Production of in-situ hard Ti/TiN composite surface layers on CP-Ti using reactive friction stir processing under nitrogen environment

Reactive friction stir processing was carried out on a commercially pure titanium substrate by blowing nitrogen gas into the stir zone. Surface layers were produced having non-uniform banded-type microstructures of TiN precipitates in a transformed titanium matrix. Using additional passes, near uniform dispersion of TiN precipitates (~ 2 μm) in a matrix of fine grains (~ 4.5 μm) with α′-Ti martensite sub-structure was achieved. Dissolution of nitrogen in the stir zone and formation of TiN-based structure were confirmed by energy-dispersive X-ray spectroscopy, X-ray and electron diffraction, respectively. The fabricated Ti/TiN (2.3 vol.%) composite layer showed an average microhardness value of ~ 972 HV, which is about six times greater than that of its underlying titanium substrate; this is attributed to the formation of hard TiN reinforcements at boundaries of fine dynamically restorated grains with α′-Ti martensite sub-structure.