Influence of Soaking Time on Deep Cryogenic Treatment of CuCoNiBe Alloy

Abstract

Deep cryogenic treatment (DCT) was investigated at different soaking times to determine the microstructural transformation and mechanical properties of copper beryllium (CuCoNiBe) alloy. Lattice shrinkage/distortion resulting from differences in thermal contraction/expansion between the alpha phase and gamma phase caused internal stress, with large atomic dislocations leading to the formation of beryllides. Average beryllide size decreased with increasing DCT time by a maximum of 37% compared to non-DCT because new small beryllides were formed. Beryllides increased and distributed in the ⍺ phase with longer soaking time. Highest beryllide number and volume fraction found at the longest soaking time of 72 h were approximately 200% and 5%, respectively higher than for non-DCT. Increasing the number of beryllides played an important role in enhancing hardness and wear resistance. Maximal increase in hardness at 12% was observed for 72 h DCT, with reduction in wear volume of 30%. Residual stress as compressive stress showed high variation, with uneven distribution over the DCT sample. Impact strength of the DCT samples decreased by 50%. Analysis of fracture surfaces suggested that beryllide shape and beryllide at the grain boundaries played important roles in reducing fracture resistance. Thermal conductivity measurements of DCT-12 h and DCT-72 h samples indicated microstructural change, with the DCT-72 h sample recording a 2% drop in thermal conductivity compared to non-DCT.