Analysis of the Effect of Nonhydrostatic Compression Conditions on Direct Phase Transformations in Carbon

We have analyzed the effect of deviations from hydrostatic compression conditions on the driving force for transformations of graphite to dense phases. We consider transformations of the hexagonal (2H) and rhombohedral (3R) modifications of graphite to lonsdaleite and diamond by martensitic mechanisms, and also a direct diffusional transformation. Comparative estimates of the driving forces were made for hydrostatic and uniaxial compression schemes, and also for shear loading. We show that a loading scheme combining hydrostatic compression with uniaxial compression is more favorable for the martensitic transformation of 2H-graphite to lonsdaleite, while shear loading and uniaxial loading are more favorable for the transformations of 3R-graphite; and lonsdaleite is formed in the first case, while diamond is formed in the second case.     

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.     

Microstructural evolution of diamond growth during HFCVD

High resolution transmission electron microscopy (HRTEM) was used to study nucleation and growth mechanism of diamond by hot filament chemical vapor deposition (HFCVD) process. A novel technique has shown a direct evidence for the formation of the diamond-like carbon layer 8–14 nm thick in which small diamond micro-crystallites were embedded. These diamond micro-crystallites were formed as a result of transformation of diamond-like carbon into diamond. The diamond micro-crystallites present in the amorphous diamond-like carbon layer provided nucleation sites for diamond growth. Large diamond crystallites were observed to grow from these micro-crystallites. The mechanism of diamond growth will be presented based on experimental findings.     

合金元素Ti对D型石墨形成条件的影响

从两个方面分析了合金元素Ti对D型石墨形成条件的影响。Ti促进了初生奥氏体枝晶的析出,影响了奥氏体枝晶的形态,为D型石墨的形核和生长创造了有利的条件;Ti对石墨形核和长大的影响与其含量有关,当Ti含量>0.1%时,它阻碍石墨化的作用占主导,有助于形成D型石墨。     

The influence of thermomechanical treatment on the transformation behaviour of steels

The effect of thermomechanical treatment on the γ – α-transformation in steel has been reviewed. It has been shown that the thermo-mechanically conditioned austenite significantly influences the kinetics of transformation due to the differences in the formation of product phases. An enhanced nucleation during the diffusion controlled transformation, as a result of austenite grain refinement and/or austenite strengthening, leads to a substantial refinement of the microstructure (ferrite grains, pearlite nodules). The deformation substructure of austenite may strongly affect the shear mechanism of the diffusionless transformation, which leads to finely fragmented martensite crystals. Such differences in the transformation characteristics result in different formation temperatures of transformation products and so to the changes in CCT diagrams.     

Nucleation on ceramic particles in cast metal-matrix composites

In order to understand the nucleation on ceramic particles in the melts of metal-matrix composites (MMCs), the effect of segregation of solute on the surface of reinforcement particles in the melt has been analyzed as a function of particle temperature and the surface energy of the particle/liquid melt. The temperature of the particle in the melt, calculated analytically, was found to become close to the melt temperature within a very short time of contact between the particle and the melt. The solute concentration near the particle surface will, therefore, primarily be influenced by the surface energy of the particle and the melt. Based on this, the undercooling due to solute segregation around the particle and the chemical free-energy change due to the formation of the new solid phase on the particle were calculated in selected hypo- and hypereutectic Al-Si alloy melts containing (1) SiC particles or (2) graphite particles. The chemical free-energy change (driving force for nucleation) due to the formation of the new phase on the particle is lower for hypoeutectic compositions than for hypereutectic compositions in the aluminum-silicon alloy systems; this is due to the higher undercooling in the hypereutectic alloys due to solute segregation on the surface of the particle. This suggests that the formation of the primary phase on the surfaces of particles in the melt should be more favorable in the hypereutectic compositions than for hypoeutectic compositions. This also indicates that even when the particle temperature is not significantly lower than the liquidus temperature, nucleation on the particles can take place due to the segregation of the solute on the particles. Experimental observations of the microstructure of several cast metal-matrix composites, including Al-Si-SiC and Al-Si-graphite, show (1) the presence of silicon in contact with the reinforcement particles in hypereutectic alloys, suggesting that nucleation and growth of primary silicon under certain conditions occurs on silicon carbide and graphite particles, possibly due to solute segregation on the surface of the particles, and (2) the presence of reinforcement particles in the last-freezing interdendritic regions of the primary phases in hypoeutectic alloys, suggesting the absence of nucleation of primary phases on the reinforcement surface, as predicted by the analysis.     

金属包膜的显微分析

人工合成金刚石的相变过程中,金属包膜的作用是很重要的。为此利用ＤＩＣ、ＳＥＭ和ＳＡＭ技术研究了金属包膜的形态、组织、成分及其对碳的溶解和结合的本质。此外,还探讨了石墨在合金触媒的作用下转变成金刚石的相变机理。结果表明,金属包膜对碳的迁移和２ｓ２ｐ电子能态的激化起到了重要的作用。     

Effect of oxygen on the mechanical properties of copper and copper-matrix composites hardened by melt-synthesized chromium carbides

Thermal analysis is used to study the saturation of the copper melt by oxygen from an oxygen-containing gas phase and the possibility of deoxidation of this melt by nanosized diamond–graphite, which enters in the reaction mixture used to synthesize chromium carbide in the production of copper-matrix composites, are studied. The oxygen and chromium carbide contents are found to affect the mechanical properties of copper and copper-matrix composites. Diamond–graphite is shown to have a high refining ability, which can substantially increase the plasticity of copper and copper-matrix composites. A low chromium carbide content is found to play a modifying role in grain refinement, and a high chromium carbide content is shown to cause the formation of a precipitation-hardened structure and an increase in the physicomechanical properties of copper-matrix composites.     

Three-dimensional finite-element analysis of the quenching process of plain-carbon steel with phase transformation

In this study, three-dimensional finite-element (FE) analyses were carried out to predict the volume fractions of various phases generated during the quenching process of plain-carbon steel and the temperature history, considering the latent heat generated due to phase transformation. Diffusional phase transformation for a nonisothermal process was described by discretizing the cooling curve into various small isothermal steps and using a time-temperature-transformation (TTT) diagram for carbon steel. For this, Sheil’s additive rule was adopted to predict the incubation time which indicates the onset of phase nucleation, and, also, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation was used to model the phase growth. For handling diffusionless transformation, Koistinen and Marburger’s equation was employed to model the austenite-martensite transformation during the rapid quenching process. Finally, temperature variations obtained from the FE simulation were compared to the experimental data available in the literature to validate the reliability of the numerical solution, and its application was made to simulate the three-dimensional forming process of a bevel gear and cam lobe.     

Variation in the structure and properties of sintered alloys under the effect of nanodimensional carbon additives

The influence of nanodimensional carbon additives on the structural-phase state and properties of sintered alloys on copper, iron, and tungsten–cobalt bases is considered. It is shown that the undissolved part of the nanodimensional additive can serve as additional nucleation centers of crystals during liquid-phase sintering and crystallization of powder compositions, which promotes structure refining. It is established that the nanodimensional diamond–graphite additive in alloys able to form solid solutions with carbon or strengthening carbide phases leads to an increase in hardness of sintered composites and an improvement in their tribotechnical properties.     

Nodular graphite formation in vacuum melted high purity Fe-C-Si alloys

This paper describes a study of the cast structure of vacuum melted high purity Fe-C-Si alloys with emphasis on hypoeutectic and eutectic compositions. Nodular graphite was observed to form at high cooling rates and coral graphite at low cooling rates. This result was also confirmed by a limited study on directional solidification of alloys prepared from the same starting materials. The formation of nodular graphite at the high cooling rates was suppressed to near zero by changing the starting iron from 99.94 pct electrolytic iron to an ultra-pure zone refined iron, or by holding the melt at a low super-heat prior to cooling. Chemical analysis showed only that the impurity responsible for nodular formation was present at the low ppm level. An attempt is made to explain the appearance of the various microstructures in terms of the nucleation and growth of nodular graphite, coral graphite and the carbide structure of white iron.     

Role of Sulfur on the Transition from Graphite to Cementite Eutectic in Cast Iron

In the current study, an analytic solution is considered to explain the influence of sulfur on the transition from graphite to cementite eutectic in cast iron. The outcome from the current study indicates that this transition can be related to (a) the graphite nucleation potential (directly characterized by the cell count and indirectly by the nucleation coefficients; (b) the eutectic graphite growth rate coefficient; (c) the temperature range between the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic; and (d) the liquid volume fraction, after pre-eutectic austenite solidification. In addition, the absolute and the relative chilling tendencies, as well as critical cooling rates including the chill width of the cast iron can be predicted from the current study. The analytic model was experimentally verified for castings with various sulfur contents. It is found that the main role of sulfur on the transition from graphite to cementite eutectic is through its effect on lowering the growth coefficient, and hence, the graphite eutectic growth rate. In addition, it is found that with the increasing sulfur content, the critical cooling rate is significantly reduced, thus increasing the absolute and the relative chilling tendency values, including the chill width.     

Phase distributions in rapidly solidified stainless steels

Phase distributions and the internal magnetic fields have been determined in rapidly solidified stainless steels (Fe-nCr-8Ni-0.05C, Fe-nCr-5Ni, and Fe-nCr withn in the range of 10 to 24) by transmission and conversion electron Mössbauer spectroscopy (TMS and CEMS). Based on these results, a modification of the phase boundaries in the Schaeffler diagram is suggested to account, in particular, for rapidly solidified stainless steels. The suggested modification is primarily an expansion of the austenite field toward higher Cr and lower Ni equivalent contents. Combining CEMS and TMS makes it possible to determine the phase distributions both in the near surface region (outmost 300 nm) and in the bulk of the ribbons. For the low-Cr alloys, the content of the bcc phase (martensite) in the surface region is higher than in the sample as a whole. In the high-Cr alloys, the content of the bcc phase (ferrite) is lower in the surface than in the bulk. This disparity is ascribed to the different mechanisms of formation of martensite (diffusionless) and ferrite (nucleation and growth) in relation to the higher cooling rates of the surface layers. The determinations of the internal magnetic field are in good agreement with earlier investigations on conventionally processed Fe-Cr steels, where it was found that the internal magnetic field decreases with increasing Cr content.     

Preparation and Characterization of Graphite Powder Covered with CeO2

In order to improve the wetting properties of graphite with Al melt and reduce the oxidation of the graphite, by which the segregation of components during the liquid-stir-casting process could be prevented. In this paper, a uniform thin nano-film of CeO2, about 20 nm thick, was successfully prepared onto graphite powder surface by heterogeneous nucleation process. The results show that an obvious chemical reaction did exit between CeO2 film and graphite with the formation of Ce-O-C bond, leading to a shift of the binding energy of C and Ce. The cover with CeO2 film illustrates a distinct change of surface state of graphite with a decrease of angle of contact.     

Relationship Between Inoculants and the Morphologies of MnS and Graphite in Gray Cast Iron

The influence of oxides and sulfides in gray cast iron on the growth morphologies of MnS and on the nucleation of graphite was experimentally investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX) analysis with evidences that shows the possible nucleation sites for graphite nucleation. Thermodynamic studies have been done on the influence of varying sulfur concentrations on the nucleation of MnS in the melt and during solidification. The consumption of dissolved oxygen and sulfur in the melt during the cooling process was analyzed and we explored how this influenced the nucleation process of oxides and sulfides. A sequential nucleation concept of oxides and MnS is proposed in relation to the growth morphology of MnS and graphite with respect to the mechanical properties of cast iron. The nucleation of new oxides and sulfides was analyzed using thermodynamics and compared to our experimental results. Graphite nucleation on substrates other than MnS, such as MoS₂ oxides and (Mo,Cr)S, was experimentally analyzed along with the influence of the substrates on graphite nucleation and growth morphology.     

金刚石工具胎体材料中碳化物形成元素的行为

研究了金刚石胎体材料中的碳化物形成元素(Cr,Ti,W)与石墨和金刚石之间的粘结行为。结果表明,碳化物形成元素与石墨或金刚石发生反应而形成碳化物层。通过碳化物层,胎体材料与金刚石之间产生冶金结合。Co基和Cu基胎体材料中加入Cr可提高胎体对金刚石的粘结强度,Cr对Co基胎体本身也有固溶强化作用。W对金刚石有一定粘结作用。     

金刚石合成过程的热力学分析

在考虑了晶胚曲面引起的附加压力而产生的自由能变化的情况下,对金刚石形核过程热力学进行了分析,在此基础上,就合成温度、压力对金刚石的形核、生长速度及最终合成效果(单产、粒度和强度)影响分别进行了分析和讨论。     

A comprehensive dynamical study of nucleation and growth in a one- dimensional shear martensitic transition

We have constructed a complete hydrodynamic theory of nucleation and growth in a one-dimensional (1-D) version of an elastic shear martensitic transformation with open boundary conditions, where we have accounted for interfacial energies with strain-gradient contributions. We have studied the critical martensitic nuclei for this problem: Interestingly, the bulk critical nuclei aretwinned struc-tures, although we have determined that the dominant route for the formation of martensite is throughsurface nucleation. We have analytically solved for the surface nuclei and evaluated exact nucleation rates showing the strong preference for surface nucleation. We have also examined the growth of martensite: There are two possible martensitic growth fronts,viz., dynamical twinning and so-called two-kink solutions. These transformation fronts are separated by adynamical phase transition. We analytically derive this phase diagram and determine expressions for the speeds of the martensitic growth fronts.     

Structural features of “Carbonado” diamond polycrystalline composite materials

A thermodynamic analysis of the graphite → diamond transformation during the synthesis of diamond polycrystals is performed. It is established that the critical nucleus size decreases as the pressure increases, being 2.3 nm for p = 12 GPa. It is established when studying the crystal structure of diamond polycrystals that their block size (coherent scattering region (CSR)) is independent of the synthesis pressure and exceeds the block size of the starting graphite. It is shown that the features for martensite transformation in metals and alloys are unobservable during the synthesis of diamond polycrystals.