Improvement of Mechanical Properties of an Al-Li Alloy by means of Additional Elements

“利用附加元素改善Al-Li合金的力学性能”项目已在国家教育、科学和文化部科研资助项目下完成。结果表明：(1)含Zr、Ti和Hf的Al-Li合金在二次时效过程中发生了复杂的析出。复合析出相由Al_3M （M:Zr， Ti或Hf）包裹在Al_3Li周围组成，Al_3Li在二次时效过程中生长。(2)所有析出相均为有序l12_2结构的亚稳相。(3)通过透射电子显微镜的显微组织观察，研究了各元素在时效过程中的作用。结果表明：Zr、Ti和Hf在高温第一次时效过程中均以Al<@23<@D2M的形态析出。该析出物作为Al-<@D23@>D2Li在时效第二阶段形成的核。Cr：在第一次时效的早期阶段，以Al_7Cr的形式在晶界析出非均相。析出物固定在晶界上，从而完全抑制了晶界反应型（不连续析出）的析出。结果表明，细小球形颗粒的连续析出覆盖面积增大，最大硬度显著提高。Si：这加速了晶粒的连续析出，因此也有助于这些时效合金力学性能的改善。

The project entitled "Improvement of Mechanical Properties of an Al-Li Alloy by Means of Additional Elements" has been completed under the Grant-in-Aid for Scientific Research by The Ministry of Education, Science and Culture. Abstract of results obtained is as follows:(1) It has been found that a complex precipitation takes place during duble aging in Al-Li alloys containing Zr, Ti or Hf. The complex precipitates consist of Al_3M (M:Zr, Ti or Hf) surrounded by the Al_3Li, which grows during the second aging.(2) All precipitates are found to be metastable phase of ordered L1_2 structure.(3) Role of each element in the aging procedure has been investigated by microstructural observation by an transmission electron microscope. Results are summarized as follows:Zr, Ti or Hf : Each of these elements precipitates as Al<@23<@D2M during g the first aging at high temperature. This precipitates acts as the nuclei of the Al-<@D23@>D2Li formed during the second step of aging.Cr : This precipitates heterogeneous at grain boundaries as Al_7Cr at very early stage of the first aging. The precipitate pinned down the grain boundary, so that the precipitation of the grain boundary reaction type (discontinuous precipitation) was completely suppressed. As a result, area covered by the continuous precipitation of fine spherical particles increased, then the maximum hardness is improved remarkably. Si : This accelerates the continuous precipitation in grains, so that it also contributes the improvement of mechanical properties of these aged alloys.