Phase transformations of disordered structures of graphite-like boron nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.     

Shock Synthesis of Ternary Diamond-Like Phases in the B ― C ― N System

Nanodispersed powders of the ternary graphite-like phases BC_1.28N and BC_2.14N have been used along with mixtures of powders of turbostratic BN and C in high-temperature shock compression followed by sharp quenching. The yield of diamond-like phases has attained 50% by volume. The excess of the graphite-like phase has been removed by treatment with molten alkalis containing nitrates and then with concentrated HClO₄. The shock and chemical treatments alter the compositions of the diamond-like phases towards BCN in both of the BC_xN specimens on account of the segregation of the excess carbon followed by the dissolution of it. Precision lattice-parameter measurements and line shape analysis indicate that the diamond-like phase is a BN ― C substitutional solid solution. Treatment of a mechanical mixture of the graphite-like phases of carbon and BN under the same p and T conditions led to the formation of a mixture of cubic boron nitride and diamond.     

Explosion energy in the synthesis of superhard phases

This article reviews works of the Institute for Problems of Materials Science evoted to the application of explosion energy for the synthesis of superhard phases (SHPs) based on phase transformations of layered structures of carbon and boron nitride at high pressures. Major attention is given to the development and application of the high-temperature shock compression method, which made it possible to synthesize a new structural form of carbon and to attain, for the first time, a 70% yield of cubic BN under shock compression. The diagram of BN phase transformations by different mechanisms is discussed, which demonstrates the dependence of the phase composition of shock compression products on the ordering degree of the starting graphite-like structure. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 3–10, 2007.     

Structure formation patterns in carbamide synthesis of nanocrystalline graphite-like boron nitride

We have used chemical analysis, x-ray diffractometry, transmission and scanning electron microscopy, and also measurement of the specific surface area to study the characteristic patterns of nitriding and structure formation in graphite-like BN powders synthesized by the carbamide method under various conditions. We have studied the effect on the synthesis process from the ratio of boric acid to carbamide in the starting mixture, preliminary grinding of the charge, the nitriding temperature and time. We show that the major factor determining the composition and structure of the product synthesized at 900–1200 °C is the ratio of the starting components. Carbon-free BN of stoichiometric composition is formed for a 1: 3 ratio of boric acid to carbamide as a result of two-hour nitriding at 1200 °C. The graphite-like BN obtained is characterized by a turbostratic nanocrystalline structure with a specific surface area of 100 m²/g and high nanoporosity of the powder particles.     

Structural studies of ceramics based on detonation-wave-treated graphite-like boron nitride

The formation of a structure during pressureless sintering of graphite-like boron nitride (BN) powders pretreated in detonation waves was studied. The treated powders contained 10–12% wurtzite BN phase and 2–3% sphalerite phase. During sintering at just 1200 °C a reverse transformation from dense BN phases to graphite-like phases and primary recrystallization of highly fragmented BN_g occur as a result of the pretreatment. Accretive recrystallization of BN_g begins at 1500 °C and at T=1700–1950 °C platy grains with a size of 1–3 µm are formed in the developed surface. At 1700 °C the density of the specimens reach 0.95 of the theoretical value. The specimens produced at 1950 °C have a higher compressive strength and radiation resistance than do those made without pretreatment.Deceased.Institute of Materials Science, Ukrainian Academy of Science, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5/6, pp. 75–80, May–June, 1995.     

Development of a Method for High-Temperature Shock Compression of Powder Mixtures to Obtain Cubic Boron Nitride

We used shock compression of a mixture of boron nitride powder with an alkali halide salt as an additive to realize the phase transformation of turbostratic BN to the cubic modification. We have experimentally studied the effect of the type and amount of the additive and also the initial density of the mixture on the yield of the cubic phase of BN. We have roughly calculated the pressure and temperature arising upon shock compression of powder mixtures in an annular cylindrical storage ampul. We have shown that melting of the additive has a substantial effect on development of the phase transformation in boron nitride under shock loading conditions. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 107–118, July–August, 2005.     

Reduction Extraction Method for Determining the Enthalpies of Decomposition Reactions of Different Boron Nitride Modifications

Pulsed (7–10 sec) reduction extraction in a helium gas carrier stream has been used for the first time to determine the standard enthalpies and the internal energies of thermal dissociation reactions for various modifications of boron nitride (BN) with and without carbon. The temperature dependences of the weight content of nitrogen extracted from nanosized (4–10 nm) and microsized (<1; 4–5 μm) BN powders in the range 2100–4000 K with a step of ~200 ± 25 K have been established. Nitrogen extraction from layered BN modifications (graphene and graphite structures) begins at 2400–2500 K and reaches 100% at 3500 K. The thermal decomposition curve for nanoand micropowders of dense BN modifications (wurtzite and sphalerite structures) is similar to that for the layered modifications and is shifted toward the high-temperature region by 900 K. The standard enthalpies of decomposition reactions for different BN modifications have been determined: 231 kJ/mole for graphene-like, 249 kJ/mole for graphite-like, 296 kJ/mole forwurtzite-like, and 391 kJ/mole for sphalerite-like modifications.     

Nature of the real structure of graphite-like BN and its transformation into a wurtsite modification under impact compression

The dependence of the interlayer distance d on the degree of unidimensional disorder γ in graphite-like BN was determined. It was established that d=0.33273±0.00005 nm at 20°C in the perfectly ordered structure (γ=0); the value of d increases uniformly with increase of γ, and at γ=1 it equals 0.343±0.001 nm. It was also established that the yield of wurtsite phase produced by impact compression of the graphite-like modification sharply decreases with increase of γ, and for γ>0.2 it becomes equal to zero. This result is explained in terms of the nature of martensitic transformations in layered structures.     

Texture Evolution in Graphite-Like Phases Recrystallizing under External Stress

It has been observed that the texture axis of graphite-like boron nitride is reoriented by recrystallization under pressure. The observed effect is due to the compression conditions for the anisotropic phase deviating from hydrostatic.     

The structural aspect of high-pressure superhard phase synthesis

Structural aspects are considered for the direct phase transitions in carbon and boron nitride at high pressures from the viewpoint of martensite and diffusion transitions. The mechanism for the transitions of graphite and graphite-type BN into superhardphases is controlled primarily by the crystalline perfection of the initial structures that show martensite transformations to metastable phases (lonsdaleite and BN wurtzite allotrope), while highly defective ones show diffusion transformation to high-pressure stable phases (diamond and cubic boron nitride). The perfection in the initial structure has a very marked effect on the transformation mechanism during shock compression, which is the main technique in the commercial production of superhard phases.Institute of Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7/8(380), pp. 83–92, July–August, 1995.     

Elastic constants and elasticity moduli of cubic and wurtzitic boron nitride

Elasticity moduli and elastic constants for sphaleritic and wurtzitic boron nitride are calculated and determined by experiment. Numerical values are given for single crystals of these materials and polycrystals of hexanite-R, elbor-R, belbor, kiborite, and other superhard materials. An approach is suggested for combining hot pressing at ultrahigh pressures and a pulsed ultrasonic technique which may be used to determine the elasticity modulus and elastic constants of the majority of refractory compounds which are difficult to determine by other techniques.Institute of Materials Science, Ukrainian Academy of Sciences, Kiev. Institute of Metal Physics, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7/8(380), pp. 92–99, July–August, 1995.     

Structural changes occurring in boron nitride during shock compression in the presence of water

Conclusions The presence of a small amount of water in the process of shock compression of graphite-like boron nitride hinders the formation of crystal structure defects in BNg and suppresses the reverse transformation of the wurtzite phase. As a result, the amount of the wurtzite phase grows compared with that of the graphitelike phase. However, the extent of this phenomenon is decreased by partial hydrolysis of boron nitride, leading to the formation of boric anhydride. The monocrystalline particles of the wurtzite phase forming during shock compression with water are much larger than the particles produced in the absence of additions. The level of internal defectiveness in BN_w particles, which determines the widths of lines in x-ray diffraction photographs, is virtually independent of compression conditions.Translated from Poroshkovaya Metallurgiya, No. 9(189), pp. 76–80, September, 1978.     

高温含水条件下BN粉体的反应动力学

针对高温含水条件下非氧化物材料经常面临的性能失效问题,以六方BN粉体(平均粒度为1.2μm)为研究对象,利用高温热重分析、X射线衍射以及扫描电镜等手段,考察了BN粉体在不同温度(1273-1373K)含水条件下f水和空气的体积比为3:7)的反应行为,并与其在干燥空气下的反应行为进行对比.BN粉体在含水条件下的反应有如下特点:在反应初期,试样质量增加率快速增加;在反应后期,试样质量增加率变缓.结合热力学分析探讨了BN材料在高温含水条件下的反应机理:质量快速增加阶段主要发生BN与O₂之间的氧化反应,试样质量增加率变缓阶段主要是由于氧化产物B₂O₃与H₂O反应生成了挥发性产物.随着温度的升高,两反应阶段试样的质量增加率均有所提高.利用周模型对BN材料在高温含水条件下的反应动力学进行了较为精确且定量的拟合,结果与实验数据吻合良好.      

料浆烧结法制备改性Si-Cr-Fe高温抗氧化涂层的研究

采用料浆法在铌合金表面制备了Ge改性的Si—Cr-Fe系高温抗氧化涂层，分析了涂层氧化前后的表面和截面形貌、组织成分、相的分布等。将两次加涂后的涂层与一次加涂后的涂层形貌、组织和抗氧化性能进行了对比。结果表明：C-103铌合金表面的Si—Cr-Fe料浆熔烧涂层中，主体层是较致密的复杂硅化物相（Cr，Fe，Ge，Nb）Si2；扩散过渡层是致密的（Cr，Fe，Ge，Nb）5Si3相和少量（Cr，Fe，Ge，Nb）Si2的混合物；二次熔烧的涂层比加涂一次的涂层系统表面组织较均匀、致密，具有更宽的过渡层，对提高涂层的高温抗氧化性能有利；过渡层可有效阻止裂纹的进一步扩展，从而提高涂层的高温抗氧化性能。     

Preparation of One-Phase Superconducting Powder of the Y – Ba – Cu – O System

We describe methods for obtaining homogeneous yttrium high-temperature superconducting powders (123 phase): solid-phase synthesis, the chemical method, and explosive compression. The effect of the starting components in the mix and the technological conditions on the phase composition of the powders has been studied. It was shown that it is possible to obtain high-temperature superconducting materials with 100% YBa₂Cu₃O_{7 _ δ} phase.     

Diffusionless Nucleation of Lonsdaleite and Diamond in Hexagonal Graphite under Static Compression

We have used transmission electron microscopy to study the initial stages of diffusionless formation of lonsdaleite and diamond crystals when pyrolytic graphite is subjected to static compression. We consider the effect of plastic deformation of the matrix phase on development of the transformation. We propose a dislocation model for nucleation of dense phases, making it possible to explain the reason for the formation of a metastable lonsdaleite phase, the nature of its structural disordering, and also the possibility of diffusionless nucleation of diamond directly from hexagonal graphite.     

Reaction of Titanium with Boron Nitride under Self-Propagating High-Temperature Synthesis Conditions

We have used the ASTRA computer program for thermodynamic analysis of the chemical reaction of mixtures of titanium with boron nitride in the composition range of 1-26 mass% BN under adiabatic conditions. We have calculated the adiabatic temperatures, the heat change, and the concentration of the reaction products. We have established that under self-propagating high-temperature synthesis (SHS) conditions, we can obtain alloys of titanium with titanium nitride and monoboride and alloys of titanium nitride with titanium borides. Experimental synthesis of the alloys under SHS conditions confirm the thermodynamic analysis results.     

Basic Research on Precipitation and Control of BN Inclusions in Steel

The influencing factors on precipitation and the suitable controlling conditions of BN inclusions were not reported until now, although it has been proven that BN could improve the cutting performance of steel. The precipitation and control of BN inclusions have been investigated in detail in this article. First, the precipitation behaviors of main inclusions in steel have been investigated by theoretical calculation, especially those of BN inclusions. Then, the directional solidification and heat treatments were applied to study the precipitation of BN inclusions realistically, and the microstructures of steels under different cooling rates have been analyzed. The variation laws of size, distribution, and area ratio of BN inclusions were discussed under conditions of quantitative control of chemical composition and cooling rate of steel. The variation laws and the main influencing factors of precipitation of BN inclusions in steel have been obtained and the suitable controlling conditions have been proposed. Finally, it is validated with a cutting experiment that the cutting performance and the chip-breaking performance of steel have been greatly improved by BN inclusions precipitated in the suitable control conditions.     

Synthesis and Structure of Nanocrystalline Powders of Ultrahard Phases

The synthesis of ultrahard nanocrystalline phases is based on the occurrence of direct phase transformations in carbon and boron nitride at high pressures during shock compression. This survey examines the relationship between features of the initial layered structures, the mechanisms of the direct transformations (martensitic, diffusional), and the actual structure of the ultrahard phases formed during shock compression.     

Formation of graded structures under high-temperature oxidation conditions with surface light-heat treatment

Complex alloying of low-carbon steel with treatment by artificial concentrated solar radiation (CSR) in air using a LaB₆-based composite is studied. The distribution of elements through the thickness of the alloyed layer along the axis of CSR entry and at the periphery of the working zone is established. On the basis of electron-probe microanalysis, x-radiographic, and metallographic data it is established that as a result of high-temperature oxidation of the composite, formation of new phases, and convective mass transfer under CSR conditions, a graded structure forms at the steel surface in the form of a dense three-layer oxide film. Its outer layer contains lanthanum and nickel chromites that provide high resistance to marine corrosion. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 119–124, March–April, 2000.