Interaction in the LaB6-HfB2 system

Conclusions The data of metallographic and x-ray diffraction analyses, determination of the melting point, and measurement of the microhardness of individual phases served as the basis for constructing the phase diagram of the quasibinary eutectic type LaB₆-HfB₂ system with a eutectic composition of 70% LaB₆ 30% HfB₂ and t_eut= 2750°K. Up to the eutectic temperature in the LaB₆-HfB₃ system the components are practically insoluble.The established compatibility of LaB₆ with HfB₂ is an indication of the possibility of the development of sintered compositions with controlled physicomechanical properties.Translated from Poroshkovaya Metallurgiya, No. 2(254), pp. 79–81, February, 1984.     

High Temperature Chemical Reaction of La2O3 in H3BO3 -C System

The high temperature chemical reaction process of La2O3 in H3BO3-C system was studied by means of XRD and TG-DTA.The results showed that dehydration reaction of H3BO3 occurred in the temperature range of 82～390 ℃;La2O3 and B2O3 reacted to form LaB3O6,LaBO3,and B4C in the temperature range of 836～1400℃;at 1450 ℃,B4C and LaBO3 further reacted to form LaB4,and partial LaB4 and B reacted to form LaB6;at 1500 ℃,LaB4 and B reacting into LaB6 was the main reaction,and the content of LaB6 increased with prolonging time.     

Diffusion Interaction in Fiber-Reinforced Composites of the Chromium Alloy – Silicon Carbide System

Diffusion interaction in the Cr – ZrC and Cr – HfC systems has been studied in the temperature range 1200-1400°C. The impurities contained in chromium or carbides were found to affect the interaction in the systems. Almost no interaction was observed when the systems were held at temperatures up to 1300°C for 100 h. At higher temperatures the carbides were reduced to ZrO₂, HfO₂, and Cr₂₃C₆ because of the presence of a slight amount of oxygen. Thermodynamic calculations indicated no interaction in these systems at temperatures up to 1600°C. A study of the interaction in the systems SiC – ZrC and SiC – HfC showed that a transition zone formed already in the stage of sample preparation by diffusion welding in vacuum (1300°C, vacuum of 10^_3 Pa, with a load applied for 20 min). During annealing (1300°C, 50 h) the transition zone stratified, forming a solid solution of silicon in Zr(Hf)C and SiC inclusions in the SiC – Zr(Hf)C solid solution. A transition zone formed on the zirconium carbide side when SiC interacted with Zr(Hf)C. The interaction in SiC – Zr(Hf)C casts doubt on the use of them as a barrier without antidiffusion layers.     

Compactability of powdered copper-lanthanum hexaboride composites under hydrostatic treatment

Conclusions The compactability of powder composites copper-lanthanum hexaboride depends largely on the content of additive. Under hydrostatic pressing of powder mixtures the hard inclusions of LaB₆ hinder plastic deformation of copper particles, and thereby they hinder densification of the composite. In sintering of compacts the additive LaB₆ hinders the increase of volume of the specimens; on the other hand, an increase of content of LaB₆ hinders shrinkage of the composite material. In hydroextrusion of sintered compacts, inclusions of lanthanum hexaboride increase the deformation resistance of the material, and that increases the extrusion pressure of the composite, causing the most intense healing of pores.Almost pore-free powder materials of the system Cu-LaB₆ were obtained by hydrostatic pressing at pressures of 800–1000 MPa, sintering in hydrogen at 950°C for 2 h and subsequent hydroextrusion with reduction =80–90%.Translated from Poroshkovaya Metallurgiya, No. 5(353), pp. 18–22, May, 1992.     

Grain refining potency of LaB6 on aluminum alloy

Al-LaB6 alloy was successfully prepared by aluminum melt reaction method.Microstructure analysis of this alloy was carried out by field emission scanning electron microscopy(FESEM),Raman spectroscopy and transmission electron microscopy(TEM).It was found that cubic LaB6 particles were highly dispersed in aluminum matrix with a uniform edge length of about 4.5 μm.Grain refining potency of LaB6 on commercial pure aluminum was also investigated.It was shown that LaB6 could act as an effective and stable nucleation substrate for α-Al during solidification process,due to their crystallographic similarity.The coarse grains of commercial pure aluminum were obviously refined to small equiaxed ones by addition of 0.5% Al-5LaB6 alloy at 720℃.     

Fundamental Understanding of the Dissolution of Oxide Film on Ti Powder and the Unique Scavenging Feature by LaB<Subscript>6</Subscript>

The surface oxide film on Ti powder consists of TiO₂, Ti₂O₃, and TiO and starts to dissolve into the titanium matrix underneath from ~ 943 K (670 °C). LaB₆ can scavenge O prior to the active dissolution of the surface oxide film. The scavenging process begins by forming an interfacial LaBO₃ layer due to reaction between surface oxide layer and LaB₆, followed by diffusion of oxygen through the LaBO₃ layer. The unique oxygen-scavenging feature of LaB₆ produces better tensile elongation.     

The evaporation of lanthanum hexaboride

Summary A mass spectrometric investigation of the evaporation of lanthanum hexaboride showed that the vapor effusing from a Knudsen cell consists chiefly of atomic lanthanum. The ion current ratio at 2100°K is 1 0.007. The heat of sublimation of LaB₆ at a mean test temperature of 2200°C, determined on the basis of the second law of thermodynamics, is estimated to be 536 ± 8.8 kJ/mole. The emission power of the tungsten cathode is found to increase as lanthanum is vapor-deposited on the latter.     

Structure formation of thin-film resistance composites based on alloyed tin dioxide

The nature of interaction between tin dioxide and some refractory borides during heat treatment of resistive composites in air is studied by differential thermal analysis and x-ray diffraction. The reaction products are β-Sn, metals and boron oxides. Addition of HfB₂, ZrB₂, and LaB₆ to composites based on Sb-doped tin oxide result in a reduction of the film temperature coefficient of resistance. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5-6, pp. 105–110, May–June, 1998.     

Sintering of alloys of zirconium diboride with molybdenum disilicide

Summary The sintering of zirconium diboride with molybdenum disilicide is accompanied by the formation of a solid solution based on zirconium diboride, formation of a liquid phase at temperatures above 1800°C, and partial vaporization of silicon in the ZrB₂+15% MoSi₂ alloy. At temperatures up to 1800°C, solidphase sintering takes place; at low temperatures, this is accompanied by specimen growth due to heterodiffusion processes resulting from the difference in the partial diffusion coefficients of the components and to the vaporization of excess silicon in the case of the ZrB₂+15% MoSi₂ alloy.At temperatures above 1800°C, shrinkage is caused by the formation of a liquid phase, which disappears during sintering. Under these conditions, grain recrystallization and growth in the solid solution of Mo and Si in zirconium diboride in the case of 15% MoSi₂ alloys are not completed even after 4-h holding at temperatures of 1800, 1900, and 2000°C.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 11–16, September, 1966.     

Study of contact interaction between metalline borides and graphite at high temperatures in a vacuum

Summary We studied the contact interaction between compacted boride specimens and boride powders: TiB₂, ZrB₂, HfB₂, NbB₂, TaB₂, Mo₂B₅, and W₂B₅ and graphite powder and graphite at temperatures up to 2200°C in a vacuum. It was established by metallographic and chemical analyses together with microhardness test that the borides of hafnium, niobium, tantalum, and tungsten were stable in contact with graphite right up to 2200°C, and that molybdenum boride was stable up to its melting point, after a contact period of up to 5 h. In the case of titanium boride, we observed a slight interaction with graphite at 2200°, as a result of which the boride loses up to 1% boron, which transfers to the graphite charge. At 2200° after 5 h soaking, clear signs of interaction with graphite are detected in zirconium boride; during contact between compacted boride and graphite powder, the weight of the boride specimen and its microhardness decline, and traces of zirconium and boron are detected in the graphite charge. When compacted graphite is in contact with ZrB₂ powder, we observe a change in the chemical composition of the boride, an increase in the microhardness of the graphite, and a transition layer is detected on the contact boundary.     

Temperature-Dependent Emission of Pr^3＋ -Doped LaB3O6 under Vacuum Ultraviolet Excitation

The vacuum ultraviolet （VUV） luminescent properties of Pr^3＋ -activated LaB3O6 were investigated with highenergetic synchrotron radiation from 20 to 300 K. In the emission spectra, the parity-forbidden 4f^2→4f^2 and parity-allowed 4f5d→4f^2 transitions were observed simultaneously. In addition, it was also observed that the intensity of 4f5d→4f^2 emission bands increased relative to the intensity of 4f^2→4f^2 emissions with increasing temperature. The thermal equilibrium model of energy levels was employed with respect to the lowest 4f5d state and ^1S0 state of LaB3O6：Pr^3＋ , as a result of which the fitted curve had a good agreement with the experiment values, which clarified the physical nature of temperature-dependent emission characteristics of Pr^3＋ in LaB3O6.     

Kinetics and mechanism of oxidation of sialons

Conlusions Oxidation at temperatures of up to 1300°C has no effect on the lattice constants of sialons. At 1000°C crystalline silica layers form on the surfaces of specimens which offer little protection against oxidation. Oxidation at 1300°C leads to the formation of a liquid silicate phase, which increases the oxidation resistance of the materials. However, the oxidation of a material containing more than 6% Al₂O₃ is accompanied by the formation of a blistered, foamed oxide film which readily disintegrates under any mechanical action.Translated from Poroshkovaya Metallurgiya, No. 9(273), pp. 54–58, September, 1985.     

Wetting and the contact reaction in the LaB6-(Cu-Ni) system

Conclusions The wetting of lanthanum hexaboride by copper is accompanied by the establishment of a bond between them which has a chemical character. Raising the temperature markedly improves wetting, full wetting being attained at a temperature of 1770°K. Nickel in the LaB₆-(Cu-Ni) system constitutes a weak interface-active addition. The chemical reaction of Cu+5% Ni, Cu+10% Ni, and Cu+15% Ni alloys with lanthanum hexaboride takes place with the formation of a new phase enriched in nickel.Translated from Poroshkovaya Metallurgiya, No. 7(223), pp. 68–72, July, 1981.     

Laser processing of LaB6-based thick film resistors

The paper studies the effect of laser processing on the structure and electrophysical properties of LaB₆-based thick film resistors (TFR). The phases and structure of the composition are investigated. The exposure to micro-and nanosecond pulses, unlike millisecond ones, is shown to insignificantly increase the electrical resistance of the film and to aid in forming more uniform and fine TFR structure. LaB₆-based TFRs are more resistant to long-term exposure to nanosecond laser radiation than BaB₆-and (BaB₆-LaB₆)-based ones.     

Phase Equilibria in the Pr - Re - B System

X-ray diffraction methods have been applied to the component interaction in the system Pr - Re - B. Isothermal sections of the phase diagram have been constructed for 600°C (region of more than 33 at.% Pr) and 800°C (region less than 33 at.% Pr). The previously known borides Pr₂Re₃B₆, Pr₈Re_13–x B₁₂, Pr₇(Re₄B₄)₆ are confirmed, which have specific structural types. A new ternary compound has been found, Pr₃ReB₆ (structure not determined).__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 46–51, January–February, 2005.     

A New Grain Refiner for Ferritic Steels

A new grain refiner, LaB₆, was identified for ferritic steels based on the crystallographic calculation using the edge-to-edge matching model. Addition of 0.5 wt pct LaB₆ led to a reduction of the average grain size from 765 to 92 μm and the proportion of the columnar structure from 35 to 8 pct in an as-cast Fe-4Si ferritic alloy. Although LaB₆ was supposed to act as an active inoculant for δ-ferrite, thermodynamic calculation indicated that LaB₆ is not thermodynamically stable in the melt of the Fe-4Si alloy. It was subject to decompose into La and B solutes. Consequently, both La and B reacted with Fe, O and S, forming different compounds. Microstructural examination at room temperature observed La₂SO₂ and La₂O₃ particles within the ferrite grains and Fe₂B along the grain boundaries in the samples. Through EBSD analysis, a reproducible orientation relationship between ferrite and La₂SO₂ was identified. In addition, the edge-to-edge matching calculation also predicted the high potency for La₂SO₂ to be an effective nucleant for δ-ferrite. It was considered that the grain refinement of LaB₆ was attributed to the enhanced heterogeneous nucleation of δ-ferrite by La₂SO₂, and the solute effect of B due to the high Q-value in ferrite.     

Emissive electric-spark coatings based on lanthanum hexaboride

Conclusions The electric-spark alloying of metals and steels with lanthanum hexaboride and its alloys results in the formation of heterophase coatings whose principal phase constituent is a lower boride of the basis metal. The coatings contain also a small amount of a LaB₆-base phase, whose presence ensures that their level of emission is close to that of LaB₆ itself. By varying the composition of the alloy and using different basis materials, it is possible to exercise control over the efficiency of deposition of coatings and their thermionic emission properties.Translated from Poroshkovaya Metallurgiya, No. 2(254), pp. 50–53, February, 1984.     

High-temperature x-ray equipment for the study of powder and dense objects

Conclusions The existing experimental models of both Debye cameras and camera attachments for x-ray diffraction apparatus enable the atomic and crystal structure of metals and alloys to be studied on heating to very high temperatures (occasionally of the order of 3000°C) in a vacuum of 10^–6 torr. The camera attachments currently produced by Soviet industry are suitable for operation at temperatures up to1500°C in a vacuum of about 10^–4 torr.It is now necessary to organize production in the Soviet Union of attachments with a temperature limit of 2000°C in a vacuum of 10^–6 to 10^–7 torr and high-temperature vacuum Debye cameras designed for temperatures up to 1700°C and a vacuum of 10^–5 to 10^–5 torr.Translated from Poroshkovaya Metallurgiya, No. 5(89), pp. 88–102, May, 1970.