Shear Enhanced Heterogeneous Nucleation in AZ91D Alloy
Intensive shearing was applied to AZ91D Mg-alloy melts at temperatures above its liquidus (600oC) by using a twin-screw mechanism. The sheared melt was then cast into a TP1 mould (Aluminium Association’s standard test mould with 3.5K/s cooling rate) for microstructural examination. Alloy melts, both with and without shearing, were also filtered using a pressurized filtration technique to collect oxides and other second phase particles. The experimental results showed a significant grain refinement of AZ91D alloy through enhancement of heterogeneous nucleation. Based on the experimental results, a multi-step heterogeneous nucleation mechanism has been proposed: (1) intensive melt shearing converts oxide films and clusters into fine and well-dispersed particles with a narrow size distribution; (2) the dispersed oxide particles act as potent sites for nucleation of Al8Mn5 intermetallic particles; (3) during the further cooling, Al8Mn5 intermetallic particles nucleate the -Mg phase, resulting in a fine and uniform microstructure. The proposed multi-step nucleation mechanism will be discussed in terms of the effect of forced wetting, particle size and particle size distribution on the enhancement of heterogeneous nucleation.
A. Roósz, V. Mertinger, P. Barkóczy and Cs. Hoó
Z. Fan et al., "Shear Enhanced Heterogeneous Nucleation in AZ91D Alloy", Materials Science Forum, Vol. 649, pp. 301-306, 2010