Prismatic Slip in Beryllium and the Relative Ease of Glide in H.C.P. Metals

The macroscopic C.R.S.S. for prismatic slip in Beryllium is found to increase with increasing temperature in the range −100 to +50 °C while the microyield limit decreases regularly. Slip is initiated on the prism planes from the edge dislocations present in the annealed material. Evidences of a thermally activated locking of screw dislocations are presented. This locking is thought to occur by the usual splitting in the basal plane. The assumption that the a dislocations can also dissociate in the prismatic plane seems necessary to account for the activation of this locking. Possible dissociations in the prismatic plane are proposed; the most probable of them is a → a/3 + 2 a/3. It gives a ribbon of stacking fault corresponding to the transformation of two {1010} h.c.p. “flat planes” into two {112} b.c.c. planes. It is suggested that there is a relationship between the energy of the stacking fault on the prismatic plane and the temperature of the phase transformation h.c.p. → b.c.c. This relationship permits to account for the relative ease of slip in h.c.p. metals.