Phase transformation of a CuZnAl alloy during friction

Changes in compositions and microstructures in the surface layer of a Cu-Zn-Al shape memory alloy during friction were investigated by means of scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffraction, and transmission electron microscopy. It has been found that in friction, single M18R martensite β′ transforms into α+β, with the shape and distribution of the new phases varying with the conditions of friction. In mild friction, Zn and Al diffuse over short distances along the sliding direction, resulting in a structure in which needlelike β and α alternate along the sliding surface. In severe friction, where higher surface temperatures and temperature gradients are expected, Zn and Al make long-distance uphill diffusion from inside toward the surface. Consequently, Zn and Al are concentrated on the surface and depleted in the subsurface, and the phase layers present from the outmost surface inward are β, α, α+β, cross-martensite, and the matrix.