Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Abstract

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe3Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH f °(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe3Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe3Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe3Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.