Surface nanocrystallization and gradient structure developed in the bulk TC4 alloy processed by shot peening

A gradient nanocrystalline structure with the grain size ranging from nanometer scale at the treated surface to micrometer scale in the matrix of the bulk coarse-grained TC4 alloy was fabricated by means of shot peening at an air pressure of 0.25 MPa and processing durations of 30-60 min. The microstructure characteristics including the constitution of deformation zones, grain size distribution and phase transformation had been systematically investigated. The experimental results showed that the thickness of deformation zones comprising equiaxed nanograins (EqN) layer, equiaxed ultrafine grains (EqU) layer, elongated ultrafine grains layer, the refined grains layer and low-strain matrix layer was up to 160 μm. Especially, the thickness of EqN layer was up to 40 μm and the average grain size of the treated surface was about 64.6 nm. Furthermore, a small amount of rare phase transformation of α phase from hexagonal close-packed crystal structure to face-centered cubic structure manifested in EqN layer and EqU layer. Microhardness test showed that a gradient variation of the microhardness (HV_0.025) with the depth from 486 at the treated surface to 315 in the matrix was obtained.