Xianglong Meng

Effect of aging on the phase transformation and mechanical behavior of Ti36Ni49Hf15 high temperature shape memory alloy

The TiNiHf alloys are newly developed as high temperature shape memory alloys with the high transformation temperatures and with lower cost in comparison with TiNiX (X 5 Pd, Pt) alloys. Recently a lot of investigations were focused on the TiNiHf alloys, in which Hf contents are ranged from 1% to 30% [1–8]. Zhu et al. [9] proposed that the decrease of the transformation temperatures of the Ti51.5-xNi48.5Hfx (x 5 15, 30) alloys in the early period of high temperature duration at 723K was due to the precipitated second phase though no further experimental results were provided. In the Ti36.5Ni48.5Hf15 alloy aged at 873K for 150h, a new precipitate phase of the composition of (Ti0.4Hf0.6)Ni of a spindle-like shape with a habit plane of (100) P//(001)M and a long axis of [001]P//[1#10]M was observed by Han et al. [10]. It is well known that, in the Ni-rich binary TiNi SMAs, the precipitation sequences are as follows [11]:

Shape memory properties of the Ti36Ni49Hf15 high temperature shape memory alloy

Abstract The shape memory properties have been studied in a Ti 36 Ni 49 Hf 15 high temperature shape memory alloy (SMA) by bending tests. The shape memory effect (SME) of the alloy is closely related to the deformation condition. It shows about 3% completely reversible strain when the TiNiHf alloy is deformed at the room temperature. The shape recovery ratio is constant at 92% when the deformation temperature is below 457 K, then rapidly decreases to zero above 590 K for the specimen deformed to 4.5%. Obvious two-way shape memory effect (TWSME) is obtained in the Ti 36 Ni 49 Hf 15 alloy aged at 973 K for various hours. As the aging time further increases, the TWSME decreases. Moreover, TWSME in the aged Ti 36 Ni 49 Hf 15 alloy is unstable and decreases rapidly after several thermal cycles.

Stress-induced martensitic transformation behavior of a Ti-Ni-Hf high temperature shape memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti36Ni49Hf15 high temperature shape memory alloy (SMA) have been investigated using tensile tests and transition electronic microscopy (TEM) observations. It shows that, compared with the TiNi SMAs, there is no stress plateau in the stress-strain curve as the TiNiHf alloy deformed in an austenite. The martensitic transformation enthalpy is calculated to be � 106.84 cal/ mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523 K. The substructure of SIM and deformed SIM in the present alloy are (001) compound twin. Martensite variants are (011) type I twin related. Further, increasing the deformation temperature and deformation strain, the preferential oriented SIM variants gradually develop to martensite variants with variant-crack/variant-intersect morphologies. The inexistence of stress plateau in the stress-strain curve of TiNiHf may mainly result from the dislocation slip during the stress-induced martensitic transformation. D 2002 Elsevier Science B.V. All rights reserved.

Two-way shape memory effect induced by martensite deformation and stabilization of martensite in Ti36Ni49Hf15 high temperature shape memory alloy

The two-way shape memory effect (TWSME) induced by martensite deformation and the stabilization of martensite of a Ti36Ni49Hf15 high temperature shape memory alloy (SMA) have been investigated. The experimental results indicated that the martensite deformation is an effective method to get TWSME in the Ti36Ni49Hf15 alloy even with a small deformation strain. During martensite deformation the martensite reorientation and the dislocation slip occur simultaneously. Thus an internal oriented stress field is easily created to induce the TWSME. The TWSME of the Ti36Ni49Hf15 alloy shows poor stability during the subsequent thermal cycling. In addition, the martensite stabilization is caused by the martensite deformation. The variation of the elastic and irreversible energies should be responsible for this. D 2003 Elsevier Science B.V. All rights reserved.

Effects of Co and Al addition on martensitic transformation and microstructure in ZrCu-based shape memory alloys

The effect of ternary alloying element Al and quaternary alloying element Co on the martensitic transformation of ZrCu-based shape memory alloy was investigated. The results show that the addition of Al and Co in ZrCu alloy decreases both the martensitic transformation temperature and the martensitic transformation temperature hysteresis. Transmission electron microscope (TEM) observations reveal that the Cm martensite structure is the preferential formation phase. The intervariant structures in ZrCuAlCo alloy are (021) type I twins, while the dominant substructures inside the martensite variant are the (001) compound twins. With the increase of Co content, tensile fracture strength and strain are improved obviously.

Empirical mapping of ZrCu-based alloys with valence electrons versus transformation temperatures

An empirical map of martensitic transformation temperatures versus average valence electrons per atom (ev/a) and valence electron concentration (cv) was developed in order to design ZrCu-based shape memory alloys (SMAs). The martensitic transformation temperatures of about 40 different alloys (Ni, Co, Hf, Ag, Ti, Al, Cr, etc.), covering nearly all possible replacements of Zr or Cu, are exhibited. The relationship between transformation temperature and cv or electron density (n) was determined. The results indicate that the transformation temperatures of ZrCu-based alloys gradually decrease until reaching an inflection point at cv = 0.218, above which the transformation temperatures go down. A linear dependence of the transformation temperatures of ZrCu-based alloys on the electron density is revealed by data-fitting. Under the guidance of these contour maps describing transformation temperatures and thermal hysteresis, a series of ZrCu-based alloys that can function under different conditions can be designed.摘要本文建立了一个关于马氏体相变温度和价电子数及电子浓度的经验图谱并设计了一系列ZrCu基形状记忆合金. 这些相变温度包括了所有报道过的经Ni、Co、Hf、Ag、Ti、Al、Cr等掺杂替换Zr或者Cu原子的约40种合金. 反映了转变温度和电子浓度或者电子密度之间的关系. 结果显示ZrCu基合金的转变温度逐渐下降, 当cv > 0.218时, 相变温度急剧下降. ZrCu基合金的马氏体相变温度与电子密度呈线性关系. 基于相变温度和滞后图谱的结果可以设计出在不同条件下工作的ZrCu基合金.

Effect of Ce Addition on Martensitic Transformation Behavior of TiNi Shape Memory Alloys

The effect of cerium addition on the martensitic transformation behavior and microstructure of Ti50-x/2Ni50-x/2Cex (x=0, 0.5, 2, 5 and 10at.%) alloys have been studied by differential scanning calorimetry (DSC) and energy dispersive spectroscopy (EDS). The results show that the addition of cerium affects the martensitic transformation temperature obviously. With the increase of Ce content, the phase transformation temperatures first increase rapidly and then decrease slightly, which may be attributed to the change of the Ni/Ti ratio in matrix. Moreover, the dispersed Ce-riched second particles with various morphologies are observed in TiNiCe alloys.

Superelasticity in TiNi Alloys and Its Applications in Smart Systems

The superelasticity is one of the most important properties of TiNi alloy, which has been widely used in the smart systems of many fields, such as aerospace, aviation, biomedicine and energy etc. The current state of superelasticity of TiNi alloy has been reviewed with emphasis on the superelasticity, phase transformations, thermo-mechanical treatment and their relationship. The mechanisms of linear superelasticity and non-linear superelasticity have been revealed. Some successful applications based on the superelasticity of TiNi alloys in smart systems have been introduced.