High Martensitic Transformation Temperature Research Articles

Electronic structure, magnetism and martensitic transformation in all-d-metal Heusler alloys Mn2PtZ (Z = Sc, Ti, V)

The atomic ordering, electronic structure, magnetic properties and martensitic transformation of three new all-d-metal Heusler alloys Mn2PtZ (Z = Sc, Ti and V) have been investigated theoretically to discover new magnetic shape memory alloys (MSMAs). Depending on the different third transition metal element Z, Mn2PtSc and Mn2PtTi tend to form XA structure while Mn2PtV forms L21 structure. This difference is related to their DOS structures. Charge density difference calculations indicate enhanced d-d hybridization in these all-d-metal Heusler alloys. Mn2PtZ are all ferrimagnets with their Mn-Mn or Mn-V local spin moments antiparallel coupled. In these alloys, Mn2PtTi is a possible new MSMA with the energy difference ΔEM between the martensite and austenite as large as −262 meV/f.u., which indicates a high martensitic transformation temperature and is related to the Jahn-Teller effect in the electronic structure. A large volume effect of 2.9% cell shrinkage occurs during the martensitic transformation. All this makes Mn2PtTi a promising candidate for MSMAs with wide applications.

Order degree and self-accommodation of γ phases in Ni-Mn-Ga alloys

High temperature shape memory alloys Ni-Mn-Ga have high martensitic transformation temperature, good thermal stability and moderate shape memory performance. The second phase γ significantly affects the alloys’ macroscopic performances, but its atomic structure and microstructural feature still remain almost unknown. Microstructure and degree of atomic ordering of γ precipitate are uncovered in Ni54Mn30+xGa16–x (x = 0, 2, 5, 9, 11) and Ni57+yMn25Ga18–y (y = 0, 1, 2, 3, 4) alloys. For as-cast Mn-rich alloys, a hierarchical “nano-lamellae within micro-lamellae” microstructure is found. Nano-lamellae are paired to form a variant, two variants constitute a micro-lamella, a pair of micro-lamellae makes up single group, and there are six groups in a grain. Single nano-lamella is a partially ordered phase with tetragonal crystal structure. All nano-lamellae are of nearly identical crystallographic orientation. This arrangement of the 24 variants is self-accommodated. In annealed state, the lamellae in large γ precipitates still exist due to the constraint from martensites, but in small grains partly or completely disappear and further evolve into nanotwins. For as-cast Ni-rich alloys, the paired nano-lamellae are disordered and another partially ordered phases, respectively. After annealing, they become single crystal with ordered structure. Models describing atomic positions of γ phases with different orderings are built. For Mn-rich alloys, the martensitic transformation temperature reaches about 600 °C, shape memory strain is up to 5%, and compressive stress and strain are higher than 2100 MPa, and 26%, respectively. Relative to Ni-rich samples, Mn-rich alloys have very high martensitic transformation temperature, moderately enhanced ductility and reasonably good shape memory strain. They are closely related to martensite’s electron concentration and γ’s atomic ordering, crystal structure, and lamellar microstructure.

The effect of systematic substitution of Nb with Ti on a potential high-temperature shape memory alloy: B2 NbRu

NbRu has been considered as a potential high-temperature shape memory alloy due to its high martensitic transformation temperature, however, it has poor superelasticity and other shape memory properties, which limits its applications. In this paper, the effect of systematic substitution of Nb with Ti on the structural, thermodynamic, and elastic properties of supercell 2x2x2 B2 Ru50Nb50-xTix spanning the entire composition range (x = 0, 6.25, 12.50, 18.75, 25.00, 31.25, 37.50, 43.75, 50 at. %) is investigated using ab initio calculations. Elasticity results showed that the mechanical stability of the B2 phase improved with increasing Ti content, which could be associated with a decrease in transformation temperature. However, a suitable composition range within which Cʹ~0, resembling that of well-known TiNi, is identified. This predicts Ti addition to be effective in improving shape memory properties of B2 NbRu alloy.

(TiB+La2O3)/Ti-V-Al lightweight high temperature shape memory composites with high strength fabricated by reaction hot pressing and hot rolling

In the present study, lightweight high-strength Ti-V-Al based high temperature shape memory composites were fabricated to broaden the application of shape memory alloys. In-situ TiB whiskers and La2O3 particles were introduced and the distribution of reinforcements was optimized by the combination of low-energy milling, hot-press sintering and subsequent hot rolling. The in-situ reinforcements formed a network structure, which evolved from an equiaxed shape to an ellipsoidal shape. Furthermore, the direction of the reinforcement was inclined to the rolling direction. In the reinforcement-lean area, the phase constitution evolved from the β parent phase to the α″ martensite phase, and the width of the α″ martensite gradually widened with increasing distance to the in-situ reinforcements. The composites maintained high martensitic transformation temperature characteristics. Excellent mechanical properties with a high tensile strength of 885 MPa and a large elongation of 13.1% are obtained in the composite with a 1.0 wt% LaB6 addition and 70% deformation ratio. Meanwhile, the composite realized 3.49% recoverable strain under 6% pre-strain conditions.

Low-cost (ZrCu)50-xTax high temperature shape memory alloys showing excellent shape memory effect

ZrCu-based alloys, as one of the most potential high-temperature shape memory alloys, show quite high martensitic transformation temperature, relatively low price for the raw materials, and good casting fluidity. In this work, the martensitic transformation (MT) and shape memory effect of (ZrCu)50-xTax high-temperature shape memory alloys were systematically investigated. Both X ray diffraction and SEM backscattered electron image demonstrated the coexistence of the main phase monoclinic martensite phase and the second phase Ta2Cu. SEM results also revealed that increasing Ta content resulted in a larger volume fraction of Ta2Cu phase. Differential scanning calorimetry results showed that the MT temperatures remarkably increased with Ta addition. The ductility of the (ZrCu)50-xTax alloys was reduced to some degree with increasing Ta content. The shape memory effect was improved significantly, in which (ZrCu)98Ta2 alloy showed the 6.19% maximum recovery strain during 8% pre-strain under compression at 25 ​°C.

The effect of vanadium and nickel on the microstructure and transformation temperatures of Ti50Pt50 alloy

Significant research has been done to produce shape memory alloys that have good shape memory properties and high martensitic transformation temperatures. The Ti50Pt50 alloys have been found to have high transformation temperature of around 1050℃ however, they exhibit negligible shape memory properties. The solid solution strengthening, and improved shape memory properties could be enhanced by ternary alloying. Therefore, this work investigates the effect of varying V and Ni contents, in the range of 6.25 to 12.5at%, on the austenitic and martensitic transformation temperatures, and hardness of the equi-atomic Ti50Pt50 alloy. Arc melting followed by casting and solution heat treatment was carried out to produce the alloys. As-produced alloys were characterized by using scanning electron microscopy, differential scanning calorimetry and hardness testing. The microstructures showed high volume fraction of second phases formed in the TiPtV alloy compared with Ti50Pt50 and TiPtNi alloys. The multiple phases formed in the TiPtV alloys could be the cause of high hardness values observed in these alloys as compared withTi50Pt50 and TiPtNi alloys. Thermal transformation studies revealed that TiPtV alloys exhibit transformation temperature close to Ti50Pt50 alloy, in contrast with TiPtNi alloys. TiPtNi alloys thermal behaviour was improved by solution heat treatment.

Brittle-to-ductile transition in Ti–Pt intermetallic compounds

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

Martensitic Transformation of High-Entropy and Medium-Entropy Shape Memory Alloys

As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti45Zr5Pd25Pt20Au5, and Ti45Zr5Pd25Pt20Co5 had the martensitic transformation. The perfect recovery was obtained in Ti45Zr5Pd25Pt20Co5 during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti45Zr5Pd25Pt20Au5 during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti45Zr5Pd25Pt20Co5 is the one possible HT-SMA working between 342 and 450 °C.

Study on microstructure and properties of TiBw/Ti-V-Al light weight high temperature shape memory composite

This study investigated a novel light weight high temperature shape memory composite with a quasi-continuous network microstructure, which greatly promoted to the application of shape memory alloy in aerospace. In-situ TiB whisker was introduced and mainly distributed along the large Ti-V-Al matrix, which contributed to the breakthrough of the mechanical properties. No obvious influence on the martensite structure, and the spear-like martensite variants still exhibited the typical self-accommodation morphology with {011} type I twin relationship. The XRD results showed that the phase constitution gradually evolved from sole α″ phase to the coexistence of the α″ phase and β phase with the increasing of TiB2 contents. The reverse martensitic transformation peak temperature increased firstly and then decreased with the increasing of TiB2 contents, owing to the influence of composition variation and stress field. It was noteworthy that the TiBw/Ti-V-Al composite with 2.0 wt% TiB2 addition exhibited a higher strength, accompanied by the superior elongation (15.7%) and high martensitic transformation temperature. The yield strength and fracture strength were 692 MPa and 932 MPa, respectively. In addition, the shape memory effect increased firstly and then decreased with the increasing of the TiB2 contents. The maximum recoverable strain of 4.4% with 6% pre-strain can be gained in 0.3 wt%TiB2 added Ti-V-Al composite.

Martensitic Transformation and Shape Memory Effect of Ti–16Nb–xZr (x = 0–8) Alloys

The influence of Zr content on the martensitic transformation behavior and shape memory effect (SME) was investigated to develop Ti–Nb–Zr high-temperature shape memory alloys. The addition of Zr decreases the peak temperature of the reverse martensitic transformation (Ap) by 19οC with 1 at % increase of Zr in the Ti–16Nb–xZr (x = 0–8) alloys, while it improves the stability of martensitic transformation by suppressing the formation of ω phase. At the same time, SME is enhanced greatly owing to the solution strengthening effect of Zr and the increase in the theoretical limit of transformation strain. The optimal Zr content for getting a high martensitic transformation temperature, an enhanced phase transformation stability and a good SME is determined to be 6 at %, where the martensitic transformation onset temperature (Ms) is 252οC and shape recovery ratio at the pre-strain of 8 is 90%.

Microstructure and Phase Transformation in Ni50Mn40Sn10 Shape Memory Alloy

In the present work, a ternary Heusler alloy of composition Ni50Mn40Sn10 with a high martensitic transformation temperature has been prepared by arc-melting technique. The crystal structure of the obtained alloy is investigated by X-ray diffraction. The microstructure of the specimen sintered at 950°C for 72 h followed by ice-water quenching shows needle-like martensitic structure. Energy dispersive spectroscopy is utilized for compositional study. The differential scanning calorimetry result shows that the alloy undergoes a martensitic transformation at 190°C upon cooling. The M–H curve suggests paramagnetic nature of the martensite phase at room temperature. The present alloy system has a great potential to be used for high-temperature shape memory applications. Experimental data of this study may be useful for developing Ga-free high-temperature shape memory alloys.

High temperature Ni45Co5Mn25 − xFexGa20Cu5 ferromagnetic shape memory alloys

We anticipated that replacing Mn by Fe in the Ni50−yCoyMn25−xFexGa25−zCuz magnetic shape memory alloys would increase their Curie temperature and spontaneous magnetization. In particular, in the set of alloys Ni45Co5Mn25−xFexGa20Cu5 (x=0, …8), a record-breaking combination of high martensitic transformation temperature (400K) and Curie temperature (458K) has been achieved for Ni45Co5Mn21Fe4Ga20Cu5. This alloy shows a non-modulated tetragonal martensite with the lowest lattice distortion ratio, c/a=1.15, near the reverse martensitic transformation temperature. Because of the high Curie temperature and low c/a ratio, the Ni45Co5Mn25−xFexGa20Cu5 and Ni50−yCoyMn25−xFexGa25−zCuz alloys are promising candidates for high temperature magnetic shape memory applications.

Effects of Grain Size and Co Addition on the Transformation Temperatures of Ti–Ni–Zr Thin Films

The transformation temperatures and temperature hysteresis of Ti–Ni–Zr films were found to be very sensitive to grain size. However, films annealed at 1223 K exhibited transformation temperatures and temperature hysteresis nearly the same as those of the corresponding bulk alloys. Annealing of Ti–Ni–Zr–Co films with 10.5–26.7 at.% Zr and 0–12.7 at.% Co at 1223 K for 10 h showed that Co substitution for Ni in the Ti–Ni–Zr films was beneficial to reducing the temperature hysteresis. Correspondingly, a Ti29.3Ni42.7Zr20.9Co7.1 bulk alloy annealed at 1223 K for 10 h exhibited a high martensitic transformation temperature (432 K) and a small temperature hysteresis compared to Ti–Ni–Zr bulk alloys (21 K). The present study verified the usefulness of sputtering for the exploration of new shape-memory alloys.

Structure and Performance Changes of Ni-Co-Al Shape Memory Alloys in Relation to Co/Al Atomic Ratio

As a potential ferromagnetic shape memory alloy, Ni-Co-Al has excellent mechanical properties, large magentic-field-induced strain and high martensitic transformation temperature. The relationship between microstructure and performance (mechanical and magnetic properties) of Ni-Co-Al with different Co/Al atomic ratios (RCo/A1) was investigated. Samples exhibit β and γ dual-phase structure. The γ phase grows coarse and the volume fraction of γ phase increases with the rise of RCo/A1. Besides, sample with high amount of γ phase content has smaller β grains owing to the pinning effect of γ phase. The martensite, transformed from β phase, is tetragonal L10 structure with a (1▪1) twinning plane. The martensitic transformation temperature of samples ascends with increasing RCo/A1 owing to more Co embedded into the cell, which makes the valence electron concentration (e/a) of system rise. The saturation magnetization (Ms) of samples increases as RCo/A1 rises because Co-rich γ phase has excellent magnetic property. Meanwhile, both compressive and micro-hardness tests reveal that the samples containing more γ phase have excellent ductility due to the intrinsic good ductility nature of γ phase.

Ti(Pt, Pd, Au) based High Temperature Shape Memory Alloys

Rising martensitic transformation temperature is required to develop HTSMAs. Then, we have focused on the alloys consisting of Ti and precious metals such as TiPt, TiPd and TiAu due to their high martensitic transformation temperatures. To strengthen these compounds and control phase transformation temperature, third elements were partially substituted to the alloys. Some of the TiPd–base compounds indicated 100% shape recovery at temperature range between 400-450°C. The effect of third elements on phase transformation and shape memory effects was investigated.

Martensitic transformation and shape memory effect of Ti–V–Al lightweight high-temperature shape memory alloys

The effects of Al content on martensitic transformation and shape memory effect of Ti–V–Al alloys were investigated. The addition of Al suppressed the formation of ω phase and improved the shape memory effect and mechanical properties. The optimal Al content for getting a high martensitic transformation temperature and a good shape memory effect is determined to be 3 at.%. The recoverable strain with 6% pre-strain was 4.2% and the martensitic transformation temperature of Ti–13V–3Al was 359 °C. The alloy showed an elongation of over 20%.

A new type of Cu–Al–Ta shape memory alloy with high martensitic transformation temperature

In this study, a new type of Cu–Al–Ta (Cu86Al12Ta2 wt%) shape memory alloy with high martensitic transformation temperature is explored. The microstructure, reversible martensitic transformation and shape memory properties are investigated by means of optical microscopy, back-scattered electron, electron probe microanalysis, x-ray diffraction, differential scanning calorimetry and tensile tests. It is proposed that Cu86Al12Ta2 alloy consists of a mixture of primarily martensite and a little martensite and some different precipitates. The tiny thin-striped Ta2(Al,Cu)3 precipitate is predominant in the as-quenched condition, whereas the particle-shaped Cu(Al, Ta) precipitate is dominant after hot-rolling. Additionally, the dendritic-shaped γ1(Cu9Al4) phase begins to appear after hot-rolling, but it disappears when the sample is re-quenched. All studied samples have reversible martensitic transformation temperatures higher than 450 ° C. The results show that two-step martensitic transformation behavior is observed for Cu86Al12Ta2 alloy in all three different conditions due to the transformations between () martensites and the austenite parent phase. The results further show that the recovery ratios are almost 100% when the pre-strains are ≤2.5%, then they gradually decrease with further increase of the pre-strains. The shape memory effects clearly increase as a result of increase of the pre-strains, up to a maximum value of 3.2%.

Effect of internal stress on sructure, martensitic transformation and magnetic properties of ferromagnetic shape memory alloy Mn2NiGa

The structures, the martensitic transformations and the magnetic properties of ferromagnetic shape memory alloy Mn2NiGa with the loading and the unloading of the external compressive stress are investigated. The plastic deformation occurring during the stressing causes a high level of dislocation defects existing in the sample. The residual internal stress results in a significant increase of the reversed transformation temperature. The threshold value of compressive pressure for causing the completely martensitic transformation at room temperature is identified to be 1.0 GPa based on the experimental measurements of XRD and magnetic coercivity. The coercivity of the martensitic phase increases from 50 Oe to 350 Oe due to the application of the external stress up to 2.0 GPa. It is also observed that the reversed martensitic transformation occurrs, when the sample is treated by heating up to about 730 K. This is attributed to the elimination of the dislocation by the annealing effect. Such a high reverse martensitic transformation temperature allows the measurement of the Curie temperature of martensitic phase and the obtained value is 530 K.

Reproducibility of martensitic transformation and phase constitution in Ni–Co–Al

Ni–Co–Al is a potential thermal and magnetic shape-memory alloy due to its high martensitic transformation (MT) temperature and good ductility. The mechanical and thermal pre-treatment of samples are shown to be crucial for a reproducible adjustment of MT. Crushing, which involves plastic deformation, suppresses MT fully. Annealing can partly restore MT, however generally, MT temperatures, crystal structure and phase constitution of the initial bulk material cannot fully be restored. MT temperatures are constant and reproducible, when keeping sample temperature below 150 °C, short time annealing at 300 °C changes MT temperatures drastically (10–40 K). A guideline is given to obtain reproducibly the desired MT and phase constitution in Ni–Co–Al. The MT temperature and phase constitution are investigated for several different alloy compositions (Ni 35.2Co 37.8Al 27.0; Ni 32.5+ x Co 39.5− x Al 28.0; Ni 33.5+ x Co 39.5− x Al 27.0; x = 0, 0.5, 1, 1.5, 2; at.%) and sample treatments. High MT temperatures around 100 °C can be obtained.

Martensitic transformation and shape memory properties of Ti–Ta–Sn high temperature shape memory alloys

Abstract The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti–Ta–Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti–Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti–Ta–Sn alloys. A Ti–20Ta–3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti–Ta binary alloy showing similar martensitic transformation temperature.