The threshold stress in a rapidly solidified Al–5Cr–2Zr alloy

In this paper, a new insight into the origin of the threshold stress is presented for a rapidly solidified Al–5Cr–2Zr (wt.%) alloy obtained by hot extrusion of powder particles of three different size fractions. The creep behaviour of this alloy at low temperature (<523 K) is characterised by a stress exponent higher than 10, which is rationalised with the introduction of a threshold stress, σ0. The value of σ0 clearly diminishes as temperature increases. The magnitude of σ0 is also strongly affected by the powder particle diameter, directly associated with the grain size: the larger the grain size, the lower the threshold stress. It is concluded that the threshold stress is related to the grain size through a Hall–Petch-type mechanism and that its temperature dependence is similar to the temperature dependence of the Hall–Petch coefficient.