Microstructural Stability and Crack Growth Behaviour of a Polycrystalline Nickel-Base Superalloy

Abstract

In this paper the microstructural instability of a recently developed powder-processed alloy, RRlOOO, is investigated and the corresponding influence on the creep-fatigue crack growth behaviour scrutinised. Equilibrium phase calculations of the alloy composition predict the presence of the TCP (Topologically Close Packed) phase sigma at the intended operating temperature of the alloy. Subsequent exposures at elevated temperatures reveal intergranular sigma phase precipitation. The kinetics of sigma phase formation has been determined experimentally by quantitative x-ray diffraction of exposed specimens. The effect of this grain boundary modification during prolonged exposure on the mechanical properties of the alloy was then investigated by testing specimens with two thermal histories: as-heat-treated (with a standard solution and ageing heat treatment developed for RRlooO) and exposed to contain 1 wt% sigma phase on the grain boundaries. To enable the discrimination of creep and environmental damage mechanisms during crack growth, fatigue and sustained load crack growth tests were performed in both vacuum and air. The influence of exposure on tensile aud creep rupture behaviour was also investigated. In order to provide a suitable reference, a single fatigue crack propagation test was performed on the as-heat-treated material at room temperature. All other tests were performed at 725 “C. From the crack growth rates recorded and fractographlc studies it is concluded creep aud environmental mechanisms are responsible for increases in crack growth rates observed when the temperature is increased from 20 “C to 725 “C. Sigma phase precipitation during alloy exposure increases the severity of creep and environmental processes leading to increases in crack growth rates. Superalloys 2ooo Edited byT.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and J.J. Schirm TMS (The Minerals, Metals &Materials Society), 2OC0