Microstructure and transformation behaviour of Ni75−XTiXPd25 high temperature shape memory alloys
Microstructure and transformation behaviour of Ni75−XTiXPd25 high temperature shape memory alloys
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DOI：
10.1016/j.jallcom.2012.11.165
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发表时间：
2013-03
影响因子：
6.2
通讯作者：
K. Ramaiah;C. Saikrishna;Gouthama;S. Bhaumik
中科院分区：
文献类型：
--
作者：
K. Ramaiah;C. Saikrishna;Gouthama;S. Bhaumik
研究了成分对铸态和均质化Ni75XTiXPd25(X=49.7，50.0和50.3at.%)−合金的组织、相变行为和热稳定性的影响。结果表明，合金元素在凝固过程中发生了明显的分凝，形成了铸态合金的核心组织。与枝晶区相比，枝晶间区Pd含量较低，而Ni含量较高。枝晶间还存在线状富钛第二相。均质态合金的显微组织为NiTiPd基体相夹杂Ti2(Ni，Pd)第二相析出物。发现析出相富Ni，贫Pd。EPMA分析表明，在均匀化过程中，Ni在基体和析出相中的浓度发生了显著的再分布。X-射线衍射分析证实，该基质相在室温下为正交晶系B19结构。研究表明，合金的相变温度强烈依赖于钛含量。化学计量比钛合金的马氏体终温度(Mf)由157℃提高到179℃，而富钛和贫钛合金则分别降低到105℃和179℃。在7~12℃温度范围内，合金的相变滞后相对较低。透射电子显微镜照片显示，合金中存在无孪晶/小孪晶比的马氏体相，使界面能最小，从而降低了相变滞后。化学计量比钛及富钛合金的无应力热循环相变稳定性优于贫钛合金。
The effect of composition on microstructure, transformation behaviour and thermal stability of cast and homogenized Ni75−XTiXPd25alloys (X=49.7, 50.0 and 50.3at.%) were studied. Results showed significant partitioning of the alloying elements during solidification, resulting in cored microstructure in the cast alloys. The interdendritic regions were depleted in Pd and richer in Ni compared to dendritic regions. The interdendritic regions also showed presence of a thread-like Ti-rich second phase. The microstructure of the homogenized alloys consisted of NiTiPd matrix phase interspersed with Ti2(Ni,Pd) second phase precipitates. The precipitate phase was found to be rich in Ni and depleted in Pd. EPMA analysis showed that significant redistribution of Ni concentration in the matrix and the precipitate phase takes place during homogenization. X-ray diffraction study confirmed the matrix phase at room temperature to be of orthorhombic B19 structure. Study showed that the transformation temperatures of the alloys were strongly dependent on Ti content. The martensite finish temperature (Mf) of 157°C for stoichiometric-Ti alloy increased to 179°C and decreased to 105°C for Ti-rich and Ti-lean alloys, respectively. Also, the alloys showed relatively low transformation hysteresis in the range 7–12°C. TEM micrographs showed the presence of twinless/small twin ratio martensite which minimizes the interfacial energy and hence lower hysteresis. The transformation stability upon stress-free thermal cycling was found to be better for stoichiometric-Ti and Ti-rich alloy than the Ti-lean alloy.