Crystallographic study on the precipitation and the phase transformation processes in alloys

(1)钢和亚稳β相钛合金中间组织的形成机理用透射电子显微镜分析了低碳镍钢中上贝氏体铁素体和渗碳体的习惯面、铁素体和奥氏体间的取向关系、渗碳体片状组织的习惯面、铁素体和渗碳体之间的取向关系。根据这些结果，提出了上贝氏体相变机制的模型--剪切辅助扩散相变机制，该模型假定从奥氏体到铁素体的晶格转变是通过保持原子-格位对应的单个原子跨越相界而发生的，但这种相变所产生的弹性应变应该通过晶格不变剪切来松弛。该模型已成功地应用于钛合金中魏氏组织的形成。Fe-Cr-C合金中常见的由针状铁素体和碳化物组成的类贝氏体状组织肯定不是贝氏体，而是一种共析组织。(2)有色合金中析出过程的晶相分析研究了亚稳β相钛合金中的欧米伽相析出及其对βα和βα的影响。此外，还用高分辨电子显微镜研究了Al-Li基合金的早期时效过程。利用高分辨率图像的傅里叶变换和逆傅立叶变换，证明了该合金存在同成分有序化过程。

(1) Mechanism of intermediate structure formation in steels and metastable beta phase titanium alloysThe habit plane, the orientation relationship between ferrite and austenite, the habit plane of the cementite platelets, the orientation relationship between ferrite and cementite in the upper bainite in low carbon nickel steels have been analysed by means of transmission electron microscopy. From these results, A model of upper bainite transformation mechanism, Shear-assisted Diffusional Transformation Mechanism, where the lattice change from austenite to ferrite is assumed to occur via individual atomic jumps across the phase interface with keeping the atomic-site correspondence but the elastic strain due to this phase change should be relaxed by the lattice invariant shear, has been proposed. This model has been successfully applied to the Widmanstatten alpha formation in the titanium alloy. It has also been shown that the bainite-like structure consisting acicular ferrite and carbide often observed in Fe-Cr-C alloys is definitely not a bainite but a kind of eutectoid structure.(2) Crystallographic analyses for the precipitation processed in non-ferrous alloysThe omega phase precipitation in a metastable beta phase titanium alloy and the effects of it on the beta*alpha and beta*alpha" have been examined. In addition to this, the early stage of ageing in an Al-Li based alloy was also been investigated by means of HRTEM.Using the Fourier and the inverse Fourier Transforms of the high resolution images, it has been shown that the congruent ordering processes occur in this alloy.