MICROSTRUCTURAL CHARACTERIZATION OF PRECIPITATES FORMED DURING HIGH-TEMPERATURE TESTING AND PROCESSING OF IRON ALUMINIDE ALLOYS

Iron-aluminide alloys were examined using analytical electron microscopy after creep-rupture testing at 600-650-degrees-C or after thermomechanical processing at 650-1250-degrees-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-degrees-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-degrees-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-degrees-C. The precipitates partially dissolved and only MC remained after solution annealing (SA) at 1100-1200-degrees-C, but finer grain boundary MC was found after a SA treatment at 1250-degrees-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-degrees-C (207 MPa for 244 h) and during aging for 9 h at 750-degrees-C.