Reviewer: In Figure 11, what was the percentage of residual salts in your sample castings?
ing an XRD. The measured diffraction angles would give the relative amounts of boride particles and residual salt. The relative amount could then be compared to the weight of particles collected after matrix dissolution to determine the amount of residual salt.
, AlB2
Authors: It is difficult to determine the concentration of the residual salts because the compositional analysis in Table 3 is a measure of total element content. For example, the concentration of titanium and boron measured is a sum- mation of the concentrations found within the matrix and residual salt. The compositional analysis cannot differen- tiate the contribution from the matrix and that from the residual salt. It is just a measure of total weight percent. A possible method to determine the amount of residual salt would be to dissolve the Al matrix, collect the par- ticles obtained (TiB2
, residual salt) and examine us-
Authors: The AZ31 castings were cast in a permanent mold and not extruded. The ratios refer to the concentrations of titanium and boron in the refiners.
Reviewer: Given that you’ve done TEM analysis, do you have any data on the crystallographic match of your nucleat- ing particles with the matrix?
Authors: The detection of the nucleating particles by TEM proved to be difficult. Unfortunately, no data on the crys- tallographic match between the nucleating particle and the matrix are available.
Reviewer: I am concerned that the solidification conditions are so atypical of “normal” cooling rates. Grain refiners function differently at different cooling rates and I would question whether the conclusions would be the same at a normal cooling rate.
Authors: More recent unpublished work utilized the same addition levels at a cooling rate of 12-17 °C/s (permanent mold casting) as compared to 1.5 °C/s during this study. The Al-Ti-B grain refiners still show appreciable grain refinement of approximately 30 % reduction as compared to a 65 % reduction obtained during this study. The very low cooling rate established during this study was used to emphasize small differences in grain size with various addition levels and holding times. The authors were con- cerned that a higher cooling rate would reduce the range of grain sizes obtained and any small variations in grain size with addition level and holding time would be lost.
Reviewer: I thought the grain refiners were prepared unusu- ally, particularly the use of hexachloroethane as a degasser. The residual carbon shown in Figure 15 could well be the carbon from the hexachloroethane and not mold material. Carbon is a powerful grain refiner in magnesium in its own right and the results may be influenced by residual carbon or the residual salts shown in Figure 11. The presence of these residual salts makes me question if these grain refiners were prepared properly.
International Journal of Metalcasting/Spring 11
Reviewer: Is your “spot size” small enough for EDX to re- solve ~1 μm particle shown in Figure 8? If your particle size is ~ 2.5 μm in diameter as in Figure 13?
Authors: The EDX analysis conducted on the 1 µm par- ticle in Figure 8 does include information regarding the matrix, hence the detected presence of aluminum and magnesium. The EDX spot analysis was only used as ver- ification for the presence of titanium. No compositional information was gathered from the EDX analysis. The de- tection of the titanium particle was no different than the result obtained by Liu et al.12
Titanium is a much heavier
element than both aluminum and magnesium. It gives a very distinct peak and is easily detected even in small concentrations.
Reviewer: What are the “cracks” in the matrix Figure 8?
Authors: The crack like features in the micrograph are a result of the etching process and are related to the inherent compositional variation (in particular, Al). More detailed explanation about the formation of the “crack like” features of the etched surface can be found in the following study: Maltais, A., Dube, D., Fiset, M., Laroche, G., Tur- geon, S., “Improvements in the Metallography of As- cast AZ91 Alloy”, Materials Characterization, Vol 52 pp.103-119 (2004)
The sample in Figure 8 was etched prior to SEM analysis to improve image contrast. The etched surface made the fine TiB2
particles easier to detect.
Reviewer: What are the striations in the matrix in Figure 12? Is it an etchant residue?
Authors: Similar to Figure 8, the striations are from the etching process. The variation in aluminum concentra- tion results in the striation lines on the sample. The etch- ing was conducted to improve image contrast. It is not etchant residue.
41
Authors: During grain refiner synthesis, the aluminum melt was liquid for approximately one hour, which could result in hydrogen gas pickup. The presence of excess hydrogen could result in relatively high porosity levels and reduce the potency of the grain refiners. Therefore, hexachloroethane was used to reduce the amount of dissolved hydrogen in the aluminum melt. The chemical analysis of the grain refiner did not show any carbon content. It is thus unlikely that re- sidual carbon was present within the castings as a result of the hexachloroethane treatment.
Reviewer: Was AZ31 mentioned in the introduction ex- truded?
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