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.Peer reviewe

Creep behaviour of a rapidly solidified Al-5Cr-2zr alloy between room temperature and 823 K

The flow behaviour of the rapidly solidified Al-5Cr-2Zr alloy investigated in this study is well described by the power law creep. Based on the stress sensitivity and the activation energy for creep encountered, three temperature ranges were defined: a low temperature range from 298 to 523 K with n > 20 and Qc = 77 kJ mol-1, an intermediate temperature range from 573 to 673 K with n close to 8 and Qc = 167 kJ mol-1, and a high temperature range from 723 to 823 K with n close to 8 and Qc = 273 kJ mol-1. In the low temperature range, creep rate is controlled by aluminium pipe diffusion. The high value of n > 20 could not be associated with any diffusion controlled dislocation creep mechanism. This behaviour can be rationalized using a substructure invariant model with a stress exponent of 10 and a threshold stress. In the intermediate temperature range, creep rate is controlled by aluminium lattice self-diffusion. The stress exponent and the activation energy obtained are consistent with the substructure invariant creep model. In the high temperature range, although the stress exponent suggests that creep may be explained by the substructure invariant model, the apparent activation energy is much higher than the activation energy for self-diffusion of pure aluminium, which may be attributed to the coarsening of dispersoids.Peer Reviewe

Flow stress of rapidly solidified Al-5Cr-2Zr alloy as a function of processing variables

The highest flow stress of gas atomised and extruded Al-5Cr-2Zr alloy in the test temperature range of 373 to 773 K was obtained for specimens processed from the smallest powder particle size extruded at the lowest temperature. As test temperature increased the beneficial effect of small powder particle diameter increasingly prevailed over that of low extrusion temperature. The major microstructural feature that contributed to the high flow stress of the Al-5Cr-2Zr alloy was the small grain size. At low test temperatures, the Hall-Petch strengthening mechanism accounted for as much as 90% of the yield strength. At high test temperatures, although degradation occurred, some strengthening still persisted up to at least 773 K.Peer Reviewe