The flow of polycrystalline cadmium under simple shear

Abstract

Cadmium has been selected as a metal with which to investigate the detailed flow behaviour of a hexagonal close packed metal under constant shear stress σ , some of the results having been confirmed with zinc. A new type of flow, here called subnormal flow, has been found to prevail at low stresses, the shear strain γ being accurately represented by γ = M (1 - e - mt ) + K M t , the transient flow strain thus reaching a definite limit, which for long times of flow is small compared to the total strain. The mechanism of the M flow is grain boundary sliding, of a type discussed, which leads to the observed variation of M with grain size. When the stress exceeds a transition stress, σ c , the flow is w hat is here called normal, being represented by γ = A + Bt 1/3 + Kt . The exponential flow exists for these larger stresses, but is in general negligibly small compared to the normal flow. There is a small t 1/3 contribution to the subnormal flow at stresses just below σ c . In the region of subnormal flow the flow constants show a sudden increase with increase of temperature at a transition temperature T c , which depends upon grain size and is the temperature which, if exceeded, leads to grain growth in the stress-free metal. The dependence on grain size, which is marked, of all the constants involved in both types of flow has been investigated in some detail. It has been found that the transition stress is proportional to d -1/2 while the excess of the transition temperature over a certain fixed temperature T 0 , which occurs in another relation, is proportional to d 1/2 , where d is the average grain diameter. In the transition stress, and in the constants which express normal flow, the expression σd 1/2 which it is proposed to call the shove, plays a prominent part. It has been found that the flow linear with time, both in the normal region, where log K varies linearly with a σ , and in the subnormal region, where K M is proportional to σ 2 , is connected with grain boundary migration, to the extent of which it is proportional. In the normal region the B flow is governed by intragranular glide, Bt 1/2 being proportional to the number of glide bands visible in photomicrographs. The B flow is conditioned by twinning, which takes place immediately on application of stress and is responsible for the immediate strain. The question of flow under reversals of stress has been investigated, among the results being that, under certain conditions, the strain in reversed flow varies with time in the same manner as it does in forward flow, both with normal and subnormal flow. The various regularities which have been found are discussed in terms of the mechanical properties of the grains and of the boundary layer. The contrast between the flow properties of hexagonal and cubic metals is largely due to the smallness of the intragranular hardening with strain of the former as compared with the latter, which finds expression in the behaviour of individual slip bands.