LAUSR

Abstract Biomedical Ti-6Al-7Nb alloy was prepared by a dedicated thermal treatment followed by equal-channel angular pressing (ECAP) and extrusion. Ultra-fine grained duplex microstructure consisting of deformed primary α-grains and fragmented α + β region was achieved. Microstructural changes during heating with the rate of 5 °C/min were studied by in-situ electrical resistance. Microstructure after deformation and also after subsequent heating was thoroughly characterized by scanning electron microscopy, X-ray diffraction, and positron annihilation spectroscopy (PAS). X-ray diffraction and positron annihilation spectroscopy proved a very high dislocation density and the presence of high concentration of vacancy clusters in deformed material. The ultra-fine grained microstructure of Ti-6Al-7Nb alloy is stable up to 440 °C, while upon heating to 550 °C and to 660 °C, the dislocation density decreases and vacancy clusters disappear. Enhanced microhardness can be achieved by ECAP followed by aging at 500 °C. Upon heating to 660 °C, the microhardness decreases due to ongoing recovery and recrystallization. Coincidence Doppler broadening (CDB), a special method of PAS, proved that dislocation cores are preferentially occupied by Al atoms that are known to cause substitutional solid solution strengthening.