UNDERCOOLING AND HETEROGENEOUS NUCLEATION IN IRON-NIOBIUM AND NICKEL-NIOBIUM ALLOYS

研究了非均质物质对铁铌和镍铌二元合金过冷度和凝固组织的影响。在该实验中使用氧化铝坩埚和覆盖有商业玻璃的坩埚。在Fe-Nb合金中加热至1823 K和在Ni-Nb合金中加热至1173 K后，将样品保持1小时，并分别在1723 K和1673 K下淬火，以保持刚凝固后的显微组织。在二元合金中加入1mass%的碳制成的三元合金也进行了实验。所得结果总结如下。（1）Fe-Nb和Ni-Nb合金的过冷度均有所增加，在2mass%Nb时达到15 K，在1mass%Nb时达到80 K。两种合金的过冷度均随Nb含量的增加而减小。Ni-Nb合金的过冷度总是大于Fe-Nb合金。（2）采用玻璃坩埚时，过冷度增大，Ni-Nb合金的过冷度可达120 K。（3）由于碳化物的形成，三元合金的过冷度比二元合金的过冷度低。即使采用玻璃坩埚，三元合金的过冷度也有所降低。（4）合金从枝晶形态到晶粒结构的形成过程与合金的溶剂不同。也就是说，Ni基合金中的晶粒形成和Fe基合金中的枝晶形成容易发生。结果表明，坩埚壁主要起非均质作用，而熔体中形成的氧化物或碳化物也起非均质作用。另外，由于Fe氧化物比Ni氧化物更容易形成，Ni基合金的过冷度总是大于Fe基合金。

The effects of heterogeneous substances on undercooling and solidified microstructure were investigated for Iron-niobium and Nickel-niobium binary alloys. An alumina crucible and a crucible covered with commercial glass were used in this experiment. The samples were kept for 1 hr after heating up to 1823K in the Fe-Nb alloy and 1173K in the Ni-Nb alloy and quenched at 1723K and 1673K, respectively, to keep microstructures just after solidification. The experiment on the ternary alloys made by adding 1mass% carbon to the binary alloys was also done. The results obtained are summarized as follows.(1)The undercooling of the Fe-Nb and Ni-Nb alloys increased and reached to 15K at 2mass%Nb and 80K at 1mass%Nb, respectively. The undercooling of both alloys decreases with increasing Nb concentration. The under cooling of the Ni-Nb alloy is always larger than that of the Fe-Nb alloy. (2)When the glass covered crucible was used, the undercooling became larger and it reached to 120K in the case of Ni-Nb alloy. (3)The undercooling of the ternary alloys became lower than those of binary alloy because of carbide formation. The undercooling in the ternary alloys decreased even if glass covered crucible was used. (4)The formation process from dendrite morphology to grain structure was different from the solvent of the alloy. That is, the grain formation in the Ni-base and dendrite formation in the Fe-base alloys are easy to occur. It is concluded that crucible wall mostly acts as heterogeneous materials and subsequently the oxide or carbide formed in the melt also acts as the heterogeneous substances. In addition, the undercooling of the Ni-base alloy was always larger than that of Fe-base alloys because Fe oxide is easy to form compared with Ni oxide.