Solid-state interdiffusion in Ni-Mo diffusion couples at high temperatures

Abstract

According to the present study, two-layered diffusion zones formed in the Ni-Mo bulk diffusion couples after annealings for various periods of time at 1250 and 1300 °C. One of the layered diffusion products formed nearby tihe MO component was determined to be γ-NiMo. The other one formed adjacent to the Ni component was identified to be Ni 3Mo. The growth rate of the layered reaction products followed a parabolic relationship with annealing time. The growth rates of γ-NiMo and Ni 3Mo at 1250 °C were determined to be 2.7 x 10 −11 and 3.6 x 10 −11 cm 2/sec, respectively. At 13OO °C, the growth rate of γ-NiMo was calculated tso be 4.5 x 10 −11 cm 2/sec. These layered reaction products were separated from each other and from the MO as well as the Ni terminal solid solution by a number of phase boundaries. The phase boundary separating the γ-NiMo phase from the MO terminal solid solution was wavy in configuration, suggesting that the nucleation/ growth of γ-NiMo phase might be heterogeneous in nature and possibly limited by short-circuit diffusion through structural defects. As a comparison, most other phase boundaries were present with linear and sharp-contrasted contours. Exceptional case was observed in the phase boundary formed at 13OO °c, which separated the Ni 3Mo layer from the Ni terminal solid solution. At this very phase boundary, extensive aggregation of voids was noted to form a banded, spongy structure. The formation of voids is attributable to the Kirkendall effect resulting from the faster diffusion of Ni species, which in turn gave rise to a reverse vacancy flux diffusing toward the Ni component. Significant interfacial melting was effected in the diffusion couples after annealing at 1300 °C. The extent of interfacial melting increased with the duration of annealing time. The inter-facial molten-metal solution solidified into a dual phase microstructure consisting of γ-NiMo and Ni 3Mo. The γ-NiMo phase was present as isolated columnar islands dispersed in a continuous matrix of Ni 3Mo. Annealing at 1350 °C resulted in overwhelming interfacial melting and complete consumption of the Ni component. Upon solidification, the molten metal solution turned into a button-like structure consisting of a typical solidified dendritic microstructure.