The role of flow properties and damage accumulation in superplastic ductility of Al-Mg-Mn alloys

It has been suggested earlier that an inhomogeneity in the initial level of cavitation could be a factor in limiting superplastic (SP) ductilities. We present a model that quantifies this effect, assuming a pre-existing level of cavitation that is inhomogeneously distributed, with cavity growth being represented by a plastic growth mechanism. The presence and magnitude of cavitation inhomogenity are shown through experimental studies in a SP Al-4.8Mg-Mn alloy. The use of a simple constitutive relation {dollar}(sigma{dollar} = K{dollar}varepsilonsp{lcub}n{rcub}varepsilonsp{lcub}rm m{rcub}){dollar} enables a comparison of predicted failure strains with experimental results in a range of cavitating alloys and also with previous models. A more representative constitutive equation is then employed that better reflects features of SPD in the 5083 Al alloy system, such as dynamic grain growth and the variation in m at different cross-sections. SPD and failure can consequently be modeled over a range of material and testing conditions (strain rate, test temperature and initial grain size) apart from the initial cavitation characteristics. The model predictions are in good agreement with experimental results over a range of strain rates from 5 {dollar}times{dollar} 10{dollar}sp{lcub}-4{rcub}{dollar} to 10{dollar}sp{lcub}-1{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar} and temperatures from 500 C to 550 C. This approach also helps determine possible sources of scatter in SP tensile elongations by evaluating the effects of an inhomogeneity in cavitation levels relative to that in grain sizes and test temperatures.; A series of tensile tests to failure on SP Pb-Sn samples with pre-machined holes indicated that the highest elongation was obtained in the sample which had the maximum number of holes (39) aligned along a single line parallel to the tensile axis, while a sample with a single isolated hole gave the least elongation. An extension of our two-section cavitation model to this case was done and the predicted failure elongations were found to be in good agreement with experiment. The results suggest that alignment and distribution of voids also play an important role in determining overall elongations. These results could also help explain the higher elongations observed in the alloy 19A (Al-4.8Mg-1.6Mn) which had long broken/cracked stringers of Mn-bearing particles aligned along the rolling direction, than in a lower Mn alloy in which the voids were more random. A case study of the cavitation characteristics in the above alloys at low strains ({dollar}<{dollar}0.5) also helps examine the assumptions of negligible diffusional void growth and dynamic void nucleation and their validity.