The phase transformation and microhardness changes of hypoeutectic grey iron subjected to rapid solidification in Helium was studied to showcase the relationship between the evolved microstructure and microhardness of this important engineering material. Droplets samples were prepared, using containerless processing via drop-tube technique. These rapidly solidified samples were collected and sieved into 8 size ranges from 850 µm to 53 µm diameters with corresponding estimated cooling rate range of ~900 K/s to ~78,000 K/s respectively. The emerged phases and evolved morphologies were characterized using optical and scanning electron microscopy and X-ray diffraction analysis; while Vickers microhardness tester was used to measure the hardness of the various samples. As the cooling rate increases, the undercooling increased and martensitic or acicular ferrite structure was observed in the very small droplets which confirms the progressive increase in microhardness of the samples from the as-cast to decreasing droplets sizes.
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机译：研究了在氦气中快速凝固的亚共晶灰铸铁的相变和显微硬度变化，以揭示这种重要工程材料的组织演变与显微硬度之间的关系。使用无容器处理通过液滴管技术制备液滴样品。收集这些快速固化的样品并将其筛分成8个尺寸范围，直径范围从850 µm至53 µm，相应的估计冷却速率范围分别为〜900 K / s至〜78,000 K / s。使用光学和扫描电子显微镜以及X射线衍射分析对出现的相和演化的形态进行表征。使用维氏显微硬度计测量各种样品的硬度。随着冷却速率的增加，过冷度增加，并且在非常小的液滴中观察到马氏体或针状铁素体结构，这证实了样品的显微硬度从铸态到减小的液滴尺寸逐渐增加。
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