The Elastic Behaviour of NiCo Alloys

The adiabatic, elastic stiffnesses of five f.c.c. Ni—Co alloys in the range from 10 to 62% Co were determined by the ultrasonic pulse‐echo technique at temperatures between 77 and 300 °K. As Co is added to Ni the bulk modulus increases linearly while (C11 − C12)/2 and C44 decrease and increase, respectively. The alloy stiffnesses were extrapolated to 100% Co to yield the following estimates for the stiffnesses of f.c.c. Co at room temperature: B = 1.89, (C11 − C12)/2 = 0.38, C44 = 1.33 (dynes/cm2 × 1012). Debye temperatures calculated from the extrapolated 0 °K stiffnesses indicate that the high temperature instability of h.c.p. Co is due to the more favorable vibrational entropy of f.c.c. structure.