Serrated flow in gold-indium alloys

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Abstract
The characteristics of serrated flow in gold-indium solid solution alloys are studied over a range of strain rates and temperatures. Two types of serrations are distinguished, the general Type A locking serrations which are similar to those observed by Russell (1963) and the Type C unlocking serrations. The Type C serrations are restricted to a narrow temperature range at the upper temperature limit for serrated flow. The activation energy of the Type A serrations is 0·77 ev. The activation energy is considered to be the same as that for the exchange of an indium atom with a vacancy in gold. By using pre-strain experiments the exponent of strain (m+β) in the Ham—Jaffrey equation is found to be independent of the indium concentration, but dependent on the stage of work-hardening in which the serrations commence. (m+β) values of 2·14 and ≃3 are determined for Stages II and III respectively. The difference in these values is thought to result from the different rates of vacancy production. For (m+β) = 2·14, m = 1·19 and β = 0·95 are determined. The importance of enhanced vacancy concentration in promoting serrated flow is confirmed by quenching and ageing experiments. The strain to the onset of serrated flow is changed markedly by variations in quenching temperature and by isothermal ageing. The relation (C V ≃ 5·2×10−3 ε1,19 is deduced for the increase in vacancy concentration during deformation of a gold-8 at. % indium alloy in Stage II hardening.