Effect of chemistry and temperature on planar defects in superalloys

Allen, Joshua D. T. (2019). Effect of chemistry and temperature on planar defects in superalloys. University of Birmingham. Ph.D.

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Abstract

Fault energies are exceptionally important, determining the activation of deformation processes in metals. This is especially true in the case of two phase γ/γ' superalloys. However there is currently no model for how the fault energies in these alloys vary with composition. This is a large issue due to the complex nature of superalloys, which typically feature more than 10 alloying elements. The main aim of this thesis is to design a model to predict fault energies for arbitrary alloying compositions of superalloys. This would provide a tool for alloy designers, allowing the choice of alloy compositions to make the alloys resist deformation, facilitating higher operational temperatures and efficiency.

In this research, first-principles calculations are undertaken using the projector augmented wave basis set in conjunction with the generalised-gradient approximation, as implemented in the Vienna ab initio simulation package. This allows the generation of input parameters for axial interaction models. Calculations are made for a large number of compounds and alloys to allow the assessment of how changing the alloy chemistry impacts the intrinsic stacking fault and superlattice intrinsic stacking fault energy. Using interpolation and fitting of these results it is possible to produce a model for arbitrary alloying compositions. Due to the high operational temperatures of superalloys the change in these fault energies with temperature was calculated (as first-principles calculations are traditionally only possible at 0 K). This was done using the quasiharmonic Debye model as implemented in the GIBBS package. The effects of temperature were found, in general, to be significantly less than the effects of alloying, providing validation for the usage of first-principles calculations for high temperature alloys.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):