Effect of carbon and boron on the precipitate microstructure and mechanical behavior of the Ni-19Si-3Nb based alloy

The Ni-19Si-3Nb base alloy was microalloyed with different combinations of boron and carbon for modifying its precipitate microstructure as well as improving its ductility. The microstructural evolution in the Ni-19Si-3Nb-xB-yC alloys was characterized by X-ray diffraction, DTA, SEM, Auger analysis, and TEM. The results of X-ray diffraction and DTA could not reveal any phase change in the Ni-19Si-3Nb based alloys with different carbon and boron contents. However, the morphology of the Nb-rich precipitate (which was confirmed to be a cubic Nb₃Ni₂Si phase by selected area diffraction of TEM) changes from plate-like shape to discrete equiaxial shape when the boron and carbon additions increase to 100 ppm or more. Meanwhile, the fracture behavior of the Ni-19Si-3Nb alloy with different carbon and boron additions changes significantly from brittle mode to ductile mode. In addition, the results of Auger analysis revealed that the boron and carbon elements would segregate on the grain boundaries and the interface between Nb-rich precipitates and matrix. The additions of boron and carbon are suggested to improve the adhesion of grain boundaries to the matrix, and to re-modify the shape of the precipitates to resist crack propagation and improve the ductility of the Ni-19Si-3Nb-based alloy.