The microstrain behavior of beryllium single crystals

Abstract

The room temperature microstrain behavior of zone-refined single crystal beryllium has been examined as a function of prestrain. Crystals oriented for basal slip were deformed in simple tension (prestrains ≤0.09) and by low frequency cyclic compression (prestrains ≤0.40). The friction stress was determined from the energy dissipated in generating closed hysteresis loops as a function of forward plastic strain and maximum stress amplitude. The corresponding dislocation substructures were characterized by transmission electron microscopy. The friction stress T f associated with forward plastic strain amplitudes ≤5 × 10 −5 is low and in the range 0.02 – 0.13 kg·mm −2, with no significant effect of prestrain; a large fraction of T f is due to dislocation-impurity interaction so that the actual lattice friction stress on the basal plane is extremely small. Closed hysteresis loops exist at stress amplitudes up to the macroscopic flow stress level with corresponding forward plastic strain amplitudes as high as 2 × 10 −3. Under these conditions, a much higher friction stress T f(2) ~ 1 kg · mm −2 is measured. Cyclic compression eliminates the region of easy glide (Stage I) in beryllium and raises the flow stress. Differences in behavior in tension and cyclic compression, and the independence of T f with prestrain, have been related to the associated dislocation substructures.