Prismatic slip in beryllium

Abstract

An in situ study of prismatic glide in beryllium is made to provide a simple model for understanding the anomalous increase in elastic limit as a function of temperature. This effect is also observed in several ordered alloys such as superalloys. In this paper, Part I, the mechanism controlling the deformation is studied at the stress peak temperature (300 K). It consists of alternate cross-slip events between the basal and the prismatic plane, leading to a ‘locking-unlocking’ mechanism which is studied in detail. This new mechanism appears to be a variant of the Peierls mechanism, where screw dislocations exhibit a metastable spreading in the prismatic plane, in addition to the classical stable spreading in the basal plane. Quantitative in situ measurements give local values of several parameters (such as stress, probabilities of locking and unlocking and strain rate), and the velocity of dislocations and the local strain rate are theoretically expressed. The exact origin of the stress anomaly is given in the companion paper, Part II.