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Strengthening Micromechanisms in Cold-Chamber High-Pressure Die-Cast Mg-Al Alloys

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Abstract

The contributions from grain boundary, solid solution, and dispersion strengthening to the yield strength of cast-to-shape specimens were calculated for seven binary alloys with compositions ranging from very dilute (0.5 mass pct Al) to concentrated (12 mass pct Al). Experimentally and theoretically determined parameters were used to explicitly account for the different microstructures at the skin and core regions of specimens’ cross sections. Microhardness maps were used to identify the specimens’ skin. The specimens’ strength was calculated as the weighted addition of the respective strengths of skin and core. The calculated strengths reproduced well the experimental values for the dilute alloys but underestimated the strength of the most concentrated alloys by as much as ~35 MPa. It is argued that the presence of the percolating network of Mg17Al12 eutectic intermetallic, particularly in the skin region, in conjunction with highly efficient dispersion hardening due to the convoluted shape of the intermetallics, accounts for the shortfall in the calculated strength.

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Notes

  1. The data of Figure 1 are given without error bars as they were collected on one set of grains, on a single specimen of each alloy. The accuracy of the data was reassured by the qualitative match with the micrographs of Figures 3 and 4, and the close quantitative agreement of the calculated values for the dilute alloys, whose strengths are largely determined by the grain size (Figure 9).

  2. The specimens were slightly etched to increase the contrast, but with care not to etch out the Mg17Al12 (the bright phase). The volume fractions measured by image analysis (Table II; Figure 2(b)) on these images matched the values obtained by other techniques, providing confidence that the etching did not affect the integrity of the intermetallic.

  3. The fraction of the cross section that remained elastic (thus defining the skin) at the point of the stress–strain curve where the core became fully plastic was determined analytically by comparing the strain hardening rate of the different alloys at yielding with that of the most dilute alloy, which does not exhibit a differentiated skin region.[22]

  4. The Al dissolved in the eutectic α-Mg was included in the calculation of c sα.

  5. Both the c ss- and the c E-values are immediate from the phase diagram, unlike the c sα, which depends on the volume fraction of ESGs, or, in geometrical terms, on the area fraction covered by the core.

  6. Any departure from the predicted solute profiles of Figure 2 implies a stronger solid solution hardening effect, hence the reference to this calculations as “lower bounds.”

  7. The strength of the solid solution calculated using the grain’s average solute concentration (c sα values in Table II) is nearly identical to the solid solution strength averaged over the grain’s diameter.

  8. Hansen showed that whether the reinforcing particles are located along the grain boundaries or uniformly dispersed inside the grains make little difference to the total strengthening.[49]

  9. Hansen and Ralph[52] also found a better agreement with the experimental data in dispersion-hardened Cu alloys using a linear addition law.

  10. The use of a linear addition of the strengths of skin and core is consistent with Kurzydlowski and Bucki’s analysis[53] of the strength of polycrystals made of subpolycrystals of different grain sizes.

  11. The stiffness of a bending dominated cellular structure scales with the square of the relative volume fraction of solid material.[35,57] From Table 2, the intermetallic at the skin increases from 11 pct for the 8.77 Al alloy to about 17 pct for the 11.6 Al alloy, increasing the local 3D network’s contribution to (7*(17/11)2) ~ 16.7 MPa.

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Author notes

  1. Kun V. Yang (Postdoctoral Research Fellow)

    Present address: Department of Materials Engineering, Monash University, Clayton, VIC, 3168, Australia

Authors and Affiliations

  1. ARC Centre of Excellence Design in Light Metals, Materials Engineering, School of Engineering, The University of Queensland, St Lucia, QLD, 4072, Australia

    Kun V. Yang (Postdoctoral Research Fellow) & Carlos H. Cáceres (Reader)

  2. School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC, 3083, Australia

    Mark A. Easton (Professor)

  3. Department of Materials Engineering, Monash University, Clayton, VIC, 3168, Australia

    Mark A. Easton (Professor)

Authors
  1. Kun V. Yang
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Correspondence to Carlos H. Cáceres.

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Manuscript submitted October 14, 2013.

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Yang, K.V., Cáceres, C.H. & Easton, M.A. Strengthening Micromechanisms in Cold-Chamber High-Pressure Die-Cast Mg-Al Alloys. Metall Mater Trans A 45, 4117–4128 (2014). https://doi.org/10.1007/s11661-014-2326-x

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