Title:

Internal stresses and the cyclic deformation of an aluminium matrix composite

The development of internal stresses in planar random Saffil fibre reinforced aluminium with a range of fibre volume fractions has been studied theoretically and in monotonic and cyclic deformation (Bauschinger) experiments at room temperature and 77K. The conventional method of analysing Bauschinger experiments is extended to allow for a separation of the mean and thermal stresses. This analysis is applied to experimental results enabling the mean stress hardening rate and the magnitude of the thermal stress to be measured. The experimental results are compared with predictions of the mean field model, which is based on the Eshelby method of determining internal stresses. For that purpose the Eshelby S tensor for a planar random array of fibres is calculated. Because the aluminium/Saffil composites are not isotropic in the transverse directions, the plastic strain used in the calculations has to be determined experimentally. A method for quantifying the anisotropic plastic flow of the aluminium/Saffil composites is proposed and the results are used in calculations of the mean stress hardening rate. A comparison of predictions for the mean stress hardening rate with results of the experimental analysis proposed here shows that good agreement is obtained for low fibre volume fractions at 77 K. The results also show that relaxation of the mean stress increases with fibre volume fraction and that at 77 K the mean stress hardening rate is about a factor of two larger than at room temperature. The measurements of the thermal stresses obtained in the Bauschinger experiments are in quantitative agreement with results obtained in monotonic tests. The magnitude of the thermal stress at room temperature or 77 K is independent of fibre volume fraction and a comparison with predictions shows that relaxation of the thermal stress increases with fibre volume fraction. Cycling in the Bauschinger experiments reduces the thermal stress and hence the separation of the mean and thermal stresses is essential for a reliable measurement of the mean stress hardening rate. Matrix hardening contributes considerably to the overall hardening of the composite, both at room temperature and 77 K. The modified OrowanWilson model, which enables the plastic friction coefficient to be measured in coppertungsten composites, has been applied to the aluminium\Saffil composites. The model requires both the mean stress and the peak stress curves obtained in Bauschinger experiments to be linear in plastic strain. Most of the peak stress curves for the aluminium/Saffil composites are nonlinear but for the curves which are linear the predictions of the model are not in quantitative accord with experimental results. This may be because relaxation reduces the mean stress and the source shortening stress in different proportions. The diameter of the Saffil fibres is also close to the lower end of applicability of the model. The temperature dependence of the mean stress hardening rate suggests that relaxation is thermally activated. A model for relaxation of the mean stress is proposed. An equation is derived for the number of Orowan loops per fibre and it is assumed that the rate controlling mechanism of relaxation is cross slip of screw dislocations. The estimated activation energy is independent of fibre volume fraction but the activation volume decreases with increasing fibre volume fraction. The magnitudes of activation energy and activation volume support the assumptions of the model. A preliminary study on the early stages of fatigue shows that persistent slip bands form in the matrix of the aluminium/Saffil composites.
