Binary systems, borides

The sintering of boride-metal composites cannot be developed here, although it allows obtaining fully dense parts. For a review, limited to MB —M pseudo-binary systems containing more than SO vol% boride and excluding infiltrated borides, see ref. 1, 6.7.5.1.4. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Metal organic chemical vapor deposition (MOCVD) is a well-established, practical technique for forming simple as well as complex solid state films (130). For binary systems the conventional approach is to use mixtures of the most readily available molecules containing the elements of interest. This approach has been employed to prepare borides of several types. For example, iron-boron alloys have been pre-... 

The present status of information about phase equilibria and the formation of borides is summarized in Table 2 (binary borides) and in Figure 2a, b (ternary systems) ". ... 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

J The concept of counter-phases. When a stable compound penetrates from a binary into a ternary system, it may extend right across the system or exhibit only limited solubility for the third element. In the latter case, any characterisation also requires thermodynamic parameters to be available for the equivalent metastable compound in one of the other binaries. These are known as counter-phases. Figure 6.16 shows an isothermal section across the Fe-Mo-B system (Pan 1992) which involves such extensions for the binary borides. In the absence of any other guide-... 

Before the present work, insufficient data were available in the literature for phase equilibria of systems based on Ti-B. There is a large body of data in literature on the properties of titanium-boride materials, but there is no systematic information regarding the specific role(s) of candidate alloying elements. Thus, the present effort was undertaken to explore the role of alloying additions through the study of phase equilibria and alloy properties, as well as their interrelation, from the binary Ti-B system to the Ti-rich portions of ternaries Ti-Al-B and Ti-B-X, further to quaternaries Ti-10 at.% Al-B-X (where X = Si, Ge, Sn, Zr, V, or Nb) and to some multi-component alloys. 

The occurrence of the binary borides of the alkaline, alkaline earth, aluminum, and transition elements has been collected in Table 1, together with boron compounds of the right main group elements (carbides, etc.). Only relatively well-established phases have been included. Noncorroborated and/or badly characterized borides lacking precise composition and structure data are not included. The reader is referred to other sources for references. There are no binary borides among the Cu, Zn, Ga, and Ge group elements with the exception of a noncorroborated early report on diborides in the Ag-B and Au-B systems. Two silicon borides have been established, namely, SiB3 4 and SiBe. 

Figure 9. (a) The binary B-Si system according to experimental results with powders of technical purity. Homogeneity fields of the silicon borides according to Ettmayer et al. [76], Lugscheider et al. [77], Armas et al. 1981 [82], (b) The binary B-Si system according to recent calculations by Lim and Lukas [52],... 

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. 

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... 

The phase equilibria in the ternary systems Fe-B-N, Co-B-N, and Ni-B-N have been determined from X-ray powder diffraction data and isothermal sections at 900°C and 1 atm Ar have been constructed. In all three systems the binary transition metal borides form two-phase equilibria with a-BN [28]. The phase equilibria are depicted in Fig. 4-29 to 4-31. 

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