Surface Engineering to Form Ultra-Fine Grains and a Hardened Surface in Ti Alloys

Mechanical treatments of surfaces to create compressive residual stress gradients and plastic deformation are commonly used to improve the mechanical properties of engineering alloys. Commercial Ti alloys contain complex grain structures based on heat treatments, however heat treatments that involve recrystallization commonly decrease the residual stresses formed from processes such as shot peening. In this study we develop ultra-fine grain structures in three difference Ti alloys with colony, basket weave, and lamellar precipitates. All base microstructures were peened to control residual stresses on the order of 200 to 800 MPa to depths of about 200 microns; the surface nanoindentation hardness after peening was 3-4 GPa (depending on the alloy). Nanoindentation was used in conjunction with x-ray diffraction to quantify the residual stresses. A thermal treatment schedule between 700-1000K for 20-30 minutes was sufficient to recrystallize the top 2-5 microns of the surface, which lowered residual stresses but formed a grain size between 350 and 700 nm. The resulting hardness increased over 1 GPa (from 4-5 GPa). This increase in surface hardness was achieved with no decrease in bulk hardness; and the compressive residual stress appears to stabilize the alpha phase in commercial beta Ti alloys, allowing for controlled precipitation and gradients in microstructure with no changes in bulk chemistry.