The skin effect and the yielding behavior of cold chamber high pressure die cast Mg–Al alloys

The volume fraction of material that remained elastic as yielding developed in cast-to-shape tensile specimens of binary alloys with Al contents between 0.47 and 11.6 mass% was calculated using the Kocks–Mecking method of analysis. In the most dilute alloys the elastic fraction decreased rapidly to zero at a well-defined stress, suggesting that yielding was uniform across the specimen, whereas in the concentrated ones it decreased gradually over a wide range of stresses, suggesting that yielding developed first in the softer core of the casting while the harder outer layer, or skin, remained elastic. Comparison with specimens of the concentrated alloys which had a surface layer removed showed that the strain hardening behavior of the core resembled that of full specimens of the most dilute alloy. The maximum amount of elastic material in comparison with the most dilute alloy was used to define the area fraction covered by the skin, for each alloy. The skin covered between ∼10% and ∼30% of the cross section, the greater values for the concentrated alloys. The skin imposed an elastic constraint that delayed the development of full plasticity at the core.