CREEP OF γ-TIAL BASED ALLOYS - EXPERIMENTS AND COMPUTATIONAL MODELING

摘要

Internietallic γ-TiAl based materials are qualified to become an important material for advanced applications especially in aeroengine and aerospace industries (Kim, 1994). Research and development have progressed significantly within the last few years and led to comprehensive understanding of fundamental correlations between alloy composition and microstructure, processing behavior and mechanical properties. It is well known that the mechanical properties of γ-TiAl alloys depend strongly on microstructure, which in turn is influenced by the alloy chemistry and the applied heat treatments. A designed fully lamellar (DFL) microstructure which consists of colonies of parallel γ-TiAl (tetragonal face centered L1_0 structure) and α_2-Ti_3Al (ordered hexagonal DO_(19) structure) laths with a colony size in the range of 150-200μm possesses superior creep resistance. Recent studies (Parthasarathy et al. 1998, Crofts et al. 1996, Maruyama et al. 1997) have shown that interface spacing has a major influence on the creep behavior. This may allow the conclusion that the interfaces γ/γ and α_2/γ must play a role in limiting creep flow or providing creep strength. In this paper differently spaced DFL microstructures were adjusted in order to investigate their influence on creep. Fine grained sheet material exhibiting a nominal composition of Ti-46.5at%Al-4at%(Cr,Nb,Ta,B) was used and short term creep tests were carried out in air at 700°C and 800°C under a load stress of 175MPa constant in time. The interface spacing was varied in the range of 1.2μm to 0.14 μm by altering the cooling rates from 1 K/min to 200K/min. A first approach in modeling the steady state creep deformation of the fully lamellar material in question is presented. A power law description for diffusion controlled dislocation creep is proposed and a structure factor is introduced which depends on the lamellar orientation with respect to the loading axis as well as on the mean lamellar interface spacing.