Japan Institute of MetalsActa Medica Okayama0021-487689112025Ti-18Nb-xAl合金の構成相と材料特性に及ぼすAl添加量の影響337343ENYoshikazuMantaniDepartment of Materials Science and Engineering, National Institute of Technology (KOSEN), Suzuka CollegeYoshitoTakemotoFaculty of Environmental, Life, Natural Science and Technology, Okayama UniversityThe Ti-18mass%Nb alloy with a quenched α” martensitic structure exhibited a high damping capacity. However, there are issues such as lower strength than annealed α+β structure and decreasing damping capacity due to heating until 400 K. Therefore, in this study, to address these issues, we investigated the effect of Al addition on the constituent phases and material properties of Ti-18Nb-xAl alloys. The crystal structure was determined by examining the lattice constant and unit volume using X-ray diffraction, and optical microscopy was also performed. The material properties were investigated by Vickers hardness, Young’s modulus, internal friction, tensile tests, and DSC measurements. Vickers hardness and tensile strength increased with increasing Al content. This is thought to be due to the combined effects of the refinement of the microstructure and solid-solution strengthening due to Al addition. The Young’s modulus increased slightly from 0Al to 1Al, but increased significantly to 4Al. Internal friction was highest for 0Al and decreased for 4Al, whereas 7Al showed a higher value than 1Al. In the DSC heating curves, there was a decrease in the exothermic peak starting temperature and an increase in the phase-transformation heat with the addition of Al, except for 1Al. It was suggested that these changes in Ti-18Nb-xAl alloys were influenced by the structure of the quenched α” phase, texture, and pseudoelasticity or phase transformation by deformation.No potential conflict of interest relevant to this article was reported.Japan Institute of MetalsActa Medica Okayama0021-487685112021Ti–10Mo–7Al合金の焼戻し誘起マルテンサイトの形成機構405412ENYoshitoTakemotoGraduate School of Natural Science and Technology, Okayama UniversityMikikoYasunoGraduate School of Natural Science and Technology, Okayama UniversityMasakiIkemotoGraduate School of Natural Science and Technology, Okayama UniversityHiroyukiAndoFaculty of Engineering, Okayama UniversityIchiroShimizuFaculty of Engineering, Okayama University of ScienceThe formation mechanism of αAA–martensite (αAAMt) induced by tempering at 450–550℃ for a short time was investigated using Ti–10Mo–7Al alloy. The solution treated and quenched (STQ) sample was composed of β phase and a small amount of αAAMq, and a large amount of αAAMt was generated by rapid tempering at 550℃–3 s using a salt bath. However, αAAMt was completely transformed into a single β phase by aging at 200℃ for 3 min. Reversibility was observed between the αAAMt transformation and the β reverse transformation. In–situ high–temperature X–ray diffraction measurements revealed that αAAMq → β reverse transformation occurred at 200℃ and that a thermally activated αAAiso was generated at
450℃ due to the slow heating rate. In–situ optical microscopic observation of STQ sample with rapid lamp heating revealed that αAAMt was formed during heating process. However, αAAMt did not generate under following conditions; that is, a slow heating rate, thin sample plate, and a small temperature difference until tempering by preheating. On the other hand, rapid tempering using thick plate from liquid nitrogen (−196℃) to 250℃ was performed to ensure a sufficient temperature difference, but αAAMt was not generated at all.
From the cross–sectional observation of the STQ plate, it was found that αAAMq was hardly formed on the surface of the sample, but was formed abundantly inside the sample. On the other hand, in the rapidly tempered plate, a large amount of αAAMt was distributed in the surface layer than inside sample. These results suggest that the thermal compressive stress induced by rapid heat treatment contributes to the formation of α''M.No potential conflict of interest relevant to this article was reported.