The effect of hydrostatic pressure on brittleness in chromium

Recrystallized chromium, which is normally brittle, shows high ductility at atmospheric pressure after subjection to a hydrostatic pressure of 10 4 atmospheres. The pressurization does not produce microscopically detectable deformation, but there is a marked drop in yield stress similar to that found in iron (Bullen et al. 1964); it is suggested that in each case the effect is due to the appearance of free dislocations during pressurization. It is argued that the stress at yield in pressurized material is insufficient to initiate cracks and that the stress required to initiate cracks is substantially increased by deformation. This accounts for the high ductility of pressurized specimens, the brittle appearance of their fracture even after ∼60% strain and the observation that specimens pre-strained during or subsequent to pressurization remain ductile after ageing or in further straining at high strain rates.