Ternary composites, borides

Uncharacterized ternary and binary metal borides, lacking precise composition or crystal chemistry data (see Fig. 2, Table 2), are not covered in this context. Further-... 

Fig. 18. The Hf-B-C isothermal ternary cross sections at three different temperatures. Points A and B represent two possible stoichiometries for processing hafnium boride/hafnium carbide composites. After Rudy [57],...

G.16 W. B. Pearson. A Handbook of Lattice Spacings and Structures of Metals and Alloys (New York Pergamon Press, 1958). A most useful source of information. Gives the crystal structures of intermediate phases, and the variation of lattice parameter with composition in solid solutions, of binary and ternary alloys. Also gives the crystal structures of metal borides, carbides, hydrides, nitrides, and binary oxides. 

It is very often difficult to establish the existence and composition of borides owing to one or more of the following reasons. The great reactivity of boron toward, for instance, the crucible material leads to the occurrence of impurities in the products. The impurities are often said to stabilize a structure type, while in reality a ternary compound has been formed although unintentionally. It is also difficult to analyze chemically or by microprobe the boron content owing to low accuracy of the methods. Indeed, in many cases a single-crystal-structure determination is, together with complementary structural information obtained by other... 

Fligh-temperature equilibria of the extraordinarily hard borides with metallic melts bring about the opportunity for a pressureless liquid phase sintering and the fabrication of hard and simultaneously tough composites similar to hard metals but are also of interest in ceramic systems or coatings. In this section emphasis is put on binary and ternary borides which are in equilibrium with transition metals. 

Cemented borides with a metallic matrix have also been fabricated successfully from the ternary transition metal borides of so-called x-, ip- and ca-types since these composites can easily be liquid phase sintered with metallic melts. 

In a similar multiphase system, transition metal carbides were used as additives for pressureless sintering of TiB2, yielding composites of binary and ternary borides [218,296]. Attrition milled powder mixtures of TiB2 with 3-10 mass-% Co or Ni and 20-35 mass-% WC have been sintered in a vacuum at temperatures between... 

As discussed before, hard metal-like composites can be prepared by pressureless sintering of ternary borides with Fe, Ni, or Co melts. Materials with x-phase (M21M2 Bg, where = Fe, Ni, or Co, and M = Zr, Hf, Nb, Ta, or W with M as the matrix phase) have not been developed for technical use but Ni-based alloys with x are in applications as wear- and corrosion-resistant coatings on steels [355]. The x phase is also used for the improvement of the creep resistance of Ni-based superalloys. 

The isothermal section of Sc-Ni-B at 800°C has been established by Stepanchikova and Kuz ma (1981). The occurrence of an earlier known ternary compound, Sc3Ni2oB6 has been confirmed and four new ternary borides have been added (table 14). Zavalii et al. (1988) used a single crystal investigation for the refinement of the composition of the ScNi4B4 compound reported by Stepanchikova and Kuz ma. According to the results of... 

While most of the binary carbides and nitrides considered above form unlimited homogeneous solid solutions, some other 5 and p elements (B, Be, Al, Mg, etc.) have only a low solubility in these phases. As their content in carbides and nitrides increases, ternary compounds with very specific crystal structures are formed which were reviewed by Alyamovsky et al (1981) and Goldschmidt (1967). It is well known (see Samsonov, Serebryakov and Neronov (1975)) that B or transition metal borides do not form unlimited solid solutions, when interacting with MX phases (X = C, N) and single-phase TiNjBj, compounds exist over a narrow composition range for example, when z + y = 0.62-0.94, y < 0.03 (see Alyamovsky, Zainulin and Shveikin (1976)). As the B/N ratio increases. 

Quaternary boro-nitrides. The well-known magnetic properties of R2T14B ternary borides (T=transition metal) are typified by the neodymium iron compound Nd[2Fei4B, discovered in 1983. Several studies aimed at introducing interstitial nitrogen in the composition of such permanent-magnet materials in order to improve their characteristics, in particular the Curie temperature. 

---