Carbon – Molybdenum – Tungsten

Abstract

The C-Mo-W system is of interest with respect to the development of hard alloys for various tools consisting of carbides in Co or Ni matrix and also for hard coatings. In the first work [1936Mol] the solid solutions between the binary carbides Mo2C and WC were found out indicating a possible interaction between the compounds of the C-Mo and C-W systems. Interaction between the Mo and W carbides was confirmed then in [1943Kie, 1950Daw]. In [1956Alb] the isothermal section of the C-Mo-W phase diagram at 1710°C was constructed using X-ray diffraction method for the samples obtained by the powder metallurgy technology. The same technology for the alloy preparation was often used in the following investigations. [1969Fri] tried to construct liquidus along a section from the C-Mo to the C-W, but the data turned out to be doubtful because the C amounts in the alloys and the carbide types remained unknown. [1970Gor] managed to establish the joint solubility of W and C in solid (Mo) and Mo and C in solid (W) at 1000-2000°C. In further work [1973Gor] the partial isothermal section at 1000°C was constructed. The work indicated a possibility of the extended solid solutions on the basis of the carbides containing approximately the same contents of C. Simultaneously the ordering phenomenon was established in the alloys in the solid state. However, the isothermal section [1973Gor] did not taken into account the existence of a number of the binary compounds. The extended solid solutions between carbides were shown by [1976Sch] for the MoC and WC carbides and by [1979Sch, 1989Tru] for the MoC1–x-WC1–x and Mo2C-W2C carbides. The formation of the extended and continuous solid solutions along the section MoC0.65-WC0.62 was indicated experimentally by [1984Ere]. The MoC0.65 and WC0.62 carbides coincide actually with the carbides MoC1–x and WC1–x of [1979Sch]. [1978Rud] presented two vertical sections of the C-Mo-W phase diagram along the sections MoC-WC and MoC0.495-W0.495 indicating a rather complicated constitution of the system and a possibility of the phase transformations during heat treatment. The vertical section MoC0.65-WC0.62 was constructed also by [1984Ere]. Certain fragments of the C-Mo-W phase diagram were calculated thermodynamically [1963Rud, 1978Thr, 1988Gus]. In [1963Rud] the higher stability of Mo2C, as compared with W2C, was established. Besides, the tie lines in the two-phase region between the continuous solid solutions Mo-W and Mo2C-W2C at 1527°C were constructed. [1978Thr] calculated isothermal sections of the phase diagram at seven temperatures from 1227 to 2227°C indicating extensions of the solutions and the tie-lines in the two-phase regions. However, in the calculated isothermal sections [1978Thr] some phases corresponding to the modern binary diagrams were missed. [1988Gus] calculated a number of the isothermal sections: at 1500, 2000 and 2500°C, isopleths for 37, 42 and 50 at.% C and a partial projection of the liquidus surface (up to ~50 at.% C). The isopleths agreed well with the experimental data after thermal analysis presented by [1988Gus]. In addition, some invariant equilibrium reactions in the system were calculated by [1988Gus]. Their temperatures are in good agreement with experimental data. The existing reviews on the C-Mo-W phase diagram [1961Eng, 1974Vel, 1976Jen, 1981Hol, 1984Hol, 1990Ere, 1994Mch] reproduce its versions in accordance with the publications appeared in the successive years. In [1961Eng] the isothermal section at 1710°C is presented according to [1956Alb]. In [1974Vel] projection of the solidus surface of the phase diagram is suggested based on the existing data by this time. In the solidus surface the continuous solid solutions Mo-W, Mo2C-W2C and MoC1–x-WC1–x are indicated. In [1990Ere] the projection of the liquidus surface and two polythermal sections, MoC0.65-WC0.62, MoC-WC, are reproduced from the later publications. Along with this, based on the published data the reaction scheme in the C-Mo-W system is presented by [1990Ere]. In the review [1994Mch] the calculated isothermal sections from [1978Thr] are reproduced without changes.