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

点击翻译按钮获取中文摘要

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.