Enhanced tensile ductility of coarse-grain Al-Mg alloys

The effective forming of near-net-shape parts from aluminum alloys is of significant interest for automotive and aerospace applications. It has traditionally been thought that the very high tensile ductilities necessary for many near-net-shape forming operations, in excess of 100%, were only available in fine-grain superplastic materials. Tensile ductilities in excess of 300%, however, have been attained in coarse-grain, non-superplastic, binary Al-Mg alloys as a result of a solute-drag-controlled dislocation creep process. Such enhanced ductilities from non-superplastic Al-Mg alloys might offer an inexpensive alternative to superplastic alloys that often require elaborate processing to develop fine grain sizes. In this investigation, tension tests at various temperatures and strain rates were used to characterize the ductility, strain-rate sensitivity, and strain-hardening behavior of two binary alloys: Al-2.8 wt.%Mg and Al-5.5 wt.%Mg. Under the proper conditions, tensile elongations of 300% were achieved. The tensile elongations in these binary alloys were governed not by the onset of tensile instability in the classical sense, but by the rate of neck propagation when failure occurred by necking to a point. Predictions of tensile ductility for the case of failure by necking were made using a simple numerical model, and the results are correlated with data from the two binary alloys.