Phase equilibria and elastic moduli of rapidly solidified Fe-Cr-Mo-B and Fe-Cr-Ni-B alloy

A study was carried out on rapidly solidified Iron-Transition Metal-Boron alloys (Fe-TM-B, TM = Cr, Mo, Ni) in order to understand the effect of microstructure on the elastic properties of the materials. The thermodynamic principles governing phase equilibria in alloy systems are presented and phase diagram calculation techniques introduced. A brief review of rapid solidification technology is given, followed by thermodynamic modelling of phase formation in gas atomised powders. The elastic behaviour of metallic alloys and mathematical modelling of their elastic modulus are discussed. Thermodynamic characterisations of the relevant phase diagrams in the Fe-TM-B systems are presented together with critical assessments of previous work in these system. A thermodynamic database is established and used in designing test alloy compositions. The criteria are defined, and subsequent calculations made, for the selection of a series of model alloys based in the Fe-Cr-B, Fe-Cr-Ni-B, and Fe-Cr-Mo-B systems. Phase formation in the gas atomised powders of the designed alloys are predicted as a function of powder diameter, using a combined thermodynamic and kinetic approach, and the undercooling and critical powder size necessary for producing the desired amorphous or supersaturated solid solutions calculated. The elastic properties of the alloys are estimated in terms of microstructural parameters using Eshelby's Equivalent Inclusion model. These predictions helped reveal the microstructural evolution in rapidly solidified alloys and understand the relations between microstructure and the properties, in particular the elastic properties of the materials. The calculations were made by taking advantage of both computerised software available, and that developed by the present author. Metallographic and mechanical examination results of the designed test alloys are discussed and analysed with respect to the theoretical calculation results. The effect of a number of variables, in particular the microstructure, on the elastic properties of the alloys is analysed, and the routes for achieving alloys with optimum microstructure and elastic properties recommended.