Effects of microstructure on mechanical fatigue crack growth characteristics of a Ti–Ni–Co shape memory alloy

Crystal structure and mechanical fatigue crack growth characteristics of a Ti–49.7at.%Ni–1.38at.%Co alloy at 295±1 K were investigated as a function of heat-treatment. The crystal structure of the alloy aged at 723 K was examined by using an X-ray diffractometer, and the lattice constant and crystallographic axis angle were determined to be a=0.3014 nm and α=β=γ=90, respectively. The fatigue crack growth characteristics of the alloy were examined for center-cracked-tension type specimens by using a computer controlled servo-hydraulic type machine. The fatigue crack growth characteristics of the 723 K aged alloy were comparable to those of the as-rolled alloy, and, for the same stress intensity factor range, the crack growth rates of the aged and as-rolled alloys were equivalent within experimental error. Therefore, the martensitic transformation temperature appears not to influence the fatigue crack growth characteristics. Fatigue fracture surfaces of the two alloys were examined by using a scanning electron microscope with a field emission electron gun, and striation patterns were observed on them. The micro crack growth rate estimated from the striation patterns was in good agreement with the crack growth rate, dl/dN, obtained from the examination of center-cracked-tension type specimens.