Analysis of Steady-state Creep and Creep Fracture of Directionally Solidified Eutectic Gamma/gamma'-alpha Alloy

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-1.6V-1.2Re (wt pet) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress is n = 7.5 +/- 0.3, and the apparent activation energy for creep of the eutectic gamma/gamma'-alpha composite is determined to be Q = 517 +/- 11 kJ mol(-1). When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creep Q(c) is calculated to be between 412 and 424 kJ mol(-1) and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation