MICROSTRUCTURAL INVESTIGATION ON THE CRYSTALLISATION AND AGEING PROCESSES OF ADVANCED ALLOYS

本文用高分辨透射电镜、差示扫描热分析、X射线衍射和振动样品磁强计研究了各种先进材料的晶化、相变和时效过程，得到以下结果。(1)Pr-Fe-B和Nd-Fe-Cr-B非晶薄带的晶化过程和磁性能在退火后期，<23>Nd_2Fe_B_3颗粒分解为Nd_2Fe_B_2颗粒<14>，其中α-Fe颗粒很细。这种结构由于Nd_2Fe_B_2粗颗粒与α-Fe细颗粒之间的交换耦合作用而具有优良的硬磁性能<14>。(2)Al-Mg-Si合金的时效过程研究表明，对沉淀硬化有贡献的β”相的结构与Andersen提出的模型相符合 ...更多信息 .实验还表明，β”沉淀后的反应不是以往所认为的β'棒的形成，而是长方体状β粒子的沉淀。β粒子与基体之间存在三种不同的取向关系，本研究发现了其中之一。这种关系可以通过考虑形态、体积弹性能和界面能来解释。(3)Fe-Ni-C系贝氏体的晶体学研究虽然贝氏体相变是通过具有原子-位置对应的扩散晶格变化和晶格不变切变发生的，但已证实贝氏体的大部分随后以扩散方式迁移。因此，界面上的位错结构不能作为Aaronson等人提出的台阶机制的证据。(4)近β Ti合金和Ti-Al合金的相变过程在Ti合金中，研究了ω相粒子对α”马氏体相变的影响。结果表明，ω相析出初期加速了β→α”马氏体相变，而析出后期则显著稳定了马氏体相变。提出了Ti-Al合金中块状γ组织和反相畴形成的模型。少

The crystallisation, phase transformation and ageing processes of various advanced materials have been investigated by means of high resolution transmission electron microscopy, differential scanning thermal analysis, X-ray diffection and vibrating sample magnetometer and the following results were obtained.(1) The crystellisation processes and magnetic properties of Pr-Fe-B and Nd-Fe-Cr-B amorphous ribbonsThe decomposition of coarse Nd_2Fe_<23>B_3 particles into coarse Nd_2Fe_<14>B_2 particles involving very fine α-Fe particles has been found in the later stage of annealing. It has also been found that this structure exhibits excellent hard magnetic properties arising from the exchange coupling interaction between coarse Nd_2Fe_<14>B_2 particles and fine α-Fe particles within them.(2) The ageing process of Al-Mg-Si alloysIn this study, it has been confimed that the structure of β" particle which contributes precipitation hardening is in good keeping with the model proposed by Andersen … More . It was also shown that the reaction following the β" precipitation is not the β' rod formation as has been believed so far but the precipitation of cuboid β particles. The existence of three different orientation relationships betwen β particles and the matrix has been recognised and one of them has been found in this study. Such relationships can be explained by considering the morphology, volume elastic energy and the interfacial energy.(3) Crystallographic study on Fe-Ni-C bainiteAlthough bainite transformation occurs via a diffusional lattice change with atomic-site correspondence and the lattice invariant shear, it has been confimed that me broad fece of bainite migrates subsequently in diffusional mode. Thus, the dislocation structures on the interface cannot be the evidence of the ledge mechanism proposed by Aaronson et al.(4) The transformation processes of near β Ti alloys and Ti-Al alloysIn Ti alloys, the effects of ω phase particles on the α" martensitic transformation has been examined. It has been shown that the initial stage of ω phase precipitation accelerates the β→α" martensitic transformation but the later stage stabilises the martensitic transformation significantly. In Ti-Al alloys, the models of massive γ structure and the anti-phase domain formation have been proposed. Less