Location specific properties in an aerospace alloy

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Yearwood, Gavin (2019). Location specific properties in an aerospace alloy. University of Birmingham. Ph.D.

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Abstract

In this work a physics based creep model is presented to predict the behaviour in the Nickel based superalloy RR1000. The model presented takes into account the strengthening mechanisms active during high temperature deformation, associated with interactions between dislocations and gamma' precipitates. In the creep conditions examined the gamma' precipitates act as pinning points to dislocations gliding in the matrix where the primary method for overcoming these obstacles is through vacancy assisted climb. This creep constitutive model is also coupled with a mean field approximation to capture how the microstructure is evolving during the creep process. The macroscopic creep predictions are obtained through a scale invariance argument where component-level creep rates are assumed to have the same functional form as the shear rates active slip system. Crystal plasticity simulations have been carried out to assess the validity of this assumption.

To validate the model, experiments were done comprising creep and tensile tests to better understand the mechanisms which are occurring during deformation and the effect of ageing and pre-straining on this behaviour. Also both SEM and TEM were carried out to better capture the ' dispersion and obtain mean sizes for both the tertiary and secondary gamma'. Model predictions of lifetimes and minimum creep rates over a range of temperature and stresses are shown to be in good agreement with the available experimental data on RR1000. Simulations of the creep response with different starting gamma' dispersions, obtained from a disc, have been carried out. The dispersions investigated had similar volume fractions and particle size, however they differed in size distribution. The mean field model predicts faster coarsening/dissolution rates for a broad distribution when compared to a narrow one. This results in a size distributions giving different creep rates and allowing the model to capture experimental creep scatter

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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