The Niyama criterion and the dimensionless Niyama criterion are used to predict shrinkage microporosity in Mg–6Gd–3Y–0.5Zr magnesium plates processed by gravity casting and low‐pressure sand casting. Instead of the Niyama evaluation condition recommended by commercial ProCAST software and precious studies, the most appropriate Niyama evaluation condition is suggested as fraction solid of 0.7 for the investigated alloys. With consideration of the actual distribution of shrinkage microporosity and the definition of unsound samples (shrinkage microporosity volume fraction is higher than 1% based on the tensile test), the corresponding threshold Niyama values are qualitatively determined as 0.44 and 0.3 °C^0.5 s^0.5 mm⁻¹ for gravity‐cast and low‐pressure sand‐cast plates, respectively. It indicates that exerting pressure contributes to the decrease in the threshold Niyama value. In addition, exerting pressure can reduce the amount of shrinkage microporosity. For the dimensionless Niyama criterion, it is capable to quantitatively predict the shrinkage microporosity volume fraction in low microporosity‐containing regions, but not in high microporosity‐containing regions.

中文翻译：

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Mg-6Gd-3Y-0.5Zr镁合金重力铸造和低压铸造的收缩微孔率预测

Niyama准则和无量纲Niyama准则用于预测重力铸造和低压砂铸处理的Mg-6Gd-3Y-0.5Zr镁板的收缩微孔率。代替商业ProCAST软件和宝贵研究建议的Niyama评估条件，建议最合适的Niyama评估条件为所研究合金的固体分数为0.7。考虑到收缩微孔的实际分布和不合格样品的定义（基于拉伸试验，收缩微孔的体积分数高于1％），定性确定相应的Niyama阈值为0.44和0.3°C ^0.5  s ^0.5  mm ^-1分别用于重力铸造和低压砂铸板。这表明施加压力有助于降低阈值Niyama值。另外，施加压力可以减少收缩微孔的数量。对于无量纲的Niyama准则，它能够定量预测低含微孔区域的收缩微孔体积分数，而不是高含微孔区域的收缩微孔体积分数。