Rapid solidification kinetics and mechanical property characteristics of Ni–Zr eutectic alloys processed under electromagnetic levitation state

Abstract

The rapid solidification mechanism of Ni–Zr alloys was investigated by electromagnetic levitation (EML) technique assisted with high speed videography. A systematic analysis of the competitive growth mode between the primary phase and eutectic structure was conducted for three types of alloys (hypoeutectic Ni-5 at.%Zr, eutectic Ni-8.8 at.%Zr and hypereutectic Ni-13 at.%Zr alloys), whose maximum undercoolings reached up to 260, 192 and 270 K, respectively. With the increase of undercooling, the primary (Ni) phase of hypoeutectic Ni-5 at.% Zr alloy transferred from well-defined dendrites to dendrite fragments. Zr atom was difficult to diffuse into the Ni lattice because the radius of Zr atom was larger than Ni atom. For eutectic Ni-8.8 at.% Zr alloy, the competitive nucleation and growth between primary (Ni) phase and eutectics could happen when the undercooling exceeded 18 K. The refined dendrites of primary (Ni) phase and various eutectic morphologies were observed at the undercooling of 192 K. For the hypereutectic Ni-13 at.% Zr alloy, the growth morphology transition of the primary Ni5Zr intermetallic compound from faceted to non-faceted crystals occurred if the liquid alloy achieved a high undercooling. Furthermore, the atomic scale structure of the primary phase was explored by the transmission electron microscopy (TEM), which revealed that the solid solubility and the lattice constant were consistent at various undercoolings. Moreover, the compressive performance of these three type alloys were determined for EML solidified samples. Due to the effects of undercooling and composition on microstructures, Ni-5 at.%Zr and Ni-8.8 at.%Zr alloys exhibited plastic deformation at the large strain condition, while Ni-13 at.%Zr alloy showed brittle fracture feature. The compressive strength and yield strength displayed a non-linear relationship with the rise of undercooling.