Effect of surface strain on discontinuous precipitation kinetics in a Cu-7.7 at % Ag alloy

Abstract

Precipitation of a two-phase aggregate behind a grain boundary advancing into a supersaturated solid solution is called discontinuous precipitation (DP) [1-3]. Usually, a large-angle incoherent boundary, called the reaction front (RF), provides a short-circuit path of solute transport for the precipitate colony growth [2, 3]. Recently, it has been demonstrated that recrystallization following prior strain due to scratching or hardness indentation at the external surface may generate grain boundaries capable of initiating DP [4-6]. These exploratory studies, however, did not attempt to quantify the effect of strain on the precipitation kinetics. Earlier quantitative studies on the effect of plastic strain on the overall kinetics of DP employed only bulk deformation like rolling [1, 7-9]. DP is likely to initiate heterogeneously at the deformation bands or sub-structures produced by such bulk deformation [10, 11]. Moreover, the overall effect on transformation kinetics is expected to be determined by the influence of strain on both the nucleation and growth stages. Hence, a study of the interrelationship between the amount of plastic strain or the applied load of deformation and the RF velocity is not practicable with bulk deformation techniques. On the other hand, determination of the velocity of the RF, generated by prior hardness indentation [5, 6], as a function of the applied load may furnish a more direct estimate of the role of plastic strain on the kinetics of DP. About 300g of a Cu-7.7 at% Ag alloy was prepared from 99.99 wt% purity Cu and Ag by vacuum induction melting. A large grained ingot of 10 mm diameter was grown from this alloy under Ar atmosphere by a vertical Bridgeman technique. Semi-circular disc-shaped specimens of about 5 mm thickness (see Fig. la) were cut from the ingot by a slow-speed diamond saw and homogenized at 1033 K for 12 h in a reducing atmosphere prior to quenching in water at room temperature. All flat faces of the specimens were mechanically polished with 0.5/~m diamond followed by electropolishing in concentrated (65%) H3PO 4 using a stainless steel cathode at a DC potential of 2 V for 20 min to remove all traces of mechanical strain from the specimen surfaces. Surface deformation with a predetermined load (P) was applied on one of the flat semi-circular faces of the specimens by means of the diamond indentor of a Vickers hardness tester (Fig. la). A selected set of specimens, after surface deformation, were cleaned with acetone and ethanol and subjected to surface coating of pure Ag (0.3/xm)