2006 Volume 46 Issue 12 Pages 1871-1876
The changes in degree of undercooling, secondary dendrite arm spacing and microstructure were investigated by using three cooling methods. The Fe–Nb binary alloys and Fe–0.1mass%C–Nb ternary alloys having various niobium concentrations ranging from 0.50 to 6 mass% were used. The Fe–C–Nb ternary alloys were used to investigate the role of niobiumcarbide (NbC) as the heterogeneous nucleation site. The degree of undercooling in both alloys changed with niobium concentration and that in the ternary alloy was always smaller than that in the binary alloys because of the presence of NbC. Dendritic microstructures were observed in both alloys quenched from 1 673 K after cooling at a rate of 0.028 K/s when Nb concentration exceeds 2 mass%. An Fe–0.1mass%C–3.0mass%Nb alloy air-cooled from 1 473 K also exhibited the dendritic microstructure, while an Fe–3.0%Nb alloy similarly air-cooled did not exhibit the dendritic microstructure. Then, grain boundary in the Fe–0.1mass%C–3.0mass%Nb alloy was formed between the arrays of dendrites, while it wasn't formed between secondary dendrite arms. Furthermore, the microstructure in an Fe–0.10%C–1.0%Nb alloy which was air-cooled from liquid state exhibited fine dendrite microstructure.
