In this study, we focused on the fabrication of titanium single crystal by the floating zone melting process under the Ar gas flow atmosphere where partial pressure of oxygen was kept to be extremely low around PO2=10-23 atm, by using the newly developed oxygen reduction system. In the obtained single crystal, distribution of solute oxygen atoms exhibited a peculiar gradient. In the portion gained in an early stage of the crystal growth, the oxygen content showed a higher value of about 1600 ppm than that in the mother ingot of 980 ppm. As the crystal growth proceeded, however, the oxygen content showed a gradual decrease, and in the final portion of the crystal growth, the oxygen content dropped down to 660 ppm. Such a gradient of oxygen composition was not detected in Ti single crystals fabricated under the standard and usual Ar gas flow condition. Owing to the unique gradient of oxygen content under extremely low oxygen partial pressure, the Vickers hardness in the single crystal showed a gradual decrease along the crystal growth direction.
