Effect Of Treatment Time On Mechanical Properties Of Pure Copper Processed By Surface Mechanical Attrition Treatment At Cryogenic Temperature

Bulk pure copper samples were subjected to surface mechanical attrition treatment (SMAT) at cryogenic temperature (liquid nitrogen environment) to obtain a gradient structure (GS) composed of GS layers on both sides and a coarse-grained (CG) layer in the core, with grain sizes varied from hundreds of nanometers to several micrometers. The grain sizes increased but the measured hardness decreased along the depth of the gradient-grained Cu samples. The GS samples exhibited high yield strength (YS) while the uniform elongation (UE) showed only a slight reduction in tensile testing. The high strength and superior UE in the GS samples were believed to be associated with the mechanical incompatibility and interaction between the GS and CG layers. Variation of SMAT processing time could obtain GS layers with different volume fractions and therefore resulting in a different mechanical performance of GS Cu samples. Thus, there was an optimal SMAT processing time associating with the volume fractions of the GS layers, which provided an excellent combination in strength and UE of the GS Cu sample. The loading-unloading-reloading (LUR) tests indicated that higher hetero-deformation induced (HDI) stress could be obtained at a longer SMAT processing time. The HDI stress is caused by hetero deformation among different layers, which increased with increasing SMAT processing time.