CREEP-FATIGUE-ENVIRONMENTAL INTERACTIONS IN NI-BASE SUPERALLOYS (FATIGUE CRACK GROWTH).

摘要

High temperature fatigue resistance of Ni-base superalloys can be reduced by time-dependent processes such as creep and oxidation. The purpose of these processes on fatigue crack growth in two hot isostatically pressed superalloys--Astroloy and Rene'95. These alloys are potential candidate materials to replace conventional cast and wrought turbine disk alloys because of their near net shape processing capability and substantial cost reduction.; Fatigue crack growth (fcg) rates for these alloys were measured under different waveforms, using the DC potential drop technique on compact tension specimens at 575-725(DEGREES)C in vacuum, argon and air. Fracture characteristics were examined by SEM, EDAX, TEM and metallographic techniques. The distribution of elemental species on the fracture surface, the nature of the oxides and the depth of the oxide layer were found using AES.; Low frequencies and the oxidizing atmospheres increased the fog rates substantially. Fcg rates did not change appreciably with temperature. Fcg rates increased by two orders of magnitude with superposition of hold times. The higher crack growth rates were associated with an intergranular fracture mode, whereas fracture mode was transgranular in the absence of hold times. The fracture surfaces were covered with a chromium oxide layer of 50-200 nm. Stress and deformation assisted oxygen penetration was approximately 30 (mu)m below the fracture surface in Astroloy with 6 min hold time.; It was found that the nature of the time-dependent damage was predominantly environmental. The role of oxygen in promoting transgranular and intergranular cracking is discussed in terms of adsorption theories, internal oxidation kinetics, grain boundary embrittlement and film rupture models. Deformation-assisted diffusion of oxygen is believed to "embrittle" the matrix ahead of the crack tip, weaken the grain boundaries and cause an intergranular failure. It is suggested that the small temperature dependence of fcg is an indication that lattice diffusion processes are inactive compared to grain boundary diffusion processes. A kinetic model is proposed on the basis of an oxygen affected zone to separate the frequency of the temperature dependence.