Effect of Grain Refinement on Room Temperature Ductility of Ferritic Heat-resistant Alloys Strengthened by NiAl Precipitates

The room temperature (RT) ductility of ferritic Fe–Al–Ni-based alloys containing B2-type NiAl precipitates was improved by the refinement of the matrix. The microstructure and tensile properties of Fe–Al–Ni–Cr–Mo–Mn alloys after annealing at 1223 K followed by air cooling were investigated focusing on Mn content. The matrix of the alloy containing 4 at% Mn consists of coarse equiaxed α (bcc) grains and fine grain regions composed of the α and γ (fcc) grains. Fine grain regions are formed through the phase transformation between the α and γ phases during annealing and air cooling. In the heat treatment process, the precipitation of (FeCr)2Mo Laves phase is effective in the refinement of the matrix grains due to grain boundary pinning effect. The alloy with fine grain regions in the matrix exhibits not only large elongation (approximately 14.7%) at RT but also high strength, compared with modified 9Cr-1Mo and 316 stainless steels in the temperature range from RT to 973 K. The fine grain regions which are fractured by ductile fracture mode are responsible for the large ductility of the alloy at RT. Moreover, the high strength of the alloy at high temperatures is attributed to the high stability of the NiAl precipitates at 973 K.