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Failure At Elevated Temperatures: Influence Of Dynamic Restoration

H.J. McQueen

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Inherent failure mechanisms at elevated temperature are primarily wedge crack growth at triple junctions arising from differential grain boundary (GB) sliding at higher stresses and pore formation on sliding boundaries due to vacancy diffusion at lower stresses and higher T. The behaviours of these mechanisms have been ascertained in creep where they can be studied over long periods. They continue to operate in hot working although their effects per unit strain have been reduced by decrease in fractional contribution of GB sliding. Dynamic recovery (DRV) significantly develops a stable substructure that strongly mitigates stress concentration. In alloys of austenitic steel, Ni and Cu, dynamic recrystallization (DRX) aids DRV in reducing stress concentrations and the migrating GB isolate fissures so they cannot propagate. Solutes and precipitates generally reduce ductility by diminishing DRV and DRX. Large particles and inclusions, notably on GB, introduce new sources of fissure nucleation, lowering ductility; solidification segregation and low melting constituents, especially if they spread along the GB, create severe problems.