Effect of substructure on intergranular cavitation at high temperature

Abstract

When loaded at high temperatures under either low strain rate or constant load conditions, metals and alloys often fail by the nucleation and growth of cavities at grain boundaries with limited ductility. It has now been accepted that vacancies can cluster at sites of high stress concentration at grain boundaries to form cavities, which would then grow under practical creep conditions to effect fracture. The ways that stress concentrations are generated at grain boundaries, however, have been the subject of much debate. This work investigates the effect of various microstructural variables, especially the substructure, on high temperature fracture behavior of single-phase metals. The materials used were copper of two different purities (99.9% and 99.99%) and an Al-5% Mg alloy. Both the annealed and subgrain-containing copper specimens were pulled to fracture at 773 K in purified argon, either at a strain rate of 8.3 [times] 10[sup [minus]5] s[sup [minus]1] or under a constant load corresponding to a nominal stress of 20 MPa. The Al-5% Mg specimens were crept in tension to fracture under constant stress conditions at 523 K (0.56 T[sub M]) in air. The creep stresses applied ranged from 100 to 230 MPa. After the tests, the fracture surfaces weremore » examined by means of SEM. Longitudinal sections of the fractured specimens were also prepared, polished and etched with 0.5 ml HF/99.5 ml H[sub 2]O solution for 80 s, and then examined under an optical microscope.« less