Formation Mechanism of Fracture Precursor Induced by Phase Transformation in beta Type Ti-Mo Alloy

这项研究主要是通过电子显微镜观察进行的。总结如下：1。众所周知，β型Ti-14Mo合金的硬化和脆化是由时效(1)相形成引起的。通过对时效温度下拉伸试验过程中形成的变形带的研究发现，带内的反向转变（(1)* β）驱动了带内的优先塑性变形（工作软化）。这被认为是微观脆化的一个起源。还观察到过时效合金的硬度下降，但没有改变(1)晶粒尺寸和脆性。阶梯时效制备的过渡态(1)* α使合金在拉伸变形过程中向基体中发射具有延展性的针状α相束。该合金的延展性优于直接时效形成大α相的合金。众所周知，淬火合金具有显著的{332}孪晶延展性。通过一个模型模拟，通过α -马氏体相变，原子位移到{332}孪晶，确定了尚未明确的孪晶机制。{332}孪晶的切割和相互作用可能在一种断裂前兆处引起局部变形，这种前兆可望为平面(1)相。目前正在进行TEM鉴定。对Ti-50.6% Ni形状记忆合金的原位观察表明，在准弹性变形过程中形成的初级马氏体之间和/或与初级马氏体之间的次级马氏体板在塑性区变形时成倍增加。板材之间的切割和相互作用导致试样断裂。用dx - x - alpha方法研究了给定结构的几个聚类模型（β, (1)， α或α”）。所得的键合顺序似乎可以有效地比较刺激工作软化的应力诱导产品的强度。

This study has been carried out mainly by electron microscopic observation. The summary is as follows :1. It is well known that hardening and embrittlement of beta type Ti-14Mo alloy are caused by aged (1) -phase formation. According to the study on deformation bands formed during tensile test at the aging temperature, the reverse transformation ((1) *beta) in the band drove preferential plastic deformation (work softening) in the band. This was recognized as an origin of the inacroscopic embrittlement. It was also newly observed that the hardness of over-aged alloy decreased without changing (1) -particle size and brittleness. The transition state, (1) *alpha, prepared by a step-aging made the alloy ductile emitting bundles of needle -like alpha-phase into matrix during tensile deformation. The ductility was better than that of the alloy with large alpha-phase formed by direct aging.2. It is well known that the as-quenched alloys show remarkable ductility with {332} twinning. The twinning mechanism which had not been cleared was determined by a model simulation of atom displacements to {332} twin from bee structure via alpha" martensitic transformation. Cutting and interaction between {332} twins maybe give rise to the local deformation at a kind of fractural precursors which are expected to be planar (1) -phase. The identification with TEM is now in progress.3. In-situ observation on Ti-50.6% Ni shape memory alloy revealed that the deformation in plastic region multiplied the secondary martensitic plates between and/or from the primary ones which were formed during quasi-elastic deformation. Cutting and interaction between the plates led the sample to fracture.4. Several cluster models of a given structure (beta, (1), alpha or alpha") were studied with DX-Xalpha method. The bond orders obtained seems to be effective in comparing the strength of stress induced products which stimulate work softening.