Chiang Mai Journal of Science

Effects of deep cryogenic treatment (DCT) on the microstructure, mechanical and wear properties of copper beryllium alloy (CuBeZr alloy) were studied. The samples were exposed to the DCT at -196 degrees Celsius (°C) for different soaking time, i.e. 0.5 h, 1 h, 4 h, 8 h, 12 h, 16 h, 24 h, 48 h and 72 h. Microstructural analysis including size, shape and number of precipitates was studied by optical microscopy (OM) and scanning electron microscopy (SEM). Results showed Ni precipitates and NiZr precipitates with different shapes (sphere and rod-shaped) in the α matrix. After DCT, the number and size of Ni and NiZr precipitates increased with longer soaking time, while for samples with long DCT periods, large precipitates were observed along grain boundaries. The microstructures transformation in Ni and NiZr precipitates were induced by internal stress developed during DCT process due to the difference in thermal contraction/expansion of different phases. Highest compressive residual stress was observed at the longest soaking time (72 h). Change in microstructure altered the material properties. Hardness and wear resistance were significantly enhanced by the DCT process. Increase in average hardness with reduction in surface sliding wear was found with increasing DCT period, with a maximum of 17% increase in hardness and 49% decrease in wear at 72 h soaking time. After DCT for over 24 h, impact strength measured by the Charpy impact test decreased by approximately 50%. Fractography analysis of the non-DCT and DCT-samples showed dimples, tear ridges and precipitate cracks on fracture surfaces in all cases showing different failure modes. The fraction area of each failure type changed with DCT period. Results suggested that major reductions in fracture resistance were caused by precipitates locating along the grain boundaries. Thermal conductivity was also examined using laser flash analysis (LFA) to determine whether this property degraded after the DCT process. Thermal conductivity is a very important property for most applications using copper beryllium alloy. Tests were performed for non-DCT and DCT samples at 24 and 72 h for temperature range of 300°C to 600°C. Small differences in thermal conductivity were observed for all cases studied, with maximum increase of approximately 5% compared with the non-DCT cases after 72 h DCT.