Interaction of cold work, recovery, recrystallization, and precipitation in heat-treatable copper alloys

Abstract

Inhibition of recrystallization in copper and its alloys can be caused by small quantities of foreign elements such as Ag, P, Cd, Mg, and Sn in solid solution in the copper. These elements segregate at dislocation and grain boundaries, thereby inhibiting migration. The effect of cold work is a simple function of the quantity of soluble addition and the degree of cold work. The situation is more complicated for precipitation-hardened alloys because cold work, precipitation, and softening interact. In starting with the supersaturated solid solution which has been severely cold worked, heat treatment will lead to both precipitation and recrystallization. However, by means of precipitation treatment before cold working, it is possible to slow down, and even halt, normal recrystallization; this treatment leads to the formation of a critical distribution of precipitates ('dispersoid') which effectively halts both dislocation and grain-boundary movement. Cu–Ti, Cu–Zr, Cu–Fe, and Cu–Co alloys were studied for different sequences of heat treatment and cold working and the following characteristics are described: (a) a very small residual quantity of soluble Zr suffices to maintain the softening temperature above 400°–500°C; (b) soluble Cr also inhibits the softening of Cu, although larger amounts are required than for Zr,’ (c) fine precipitates of Cr in the form of a critical dispersion are also effective for retaining cold working; (d) iron in solid solution has practically no effect on the softening temperature of copper.