Residual Pressure Effects in Polycrystalline Aluminum

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Abstract
Internal friction measurements following application of hydrostatic pressure were used to study residual pressure effects in polycrystalline aluminum. Application of 4‐kbar pressure was observed to increase irreversibly the amplitude‐dependent damping of 99.99% and 99.996% Alcoa aluminum at a strain amplitude of approximately 3×10−5 by amounts up to 60 and 35 times the initial values, respectively. With successive pressure‐anneal cycles an abrupt disappearance of the residual pressure effect was sometimes observed. The effect was studied as a function of grain size by annealing cold‐worked aluminum at different temperatures. Measurements before and after application of 6‐kbar pressure to zone‐refined polycrystalline aluminum showed no changes in damping that could be ascribed to the effect of pressure. Polycrystalline alloys prepared by doping zone‐refined aluminum to 99.99% purity by addition of copper, iron, or silicon showed no residual pressure effect. Vacuum‐melted polycrystalline 99.996% aluminum showed no residual pressure effect. Zone‐refined aluminum melted in air or under a hydrogen atmosphere showed residual pressure effects. The residual pressure effect observed in refined aluminum is attributed to quenched‐in hydrogen present in the lattice. The pressure‐sensitive defects are considered to be hydrogen‐filled cavities.

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