Kinetics of flow stress in crystals with high intrinsic lattice friction

Abstract

A relatively simple theoretical model, based on the concept of kink-pair mode of escape of screw dislocations trapped in Peierls valleys, has been developed to account for the observed temperature dependence of the critical resolved shear stress (CRSS), τ, and of the associated activation volume, v, in crystals with high intrinsic lattice friction at rather low temperatures. In this model, the CRSS varies with temperature T as τ^1/2 = A–BT, and the associated activation volume v depends on temperature T as v ⁻¹ = C–DT, where A, B, C and D are positive constants. Moreover, the activation volume v is found to be a function of τ such that vτ^1/2 is constant for a given slip system. Data analysis of the temperature dependence of the CRSS of W, α-Fe, Cr and V metal crystals shows excellent agreement between theory and experiment in both regime III (low temperature or high stress) and regime II (intermediate temperature/stress). However, the predicted temperature and stress dependence of the activation volume are borne out by experiment in regime II, but lack quantitative agreement in regime III. On the other hand, the CRSS of CdTe crystals at low temperatures (T ≤ 200 K) is determined by the Peierls mechanism, whereas the weak temperature dependence of the CRSS above 200 K is probably governed by the breakaway of edge dislocation segments from arrays of pinning points due to localized defects in the crystal.

Acknowledgements

This paper is dedicated to Prof. P. Feltham – my learned teacher – from whom I benefited a lot during my stay (1975–1978) at Brunel University, Uxbridge, Middlesex, England, and even afterwards till his sad demise in 2006. Special thanks are also due to the Pakistan Council for Science and Technology, Government of Pakistan, Islamabad, for financial assistance under the Research Productivity Allowance Scheme.