Solidification Of Peritectic Alloys

Abstract

<p>This thesis presents the results of a study of two aspects of peritectic alloy solidification: 1. an experimental<br />and theoretical study of the phenomenon of banding (alternate<br />deposition of layers of primary (α) and peritectic (β) phases)<br />during plane front solidification and 2. a theoretical study<br />of microsegregation during the non-isothermal diffusion<br />controlled peritectic transformation in an alloy system where<br />the solute diffusivities differ greatly, ie. Fe-C-Mn.</p> <p>Plane front solidification experiments using Sn-Cd<br />alloys produced specimens which solidified as metastable α<br />phase. The β phase was observed in only those specimens which<br />were purposely disturbed during solidification; the α phase<br />did not re-nucleate at the interface once β phase<br />solidification was established. Banding was observed in one of<br />these specimens, however, the α phase never entirely<br />disappeared from the interface. Mathematical model predictions<br />of the growth transient of the β phase and nucleation<br />considerations showed that the α phase would not be renucleated<br />at the β/liquid interface. It is thus expected that<br />when banding occurs in Sn-Cd peritectic alloys, α is never<br />completely removed from the growth front.</p> <p>The treatment of microsegregation during the non-isothermal<br />diffusion controlled peritectic transformation used<br />in this work exposed several aspects of the problem which are<br />obscured by some of the more sophisticated mathematical<br />treatments of this problem; the transformation was modelled as<br />a series of non-isothermal steps. For the binary Fe-C system,<br />these calculations showed that, while cooling rate and solute<br />diffusivity are important, closure of the δ-ferrite/austenite<br />two phase field ultimately determines the temperature at which<br />the transformation ends and what phases exist at that point.<br />In the ternary case, Fe-C-Mn, the combined influence of<br />constitutional and diffusional solute interactions promotes<br />the following: 1. the concentration gradient of the fast<br />diffusing solute in austenite is minimized with a<br />corresponding greater segregation of the slow diffusing solute<br />in all phases, compared to predictions based on no diffusion<br />in the austenite or complete diffusion in all phases. 2.<br />diffusional solute interaction may increase or decrease the<br />effect of constitutional solute interaction on<br />microsegregation. 3. At the liquid/austenite interface, the<br />possibility of metastable austenite solidification or<br />nucleation of δ-ferrite exists throughout the transformation.</p>