Aging effect on transformation characteristics and tensile properties of a Ti-Ni-Co alloy

Abstract
Transformation characteristics, microstructures and tensile properties of a Ti-49.7at.%Ni-1.38at.%Co alloy have been investigated as a function of aging temperature and period. After a solution-treatment, the alloy was aged at 623 K and 723 K for 1.8, 14.4, 115.2 and 230.4 ks in argon atmosphere, followed by water cooling. Microstructures of the aged alloys were observed by using a transmission electron microscope operated at 200 kV. Fine Ti₃Ni₄ precipitates were observed in all the 623 K and 723 K aged alloys, and their size increased with increasing aging temperature and time. For the same aging time, the number of Ti₃Ni₄ precipitates per unit area for the 623 K aged alloy was larger than that for the 723 K aged alloy. The R-phase products were also observed in the aged alloys at 623 K and 723 K for 115.2 and 230.4 ks. Tensile properties of the aged alloys at 623 K and 723 K for 1.8 ks were examined at temperatures higher than A_f' by 15 K with tensile speed of 8.3x10^-6 m/s. Critical stress for inducing B19' martensites, σ_M, of the aged alloy at 723 K for 1.8 ks was nearly equal to the aged alloy at 623 K for 1.8 ks, however, elongation at tensile fracture point of the former alloy was about 2 times longer than that of the latter one. These tensile properties were the same as those previously observed by the present authors, and they seemed to be interpreted by considering that the stress induced martensits were not developed in the aged alloy at 623 K for 1.8 ks although they were done in the aged alloy at 723 K for 1.8 ks, because the number of Ti₃Ni₄ precipitates per unit area were larger in the former alloy than the latter alloy, as verified by TEM observation.