Effect of composition and structure of alloys on the change of shape and crack nucleation in heat cycling

Abstract

1. It was established that the length of cylindrical specimens of copper and nickel, preliminarily annealed in the process of HC in the temperature range 953–293°K, increased after heating in a salt bath and cooling in water, and also after heating in air and cooling in water. 2. When specimens of copper M1 are subjected to HC, pores form inside the grains and on their boundaries, and twins also originate, and when they merge, they form discontinuities on the grain boundaries. An increase of the amount of impurities in copper M2 (from 0.1 to 0.5%) intensifies the process of formation of pores, their merger and formation of cavities on the grain boundaries. The nature of the increase in length of the specimens is nonlinear; this is possibly due to the grains coming closer together where there are discontinuities, and also to the turning of the grains at the joints. 3. When specimens of nickel are subjected to HC, cracks appear in the central part; they are situated on the grain boundaries perpendicularly to the axis of the specimen. Grain boundaries are apparently one of the weak spots where cracks (in nickel specimens) and discontinuities (in copper specimens) form. 4. A radical way of reducing the effect of change of length in HC is to add high melting elements to the alloys, thus increasing the strength of the alloy in the heated state; another device is to reinforce the grain boundaries by a carcass of intermetallic phases.