lattice strains increased from 0.24% to 0.69% with increase of Aluminium. With an increase in aluminium content, the martensite transformation temperature decreased from 386°C to 246°C, and the shape memory effect increased from 81 to 90% strain recovery. The ability to control shape memory characteristics by changing Al composition will improve the scope for this alloy in various shape memory applications.

Dynamic control and increase of working temperature range in Ni-Mn-In-Co MCE materials by hydrostatic pressure or biaxial stress

In order to achieve carbon neutrality, solid-state phase-change refrigeration technology based on magnetocaloric effects (MCE) is considered one of the most potential alternatives to traditional refrigeration technology. At this stage, the fixed and relatively narrow operating temperature window still remains a major disadvantage for future refrigeration applications. Hence, this work aims to show that working temperature in Ni-Mn-In-Co MCE materials can be dynamically tuned by hydrostatic pressure or biaxial stress. Our results show that Ni21Mn18In6Co3 presents adjustable operating temperature range (from 310 K to 328 K) under pressure (0–3 GPa). Unlike hydrostatic pressure, biaxial stress with compression or tension can achieve bidirectional control of martensitic transformation temperature, further widening the operating temperature range. The biaxial strain from − 1.5–1.5 % can tune the operating temperature range from 292 K to 344 K in Ni21Mn18In6Co3 alloys. Also, the physical mechanism of dynamic control and increase of working temperature range in Ni-Mn-In-Co using physical pressures is revealed detailly. Moreover, the results show that both hydrostatic pressure and biaxial stress do not decrease the curie temperature (TC), and improve the magnetization difference (ΔM) between austenite and NM martensite phases. It further proves that applying physical pressures can be an effective strategy with simultaneous enhancement of working temperatures and magnetic properties. Data availabilityThe data that supports the findings of this study are available within the article.

Thermal-induced martensitic transformation of NiTi shape memory alloy promoted by pulse current

The effect of transient and high-energy pulse current (THEPC) on the phase transformation behavior of the Ti-50.8 at.% Ni shape memory alloy was investigated in this study. The results show that the THEPC increases the start temperature of martensitic transformation, which indicates the facilitation of the thermal-induced martensitic transformation. Meanwhile, the transient and high-energy effect of the pulse current changes the phase transformation sequence of NiTi alloy during cooling, making it from one-stage phase transformation of A→M to two-stage of A→M1 and A→M2.

Room temperature huge magnetocaloric properties in low hysteresis ordered Cu-doped Ni-Mn-In-Co alloys

The reduction of the thermal hysteresis in first order magnetostructural transition is a determining factor to decrease energy losses and to improve the efficiency of magnetocaloric cooling based systems. In this work, a Cu doped NiMnInCo metamagnetic shape memory alloy (MMSMA) exhibiting a narrow thermal hysteresis (around 5 K) at room temperature has been designed. In this alloy, the induced L21 ordering process affects the phase stability in an unusual way compared to that observed in NiMnInCo and other NiMn based alloys. This ordering produces an increase in the Curie temperature of the austenite but hardly affects the martensitic transformation temperatures. As a consequence, the ordering increases the magnetization of the austenite without changing the transformation temperatures, doubles the sensitivity of the transformation to magnetic fields (the Claussius-Clapeyron slope goes from 2.1 to 3.9 K/T), improves the magnetocaloric effect, the reversibility and finally, enhances the refrigeration capacity. In addition, the magnetic hysteresis losses are among the lowest reported in the literature and the effective cooling capacity coefficient RCeff reaches 86 J/Kg for 2 T (15 % higher than those found in Ni-Mn based alloys) and 314 J/Kg for 6 T fields. Therefore, the ordered alloy possesses an excellent combination of low thermal hysteresis and high RCeff, not achieved previously in metamagnetic shape memory alloys near room temperature.