Change in Lamellar Boundary Structure and Saturation of Yield Stress in Nano Lamellar TiAl Alloy

摘要

In general yield stress σ_y of polycrystalline aggregates shows a Hall-Petch relation in a large grain size regime, whereas it saturates to an upper limit below a critical grain size. The present paper aims at examining in detail the transition behavior from the Hall-Petch relation to the saturation by using a fully lamellar Ti-39.4mol%Al alloy tested at room temperature. Its lamellar boundary structures were analyzed with transmission electron microscopy. The γ/α_2 lamellar boundaries in the alloy are perfectly coherent in thin lamellar structures formed at low aging temperatures. Misfit dislocations are introduced into the γ/α_2 lamellar boundaries to relieve the lattice misfit between γ laths and α_2 matrix in thick lamellar structures formed at high aging temperatures. The critical thickness of γ laths is 40nm. The resistance (strength) of lamellar boundary to the propagation of yielding increases when the misfit dislocations are introduced. In the range of λ < l00nm (coherent boundary) where the lamellar boundary strength is low, yield stress shows a transition from Hall-Petch relation to its upper limit. Another Hall-Petch relation of a large Hall-Petch slope (a high boundary strength) holds in the range of λ > 170nm where misfit dislocations are present on all the γ/α_2 boundaries.