Some applications of CALPHAD techniques to diffusion reactions in gradient materials

Abstract: In gradient materials, i.e. materials that have varyingproperties due to an inhomogeneous chemical composition, thereis usually a strong driving force for diffusion reactions inthe gradient zone. Such reactions may have a strong influenceon the properties of the material. In this doctoral thesisCALPHAD techniques are applied to some problems which arerelevant for diffusion reactions in gradient materials. It isshown that these techniques can give information which isindustrially highly relevant as well as scientifically veryinteresting.Part of this work consists of diffusion simulations whichare based on thermodynamic and diffusion data that have beenassessed by means of CALPHAD techniques. Experimental work ondiffusion couples between stainless steels and low alloy steelshave been carried out for comparison with the simulations and asatisfactory agreement is found between simulations andexperiments.In order to make diffusion simulations for problemsinvolving the ordered B2-BCC phase possible, a phenomenologicalmodel for diffusivities in this phase has been developed. Themodel is suitable for implementation in the DICTRA software fordiffusion simulations and can be used for multicomponentsystems. In order to test the model it has been applied to theAl-Fe and Al-Ni binary systems and the ternary Al-Fe-Ni systemand it is found that the model can represent the experimentalobservations in these systems.An experimental investigation in the Al-Cr system has alsobeen performed. A previously unreported A2→B2-BCCordering reaction has been found. This experimental informationis of interest in order to accurately establish the phasediagram.Keywords:Diffusion, gradient materials, CALPHAD,multicomponent alloys, multiphase alloys, B2-ordering,simulations, experiments, modeling, Al-Ni, Al-Cr, Al-Fe,Al-Fe-Ni, steel joints, assessment