initiation mechanism. As Figure 13 details, the fracture surfaces showed different appearances depending on the elongation-to-failure value. Sample 11-2 (Casting #11 –Location #2) had the highest elongation-to-failure (11.3%) recorded in the Location #2 samples. The frac- ture surface associated with Sample 11-2 showed pre- dominately ductile failure features (Figure 13[a]); these features exhibit a large amount of plastic deformation and tearing; additionally, a small amount of second-phase particles (mostly Mg17
Al12 ) were also observed.
In contrast, Sample 7-1 failed due to the presence of a knit line; it had the lowest (0.30%) elongation-to-failure recorded in the Location #1 samples. The features on the fracture surface were distinctly different from those seen on 11-2; this indicated that the surface did not fail in a ductile manner. These features were more similar to those observed when a “cold shut’” was present. Figure 13(c) shows that these features are characterized by what appear to be dendrites and/or cellular tips formed during solidi- fication. The features are very clear and distinct; there is no evidence of tearing or plastic deformation that would indicate these features were formed when the tensile crack propagated through the material.
Sample 4-9 had an intermediate level of elongation-to-fail- ure (3.5%). As Figure 13(b) shows, the fracture surface is characterized by a mixture of ductile features as well as the “dendritic” features seen in Sample 7-1.
Finally, the fracture surfaces were examined to determine if the fracture path was through the Mg cells or followed the Mg17
ysis found that there was not a significant percentage of the second-phase particles on the fracture surface. There- fore, the fracture path for the tensile failure appears to go through the Mg cells.
Al12 second phase. Energy-dispersive x-ray anal-
• •
conclusions
A study was performed to better understand the relationship between microstructure and mechanical properties in a high- pressure die-cast AM50 Mg. The results from this study are as follows:
•
Tensile bars excised from the AM50 castings indi- cated a statistical difference in the elongation-to- failure based upon sample location in the casting.
There was also no difference in cell size from the edge of these samples to the middle.
Externally Solidified Cells (ESCs) were present in large numbers in the sample microstructures. The ESCs tended to be grouped toward the middle of the cross-sections. No correlation could be deter- mined between the location of the samples and the size and number of the ESCs.
•
Porosity distribution was random across the sam- ple cross-section.
• A plot of elongation-to-failure versus bulk porosity indicated that porosity did influence the total fail- ure strain above 1.5% porosity; below this level, the influence of bulk porosity was minor.
•
Examination of the fracture surfaces indicated that the fracture did not preferentially occur along the Mg17
Al12 references
1. Friedrich, H.E., Mordike, B.L., Magnesium Technology: Metallurgy, Design Data, Application, Springer-Verlag, Berlin, Germany (2008).
2. Schumann, S., “Paths and Strategies for Increased Magnesium Applications in Vehicles,” Materials Science Forum, 488-489: pp. 1-8 (2005).
eutectic in the AM50 alloy.
Figure 13. Typical fracture surfaces observed on the failed tensile samples from the AM50 HPDC castings: a) Sample with 11.3% elongation; b) Sample with 3.52% elongation; c) Sample with 0.30% elongation.
24 International Journal of Metalcasting/Winter 2012
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