Since a period of time at a high temperature was required before failure occurred, we might first suspect a creep or stress corrosion mechanism for failure. Microscopic examination of the material near the fracture surface would be advisable. If many tiny, branched cracks leading away from the surface are noted, stress corrosion is a strong possibility. If the grains near the fracture surface are elongated, with many voids between the grains, creep is a more likely culprit.