Microstructural characterization of precipitates formed during high temperature testing and processing of iron–aluminide alloys

Abstract

Iron–aluminide alloys were examined using analytical electron microscopy after creep–rupture testing at 600–650 °C or after thermomechanical processing at 650–1250 °C. Various precipitates were found in several different alloys after testing or heat treatments and most were identified as carbide phases. Small Mo2C grain boundary precipitates and occasional coarse Fe–Al–Mo Laves phase particles were found after creep testing at 650 °C in a ternary Fe3Al + 2Mo alloy, but only after 0.3 h at 275 MPa and not after 90 h at 69 MPa. Large and small niobium- and zirconium-rich MC phase particles were found in the matrix and grain boundaries of a ternary Fe3Al + 1Nb alloy (zirconium and carbon are unintentional impurities) creep tested for 1932 h at 650 °C and 69 MPa, whereas no such precipitates were observed when creep–rupture occured after only 0.5 h at 275 MPa. Two different precipitate phases were found in a complex Fe3Al + Cr, Mo,Ti,Nb,V,C alloy after high temperature processing or mechanical testing, a titanium-rich MC phase and a Cr–Mo–Fe phase (Fe2MoC?) . Coarse matrix particles of both phases were found after hot rolling at 650 °C. The precipitates partially dissolved and only MC remained after solution annealing (SA) at 1100–1200 °C, but finer grain boundary MC was found after a SA treatment at 1250 °C. The Cr–Mo–Fe phase was not found after the SA treatments, but was found to form around matrix MC particles during creep at 600 °C (207 MPa for 244 h) and during aging for 9 hat 750 °C.