Upper Limit of Strengthening and Its Improvement in Nano-lamellar Materials

TiAl合金是最有前途的高温结构用轻质材料。合金为由α_2Ti_3Al和γ-TiAl相组成的片层结构，强度随片层厚度的减小而增加。然而，人民币汇率走强是有上限的。本文分析了产生该上限的原因，并提出了提高该上限的方法：1.当片层厚度α减小到50 nm以下时，γ_2/λ晶界上的失配位错消失，导致晶界对位错运动的阻力丧失。因此，屈服应力与1/√&lt；λ&gt；曲线偏离了霍尔-佩奇关系，并且屈服应力在细小的λ处有一个上限。2.如果能在厚度小于50 nm的α_2/γ片层中引入失配位错，则屈服应力的上限可以提高。晶格失配决定了引入失配位错的片层厚度下限，需要大于1.5%的晶格失配才能提高上限。3.晶片的粗化和球化是导致变形抗力损失的主要微观组织退化。退化由界面能驱动，层状组织越细密，退化越快。TiAl合金具有四种层状晶界，α_2/γ晶界具有最低的晶界迁移率。结果表明，具有高密度α_2/γ晶界的材料具有更好的蠕变变形抗力，这是由于其改善了组织的稳定性。

TiAl alloys are the most promising light material for high temperature structural applications. The alloys take a lamellar structure consisting of α_2Ti_3Al and γ TiAl phases, and their strength increases with decreasing lamellar thickness. However there is an upper limit in the strengthening. In this study the causes of the upper limit were examined and it was proposed how to improve the upper limit.1.Misfit dislocations on α_2/γ lamellar boundaries disappear when lamellar thickness λ is reduced below 50nm, resulting in loss of boundary resistance to dislocation motion. Therefore, the yield stress vs. 1/√<λ> curve deviates from the Hall-Petch relation and yield stress has an upper limit at fine λ.2.The upper limit of yield stress can be improved if we can introduce the misfit dislocations to α_2/γ lamellae less than 50nm thickness. Lattice misfit determines the lower limit of lamellar thickness for the introduction of misfit dislocations, and a lattice misfit larger than 1.5% is required to improve the upper limit.3.Coarsening and sphoroidization of lamellae are the major microstructural degradation that cause loss of deformation resistance. The degradation is driven by the interfacial energy and is faster in finer lamellar structure. The degradation proceeds by the migration of lamellar boundaries, and can be retarded by reducing the mobility of lamellar boundaries.4.TiAl alloys contain four types of lamellar boundaries, and the α_2/γ boundaries have the lowest mobility. It was confirmed that a material having a high density of α_2/γ boundaries shows better creep deformation resistance due to its improved microstructure stability.