Improvement of Tensile and Fatigue Properties of β-Titanium Alloy while Maintaining Low Young's Modulus through Grain Refinement and Oxygen Addition

To investigate methods of improving the tensile and fatigue properties of a solutionized Ti-29Nb-13Ta-4.6Zr (TNTZ) alloy without increasing its Young' modulus, two types of TNTZ alloys having oxygen contents of 0.06 and 0.14 mass% (TNTZ-0.06O and TNTZ-0.14O), respectively, were subjected to cold swaging and a subsequent heat-treatment. The effects of the grain refinement caused by the cold swaging and the subsequent heat-treatment as well as those of oxygen addition on the microstructures, Young's moduli and tensile and fatigue properties of the two alloys were investigated. The grain diameters of the TNTZ-0.06O and TNTZ-0.14O decrease from 27 mu m (as-received) to 1.7 mu m and from 33 mu m (as-received) to 1.0 mu m, respectively, after subjected to cold swaging and the subsequent heat-treatment. These results suggest that cold swaging, followed by heat treatment, is effective in refining the grains of TNTZ alloys. However, a beta (110) texture develops in the alloys as a result of the cold swaging. Young's moduli of as-cold swaged and heat-treated TNTZ-0.06O and TNTZ-0.14O are within the range of 61-68 GPa and as low as those of solutionized TNTZ-0.06O and TNTZ-0.14O. The tensile strengths and elongations of the as-cold swaged, heat-treated and solutionized TNTZ-0.14O are approximately 30% higher and 20% lower, respectively, than those of the corresponding TNTZ-0.06O. Moreover, the 0.2% proof stresses of the heat-treated TNTZ-0.14O are approximately 110% higher than that of the corresponding TNTZ-0.06O. On the other hand, the values of the Hall-Petch constant (k) for the TNTZ-0.06O (k(TNTZ-0.06O) = 0.02) and TNTZ-0.14O (k(TNTZ-0.14O) = 0.005) are much smaller than those for pure Ti and another beta-type Ti alloy (Ti-15.2Mo). These results indicate that the addition of oxygen can improve the tensile properties of TNTZ alloys. However, the grain refinement caused by cold swaging and a subsequent heat-treatment does not have a significant effect on the tensile properties of TNTZ. The fatigue limit of the heat-treated TNTZ-0.14O (540 MPa) is much higher than those of the heat-treated and solutionized TNTZ-0.06O (290 and 230 MPa) and solutionized TNTZ-0.14O (330 MPa). These results indicated that it is possible to improve the fatigue properties of solutionized TNTZ using grain refinement, which can be induced by cold swaging and a subsequent heat-treatment and by the addition of oxygen.