Effect of Heat and Cryogenic Treatments on Microstructure and Properties of 12% Tin Bronze

Abstract In the present work, 12% tin bronze (CuSn12) was subjected to solutioning, artificial aging, and cryogenic treatment. The as-received CuSn12 material contained the α-solid solution and the δ-eutectoid (a mixture of the α- and δ-phases). Solutioning induced complete dissolution of the δ-phase. Subsequent artificial aging evoked discontinuous precipitation of the ε-phase at the grain boundaries. Cryogenic treatment accelerated discontinuous precipitation. Moreover, these treatments resulted in a substantial increase in the number of fine deformation twins inside the α-solid solution grains, which may act as preferential sites of continuous precipitation. Because grain boundary diffusion is much faster than grain interior diffusion, precipitation at the grain boundaries was much more pronounced. Consistently, microhardness increased substantially in the nearby grain boundary regions. All treatments reduced hardness and the yield strength but increased ductility and impact toughness. Aging had a slight but indisputable effect on all mechanical properties (an increase in hardness and yield strength but a decrease in ductility and toughness).