Effects of stacking fault energy and reduction ratio on mechanical properties geometry size effect of cold-rolled pure Cu and CuAl alloy

Abstract

With the trend of increasing product miniaturisation, it has become necessary to consider the service performances of microparts. If the material exhibits the ‘smaller is stronger’ geometry size effect (GSE), the mechanical properties data of conventional dimension specimens can be used to design the microparts, obtaining improved service safety of the microparts. The surface layer model shows that when the exterior mechanical properties of material are higher than these of the interior, the ‘smaller is stronger’ GSE can be obtained. Stacking fault energy (SFE) and reduction ratio (RR) have been reported to affect the distribution of mechanical properties along the thickness direction of a rolled material, so that they are expected to affect the GSE of rolled sheets. Therefore, the tensile mechanical properties of different SFE Cu-Al alloys cold rolled at different RR were examined in this work. The average grain size of the surface layer of the rolled specimens was smaller than that of the interior, whereas the average strain and microhardness of the surface layer were larger than those of the interior. The difference in the strain between the exterior and interior of the specimen was found to decrease with the specimen thickness t, and increases with RR and SFE. The ‘smaller is stronger’ GSE trend was found in pure Cu and CuAl alloy rolled at 30 –50% RR, while GSE changed to ‘smaller is weaker’ at RR = 70%. The magnitude of the GSE of the rolled specimens increased with increasing RR and SFE, resulting from the effects of RR and SFE on the microstructure, microhardness and strain distribution. New models for the relationship between the strengths and t and SFE were established for three RRs. The results predicted by the models were in good agreement with the experimental values.