Rapid solidification and physical properties of a refractory Mo-Ni alloy under containerless microgravity condition

Abstract

<p indent="0mm">The solidification kinetics and physical properties of Mo-48%Ni alloy were studied using the free fall technique. The cooling rate and undercooling of this alloy both increased exponentially as the droplet diameter decreased, and they could reach <sc>2.41×10⁴ K s⁻¹</sc> and <sc>322 K</sc> (0.19 <italic>T</italic>_L), respectively. The high cooling rate significantly suppressed the subsequent solid state transformation process. The primary NiMo phase exhibited a morphological transition from coarse dendrite to a remarkably refined structure, and the regular lamellar (Ni+NiMo) eutectics changed into anomalous eutectic. The maximum length of the primary NiMo phase fell from 142.2 to <sc>28.4 μm</sc>, whereas its volume fraction initially rose and then fell, with a maximum of 60.1%. Moreover, the variation in the Mo content of the primary phase coincided well with that of the eutectic structure. As regards mechanical properties, the nanoindentation measurements indicated that the elastic moduli of the primary phase and the eutectic structure both showed a downward trend along with the microstructure evolution. The eutectic hardness decreased monotonously from 14.1 to <sc>8.7 GPa</sc>, whereas the primary phase hardness increased slightly from 8.0 to <sc>8.5 GPa</sc> and finally fell to <sc>5.4 GPa</sc>. In addition, the growth morphology of the (Ni) solid solution within the eutectic structure was shown to have an obvious effect on the magnetic properties of the Mo-48%Ni alloy. As the rate of cooling increased, the saturation magnetization and residual magnetization diminished. The alloy’s coercivity increased from 11.8 to <sc>25.7 A m⁻¹</sc>, indicating enhancement of the temperature resistance induced by microstructure refinement. The rapid solidification under space simulation conditions affected the mechanical and magnetic properties of the Mo-48%Ni alloy to a certain degree.