The effect of tin and nitrogen on liquid phase sintering of Al-Cu-Mg-Si alloys

Alloys of Al-3.8Cu-1Mg, Al-3.8Cu-1Mg-0.7Si, Al-3.8Cu-1Mg-0.1Sn and Al-3.8Cu-1Mg-0.7Si-0.1Sn were made using elemental powders and conventional press-and-sinter powder metallurgy techniques. The sintering of these alloys was studied at 590 degrees C under atmospheres of nitrogen or argon using a horizontally aligned, push-rod dilatometer. Sintered samples were characterized using scanning electron microscopy and positron annihilation lifetime spectroscopy, Under argon, shrinkage occurs in a single stage and the shrinkage rate decays monotonically over time. Sintering shrinkage under nitrogen occurs in three distinct stages and the rate increases over time. Tin decreases shrinkage under argon but activates shrinkage under nitrogen. Positron spectroscopy indicates that the presence of tin does not alter the vacancy concentration in the sintered alloys, suggesting that tin does not bind preferentially to vacancies in the aluminum matrix in these AlCuMgSi alloys. Rather, it appears that tin may limit the formation of aluminum nitride on specific surfaces. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.