The effect of strain rate changes on the flow stress of Au–Cu alloys in the region of strain prior to the start of serrated yielding has been investigated. The measurements of Δσ/Δ Δσ/Δ were found to depend on strain, strain rate, temperature, and composition. Negative values of Δσ/Δ ln ϵ were observed prior to the onset of serrated yielding. The effect of base strain rate on the strain rate sensitivity of the flow stress was correlated with the effect of ϵ on the strain at the onset of serrated yielding. The results are interpreted in terms of a recent model for dynamic strain ageing based on the diffusion of solute atoms to temporarily arrested dislocations. The occurrence of pronounced flow stress transients accompanying strain rate changes was also investigated.

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The effect of specimen dimensions and tensile machine stiffness on the characteristics of type A and B serrated yielding in an AI-Cu alloy is investigated. The strain a t the onset of type B yielding, eB is found to exhibit a pronounced dependence on specimen dimensions, increasing with increasing specimen diameter and decreasing gauge length. A decrease in machine stiffness resulted in a decrease in eg. Measurements of the band parameters are also found to be dependent on specimen dimensions and machine stiffness. The results are discussed in terms of recent models of the Portevin-Le Chatelier effect. E s wird der EinfluB der Probendimensionen und der Spannmaschinensteifigkeit auf die Charakteristiken vom Typ A und B des sagezahnartigen FlieBens in einer Al-Cu-Legierung untersucht. E s wird gefunden, da13 die Dehnung beim Einsetzen des FlieBens vom Typ B, eg, eine ausgepragte Abhangigkeit von den Probendimensionen hat, die mit zunehmendem Probendurchmesser und abnehmender MeDger&telange zunimmt. Eine Verringerung der Maschinensteifigkeit ergibt eine Verringerung von eB. E s wird gefunden, daB die Messungen der Bandparameter ebenfalls von den Probendimensionen und der Maschinensteif igkeit abhangen. Die Ergebnisse werden mit neueren Modellen des Portevin-Le-Chatelier-Effekts diskutiert.

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