Order hardening in equiatomic CuPt

Abstract

Samples of equiatomic CuPt were quenched from above the ordering temperature into icy brine, annealed at 300°, 500°, and 700°C, and examined by microhardness, optical microscope, electron microscope, and X-ray diffraction techniques. In samples annealed for short times at 300° and 500°C a fine-domained mottled structure consisting of all four orientation variants of the ordered cell is seen. Continued annealing produces a coarsedomained grain boundary component which X-ray diffraction shows to have a higher degree of order than the mottled structure. At lower temperatures the grain boundary component grows and replaces the mottled structure to complete the ordering transition. At high temperatures the mottled structures coarsen into order twins lying on (110) and (100) type planes. The transformation to order and the resulting hardness changes are seen to depend largely on the internal strains resulting from the cubic to rhombohedral distortion.