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

Abstract

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 µm (as-received) to 1.7 µm and from 33 µm (as-received) to 1.0 µm, 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 β (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 β-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.