Special aspects of microstructure formation in Cu–Cr–Zr–Y bronze under low-temperature friction stir processing

The use of friction stir treatment (FST) to modify the physical and mechanical properties of age-hardenable low-alloyed bronzes is a promising and at the same time complex task due to the wide temperature range of its implementation. The difficulty is that friction stir treatment of bronzes can result in the formation of fundamentally different types of microstructures with a wide range of grain sizes and various combinations of types of strengthening phases and their various morphologies. Moreover, options are possible when friction stir treatment leads to degradation of properties of bronzes. A favorable combination of properties can be achieved by low-temperature friction stir treatment. In this work, the main microstructural changes in promising Cu–Cr–Zr–Y bronze were analyzed during low-temperature friction stir treatment with a tool rotation speed of 1000 rpm and a feed rate of 25 mm/min (ensuring a temperature in the stir zone of ≈350 °C). Scanning electron microscopy and EBSD analysis revealed the mechanisms of formation of an ultrafine-grained structure with predominantly high-angle boundaries, as well as the development of two types of simple shear crystallographic textures. It is shown that the Cux(Y,Zr) phase observed in the initial structure can undergo mechanical destruction or retain its geometric parameters depending on its initial morphology and location. It is shown for the first time that excess Cr particles (the equilibrium fraction at the heating temperature for quenching) may not be destroyed, but plastically deformed with a strong change in their morphology. During friction stir treatment of the bronze under study, particles of a new Y-containing phase are released. The paper considered the relationship of the distribution of microhardness and electrical conductivity and the observed changes in the microstructure of a new promising material.