Quantitative Evaluation of Solute Oxygen Effect on the Young’s Modulus and Hardness of β-Ti, α₂-Ti₃Al, and γ-TiAl Phases in Ti-Al-Cr-O Alloys

Introducing the bcc β-Ti phase has been found to be effective in improving the strength and toughness of conventionally processed TiAl alloys. In recent years, powder processed fabrication technologies such as additive manufacturing and metal injection molding have been focused for TiAl alloys. In such processes, a certain amount of oxygen contamination is inevitable in comparison to conventional ingot metallurgy. It is believed that the solute oxygen makes the alloys less ductile and more brittle, so that the oxygen was avoided for alloy design. However, from the microstructure viewpoint, the following information is needed how the oxygen affects the phase equilibria among the constituent phases and affects the mechanical properties of the phases, but no information is available. In this study, thus, we have investigated the effect of oxygen on the Young’s modulus, <i>E</i>, and hardness, <i>H</i>, of the constituent phases of β-Ti, α₂-Ti₃Al and γ-TiAl phases in Ti-Al-Cr-O quaternary alloys quantitatively in terms of the chemical composition analysis and nano-indentation method. The alloys used are Ti-43Al-3Cr and Ti-44Al-4Cr-1.0O (at.%). These alloys consist of the three phases after the equilibration at 1373 K. The equilibrium compositions of each phase were analyzed by EPMA and the oxygen content in each phase was analyzed by soft X-ray emission spectroscopy.<br/>Regardless of the oxygen levels in the alloys, oxygen does not dissolve in the γ phase, and its concentration is nearly the same of less than 0.2 at.%. Most of the oxygen are partitioned into the α₂ phase with partition coefficients of <i>k</i>_O(α₂/γ) =18 and <i>k</i>_O(β/γ) =2.3 against γ phase in the O-doped alloy. In addition, the oxygen addition also changes the Cr concentration of each phase, especially in the β phase, although Al concentration of each phase remains almost unchanged; the Cr concentration in the β phase was changed by 8 at.%, whereas those in the α₂ and γ phases was changed by only about 2 at.%. These composition changes result in a significant increase in <i>E</i> of the β phase from 135 GPa to 160 GPa, whereas it does not affect the <i>E</i> of α₂ (185 GPa) and γ (178 GPa) phases. On the other hand, the composition changes harden the β and α₂ phases from 6.1 to 7.1 GPa and 7.2 to 8.1 GPa, respectively. However, the <i>H</i> of the γ phase remains almost the same of 4.3 GPa. Based on these results, the dependency of<i> E</i> and <i>H</i> on oxygen was separately evaluated and found that the mechanical properties of the β phase are extremely sensitive to the oxygen in solution, and those in α₂ phase are less but somehow sensitive than those of the β phase. Surprisingly, however, the oxygen does not affect the mechanical properties of the γ phase at all, since solubility of oxygen is very limited.<br/>From these results it is suggested that the oxygen should be treated as one of the alloying elements, rather than contamination element, and that one can design the appropriate microstructures to meet the required properties in TiAl alloys even with the high amount of oxygen.<br/>This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials Integration for revolutionary design system of structural materials” (Funding agency: JST).