Solute redistribution and micromechanical properties of rapidly solidified multicomponent Ni-based alloys

The solute trapping effect and microhardness enhancement of quaternary Ni-5%Cu-5%Ag-5%Sn and quinary Ni-5%Cu-5%Ag-5%Sn-5%Ge alloys during rapid dendritic solidification are investigated by glass fluxing technique. In these two alloys, the experimental maximum undercoolings of 310 K (0.18T_L) and 220 K (0.13T_L) have been achieved and the rapidly solidified microstructures are composed of α(Ni) and (Ag) solid solution phases. The morphological transition from coarse dendrite into equiaxed structure is observed for α(Ni) phase with the increase of undercooling. The dendritic growth velocity of α(Ni) phase in quaternary Ni-5%Cu-5%Ag-5%Sn alloy is larger than that in quinary Ni-5%Cu-5%Ag-5%Sn-5%Ge alloy, which increases firstly and then decreases with the enhancement of bulk undercooling. The Vickers hardness of α(Ni) phase in these two alloys is enhanced with the increase of undercooling, which is attributed mainly to the grain refinement effect. Meanwhile, the solute trapping effect of Cu, Sn and Ge elements in the α(Ni) phase also contributes to the microhardness enhancement under large undercoolings. The addition of Ge element effectively increases the microhardness of α(Ni) phase due to solute strengthening mechanism.