Phase transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys

Transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys have been investigated by means of differential scanning calorimetry (DSC) measurements, X-ray diffraction, electron microscopy, tensile tests and wire drawing tests. Mo addition to a Ti-Ni binary alloy induced the R phase transformation, and consequently Ti-Ni-Mo alloys showed two stage transformation, i.e., from the B2(cubic) parent phase to the R(rhombohedral) phase, and then from the R phase to B19(monoclinic) phase. In the thermo-mechanically treated 51Ti-48.3Ni-0.7Mo alloy, reverse transformation temperature, Af, kept constant, irrespective of thermo-mechanical treatment conditions, while it changed in the thermo-mechanically treated 51Ti-49Ni and 51Ti-48.5Ni-0.5Mo alloys. Mo addition to Ti-Ni binary alloy decreased wire drawing stress. Wire drawing stress decreased with raising intermediate annealing temperature monotonously when the annealing treatment was made in vacuum. When the annealing treatment was made in air, however, it decreased with raising annealing temperature up to 923 K, and then increased. Optimum intermediate annealing temperature of Ti-Ni-Mo alloys for wire drawing was 823 K, above which a thick oxide film which reduced the drawability of the alloys was formed on the surface of alloy wires.     

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.     

生物用形状记忆Ti-10Nb-5Sn合金加热相变的TEM原位观察

用透射电镜加热台对生物用金属材料Ti-10Nb-5Sn合金进行反复的加热、冷却原位观察。结果表明，该合金升温至355K时马氏体开始溶解，385K时溶解完毕：降温到353K时开始形成马氏体，333K时转变结束。该合金高温相为体心立方β相；室温马氏体以正交晶系NbTi4相为主。该合金能记忆室温以及高温的组织形态，具有双程记忆功能，并且记忆精度在50nm范围内薄膜样品的弯曲在10之弧度数量级上无变化。     

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。     

Effect of pre-strain on shape memory characteristics in a Ti-17Nb-6Zr-1.0at.%O biomedical alloy

The effect of pre-strain on the shape memory characteristics of a biomedical alloy, Ti-17Nb-6Zr-1.0at.%O, was investigated using various tensile tests. A 3% cyclic test on a solution-treated (ST) specimen showed that the critical stress for inducing martensitic transformation (σ_SIM) decreases with an increase in the number of cycles. From the 9th cycle, the shape perfectly recovers after unloading. In the case of the 4% pre-strained specimen, a 3% cyclic test showed that σ_SIM dramatically decreases with increase in the number of cycles, and the shape perfectly recovers from 4th cycle. For the 5% pre-strained specimen, 3% and 4% cyclic tests showed superior superelasticity from the 5th cycle. The ST specimen and pre-strained specimens showed similar values of the critical stress for slip and maximum superelastic recovery strain. These results indicate that pre-strain has a small effect on the basic shape memory characteristics of the alloy.     

Ti-50．75at％Ni形状记忆合金绞线超弹性试验研究

为了在常温条件下更充分地利用TiNi形状记忆合金绞线的超弹性性能，以Ti-50．75％（原子分数）Ni形状记忆合金（SMA）丝和绞线两种形式试件进行了室温条件下，加载速率、应变幅值及循环次数等因素对SMA丝和SMA绞线超弹性性能影响的试验。试验表明SMA以绞线的形式使用时能够大幅度提高其极限应变能力，相对SMA丝来说更适合应用于大变形时提供足够的恢复力；SMA绞线相对SMA丝具有更强大的超弹性性能；SMA绞线的残余应变量要比SMA丝的小得多，这也是使它具有较高的超弹性性能的主要因素之一．从而验证了SMA以绞线形式使用可以提供更强的超弹性性能．     

The electro-discharge machining characteristics of TiNi shape memory alloys

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current I _P and pulse duration _P. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration _P and a lower pulse current I _P should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO₂, TiNiO₃ and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration _P. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti₄₉Ni₅₁, Ti₅₀Ni₅₀ and Ti₅₀Ni₄₀Cu₁₀ SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.     

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.     

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.     

Co-Ni磁性形状记忆合金的显微组织和力学行为

采用光学显微分析、X射线衍射分析及拉伸试验研究了合金成分对Co-Ni磁性形状记忆合金的显微组织和力学行为的影响.结果表明,Co-Ni合金均匀化退火后其热致马氏体为密排六方结构,点阵参数为a=0.251nm,c=0.407 nm,热轧后马氏体组织呈透镜状.母相为面心立方结构,其点阵参数为a=0.355 nm.试验合金在弹性阶段过后,呈现连续的加工硬化,没有明显的屈服平台,随Ni含量的增加,σ02、E值下降,δ增加.     

Transformation characteristics of shape memory alloys in a thermal cycle

This paper presents a systematic study on the transformation characteristics of shape memory alloys in a thermal cycle with/without a load. The methods to determine some key material properties are proposed. The expressions for estimating the hysteresis and transformation interval under different types of loads are obtained. The influential factors in real experiments are identified and discussed.     

变形对 Cu-Al-Mn记忆合金相变滞后的影响

为了研究CuAlMn形状记忆合金的相变滞后效应,以Cu-Al10.61-Mn6.95(ω（B）/%）合金（A合金）与Cu-Al11.05-Mn6.81-Zn0.21(ω（B）/%）合金（B合金）为研究对象,应用电阻法测定其相变点,并利用X射线衍射谱标定其结构。结果表明,A合金主要由18R马氏体构成,B合金主要由9R马氏体构成,而9R马氏体结构的合金相变滞后温度远大于18R马氏体结构的合金。为了进一步研究变形对相变滞后温度的影响,对B合金进行了拉伸变形试验。研究发现,通过增大预变形量可以有效地提高滞后宽度。     

Multi-strengthening effects on the martensitic transformation temperatures of TiNi shape memory alloys

The multi-strengthening effects, introduced by combining cold rolling, aging and thermal cycling, on the martensitic transformation temperatures of Ti₅₀Ni₅₀ and Ti₄₉Ni₅₁ shape memory alloys have been studied by using DSC and microhardness measurements. Experimental results show that both pre-cold-rolling of Ti₅₀Ni₅₀ alloy and aging treatments of Ti₄₉N₅₁ alloy can impede the further introduction of dislocations during the thermal cycling and hence effectively reduce the effect of thermal cycling on transformation temperatures. For Ti₄₉Ni₅₁ alloy, whether the cold rolling is conducted before or after the aging treatment, the strengthening of cold rolling can significantly increase the specimen hardness but only slightly affect the following thermal cycling effect. The multi-strengthening effects on the martensitic transformation temperatures are also found to follow the equation of Ms = T₀–K_y.     

预变形和热循环对Ni50Ti45Ta5形状记忆合金相变行为的影响

利用示差扫描量热仪研究了预变形和热循环对Ni54Ti45Ta5形状记忆合金相变行为的影响。研究结果表明：预变形后合金的马氏体发生了稳定化现象。形变后第一次热循环．逆相变温度增加。正相变温度降低；而第二次相变循环。马氏体的稳定化现象消失。热循环使相变温度降低。减少了相变热。     

Tailoring the microstructure,martensitic transformation and strain recovery characteristics of Ti-Ta shape memory alloys by changing Hf content

In the present work,the microstructure features,martensitic transformation,mechanical properties and strain recovery characteristics of Ti-Ta based shape memory alloys were tailored by changing Hf contents.The single α"martensite phase was dominated in Ti-Ta alloy with 2 at.％H f.Upon Hf content exceeded 2 at.％,β phase started to appear.Moreover,the amount of β phase gradually increased with Hf content increasing.The martensitic transformation temperatures continuously decreased with the increased Hf content,which was attributed to the rising of valence electron concentration.Meanwhile,Hf addition improved the thermal cycling stability of Ti-Ta alloys due to the suppression of ω precipitation.The yield stress of Ti-Ta based alloys firstly decreased and then increased with Hf content increasing.In addition,the completely recoverable strain of 4％can be obtained in Ti-Ta alloy with 6 at.％Hf as a consequence of the higher critical stress for dislocation slip.Besieds,the Ti-Ta based alloy containing 8 at.％Hf had the superior superelasticity behavior with the fully recoverable strain of 2％at room temperature.