Effect of heterogeneous microstructure on the mechanical properties and deformation behavior of rotating backward extruded ZK61 alloy

The rotating backward extrusion (RBE), as a novel severe plastic deformation process, has prosperous industrial application prospect. However, the capability and potential of the RBE process to prepare samples with different heterogeneous microstructures has not attracted attention. In order to explore the relationship between different microstructural compositions and mechanical properties of tubular component prepared by RBE process, the grain size distribution, grain orientation and mechanical properties of a ZK61 Mg alloy tubular component prepared by the RBE process in different regions (along the wall thickness and height directions) were investigated. The results show that the microstructure evolution in different regions is closely related to the equivalent strain distribution. The grain refinement effect is positively correlated with the equivalent strain, but a higher applied strain also leads to a strong basal orientation and high residual stress. Furthermore, the tensile deformation behavior of different samples revealed that samples with preferentially microstructural compositions (volume fraction of fine grains: 40∼60%) have better work hardening capability and strong-toughness synergy. This is mainly attributed to more effective coordinated deformation between different regions (fine grain region and coarse grain region), which can relieve local stress concentration and accommodate more dislocations. In contrast, an excessively high volume fraction of fine grains in samples limits their ability to store dislocations that saturate rapidly with increasing strain, while an excessively low volume fraction of fine grains in samples has difficulty accommodating the high density dislocations transferred from the coarse grain region, resulting in accumulation of high local stress at the interfaces of different regions (fine grain region and coarse grain region). Both of the above can deteriorate the work hardening ability and comprehensive performance, causing the sample to fail before sufficient work hardening occurs.