Abstract
High-temperature crack propagation in nickel base Alloy 600 was investigated at 400 and 550 °C in various environmental and cyclic loading conditions. At 550 °C, crack propagation is rapid and intergranular (IG) for creep fatigue cycles whatever the environment. For continuous fatigue cycles at 400 or 550 °C, crack propagation is transgranular in vacuum and IG in air ( P>10 −1 to 1 Pa). In order to estimate the evolution of the stress and strain fields near the crack tip, finite element calculations of the compact tension specimens were carried out using constitutive equations determined on an experimental basis. IG crack growth at 550 °C during creep fatigue cycles appears to be due to a ‘pure’ creep cracking mechanism even in vacuum. For continuous fatigue cycles, calculations and creep tests on thin specimens suggested that an oxidation-assisted creep phenomenon occurring near the crack tip could possibly account for the observed crack propagation behaviour. A schematic representation of the crack propagation results is proposed for comparison with the case of nickel base superalloys.
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