Creep properties and microstructure of a precipitation-strengthened ferritic Fe-Al-Ni-Cr alloy

The ferritic alloy Fe-10Cr-10Ni-5.5Al-3.4Mo-0.25Zr-0.005B (wt.%), strengthened by coherent B2-structured (Ni,Fe)Al precipitates with a volume fraction of 13 vol.% and average precipitate radius of 62 nm, was subjected to creep in the stress range 30-300 MPa and the temperature range 600-700 degrees C. The stress dependence of the steady-state strain rate can be represented by a power law with high apparent stress exponents of 6-13 and high apparent activation energies of 510-680 kJ mol(-1). Threshold stresses at all studied temperatures were observed, ranging from 69 to 156 MPa, from which a true stress exponent of similar to 4 and a true activation energy of 243 +/- 37 kJ mol(-1) were determined, which are equal to those for dislocation creep and lattice diffusion in the ferritic matrix, respectively. Based on these mechanical results and detailed electron microscopy observations, the creep mechanism was rationalized to be general dislocation climb with repulsive elastic interaction between coherent precipitates and the matrix dislocations. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.