a solid particle can affect its melting point: the larger the curvature, the lower the melting point.24
So, the corners and
edges with large curvature of the particles preferentially melt, which does not only decrease the particle size, but also leads them to become spheroidal (comparing Figs. 3a and b). At the same time, the liquid phase amount obvi- ously increases. Figure 3(b) shows that a semisolid mi- crostructure with small and spheroidal primary particles is obtained after being heated for 20 minutes. When the heating time is further prolonged, the coarsening of the spheroidal particles occurs. Their size increases and their shape slowly becomes irregular again (comparing Figs. 3b, c and d). It also can be found that the liquid amount does not change with the heating time and thus the semisolid system reaches its final solid-liquid equilibrium state after 20 minutes. Previous investigation indicated that for the AZ91D specimen with dimensions of Φ16mm x 10 mm, its temperature is up to the final temperature of 580C (1076F) after heating 20 minutes.25
Therefore, it can be suggested
that the main phenomenon occurring during the period of 10-20 minutes is the spheroidization of the polygonal par- ticles and that the coarsening of the spheroidal particles occurs after 20 minutes.
Figure 4 presents the variations in primary particle size and shape factor with regard to heating time after heating for 10 minutes. It indicates that both the size and shape factor decrease during the 10-20 minute period. After that time, the size continuously increases while the shape factor increases during the 20-60 minute period and then decreases. As discussed, during the 10-20 minute period, the spheroidization of the polygonal particles occurs, which does not only decrease the particle size, but also causes the particles to spheroidize (i.e., leads the shape factor to decrease). After 20 minutes, the coarsening pro- gresses and thus the particle size continuously increases, but the change in shape factor depends on the coarsen-
ing mechanisms. Figure 3(b) shows that the particles are quite small and the distance between them is very short. Under these conditions, the solid/liquid interfacial energy of the semisolid system is relatively large. To promptly decrease the interfacial energy, the coalescence of neigh- boring particles operates through boundary migration or particle rotation (that minimizes misorientation).21
It can
be expected that this coalescence must lead the particles to become irregular and result in the obvious increase in shape factor. Figure 3(c) indicates that there are many agglomerates (marked by arrows) in which two or more particles are connected. This implies that the coarsening during the 20-60 minute time period is mainly a result of coalescence.
As the coalescence proceeds and the interfacial energy decreases, the coalescence becomes weaker and weaker while the contribution of Ostwald ripening to the coars- ening becomes more and more obvious. Ostwald ripening does not only mean the dissolution of small particles and subsequent reprecipitation on large particles, but also im- plies the dissolution of particles’ corners and edges and subsequent reprecipitation at sunken zones.26,27
The former
mechanism results in the increase of the particle size and the latter leads the particles to become more spheroidal (i.e., causes the shape factor to decrease). In addition, in the regime of Ostwald ripening, the variation of particle size with holding time should obey the formula:
D3 (t) - D3 (0) = Kt (where D(t) ticle size and K is the coarsening rate).26 is the particle size at time t, D(0)
is the initial par- The present results
shown in Fig. 5 indicate that the variation after being heated for 60 minutes obeys this formula. Furthermore, in compar- ing Fig. 3(c) and (d), it can be seen that the number of the agglomerates in the microstructure heated for 120 minutes is
Figure 4. Variations of the primary particle size and shape factor of the AZ91D alloys with heating time after being heated for 20 minutes at 580°C (1076 ºF).
International Journal of Metalcasting/Winter 2012
Figure 5. Cube of primary particle size versus holding time, using 20 minutes as starting time, t=0.
47
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73