CREEP STRENGTH AND MICROSTRUCTURE OF Ti-46Al-2W-0.5Si BASE ALLOYS

摘要

The present study summarizes data on the tensile creep strength of ABB-2 and ABB-23 alloys (base chemical composition Ti-46Al-2W-0.5Si (at-%)) collected at 700 and 750°C within the framework of the COST 522 -^sGT -^sWP 1.2 γ-TiAl action. Participating laboratories tested different cast parts including test pieces cut directly from gas turbine blades. Both, uniaxial and notched bar tests were performed. It is shown that the alloy microstructure in the as-received condition (after casting, HIPping and heat treatment) varies with the position in the cast part. While regions below the casting surface exhibit columnar grains and a nearly lamellar microstructure, the central part is often composed of globular grains and a duplex microstructure in which the fully lamellar grains coexist with the equiaxed-γ grains free of lamellae. These microstructural variations represent the important contribution to the scatter of the creep strength data. In agreement with earlier studies, it has been found that the creep strength of the duplex material is inferior to the creep strength of the fully lamellar parts. This effect should be considered in the case of thick wall cast sections where redistribution of stresses during service may occur due to the microstructurally based variations of the creep strength. Boron additions are shown to accelerate creep in the range of low applied stresses.