5 - Thermal effects and high temperature behaviour

Summary

The behaviour of a metal matrix composite is often sensitive to changes in temperature. This arises for two reasons; firstly, because the response of a metal to an applied load is itself temperature dependent and secondly, because changes in temperature can cause internal stresses to be set up as a result of differential thermal contraction between the phases. In the previous chapter, the thermal stresses were shown to result in yielding asymmetries. Here the implications of these thermal stresses are further explored, both in situations where the misfit strain is elastically accommodated, and when inelastic deformation can occur. This leads to an examination of the creep behaviour of MMCs and allows an understanding of the dramatic effects induced by thermally cycling the material while under load.

Thermal stresses and strains

Differential thermal contraction stresses

As is displayed in Fig. 5.1, metals generally have larger thermal expansion coefficients (α) than ceramics. Since fabrication of MMCs almost inevitably involves consolidation at a relatively high temperature, it is not surprising that they often contain significant differential thermal contraction stresses at ambient temperatures (e.g. see Fig. 5.17). Assuming the material to be effectively stress-free at some (high) temperature, T_esf, the stress state at a lower temperature can be envisaged as arising from the fitting of an oversized inclusion into an undersized hole in the matrix. The misfit strain is then simply Δα ΔT, where ΔT = T_esf - T₀, the ambient temperature.