Preparation of ultrafine-grained copper alloy by annealing treatment after multi-directional compression

Abstract

Ultrafine grained(UFG) metallic materials arouse a great interest due to their excellent mechanical properties.The UFG materials,produced by severe plastic deformation(SPD) technique such as channel angular pressing(ECAP) and high-pressure torsion(HPT),exhibit obvious improvement in strength but a large decrease in their thermal stability and ductility.In this article,an UFG QBel.7 copper alloy with good comprehensive properties was prepared by multi-directional compression (MDC) at room temperature and subsequent annealing at high temperature.The deformed and subsequent annealed microstructures were investigated by OM.TEM and SEM/EBSD techniques.The integrated flow curves plotted over a number of compression passes increase to a maximum at moderate strains of 1 to 2 followed by steady-state-like flow at high cumulative strains.Fine grains were not observed even at a higher cumulative stain of￡￡=4.8,although there were many sub-grains when the samples were deformed to￡s=2.4.This indicates that the dynamic recrystallization or recovery was completely inhibited by fine precipitates.Static recrystallization(SRX) of the MDCed structure at 973 K was also investigated.With the increment of cumulative strains,the effect of grain refinement became more obvious,but the thermal stability was getting worse.At a medium strain of∑e=2.4. the minimal grain size of 0.8μm could be developed with an excellent combination of properties.The formation of ultrafine grains is attributed to the development from low-angle boundaries to medium angle boundaries to large angle boundaries.The change of the average grain size with annealing time could be divided into three stages:a recovery period for grain refinement,rapid grain refinement and normal grain growth.