Abstract
The heat-treatment of tin-electrodeposited bronze has been studied to improve the surface hardness and corrosion resistance by the formation of Cu-Sn alloy layers. The formation of the alloy layers proceeds by the diffusion of tin atoms from the deposited layer inwards and of cupper atoms in the substrate outwards. Cu-Sn alloys with 0, 6, and 12wt% of tin content were coated by an electrodeposited tin layer of 10μm thickness, prior to the heat-treatment of the specimens at different temperatures of 400, 450, and 500°C for 1, 2, and 3 hours. The vertical cross sections of the heat-treated specimens were analysed by EPMA and micro hardness measurements. By the heattreatment at 400°C, the surface layer on all specimens consists of an η-phase (Cu6Sn5) at the outer part, and an ε-phase (Cu3Sn) at the inner part. The η-phase transforms to ε-phase during the treatment, and the transformation occurs more easily with Cu-Sn alloy substrates than with Cu substrate. After 3 hours of the treatment the surface layer of all specimens consists of only the ε-phase with a hardness of 360-420Hmv. By the treatment at 450°C, all the specimens shows the existence of the ε-phase at the outer part and a δ-phase (Cu4Sn) at the inner part of the surface layer. The hardness of the δ-phase changes from 370-405Hmv after a 1 hour treatment, through 400-410Hmv after 2 hours, to 430-440 Hmv after 3 hours. The hardness of the δ-phase could not be measured after the 1 hour treatment because of the thinness of the δ-phase, but it is measured to be 420-500 Hmv after 3 hours. The change in the hardness of the two phases is due to the relatively thin surface layer. At 500°C, both ε-and δ-phase are found after 1 hour treatment and only δ-phase after 2 or more hours treatment. The hardness of the δ-phase after 2 hours treatment is 500-530 Hmv. The activation energy for the formation of δ-phase is obtained to be 81.7KJ/mol.
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