Microstructural evolution and mechanical properties of a friction-stir processed Ti-hydroxyapatite (HA) nanocomposite

In this research, a new metal matrix nanocomposite with enhanced capability for biomedical applications was fabricated by incorporation of nano-sized hydroxyapatite (HA) particles within the titanium substrate using multi-pass friction-stir processing (FSP). These n-HA particles were dispersed effectively within the titanium matrix. Titanium metal-matrix was processed without introducing the HA nanoparticles, as well, for the aim of comparison. The results showed the formation of different regions with various microstructural features and mechanical property across the processed materials. A thin layer with ultra-fine grain structure and indentation hardness value of up to ~400 HV was formed on the surface after FSP modification of coarse-grained titanium substrate. This was due to severe shear deformation induced by the rotating shoulder as well as the surface absorption of N and O elements from the atmosphere inside the layer. Incorporation of nanoparticles and subsequent grain structural refinement owing to operative dynamic recrystallization mechanisms leads to a maximum hardness improvement of up to ~250 HV in the lower regions (as compared to the average hardness value of base metal ~150 HV). The FSP modified pure titanium exhibited a good combination of strength and ductility by refining the grain structure with a well-developed dimple-like structure on the fracture surface. For the nanocomposite specimen, the trend of the tensile property was found deteriorative showing the impaired features on the fracture surface. This is attributed to the complex structure of HA compound and low quality of interfacial bonding between the nanoparticles and titanium matrix.