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

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 QBe1.7 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 ∑ ε=4.8, although there were many sub-grains when the samples were deformed to ∑ ε=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 ∑ ε=2.4, the minimal grain size of 0.8 μm 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. © Right.