A study on creep behavior of superalloy 617 M using a Wilshire approach

Abstract

Tensile creep tests were performed on forged 617 M alloy. Classical creep characterized by an extended steady-state (secondary stage) is exhibited during creep at 650°C. Formation of dislocation entanglements limit creep elongation (∼1.5 %). In contrast, limited secondary creep and extended tertiary creep was observed in samples tested at 700°C/750°C. All these samples exhibited extensive creep elongation (7 to 25 %). At 272 MPa/700°C, local climb dominates, increasing dislocation length and enhancing dislocation-precipitate interactions. Instability in dislocation structure prevails as outcome of intensified dislocation interactions during short-term creep. During long-term creep at 190 MPa/700°C, significant growth of precipitates and carbides takes place, facilitating dislocation cross-slip and looping which accounts for extended tertiary creep. At 750°C, precipitate instability causes prolonged tertiary creep. Moderate creep strength reduction on increasing the temperature from 650°C to 700°C reflects microstructural stability at 700°C and a sharp drop at 750°C is ascribed to microstructural degradation. Analysis of creep data was carried out using Wilshire approach. Creep activation energy is calculated to be ∼ 259 kJ/mole which is shown to be consistent with activation energy for self-diffusion. The Wilshire equation predicted stresses for creep life of 1,00,000 h to be 130 MPa at 710°C and 101.5 MPa at 750°C.