The Yield Stress of Single Crystals of $\alpha$-Phase Copper-Aluminium Alloys

The composition dependence of the yield stress, σ, of single crystals of α-phase copper-aluminium alloys has been measured at 293 and 4.2 K under precisely defined experimental conditions to determine whether the Friedel relation with σ ∝ c1/2 or the Mott-Labusch relation with σ ∝ c2/3 applies. The best statistical fit gives a dependence of σ on c1/2 for values of c between 0 and 0.15. A curve with σ ∝ c2/3 can be fitted to the results for 0.003 ≤ c ≤ 0.15 with a small increase in the standard deviation, but a segment with σ ∝ c1/2 must be introduced for 0 ≤ c ≤ 0.003. On the basis of these measurements it is not possible to decide between the validity of Friedel and of Mott-Labusch statistics for the concentrated alloys.