Carbon Phase Diagram Research Articles

A STUDY OF THE EFFECT OF ANNEALING HEAT TREATMENT ON THE FATIGUE PROPERTIES OF 0.17%C HSLA STEELS: A STATISTICAL APPROACH

The present study investigated the effect of annealing heat treatment on the fatigue properties of 0.17%C of HSLA steels using a statistical approach. The Carbon equivalent (CE) was first calculated to determine the ideal starting point for annealing which is 840oC from the Iron Carbon phase diagram. The steel samples were then exposed to multi-régimes of annealing temperatures namely; 840 oC, 870 oC, 900oC, 930oC, 960oC and 990oC at 30oC interval they were then soaked for 30 minutes. Some of the samples were thereafter machined into fatigue samples before being subjected to fatigue test. Quantitative metallography was also carried to examine its microstructure; results showed that for the steel under study, higher annealing temperature improved the fatigue properties significantly. Highest number of cycles being recorded at 3.9 Ⅹ 103 and 1.3 X 103 at 321.3 MPa and 1606.57 MPa stress levels for samples annealed at 990 oC, this is followed closely by samples annealed at 960 oC which had 3.5 X 103 and 0.8 X 103 number of cycles at the same stress levels before failure. The control had the least number of cycles of about 0.9 X 103 and 0.1 X 103 at the same stress levels, and, standard deviation of 5.960 and a mean of 26.881 which shows that annealing greatly improved the fatigue life of the steel. The implemented software (ANOVA) test confirmed the results at 95% confidence which further showed that there was significant difference amid the annealing parameters and, it shows promising results.

The effect of kinetic and photoinduced processes at high pressure on cluster structure of ultrahard fullerite

We have studied the kinetic and photoinduced aspects of the transformation of the cluster structure of ultrahard fullerite in the pressure ranges of a new phase diagram of carbon corresponding to the stability regions of diamond (up to 55 GPa) and onion-like structures of the fullerene type (55–115 GPa). A long exposure (for 3 months) at 25 GPa leads to the beginning of the transformation of the sample of ultrahard fullerite into diamond. A further increase to 70 GPa, under the influence of laser irradiation at wavelengths of 405 and 532 nm at doses of 5*1013 J/m2, initiates the formation of onion-like particles in ultrahard fullerite samples, which is accompanied either by a broadening of the high-frequency mode of the Raman spectrum, or its disappearance under high pressure. After pressure release, the high frequency mode of the Raman spectrum is located at 1570–1605 cm−1 at excitation wavelengths of 532 and 405 nm, respectively, which is typical for ultrahard fullerite. Bulk modulus of such an irradiated sample corresponds to a typical value for ultrahard fullerite of about 600 GPa. In addition to the formation of nanodiamonds and onion–like particles, the synthesis of ultrahard fullerite is also accompanied by the appearance in the sample of systems with double bonds of the C=C=C type with a line around 1935 cm−1, the nature of which is not entirely clear.

Type Ia supernovae from chemically segregated white dwarfs

Type Ia supernovae are the outcome of the explosion of a carbon–oxygen white dwarf in a close binary system. They are thought to be the main contributors to the galactic nucleosynthesis of iron-peak elements, with important contributions to the yields of intermediate-mass elements. Recent analyses of the phase diagram of carbon and oxygen containing impurities such as 22Ne and 56Fe in conditions relevant to white dwarf interiors suggest that both isotopes can partially separate when the temperature of the star is low enough to start solidifying. The purpose of the present paper is to examine the impact of this separation on the yields of the different chemical species synthesized during explosions. We used a one-dimensional supernova code to evaluate the impact of the sedimentation assuming different degrees of chemical separation. We find that the main properties of the ejecta, the kinetic energy, and the ejected mass of 56Ni only vary slightly when the separation is taken into account. However, the yields of important isotopes that are used as diagnostic tools, such as manganese, can be strongly modified. Furthermore, the chemical separation studied here is able to change several indicators related to the metallicity of the progenitor (such as the mass ratio of calcium to sulphur in the ejecta or the UV flux of the supernova) and to its mass, whether it is a Chandrasekhar-mass white dwarf or a substantially lighter one (such as the imprint of stable nickel on late-time infrared spectra or that related to the presence of radioactive nickel at the center of the ejecta).

INTRINSIC STRESS IN CARBON COATINGS DEPOSITED FROM HYDROCARBON ION FLOW

Within the model of the nonlocal thermoelastic peak (NTP) of low-energy ion, the emergence of intrinsic stress in carbon coatings deposited from the flow of ions of hydrocarbon molecules is theoretically studied. The expression is obtained for intrinsic stress depending on species and ion energy. The time dependence of temperature and pressure in the NTP of CH4+, C2H2+ and C6H6+ ions was studied The position of NTP of ions of different species and energies on the P,T phase diagram of carbon was found and their trajectories were constructed. The position of the NTP trajectories of various ions with respect to the regions of structural stability of carbon was analyzed. The conditions for the predominant formation of sp3 bonds in the NTP of ion are discussed.

Extreme Metastability of Diamond and its Transformation to the BC8 Post-Diamond Phase of Carbon.

Diamond possesses exceptional physical properties due to its remarkably strong carbon-carbon bonding, leading to significant resilience to structural transformations at very high pressures and temperatures. Despite several experimental attempts, synthesis and recovery of the theoretically predicted post-diamond BC8 phase remains elusive. Through quantum-accurate multimillion atom molecular dynamics (MD) simulations, we have uncovered the extreme metastability of diamond at very high pressures, significantly exceeding its range of thermodynamic stability. We predict the post-diamond BC8 phase to be experimentally accessible only within a narrow high pressure-temperature region of the carbon phase diagram. The diamond to BC8 transformation proceeds through premelting followed by BC8 nucleation and growth in the metastable carbon liquid. We propose a double-shock compression pathway for BC8 synthesis, which is currently being explored in experiments at the National Ignition Facility.

Exploring the High-Pressure Phases of Carbon through X-ray Diffraction of Dynamic Compression Experiments on Sandia’s Z Pulsed Power Facility

The carbon phase diagram is rich with polymorphs which possess very different physical and optical properties ideal for different scientific and engineering applications. An understanding of the dynamically driven phase transitions in carbon is particularly important for applications in inertial confinement fusion, as well as planetary and meteorite impact histories. Experiments on the Z Pulsed Power Facility at Sandia National Laboratories generate dynamically compressed high-pressure states of matter with exceptional uniformity, duration, and size that are ideal for investigations of fundamental material properties. X-ray diffraction (XRD) is an important material physics measurement because it enables direct observation of the strain and compression of the crystal lattice, and it enables the detection and identification of phase transitions. Several unique challenges of dynamic compression experiments on Z prevent using XRD systems typically utilized at other dynamic compression facilities, so novel XRD diagnostics have been designed and implemented. We performed experiments on Z to shock compress carbon (pyrolytic graphite) samples to pressures of 150–320 GPa. The Z-Beamlet Laser generated Mn-Heα (6.2 keV) X-rays to probe the shock-compressed carbon sample, and the new XRD diagnostics measured changes in the diffraction pattern as the carbon transformed into its high-pressure phases. Quantitative analysis of the dynamic XRD patterns in combination with continuum velocimetry information constrained the stability fields and melting of high-pressure carbon polymorphs.

Machine learning interatomic potential for simulations of carbon at extreme conditions

A spectral neighbor analysis (SNAP) machine learning interatomic potential (MLIP) has been developed for simulations of carbon at extreme pressures (up to $5\phantom{\rule{0.16em}{0ex}}\mathrm{TPa}$) and temperatures (up to 20 000 K). This was achieved using a large database of experimentally relevant quantum molecular dynamics (QMD) data, training the SNAP potential using a robust machine learning methodology, and performing extensive validation against QMD and experimental data. The resultant carbon MLIP demonstrates unprecedented accuracy and transferability in predicting the carbon phase diagram, melting curves of crystalline phases, and the shock Hugoniot, all within 3% of QMD. By achieving quantum accuracy and efficient implementation on leadership-class high-performance computing systems, SNAP advances frontiers of classical MD simulations by enabling atomic-scale insights at experimental time and length scales.

Is Pbam-32 thermodynamically stable compared with diamond and graphite under variable pressure and temperature conditions?

Pbam-32 is a new carbon allotrope formed by hybrid sp3 atoms. It is promising for application as a super-hard material owing to its remarkable Young's modulus and Vicker's hardness. However, thermodynamic properties related to the stability, production and practical performance of Pbam-32 are still unclear, so further study is required to clarify these details. With the help of ab initio simulations, we systematically explore the structure, atomic environment, phonon dispersion, entropy and enthalpy changes of Pbam-32 compared with diamond and graphite. As an allotrope consisting of five-membered and seven-membered rings, Pbam-32 is a meta-stable structure without imaginary frequencies in phonon dispersion. An acoustic band gap between 39.58 THz and 42.57 THz indicates potential applications of Pbam-32 as a terahertz phononic crystal. The entropy and enthalpy of Pbam-32 are similar to those of diamond. Besides, Pbam-32 exhibits an excellent ability to resist pressure. Unfortunately, the free energy of Pbam-32 is higher than that of diamond over a wide range of pressure and temperature conditions, which means that Pbam-32 is absent in the phase diagram of the three allotropes. Therefore, the production of Pbam-32 must be kinetically controlled. Our results are helpful for understanding the thermodynamic properties of Pbam-32.

Submicrosecond Aggregation during Detonation Synthesis of Nanodiamond.

Detonation nanodiamond (DND) is known to form aggregates that significantly reduce their unique nanoscale properties and require postprocessing to separate. How and when DND aggregates is an important question that has not been answered experimentally and could provide the foundation for approaches to limit aggregation. To answer this question, time-resolved small-angle X-ray scattering was performed during the detonation of high-explosives that are expected to condense particulates in the diamond, graphite, and liquid regions of the carbon phase diagram. DND aggregation into low fractal dimension structures could be observed as early as 0.1 μs, along with a separate scattering population also observed from an explosive that produces primarily graphitic products. A counterexample is the case of a high-explosive that produces nano-onions, where no hierarchical scattering was observed for at least 10 μs behind the detonation front. These results suggest that DND aggregation occurs on time scales comparable to particle formation.

Impulse laser cutting of diamond accompanied by phase transitions to fullerene-type onions

We studied a process of diamond cutting by a focused pulsed laser beam in an explosion inside solid matter mode (a 100 ns laser beam impulse focused in diamond generates a shock wave). A cavity size created in diamond by the laser impact is described by a conception of a blast cavity formation after explosion inside solid matter. Diamond cutting is also accompanied by a phase transition of diamond to fullerene-type onions composed of 2 to 5 shells. According to a new phase diagram of carbon (which contains a zone of diamond instability in the 55–115 GPa pressure range), the observed phase transition is possible under ~70 GPa pressure and ~2400 K temperature, which indicates pressure and temperature values during laser cutting. The pressure estimated based on the new phase diagram corresponds to the pressure estimated from the known blast cavity formation model.

Phase Equilibria in the Carbide Region of Iron–Carbon Phase Diagram

Phase Equilibria in the Carbide Region of Iron–Carbon Phase Diagram

Transformation of diamond to fullerene-type onions at pressure 70 GPa and temperature 2400 K

We report the observation of a phase transition of diamond to denser than diamond carbon phase composed from 2 to 3 fullerene-type shells of onions. Raman spectra indicate the fullerene-type of the onions shells. The onions phase is a stable phase in a diamond instability zone of a phase diagram of carbon at pressure 70 GPa and temperature 2400 K. A mixture of diamond and Ni powders was heated by a laser beam under pressure in a diamond anvil cell. Both direct and catalytic diamond to onions transitions were observed during heating. The catalytic transformation includes the following steps. Melting of Ni during the laser heating at pressure 70 GPa, a ‘diamond solution’ (a transfer of carbon atoms from diamond) in liquid Ni and the formation of an equilibrium carbon phase from the supersaturated solution upon cooling. The catalytic process is a reverse one relative to the catalytic synthesis of diamond in a diamond stability zone at pressure around 6 GPa. The main result of our study is the presence of fullerene-type structures in the phase diagram of carbon in the region of diamond instability under high sub-Mbar pressure and wide range of temperatures.

Observation of Variations in Condensed Carbon Morphology Dependent on Composition B Detonation Conditions

AbstractCarbon particulates generated during detonation depend upon high explosive type, composition, and detonation conditions. Although explosive composition greatly affects particulates, the focus of this work is on how detonation geometries that induce much higher temperatures and pressures in the high explosive lead to differing particulate morphologies. In this study, two geometries were used: Detonations were initiated in Composition B cylinders at one end in conventional detonations and initiated at both ends to produce colliding detonations. Each of these detonations was observed on the sub‐μs timescale using fast radiography capturing images of the front moving through the cylinder, and colliding detonation fronts in real‐time. These imaging experiments were complemented with time‐resolved small‐angle x‐ray scattering (SAXS) experiments that were able to observe and determine the varying condensed carbon morphologies at different locations and times in each detonation. The detonations could be timed in such a way that the spatial and temporal dependence of the carbon morphology could be superimposed onto radiography images collected at the same point in time. The complementary approach is able to show that the carbon condensates are much larger when formed in the elevated temperature and pressure conditions near the location of colliding detonation fronts. Thermochemical modeling suggests that these larger particulates form either in the diamond phase or on the liquidus line of the carbon phase diagram. The increase in size observed by SAXS may correlate well with the increased residence time deeply in the diamond phase. These particulates can be described as nano‐sized phases with some surface texture or otherwise near‐surface intra‐particle heterogeneity.

Detonation synthesis of carbon nano-onions via liquid carbon condensation

Transit through the carbon liquid phase has significant consequences for the subsequent formation of solid nanocarbon detonation products. We report dynamic measurements of liquid carbon condensation and solidification into nano-onions over ∽200 ns by analysis of time-resolved, small-angle X-ray scattering data acquired during detonation of a hydrogen-free explosive, DNTF (3,4-bis(3-nitrofurazan-4-yl)furoxan). Further, thermochemical modeling predicts a direct liquid to solid graphite phase transition for DNTF products ~200 ns post-detonation. Solid detonation products were collected and characterized by high-resolution electron microscopy to confirm the abundance of carbon nano-onions with an average diameter of ∽10 nm, matching the dynamic measurements. We analyze other carbon-rich explosives by similar methods to systematically explore different regions of the carbon phase diagram traversed during detonation. Our results suggest a potential pathway to the efficient production of carbon nano-onions, while offering insight into the phase transformation kinetics of liquid carbon under extreme pressures and temperatures.

Development of a High-Strength Low-Carbon Steel with Reasonable Ductility through Thermal Cycling

In this research work, cost-effective structural material synthesis routes have been adopted in the form of thermal cycling designed and applied on a low-cost material (AISI 1010 steel). The main synthesis route consists of two-step thermal cycling (designed with respect to iron–carbon phase diagram) combining short-duration holding in upper critical and intercritical temperature regimes and different cooling conditions (forced air cooling and ice-brine quenching). Accordingly, the adopted hierarchical design approach (from nanoscale onwards) with a combination of hard and soft phases (to attain an adequate balance between mutually exclusive strength and ductility properties) results in origin of a novel microstructure. The microstructure consists of dislocation-enriched martensite regions (possessing plate and lath morphologies in 1:4 ratio) containing nanosized cementite particles and α-ferrite regions containing submicroscopic cementite particles. As a consequence, an excellent combination of strength (UTS = 905 MPa) and ductility (%Elongation = 17) is achieved in the synthesized material.

Phase diagram of carbon and the factors limiting the quantity and size of natural diamonds

Phase diagrams of carbon, and those focusing on the graphite-to-diamond transitional conditions in particular, are of great interest for fundamental and applied research. The present study introduces a number of experiments carried out to convert graphite under high-pressure conditions, showing a formation of stable phase of fullerene-type onions cross-linked by sp3-bonds in the 55–115 GPa pressure range instead of diamonds formation (even at temperature 2000–3000 K) and the already formed diamonds turn into carbon onions. Our results refute the widespread idea that diamonds can form at any pressure from 2.2 to 1000 GPa. The phase diagram built within this study allows us not only to explain the existing numerous experimental data on the formation of diamond from graphite, but also to make assumptions about the conditions of its growth in Earth’s crust.

Phase diagram of carbon

We have investigated a transition of graphite to diamond and diamond phase stability under high-pressure conditions. Graphite and diamond samples were subjected to pressure and shear deformation in the range from 25 GPa to 130 GPa. We propose a new phase diagram of carbon. The diagram includes a diamond instability zone in the pressure range from 55 to 115 GPa. It has been experimentally proved that diamond formation ceases in the pressure range from 55 to 115 GPa, while the already formed diamonds turn into carbon onions

Structural prediction of two-dimensional materials under strain

We develop a procedure for the investigation of the phase diagram of materials under strain. This is based on a global structural prediction method where the volume is constrained to predefined values. Our method is more general than other available techniques, and it avoids at the same time numerical instabilities. As a first example, we investigate the phase diagram of two-dimensional carbon as a function of the area per atom. As expected, we find that graphene is stable for a large range of biaxial strains. However, at large areas there appear novel carbon allotropes containing decagons and higher order polygons. These phases are thermodynamically stable for strains below the breaking point of graphene, indicating that they could be accessible experimentally.

A theoretical approach to the nanoporous phase diagram of carbon

Nanoporous carbon has been extensively used in a wide range of applications from water treatment to electrochemical applications, such as in energy storage devices. However, an attempt to relate structural and thermodynamical properties has not been explored from an atomistic approach. In this work we present a methodology to produce nanoporous carbon structures, using molecular dynamics simulations and a many-body potential. We designed a heating-quenching procedure in a thermodynamic region limited between the critical and triple point densities of carbon to study a sample of 1750 atomic arrangements produced at different densities, quench rates, and using graphite and diamond unit cells as precursor structures. All these samples were numerically characterized through the calculation of the free volumes, surface areas, radial distribution functions, and structure factors. We found particularly useful the potential energy dependence with sp3 hybridization content, to compute structural phases through clustering methods. Three phases were found that comprise graphite-like, sponge-like, and unstable states. From our results we can conclude that our methodology, based on Tersoff potential, is compatible with available experimental data and different theoretical findings. In such case, our methodology is a reliable choice to produce nanoporous structures with low computational cost.

Retraction Note to: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

We review the discovery of new phases of carbon (Q-carbon) and BN (Q-BN) and address critical issues related to direct conversion of carbon into diamond and hBN into cBN at ambient temperatures and pressures in air without any need for catalyst and the presence of hydrogen. The Q-carbon and Q-BN are formed as a result of quenching from super undercooled state by using high-power nanosecond laser pulses. We discuss the equilibrium phase diagram (P vs T) of carbon, and show that by rapid quenching, kinetics can shift thermodynamic graphite/diamond/liquid carbon triple point from 5000 K/12 GPa to super undercooled carbon at atmospheric pressure in air. Similarly, the hBN-cBN-Liquid triple point is shifted from 3500 K/9.5 GPa to as low as 2800 K and atmospheric pressure. It is shown that nanosecond laser heating of amorphous carbon and nanocrystalline BN on sapphire, glass, and polymer substrates can be confined to melt in a super undercooled state. By quenching this super undercooled state, we have created a new state of carbon (Q-carbon) and BN (Q-BN) from which nanocrystals, microcrystals, nanoneedles, microneedles, and thin films are formed depending upon the nucleation and growth times allowed and the presence of growth template. The large-area epitaxial diamond and cBN films are formed, when appropriate planar matching or lattice matching template is provided for growth from super undercooled liquid. The Q-phases have unique atomic structure and bonding characteristics as determined by high-resolution SEM and backscatter diffraction, HRTEM, STEM-Z, EELS, and Raman spectroscopy, and exhibit new and improved mechanical hardness, electrical conductivity, and chemical and physical properties, including room-temperature ferromagnetism and enhanced field emission. The Q-carbon exhibits robust bulk ferromagnetism with estimated Curie temperature of about 500 K and saturation magnetization value of 20 emu g−1. We have also deposited diamond on cBN by using a novel pulsed laser evaporation of carbon and obtained cBN/diamond composites, where cBN acts as template for diamond growth. Both diamond and cBN grown from super undercooled liquid can be alloyed with both p- and n-type dopants. This process allows carbon to diamond and hBN to cBN conversions and formation of useful nanostructures and microstructures at ambient temperatures in air at atmospheric pressure on practical and heat-sensitive substrates in a controlled way without need for any catalysts and hydrogen to stabilize sp3 bonding for diamond and cBN phases.