MASAHIKO IKEDA, SHIN-YA KOMATSU, ISAO SOWA, and MITSUO NIINOMI The aging behavior of the Ti-29Nb-13Ta-4.6Zr alloy quenched from 1033 K was investigated by employing electrical resistivity () and Vickers hardness (HV) measurements, optical microscopy (OM), and X-ray diffraction (XRD). Upon aging at 573 K, the value at room-and liquid-nitrogen temperatures initially increased to 120 and 30 ks, respectively, and decreased with continued aging time. The HV value started to increase from 0.06 ks and the isothermal phase was identified at 300 ks of aging by XRD. On aging at 773 K, an incubation period of 6 ks was observed for the change at both temperatures, following which decreased. An incubation period for the HV change was extended up to 12 ks, and then HV increased. Precipitated ␣ was observed and identified by OM and XRD, respectively, whereas no reflections of isothermal phase were identified during the whole aging time at 773 K. The upper-limit temperature of isothermal precipitation is situated at around 673 K.

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Using Ti-50Ta, Ti-50Ta-Sn and Ti-50Ta-Zr alloys, the effects of Sn and Zr additions on phase constitution in the solution treated and quenched state and aging behavior were studied by electrical resistivity and Vickers hardness measurements and X-ray diffactometry. All alloys were solution treated at 1173 K for 3.6 ks and then quenched into ice water (STQ). STQed specimens were isochronally aged at temperatures up to 1323 K in Ti-50Ta-Sn alloys and 1173 K in Ti-50Ta-Zr alloys. Shape recovery test was also performed in bent Ti-50Ta, Ti-50Ta-Sn and Ti50Ta-Zr alloy specimens. In the STQed state, only reflections of orthorhombic 00 martensite were observed by XRD in Ti-50Ta and Ti-50Ta-4Sn alloys. In STQed Ti-50Ta-10Zr alloy, coexistence of 00 and (bcc) phases were found by XRD. In STQed Ti-50Ta-8Sn and Ti-50Ta-20Zr alloys, only phase was identified. In Ti-50Ta, Ti-50Ta-4Sn and Ti-50Ta-10Zr alloys, resistivity at liquid nitrogen temperature and resistivity ratio increased with isochronal aging up to a certain temperature. It is considered that these increases are due to reverse-transformation of 00 into metastable phase. Shape-recovery test confirmed the shape memory effect of Ti-50Ta, 50Ta-4Sn and 50Ta-10Zr alloys.

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