α dendrites without renucleation and only the β phase is left between the dendrites or the arms. From Fig. 1, it also can be seen that the amount of the eutectic, especially the β phase, between the dendrites is more than that between the arms. In addition, there are no distinctly branched third arms on the secondary arms and the secondary arms have obviously necked at their roots.
Figure 2 shows the microstructures of the AZ91D alloys heated for different durations at 580C (1076F). It shows that the dendrites coarsen through the coalescence of the dendrite arms and the original equiaxed dendrite morphol- ogy becomes indistinct after being heated for two minutes (comparing Figs. 1[a] and 2[a]). According to the Mg-Al binary diagram,22
AZ91D alloy will experience a single α
phase interval as the temperature rises. So the eutectic β phase will dissolve towards the surrounding α phase dur- ing heating and transforms into the α phase. Due to the dis- solution or disappearance of the β phase, the equiaxed den- drites coarsen through coalescence of the arms and become
into near spheroidal particles. As shown in Fig. 2(b), the original dendrites have become near spheroidal particles after being heated for 3 minutes. So, it can be concluded that the main event occurring during the heating period of 0-3 minutes is the coarsening of dendrites through the co- alescence of dendrite arms.
Figure 2(b) also shows the other two phenomena: part of the dendrite morphology can be clearly seen in the par- ticles and some liquid phases form between the particles. The former should be attributed to the composition inho- mogeneity and the latter is ascribed to the melting of the residual β phase. Although most of the eutectic β phase has transformed into the α phase, the compositions of the transformed α phase are different from those of the origi- nal eutectic α phase and primary α dendrites because there is not enough time (only heated for 3 min) for composi- tion homogenization. So, the partial outline of the original primary dendrite morphology is still visible. In addition, the temperature rise of the specimen is very rapid during
(a)
(b)
(c)
(d)
Figure 2. Microstructures of the AZ91D alloys heated at 580°C (1076ºF) for (a) 2 minutes, (b) 3 minutes, (c) 5 minutes and (d) 7 minutes and then water quenched.
International Journal of Metalcasting/Winter 2012 45
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