PRECIPITATION PROCESSES IN SUPER-SATURATED TANTALUM-OXYGEN SOLID-SOLUTIONS - HARDNESS CHANGES-PRECIPITATES

Ta-O solid solutions containing 1-4 at.% O were aged at 423-1173 K for 6-6 × 104 min. The precipitation processes were followed by measuring the hardness of the aged specimens in dependence on aging time, oxygen concentration and aging temperature. Furthermore, metallographic examinations and X-ray diffraction measurements were carried out. At temperatures of 423 and 473 K the precipitation of the α′ suboxide in a tweed-like microstructure pattern is observed and found to obviously be the tetragonal TaOx suboxide (according to transmission electron microscopy and diffraction the α′ phase has a long-period tetragonal superlattice structure). The coherent precipitation of the TaOx phase causes a marked age-hardening at 423 and 473 K. At higher temperatures no hardness maximum is observed and the hardness decreases with aging time. At 473 K the TaOy orthorhombic suboxide is formed after long aging of Ta-4 at.% O alloys. At 523 and 573 K the TaOx phase is precipitating at short aging periods whereas upon longer aging a TaOy precipitate consisting of small platelets is forming. Aging at 673 K results in the formation of large oriented TaOy plates which decompose at medium aging times in favor of TaOz lamellae. At T ≥ 773 K only the TaOz suboxide is precipitated forming oriented platelets. The TaOz phase is found to have a tetragonal structure. © 1979.