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

TiAl合金是最有前途的轻质高温结构材料。合金呈由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提高。然而，强化有上限。本文分析了上限值产生的原因，并提出了提高上限值的方法：1.当片层厚度λ小于50 nm时，α_2/γ片层界面上的失配位错消失，导致界面对位错运动的阻力减小。因此，屈服应力与1/λ <λ>的关系曲线偏离Hall-Petch关系，屈服应力在λ 2处有一个上限。在厚度小于50 nm的α_2/γ层中引入失配位错，可以提高屈服应力的上限。晶格错配决定了引入错配位错的片层厚度下限，而要提高片层厚度上限，晶格错配必须大于1.5%。3.片层组织的粗化和球化是导致变形抗力损失的主要原因。降解是由界面能驱动的，并且在更精细的层状结构中更快。TiAl合金中存在四种类型的片层晶界，其中α_2/γ晶界的迁移率最低。结果表明，具有高密度α_2/γ晶界的材料由于其组织稳定性的提高而表现出更好的抗蠕变性能。

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.