The influence of carbon on the interdiffusion of Mo and Ni
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Duar~Gour solid-state
diffusion investigation into the binary Mo-Nt system we have come a c r o s s the important changes in diffusion behavior which are caused by small amounts (0.06 to 0.4 wt pct) of carbon present In the nickel. Totally different diffusion zones were built up at some temperatures, and two-phase diffusion layers often occurred. It turned out that the ~?-carbtdes (Mo, Nt)eC and (Mo, Nt)zaC were formed. These carbides are also v e r y important as precipitates in many heatresistant Nt-based superalloys, which contain roughly equal amounts of carbon. We have, therefore, Investigated in more detail the relevant part of the t e r n a r y phase diagram Mo-NI-C by the diffusion couple technique to get more insight in the kinetics of carbide f o r mation. Data about diffusion in this kind of systems are lacking in the literature. The general features of this study can be extended to systems as (Ni, Fe, Co)W-C and Co-Mo-C with, of course, appropriate differences. Fig. 1 shows an isothermal section at 1000~ of the Mo-Ni-C system according to Fraker and Stadelmaier. 1 Fraker et al and Ettmayer and Suchentrunk 2 investigated the phase boundaries of stable ~}-carbtdes in the Mo-NI-C system. There are two 7}-carbide phases which are basically (Mo, Nt)eC and (Mo, Ni)~aC. MosNi~C and Mo4NiaC are the boundary formulas of the MaC c a r bide which is saturated with Ni and Mo, respectively. So the metal ratio in M6C varies widely but there is only a small variation in the carbon content. The 7/carbides belong to the space group Fd3m, TiaNt-type , having 96 metal and 16 carbon atoms in a fcc lattice, with variable cubic spacings. The structure consists of octahedral and tetrahedral units. The carbon occupy the centers of slightly distorted Mo octahedra. Lectejewisz 3 has studied the (Mo, Ni)z~C compound with neutron diffraction and found as its space group again Fd3m. The possibility of substitution of Mo by W and of Nt by Co and Fe clearly exists, similarly that of C by N and O for corresponding variation of metal ratios. F r a k e r et al 1 have given the variation in lattice p a r a m e t e r s with the composition of the ~/-carbides (Table I). C. P. HEIJWEGEN and G. D. RIECK are Scientist, Laboratory for Physical Chemistry, and Professor in Physical Chemistry, respectively, University of Technology, Eindhoven, Netherlands. Manuscript submitted December 19, 1972. METALLURGICAL TRANSACTIONS
Mo
20
Mo2C 40
60
At.~ C Fig. 1--Mo-Ni-C, i s o t h e r m a l section at 10000C, according to F r a k e r and Stadetmaier. 1
EXPERIMENTAL P R O C E D U R E S Mo-Ni diffusion couples were prepared from highpurity molybdenum sheet and ~ in. NI rod. Spectrographic anatysls of the Ni rod indicated the following impurity levels (in ppm), (the carbon content is determined by a conductimetric technique)* Si: 40, AI: 40, *The analysesare performedby the AnalyticalLaboratory ofN. V. Philips, Eindhoven,Netherlands.
Fe: 250, Mg: 10, Cr: 150, Cu: 10, O: 20, C: 600 ppm. The Mo sheet had a thickness of 0.5 mm and a purity of 99.99 pct.
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