It was noted that some calorimetric results for an Al-Mg-Cu alloy did not agree with recent interpretations of positron annihilation experiments, but instead supported earlier proposals. The first hardening stage in Al-Cu-Mg alloys was caused by the formation of Guinier-Preston-Bagaryatsky zones; their replacement by more stable phases increased the amount precipitated and led to a second increase in hardness with increasing temperature.

Comment on ‘Positron Lifetime Study of an Al-1.7Mg-1.1at%Cu Alloy’. A.M.Zahra, C.Y.Zahra: Philosophical Magazine Letters, 2002, 82[1], 9-12