Portevin-LeChatelier Effect of Al-7 mass%Si-Mg Cast Alloys

Abstract

Al-7 mass%Si-Mg cast alloys can exhibit the Portevin-LeChatelier (PL) effect if excessive aging is prevented. In the present study, the PL effect of three Al-7Si-Mg cast alloys with 0.11, 0.26 and 0.46 mass% magnesium was compared with that of an Al-7 mass%Si cast alloy. Before conducting the tension test, part of the test alloys were solution treated at 813 K and the remaining were aged at 273 K. The critical strain for the onset of flow instability could be raised by increasing the magnesium content. For the solution-treated specimens tested in the low temperature regime where the critical strain decreases with temperature, the above result should be related to the attractive interaction between silicon and magnesium solutes since the apparent activation energy for the onset of flow instability increases with increasing magnesium content. For the solution-treated specimens tested in the high temperature regime, there occurs an aging effect which is more pronounced with increasing magnesium content and temperature. Consequently, the rate of increase of the critical strain for the ternary test alloys with respect to temperature is larger with higher magnesium content. For those specimens pre-aged at 273 K, the longer the aging time, the larger the critical strain. Also, the limiting aging time for which the flow instability still exists decreases with increasing magnesium content. Aging should play the role in depleting the quenched-in vacancies and solute atoms.