The influence of cobalt on the tensile and stress-rupture properties of the nickel-base superalloy mar-m247

The influence of cobalt on the mechanical properties of MAR-M247, a cast nickel-base superalloy, was investigated. Nickel was substituted for cobalt to produce 0, 5, and the standard 10 pct cobalt versions of MAR-M247. Tensile tests were performed between 649 and 982 dgC; stress-rupture tests were conducted at temperatures ranging from 760 to 982 dgC. The tensile properties were not significantly affected by cobalt level, but a slight peak in strength at 5 pct Co was apparent. A -80 °C shift in the peak yield strength temperature as Co level was reduced from 10 to 0 pct was also evident. This behavior was related to a reduction in the γ′ volume fraction, an increase in γ′ particle size, an increase in W and Ti concentrations in the γ′, and a decrease in Cr and Al concentrations in the γ as Co level in MAR-M247 was reduced. Stress-rupture properties, however, were more significantly affected by Co level. The 10 pct Co alloy exhibited rupture lives typically 1.2 times greater than that of the 5 pct Co alloy and 3 times greater than that of the 0 pct Co alloy. The steady state creep rate of the 10 pct Co alloy was generally equal to that of the 5 pct Co alloy, but was only one third as large as the creep rate of the 0 pct Co alloy. This behavior was associated with a decrease in γ′ volume fraction and the formation of a grain boundary carbide film as cobalt level was reduced.