HIGH-TEMPERATURE BEHAVIOUR OF MG ALLOY AS21X

摘要

The paper presents the high temperature behaviour of a die casting Mg alloy, AS21X (chemical composition given in Table 1), modified by rare earth addition in order to improve corrosion resistance with respect to the conventional high-temperature AS21 alloy developed for the motor of VW 'beetle'. As die-cast specimens were supplied by Hydro Light Metal Research Centre (Pergsum, Norway). Their microstructure (Figure 1a and 1b) consisted of primary grains of solid solution of Al in Mg surrounded by divorced eutectic formed by the same solid solution (richer in Al) and by the intermetallic phases Mg_2Si and Mg_(17)Al_(12). Constant load creep and tensile tests were performed in the 20-240 deg C temperature range. Experimental results in terms of tensile characteristics vs. test temperatures are depicted in Figure 2, while the stress strain curves are presented in Figure 3. Representative creep curves expressed in terms of strain rate vs. strain are reported in Figure 4, where the presence of extended primary creep at low test temperature and high applied stress, the occurrence of a stationary creep stage and a late onset of tertiary creep (except for tests performed at 210 deg C) are clear characteristic features. In the strain rate vs. stress diagram in Figure 5 the minimum creep strain rate are aligned with applied strain rate-maximum stress data from tensile tests, meaning that under experimental test conditions steady state was reached also during tensile tests.