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 Mo 2C grain boundary precipitates and occasional coarse FeAlMo Laves phase particles were found after creep testing at 650°C in ternary Fe 3Al+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 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 Fe 3Al+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 (Fe 2MoC?). 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 h at 750°C.