The effect of pressurization on yield by twinning in Armco iron

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Abstract
The transitions in the slopes of the yield-stress/temperature curves of many b.c.c. metals at low temperatures are thought to occur when twinning replaces slip as the mechanism of yield. The present work shows that the transition shifts to lower temperatures and higher stresses when recrystallized specimens are subjected to hydrostatic pressures of several kilobars before testing at atmospheric pressure. A decrease in transition temperature is consistent with the decrease in glide stress resulting from ‘pressurization’ (Bullen et al. 1964). The increase in transition stress appears to be related to a decrease in the frequency of twinning at low temperatures after pressurization. The transition stress of pressurized-aged specimens was found to be higher than for recrystallized specimens, leading to the conclusion that twins in Armco iron initiate at the same inclusions at which free dislocations are formed by pressurization. In pressurized-aged specimens the locked dislocations decrease the stress concentration of the inclusion by a fixed amount. Hence the same number of twins are observed as in recrystallized specimens, but the transition stress is increased. By contrast, the free dislocations in pressurized specimens can re-arrange themselves during loading to relax the stress concentration further. Hence few twins are initiated in pressurized specimens, even at increased stress.