Influence of heat treatment on the physical, mechanical and tribological properties of a zinc-based alloy

An attempt has been made in this study to see the influence of ageing duration on the microstructure, tensile properties and dry wear characteristics of a zinc-based (bearing) alloy. The microstructure of the as-cast alloy revealed primary a dendrites surrounded by (eutectoid and eutectic) alpha+eta along with metastable epsilon phase. Solutionizing caused the dendritic structure to break and the alloy to homogenize while ageing produced the stable T' phase. Density and electrical resistivity of the alloy were practically unaffected during ageing for different durations. However, the aged specimens attained lower electrical resistivity. Hardness of the alloy after ageing was significantly lower than of the as-cast specimen. Moreover, increasing ageing duration caused the hardness to decrease up to a specific period. This was followed by attaining a steady state value at longer ageing durations. As a result of ageing, the specimens experienced lower tensile strength but higher elongation. This was also supported by their fractographic features. Sliding wear properties of the alloy improved after the ageing treatment. Further, ageing (at 180 degrees C) for 8 h led to the attainment of minimum wear rate and frictional heating. The wear data have been further supported with the help of wear surface, subsurface and debris analyses. The study indicates that T6 treatment (i.e. solutionizing followed by artificial ageing) deteriorates the mechanical properties (hardness and tensile strength) of the alloy while its sliding wear properties improve to some extent. Moreover, ageing at 180 degrees C for 8 h produces the best result in terms of its tribological response.