Effect of normal stress on yield asymmetry in high purity tantalum crystals

Abstract

Resolved yield stresses were determined in tension and compression for ultra high vacuum degassed tantalum crystals as a function of orientation at 77 K and 197 K. The yield stress at 77K was a minimum for specimens oriented to slip on the (112) [111] system, and a maxi-mum for specimens oriented to slip on the (–112) [1–11] system. The yield stress in tension was less than the yield stress in compression for all orientations. However, at 197 K, the yield stress in tension was found to be greater than in compression for specimens oriented to slip on the (121) [111] system in agreement with the results of Lachenmann and Schultz,1 who also found that, for specimens oriented to slip on the (•112) [1–11] system, the yield stress in compression was higher than the yield stress in tension, as occurs at 77 K. This behavior may be explained by the influence of a normal stress on a lattice expansion which occurs when the leading 112 stacking fault of a dissociated core dislocation as described by Dues-beryet al2 begins to move.