HIGH-TEMPERATURE CREEP-BEHAVIOR OF A 15M2B-FE-18CR-12NI COMPOSITE

The creep behaviour of a 15M2B-Fe-18Cr-12Ni in situ composite was investigated at temperatures ranging from 873 to 1173 K and applied stresses in the range 2.01 x 10(-4) G-7.78 x 10(-3) G (where G is the shear modulus). The apparent activation energy of creep, Q(c), was found to increase slightly with increasing applied stress. By neglecting this stress dependence of Q(c) and using its weighted average value Q(c)av, a slight temperature dependence of the stress sensitivity parameter of the minimum creep rate, m, was avoided. This parameter was found to depend on the applied stress sigma, increasing from a value close to 3.5 at sigma = 20 MPa to a value close to 10 at sigma = 400 MPa. The creep in the composite is most probably lattice diffusion controlled, the difference between the value of Q(c)av = 359 kJ mol-1 and that of the activation enthalpy of lattice diffusion, DELTAH(L)BAR = 290 kJ mol-2, being accounted for by the temperature dependence of the shear modulus. The creep strength of the composite is nearly the same as that of the matrix solid solution (low carbon 18Cr-12Ni steel). Possible reasons for the negligible strengthening effect of M2B particulates are discussed. The creep-rate-controlling dislocation mechanism is most probably the same in the composite and the 18Cr-12Ni steel, namely recovery of the dislocation structure dependent on lattice diffusion and associated with subgrain or cell structure formation. It is shown that the stress sensitivity parameter m increasing with increasing stress and reaching a value as high as 10 cannot be explained in terms of the measured internal stress as a back stress.