Ceramic materials of the quasi-binary LaB 6 – MoB 2 system

Alloys of the quasi-binary LaB6-MoB2 system synthesized by melting in an atmosphere of helium have been studied. It has been established that the phase diagram of the system is eutectic and the phases do not interact. The eutectic colony crystallizes in form of the platelike eutectic on the basis of the LaB6 crystal of a cubic structure. It has been also shown that the crystallization from melt under conditions of crusibleless zone melting is accompanied by the MoB2 phase decomposes during cooling with precipitation of the submicron Mo2B5 and α-MoB crystallites. This results in an increase of hardness. According to the Mo – B phase diagram, the decomposition temperature of the MoB2 increases with increasing the boron content. In addition, the degree of the decomposition is higher if the melting point of alloy is higher, since the intensity of diffusion process increases at high temperatures.


I. Introduction
Boride-based ceramics are widely used in engineering as wear resistant materials for sputtering and melting of machinery surfaces or fabrication of nozzles for sand blasters.Materials on the basis of lanthanum hexaboride are ideal for cathodes, which provide a strong base for the development of modern electron-beam technologies for sputtering, welding and dimensional processing of metals and alloys.
The principal shortcoming of boride-based materi als as materials for cathodes is low impact toughness, 2-5 MPa•m 1/2 .This can be improved at the expense of reinforcement with fi bers or through synthesis of materials with eutectic composition, which, in general, are considered as natural composites.Since, in work ing conditions cathode is heated with a rate of 1000-1500°C/ min, it is necessary to increase the heat conductivity, strength, and fracture toughness of ca thode materials.Strengthening with the fi bers as well as rods formed in eutectic alloys under conditi ons of directed crystallization provides the best results [1][2][3].However, strength and fracture toughness of such composite materials are infl uenced by the nature of matrix and fi bers of composite materials.Therefore the search of quasi-binary systems with an eutectic character of the phase diagram and a rodlike form of one of the eutectic phase components is an urgent pro blem.Seven quasi-binary systems LaB 6 -MeB 2 where Me -Ti, Zr, Hf, V, Nb, Ta, Cr have been studied by many researchers and reviewed in ref. 4. In the present work the phase diagram, the process of structure formation and properties of the eutectic alloy of the LaB 6 -MoB 2 system have been studied for the fi rst time.

II. Experimental
According to the Mo -B phase diagram [5,6], at high temperatures molybdenum diboride exists as a phase of variable composition, namely MoB 1.9-2.49(66.6 at% B -71.4 at% B).In this work, a diboride of intermediate composition in the homogeneity region namely MoB 2.2 with a melting point of 2230°C was used, since insi gnifi cant changes in its chemical composition cannot affect its properties and process of crystallization of the LaB 6 -MoB 2 alloys.MoB 2.2 was synthesized from mixture of elements: 19.9 mass% (68.8 at%) boron and the rest was molybdenum.Polyvinyl alcohol as a plasticizer was added to the mixture.The prepared mixture was pressed under a pressure of 50 MPa and dried at about 100°C for 5 h.The water evaporated, and polymerization of the polyvinyl alcohol occurred, which led to an increase in the strength of samples.This procedure was also used in the preparation of samples for melting of the LaB 6 -MoB 2.2 alloys.The samples were heated in a vacuum furnace up to 1600-1700°C and held for 1 h.Then they were milled to fractions under 40 μm.The X-ray analysis showed that the product is MoB 2.2 with the following lattice parameters: a.=.0.3032 nm, c.=.0.3096 nm and c/a.=.1.021(for comparison, the parameters of the stoichiometric molybdenum diboride are: a.=.0.3026 nm, c.=.0.3060 nm and c/a.=.1.014).
Ceramic materials of the LaB 6 -MoB 2 system were produced using a method of crucibleless melting of pressed compacts-LaB 6 , MoB 2.2 and their mixtures (with 20, 40, 60, and 80 mass% MoB 2.2 and in the eutectic region 62, 63, and 64 mass% MoB 2.2 ).Before melting all samples were pressed under a pressure of 100 MPa.Temperature was measured by an optic pyrometer with an accuracy of ± 50°C on three samples of each composition.
Upon melting, samples were cut and prepared for metallographic examination.The structure of alloys was studied on an optic microscope Neophot-21.Microhardness and crack resistance were determined using a microhardnessmeter PMT-3 with a Vickers pyramid (load on indentor 1.5 N).X-ray diffraction spectra were obtained on a DRON-3M unit with a graphite monochromator in Cu Kα radiation.

III. Results and Discussion
Lanthanum hexaboride has a cubic structure of the space group O 1 h-Pm3m of the structure type CaB 6 , and molybdenum diboride has a hexagonal structure of the space group D 1 6h-C6/mmm of the structure type AlB 2 .The distances between boron atoms in these compounds differ insignifi cantly.
The peculiarities of changes in the melting point and microstructure of the LaB 6 -MoB 2 alloys of different compositions evidence to an eutectic character of the alloys crystallization (Fig. 1).The starting temperature of melting was evaluated as an average of the corresponding values for three samples.
In the eutectic the content of MoB 2.2 is 63 mass%.The temperature of the eutectic transformation is 2130°C.The phases do not interact.The structure of the alloys is shown in Fig. 2. In the undereutectic alloys the molybdenum diboride phase crystallizes along the boundaries of lanthanum hexaboride cry stals, whereas in the overeutectic alloys it precipitates in the form of cubic crystals and the eutectic has a fi ne-grained structure composed of hexaboride lanthanum and diboride molybdenum layers.
The structure of the alloys can be easily seen from the thin sections of eutectic colonies (Fig. 3).Firstly, one can conclude that the principal part in the formation of eutectic colony is played by the LaB 6 phase, which initiates eutectic crystallization.As seen, eutectic colonies are crystallized on primary single crystals, i.e.LaB 6 cubes.This is natural as this phase has relatively high entropy of melting (468.16kJ/mol compared to 96.14 kJ/mol for MoB 2 ).
Secondly, in order to imagine the process of the eutectic colony crystallization, let us examine the structure shown in Fig. 3, where cross sections of the colonies formed on the basic cubic LaB 6 crystal are presented.It can be seen that there is (Fig. 3a) a square of purple colour, which is a LaB 6 plate.From its corners dark (cherry colour) diagonals originate, which are sections of the eutectic pyramids faces.This indicates that pyramids are separated from each other by LaB 6 plates.So, in the section there are four trapeziums, small bases of which are located on the square sides.The trapeziums have a lamellar structure composed of LaB 6 and MoB 2 plates parallel to the LaB 6 cube faces.The partition of this colony is made by the cube (001) plane.In Fig. 3b another colony can be seen, namely a three-beam star.It is cut by the octahedron (111) plane and, naturally, has the structure of LaB 6 or MoB 2 plates inclined to the pyramid faces with an angle of 45°.In the fi rst case, four trapeziums with a small base on the basic cube crystal plane (001) are dissected, in the second case-three on the (111) plane.
Thus on the basis of examination of the colony structure layers, one can draw a conclusion that the colony is composed of six four-face pyramids on the basic cubic LaB 6 crystal.Herein, the eutectic pyramids are cut by plates of lanthanum hexaboride, which are formed owing to growth of the basic LaB 6 cube edges and at the intersects of the pyramids faces the eutectic of a platelike structure grows.As seen, as a whole, the eutectic colony consists of a two-phased cube and has a structure of the basic crystal with high melting entropy.
The structure of the eutectic LaB 6 -MoB 2 colony exactly corresponds to the structure of the classic eutectic Bi-Sn colony, in which eutectic grows in the form of rodlike-structured pyramids on the basic bismuth rhombohedron crystal [7].Properties of the LaB 6 -MoB 2.2 alloys are consis tent with those of eutectic systems with no phase-phase interaction (Table 1).
As follows from these data, the microhardness of lanthanum hexaboride remains the same in all of the alloys while that of molybdenum diboride changes in under-and overeutectic alloys: the higher the melting point of alloy, the higher its microhardness.Such dependence can be interpreted in the only way: according to the phase diagram Mo-B.Upon melting the MoB 2.2 phase decomposes into MoB 2 and Mo 2 B 5 in the temperature range 2130-1500°C and below 1500°C the eutectic decomposition of the diboride into MoB and Mo 2 B 5 is occurred.The amount of the formed Mo 2 B 5 increases with increasing the melting point of alloy, so in such a case the hardness of the phase (which actually is nonequilibrium since under rapid cooling diffusion processes does not have enough time to be realized) increases as well.This is confi rmed by X-ray diffraction analysis (Fig. 4), which identifi es the phase as a mixture of three phases MoB 2 , Mo 2 B 5 , and MoB, in which the interplane distances practically correspond to the reference ones.None of these phases in a pure state has such a high microhardness (microhardness of the Mo 2 B 5 phase is the highest and equal to 23.5 GPa.

IV. Conclusions
The results obtained indicate that the phase diagram of the LaB 6 -MoB 2.2 alloys has an eutectic character with coordinates of the eutectic point T m eut.K -2130°C, 75 mol % MoB 2.2 .In the ternary La-Mo-B system this is a quasi-binary eutectic form.
The data of metallographic and X-ray analyses evidence to the absence of components dissolution.
The eutectic has a platelike structure.The eutectic colonies grow in the form of four-face pyramids on the (001) planes of the basic LaB 6 crystal.
The microhardness of the LaB 6 phase in the alloys studied does not change, whereas that of the MoB 2.2 phase increases with rising the melting point of the alloys due to its decomposition into submicron Mo 2 B 5 and α-MoB crystallites during rapid cooling under conditions of cruciblelles zone melting.According to the Mo -B phase diagram, the decomposition temperature of MoB 2 increases with increasing the content of boron.The higher the melting point of alloy, the higher the de-gree of decomposition since the intensity of diffusion processes increases at high temperatures.
The microhardness of the alloy of eutectic composition is higher than that of pure phases (similar to uppermost eutectic alloys).