ABSTRACT

The mechanisms of failure in a sample of a superalloy in a tensile or creep test are numerous and synergistic. They differ between samples which are 'single crystals', directionally solidified, or cast or wrought polycrystals. At high temperatures and low strain rates, the alloy fails by microvoid coalescence and transverse grain boundary cavitation, leading to a rough fracture surface perpendicular to the tensile axis. Polycrystalline superalloys show only small elongations to fracture, and this lack of ductility causes serious engineering problems. The time to fracture and the elongation to fracture during creep are influenced independently by many microstructural features. The fracture stress and the rate of work hardening at room temperature depend little on the grain size, but, because the flow stress increases with decreasing grain size, the ductility decreases with decreasing grain size. Intergranular fracture is common in creep tests, where voids may form and grow in the grain boundaries by diffusional processes.