Shock synthesis and characterization of ultrafine grained NiTi shape memory alloy
Shock synthesis and characterization of ultrafine grained NiTi shape memory alloy
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DOI：
10.1016/s1359-6462(01)00920-4
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发表时间：
2001-05
影响因子：
6
通讯作者：
Xiao Xu;N. Thadhani
中科院分区：
文献类型：
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作者：
Xiao Xu;N. Thadhani
基于形状记忆效应和超弹性效应，近等原子NiTi合金被认为是最重要的金属间化合物。在良好退火等原子NiTi合金中观察到的形状记忆效应是由于从最初的高密度B2-Cscl结构到低密度单斜B19结构的马氏体相变[1，2]。在热循环或热机械处理的NiTi合金中，也经常观察到所谓的“预马氏体相变”，它出现在主马氏体相变之前，遵循B23R3B19[3-6]的路线。形状恢复的程度和马氏体相变特征已经被证明受到增益尺寸的影响[7]；然而，这些影响是否在纳米尺度范围内还不得而知。纳米晶合金具有传统晶粒度材料所不具备的独特性能，这是因为当晶粒度接近纳米级时，大量原子存在于晶界和晶界附近。特别是这种纳米晶材料的机械、化学和物理性能有望显著提高[8]。包括块状固体的等通道加工[9]和元素粉末混合物的机械合金化[10]在内的机械制造技术已被用于制造超细晶和纳米晶合金。冲击压缩已被用来固结超细晶[11]和非晶合金[12]的粉末，以保持细晶尺寸或亚稳态非晶态。在非晶态粉末的情况下，压坯随后也可以被反玻璃化以获得纳米晶结构[13，14]。反玻璃化的特征是成核和生长过程，在这个过程中，微晶在非晶态材料中的有利位置随机成核并生长，直到它们相遇并消耗掉所有材料。在冲击压实过程中，会产生大量的缺陷，这些缺陷为后冲击去玻璃化过程中的异质形核提供了场所。因此，高密度的成核位导致微晶的撞击，从而限制了晶粒的生长，并允许保留超细颗粒或纳米晶体结构[14]。本文采用冲击压实方法对球磨预合金化NiTiO粉末制备的机械非晶NiTi合金进行致密化处理。压坯随后在反玻璃化温度以上进行退火热处理，以获得纳米晶合金。利用差示扫描量热法测定了纳米晶NiTi合金的相变温度，确定了纳米晶NiTi合金的相变特征。
Near-equiatomic NiTi alloys are considered to be the most important intermetallic compounds for applications based on shape-memory and super-elasticity effects. The shape memory effect observed in well-annealed equiatomic NiTi alloys is due to the martensitic transformation from an initial highdensity B2-CsCl structure to a low-density monoclinic B19 structure [1, 2]. A so-called “premartensitic transformation” into a commensurate phase (R phase) that appears prior to the main martensitic transformation, proceeding the route of B23R3B19 [3–6], is often also observed in thermally cycled or thermo-mechanically treated NiTi alloys. The extent of shape recovery and the martensitic transformation characteristics have been shown to be influenced by gain size [7]; however, it is not known if these effects of grain size extend in the nano-scale range. Nanocrystalline alloys possess unique properties that materials with conventional grain sizes do not have, because of the large number of atoms residing at and near grain boundaries as the grain size approaches the nano regime. In particular, the mechanical, chemical and physical properties of such nanocrystalline materials, are expected to be significantly enhanced [8].Mechanical fabrication techniques including equi-channel processing of bulk solids [9] and mechanical alloying of elemental powder mixtures [10] have been used for fabrication of ultra-fine grained and nanocrystalline alloys. Shock compression has been employed to consolidate powders of ultrafine grained [11] as well as amorphous alloys [12], to retain the fine grain size or metastable non-crystalline state. In the case of amorphous powders, the compacts can also be subsequently devitrified to obtain nanocrystalline structure [13, 14]. Devitrification is characterized by a nucleation and growth process, in which the crystallites nucleate randomly at favorable sites within the amorphous material and grow until they meet and consume all the material. During shock compaction, large numbers of defects are generated which provide sites for heterogeneous nucleation during post-shock devitrification. Consequently, the high density of nucleation sites results in impingement of crystallites that limits grain growth and permits retention of ultrafine grained or nanocrystalline structure [14]. In the present work, shock compaction was used to densify mechanically amorphized NiTi alloy prepared by ball-milling pre-alloyed nitinol powder. The compacts were subsequently annealed above the devitrification temperature to obtain a nanocrystalline alloy. The transformation characteristics of the nanocrystalline NiTi alloy were determined based on measurements of transformation temperatures using differential scanning calorimetry.