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Date of Award

9-1992

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Metallurgy

Supervisor

G.R. Purdy

Abstract

This thesis presents the results of a study of two aspects of peritectic alloy solidification: 1. an experimental
and theoretical study of the phenomenon of banding (alternate
deposition of layers of primary (α) and peritectic (β) phases)
during plane front solidification and 2. a theoretical study
of microsegregation during the non-isothermal diffusion
controlled peritectic transformation in an alloy system where
the solute diffusivities differ greatly, ie. Fe-C-Mn.

Plane front solidification experiments using Sn-Cd
alloys produced specimens which solidified as metastable α
phase. The β phase was observed in only those specimens which
were purposely disturbed during solidification; the α phase
did not re-nucleate at the interface once β phase
solidification was established. Banding was observed in one of
these specimens, however, the α phase never entirely
disappeared from the interface. Mathematical model predictions
of the growth transient of the β phase and nucleation
considerations showed that the α phase would not be renucleated
at the β/liquid interface. It is thus expected that
when banding occurs in Sn-Cd peritectic alloys, α is never
completely removed from the growth front.

The treatment of microsegregation during the non-isothermal
diffusion controlled peritectic transformation used
in this work exposed several aspects of the problem which are
obscured by some of the more sophisticated mathematical
treatments of this problem; the transformation was modelled as
a series of non-isothermal steps. For the binary Fe-C system,
these calculations showed that, while cooling rate and solute
diffusivity are important, closure of the δ-ferrite/austenite
two phase field ultimately determines the temperature at which
the transformation ends and what phases exist at that point.
In the ternary case, Fe-C-Mn, the combined influence of
constitutional and diffusional solute interactions promotes
the following: 1. the concentration gradient of the fast
diffusing solute in austenite is minimized with a
corresponding greater segregation of the slow diffusing solute
in all phases, compared to predictions based on no diffusion
in the austenite or complete diffusion in all phases. 2.
diffusional solute interaction may increase or decrease the
effect of constitutional solute interaction on
microsegregation. 3. At the liquid/austenite interface, the
possibility of metastable austenite solidification or
nucleation of δ-ferrite exists throughout the transformation.

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