Effect of the surface oxide layer on transformation behavior and shape memory characteristics of Ti-Ni and Ti-Ni-Mo alloys

Oxidation behaviors of Ti-49Ni and Ti-48.3Ni-0.7Mo (at.%) alloys in dry air from 723 K to 1273 K have been investigated by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, and then effects of oxidation on transformation behavior and shape memory characteristics of the alloys were studied by means of differential scanning calorimetry and thermal cycling tests under constant load. Three-layered surface scale was formed in both alloys oxidized at temperatures higher than 1023 K, consisting of an outer TiO₂ layer, an intermediate layer of mixture of TiO₂ and Ni and an inner TiNi₃ layer. Thickness of the surface oxide layer increased from 1 m to 50 m with raising oxidation temperature from 923 K to 1273 K. The surface oxide layer raised transformation temperatures associated with the B2-B19 and the R-B19 transformation, while it did not almost change transformation temperatures associated with the B2-R transformation. Recoverable elongation was not changed in the alloys oxidized at temperatures below 823 K with raising oxidation temperature, whereas it decreased in the alloys oxidized at temperatures above 923 K. Transformation hysteresis was not almost changed by oxidation in a Ti-49Ni alloy, but it decreased largely in a Ti-48.3Ni-0.7Mo alloy.     

Phase transformation and microstructure and mechanical properties of as cast NiTiRe shape memory alloys

Abstract

The microstructure, martensitic transformation and mechanical properties of as cast Ni₅₂Ti_48?xRe_x shape memory alloys (SMAs) were investigated. The microstructure of these alloys consists of B19′ martensite phase as a matrix and B2 austenite in small percentages in addition to some precipitations of NiTi intermetallic compounds. There are two types of NiTi precipitates: the first one is Ti₂Ni, which can be seen in the all microstructures of the three alloys, and the other is Ni₂Ti, which is found only in the microstructure of Ni₅₂Ti_47·7Re_0·3 alloy. According to differential scanning calorimetry measurements, one stage of transformation reaction B2 to B19′ accompanied with forward and backward martensitic transformation was observed. The martensitic peak and the austenitic peak were increased with the addition of rhenium. Both are increased as the number of valence electron per atom increase and the valence electron concentration decrease. Hardness measurements of Ni₅₂Ti_48?xRe_x SMAs are improved by the Re additions.     

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

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.     

Phase engineering and supercompatibility of shape memory alloys

In recent years examples of unprecedented functional and structural fatigue resistance and lowered hysteresis in shape memory alloys have been achieved by combining conditions of supercompatibility between phases with suitable grain size and a favorable array of fine precipitates. We collect, review and compare these examples to elucidate the relative roles of these factors, especially in the case of the more demanding stress-induced phase transformations, and we pose key open questions. The control of these factors lends itself to systematic alloy development. Taken together, these results point to significant opportunities to discover improved shape memory alloys as well has new reversible transforming multiferroics.     

Comparative study of martensitic transformation behavior of NiAlMn and NiAlMnFe high temperature shape memory alloys

A comparative study of microstructure and martensitic transformation (MT) behavior of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ high temperature shape memory alloys (SMAs) has been performed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), optical microscopy, and micro-hardness testing. The MT temperature (MTT) of Ni₅₉Al₁₁Mn₃₀ alloy is higher than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy, and both alloys’ MTT increases with increasing annealing temperature. The temperature hysteresis and hardness of Ni₅₉Al₁₁Mn₃₀ alloy are smaller than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy. The MT behavior of Ni₆₀Al₁₉Mn₁₆Fe₅ is sensitive to aging temperature and its MTT and hysteresis decrease with increasing aging temperature. However, the MT behavior of Ni₅₉Al₁₁Mn₃₀ alloy is not sensitive to aging temperature. The MT stabilization effects appear in both alloys during thermal cycles. This stabilization effect vanishes from the second thermal cycle. The quenched microstructure of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ alloys is M plus gamma phase, in which the volume fraction of gamma phase is about 40 and 20%, respectively, and the microhardness of M is higher than that of gamma phase. No aging effects were found in both alloys after aging at 400 °C.     

Characterization and properties of ferromagnetic shape memory alloys

Ni–Mn–Ga shape memory alloys are employed for applications in actuators and sensing devices. These alloys exhibit ferromagnetic shape memory effect with large reproducible strains in moderate magnetic fields. This work presents a study of the effect of composition and annealing treatment on the microstructure and magnetic properties in Mn-rich off-stoichiometric Ni–Mn–Ga alloys. Modulated martensitic structure (c/a < 1) with hierarchical twins was found at room temperature in alloys with Mn ≥ 28 at.% whereas the alloy containing higher Ga (> 22 at.%) revealed austenitic structure at room temperature. Ferromagnetic nature of the alloys was confirmed by the magnetization curves. It is demonstrated that a maximum of 400 parts per million strain was measured in the alloy with 7 M martensitic structure at room temperature.     

Co-Ni磁控形状记忆合金的相变及显微结构

Ｃｏ基合金中存在从ｆｃｃ到ｈｃｐ的马氏体相变，其奥氏体－马氏体界面完全共格，层错能较低，马氏体相变的驱动力很小，易通过外磁场诱发ａ／６〈１１２〉Ｓｈｏｃｋｌｅｙ不全位错的移动引起相变而产生对外的应变输出，是一种理想的磁控形状记忆候选材料。本文研究了Ｃｏ－Ｎｉ合金的金相组织和透射电子显微结构，测定了一组Ｃｏ－Ｎｉ合金的马氏体相变及其逆转变温度。研究表明：Ｃｏ－Ｎｉ合金的马氏体组织呈有规则的条片状，马氏体相变温度随Ｎｉ含量的增加而降低，其母相中存在层错，母相的饱和磁化强度高达１２４Ａｍ２·ｋｇ－１，是典型磁控形状记忆材料Ｎｉ２ＭｎＧａ的近两倍。     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Modeling of superelastic behavior of porous shape memory alloys

International Journal of Mechanics and Materials in Design - In this study the superelastic behavior of porous shape memory alloys has been simulated in a finite element procedure. The Boyd and...     

Dynamic impact behavior of Ni-rich and Ti-rich shape memory alloys

Stress vs. strain curves of two kinds of Ti–Ni shape memory alloys under dynamic compressive loading condition were obtained by the split Hopkinson pressure bar experiment. One is Ni-rich, 50.75Ni–49.25Ti (at%), alloy and the other is Ti-rich, 49.5Ni–50.5Ti (at%). The former exists as austenite phase to show superelastic behavior at the room temperature, and the latter is martensitic phase at the room temperature. The dynamic stress–strain curves were compared with quasi-static compressive test results. The Ni-rich specimen displayed a much higher stress level at the high strain rate than at the quasi-static test, but the Ti-rich specimen exhibited a stress level alike in the two types of tests. Temperature measurement on the surface of the impact specimens was carried out during the impact test. The temperature rising was about 3–5 °C in the considered test conditions.     

Phase transformation in NiTiHf shape memory alloy thin films

In the present study, amorphous NiTiHf thin films with different compositions were deposited on silicon wafers by using D.C. magnetron sputtering. Crystallization and martensitic transformation characteristics were studied. Crystallization temperatures and activation energy increased with increasing Hf content. The addition of Hf caused larger atomic size mismatch and stronger interactions among constituent elements, thus, increasing the thermal stability of amorphous thin films. With increasing annealing temperature or Hf content, martensitic transformation temperature increased. The results imply that NiTiHf thin films may be used as the potential candidates for high temperature applications in microactuators.     

Ni-Mn基磁性形状记忆合金第二相的形成及其对相变和性能的影响

Ni-Mn基磁性形状记忆合金具有良好的温度场和磁场诱发的形状记忆效应、超弹性、磁热效应、磁阻效应、弹热效应、交换偏置效应等功能特性。作为一种新型多功能材料,有望应用于驱动器、传感器等多个工程领域。本文详细阐述了包含第二相的Ni-Mn基磁性形状记忆合金的研究现状,梳理和总结了第二相的形成及其对马氏体相变、功能特性和力学性能的影响,提出了一些有待解决的问题,如第二相对包括磁性形状记忆效应在内的磁功能特性的影响,并指出未来应着重于研究第二相形成与演化过程的热力学/动力学因素,对第二相进行合理调控,从而优化合金功能特性。     

Dilatometric study of martensitic transformation in NiTiCu and NiTi shape memory alloys

Dilatometric measurements have been carried out for the study of nature of martensitic transformation in the NiTiCu and NiTi shape memory alloy wire samples. Investigation has been done in the heat-treat temperature range 300–800°C. NiTiCu exhibits only single stage A M martensitic transformation in the entire heat-treat temperature range indicating the suppression of R-phase by Cu substitution. NiTi shows the two-stage A R M martensitic transformation in the heat-treat temperature range 340–410°C and the single-stage A M martensitic transformation above heat-treat temperature 410°C. The extent of dilation during phase transformation decreases with increasing heat-treat temperature in both the alloys. Effect of first 15 thermal cycles on transformation temperatures in both the alloys has been studied. It is found that transformation temperatures are unaffected with thermal cycles in NiTiCu whereas considerable decrease in transformation temperatures has been observed in the case of NiTi. The stability of transformation temperatures in NiTiCu during M A transformation against thermal cycling may be attributed to the associated smaller thermal hysteresis compared to NiTi.     

不同拉伸比的形状记忆纤维的制备及性能研究

用冻胶法直接纺制热致感应型形状记忆纤维,并对初生纤维进行了拉伸处理,通过各种测试手段研究了不同拉伸比下纤维的吸水性能、热性能、动态力学性能以及形状记忆特性。结果表明,随着拉伸比的提高,纤维的断裂强度增大,吸水性降低,纤维的模量提高,且显示出更好的热致感应形状记忆性能。     

Phase transformation behaviours and shape memory characteristics of Ti-45xNi-5Cu-xMo x=0.3-1.0 alloys

Abstract

Transformation behaviours and shape memory characteristics in Ti-45xNi-5Cu-xMo x=0.3, 0.5, or 1.0 at.- alloys were investigated by means of electrical resistivity measurements, differential scanning calorimetry, X-ray diffraction, thermal cycling tests under constant load, and tensile tests. The two stage transformation B2-B19-B19 occurred in Ti-45xNi-5Cu-xMo alloys, and complete separation of the B2-B19 transformation from the B19-B19 transformation was observed in Ti-44.0Mo-5Cu-1.0Mo at.- alloy. Substitution of Mo for Ni in Ti-45Ni-5Cu at.- alloy increased the critical stress for slip deformation and the pseudoelastic recovery. The maximum recoverable elongations of Ti-44.7Ni-5Cu-0.3Mo at.- and Ti-44.5Ni-5Cu-0.5Mo at.- alloys were 6.4 and 7 respectively.     

NiTiHf-based shape memory alloys

Abstract

NiTiHf-based shape memory alloys have been receiving considerable attention for high temperature, high strength and two-way shape memory applications since they could have transformation temperatures above 100°C, shape memory effect under high stress (above 500 MPa) and superelasticity above 100°C. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf-based alloys have some drawbacks such as low ductility and high slope in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing. It has been revealed that microstructural control is crucial to govern the shape memory properties (e.g. transformation temperatures, matrix strength, shape recovery strain, twinning type, etc.) of NiTiHf-based alloys. A summary of the most recent improvements on selected NiTiHf-based systems is presented to point out their significant shape memory properties, effects of alloying, aging and microstructure of transforming phases and precipitates.