A correlation of eutectic growth mechanism with resultant microhardness enhancement for undercooled Ni-47.7%Mo alloy

•A “lamellar-anomalous” transition is observed in undercooled Ni-Mo eutectic.•Eutectic structure is significantly refined by rapid solidification.•The dependence of Vickers hardness upon eutectic spacing is established. Liquid Ni-47.7 % Mo alloy was undercooled by 328 K (0.21 TE) and solidified under microgravity condition to investigate the correlation of microhardness enhancement with eutectic growth mechanism. The microstructural morphology transformed from (Ni) + NiMo lamellar eutectic to a duplex mixture of lamellar and anomalous eutectics, and finally into anomalous eutectic. With the increase of alloy undercooling, the eutectic interphase spacings decreased significantly, leading to a remarkable structural refinement and a conspicuous rise of Vickers hardness. The dependence of microhardness upon eutectic spacing was revealed for lamellar and anomalous eutectics, respectively.