Rapid Diffusion Of Carbon Research Articles

Dynamic strain aging mechanisms in a metastable austenitic stainless steel

Discontinuous plastic flow due to dynamic strain aging (DSA) in a Fe–13Cr–3.4Mn–0.47C metastable stainless steel was studied by uniaxial tensile tests in the temperature range from 20 °C to 500 °C. A complex picture of the discontinuous flow and its disappearance in an intermediate temperature interval was interpreted in terms of an existing model, which had to be modified. Different mechanisms of DSA were proposed for different temperature intervals. DSA at temperatures up to 200 °C was attributed to deformation-induced martensitic transformation within glide bands. Immobilization of dislocations at martensite/austenite boundaries and emission of fresh dislocations into the adjacent austenite was considered as a major process in this temperature regime. Rapid diffusion of carbon from martensite to the martensite/austenite boundaries was considered as an additional pinning mechanism contributing to discontinuous plastic flow. Serrations on the deformation curves at temperatures between 200 °C and 300 °C were associated with dislocation pinning due to carbon diffusion in austenite. This regime of DSA did not involve high diffusivity martensite. In spite of an increase in the diffusivity of carbon in the temperature range of 350-450 °C, a serration-free regime was observed there. The absence of intermittent dislocation pinning/unpinning events responsible for serrations implied continuous solute drag by carbon atoms according to the Cottrell mechanism. Serrations reappeared at 500 °C and were attributed to dislocation pinning by substitutional chromium and manganese atoms.

High-Temperature Drive Shaft Failure with a Twist

An unusual high-temperature failure is presented involving the bearings and drive shaft of a centrifugal slurry pump. The unique characteristics of the failure are explored including localized heating and interactions between the ductile iron bearing collar and the shaft material. Maximum surface temperature of the shaft at the time of failure is estimated to be 2100–2300 °F. Partial melting of the ductile iron bearing collar and softening of the shaft appears to have resulted in both the inclusion of ductile iron in the shaft material in the molten state and a form of friction stir welding which joined the two components. Hardness values in excess of 900 Vickers were recorded around the material interface. It is theorized that rapid diffusion of carbon into the shaft material occurred during failure facilitated by mechanical mixing due to severe high-temperature deformation. Graphite nodularity and phase analysis of the ductile iron collar and an embedded ductile iron fragment were conducted by means of microscopy and image analysis.

Growth and morphology of carbon nanostructures on nickel oxide nanoparticles in catalytic chemical vapor deposition

The present study explores the conditions favorable for the growth of cylindrical carbon nanostructures such as multi-walled carbon nanotube (MWCNT) and carbon nanofiber by catalytic chemical vapor deposition (CCVD) method using nickel oxide-based catalyst nanoparticles of different average sizes as well as different levels of doping by copper oxide. The role of doping and the average size have been related to the observed melting behavior of nanoparticles of nickel oxide by thermal and diffraction analysis, and the importance of melting has been highlighted in the context of growth of cylindrical nanostructures. In the reducing environment prevailing in the CCVD chamber due to decomposition of flowing acetylene gas at elevated temperature, there is extensive reduction of oxide nanoparticles. Lack of melting and faster flow of carbon-bearing gases favor the formation of a carbon deposit cover over the catalyst nanoparticles giving rise to the formation of nanobeads. Melting allows rapid diffusion of carbon from the surface to inside catalyst particles, and reduced flow of gas lowers the rate of carbon deposit, both creating conditions favorable for the formation of cylindrical nanostructures, which grows around the catalyst particles. Smaller particle size and lower doping favor growth of MWCNT, while growth of fiber is commonly observed on larger particles having relatively higher level of doping.

Effect of film thickness and annealing temperature on the carbon induced interfacial charge of tetraethoxyorthosilicate based oxide films on p-type Si(100)

Tetraethoxyorthosilicate (TEOS) based silicon oxide films were deposited by plasma enhanced chemical vapor deposition and subsequently annealed in an oxidizing ambient at 950 and 1070 °C. Carbon atoms were found to segregate at the SiO2/Si interface inducing interfacial charges in excess of 1×1011cm−2 in the presence of hydrogen. The interfacial carbon and charge concentrations were dependent on the annealing temperature and film thickness. A mechanism based on the rapid diffusion of carbon toward the interface during the early stages of annealing followed by oxygen diffusion toward the interface and the subsequent oxidation of interfacial carbon is proposed as an explanation for the charge dependence on the film thickness and annealing temperature. Additional data are presented concerning the composition of interfacial carbon species as well as hydrogen diffusion through the TEOS based oxide layer.

Characterization of single crystal films of molybdenum (011) grown by molecular beam epitaxy on sapphire (112̄0) and studied by low-energy electron microscopy

Films of molybdenum grown on the (112̄0) plane of sapphire (Al 2O 3) are characterized using low-energy microscopy and low-energy electron diffraction. Stress fields observed on the Mo surface originate at dislocations and at miscut steps of the buried molybdenum–alumina vicinal interface. As-grown films contain small-angle grain boundaries. These are largely eliminated upon heating to 1700 K as edge dislocations that form the boundaries become extremely mobile. Edge dislocations attract and annihilate one another, and the small-angle grain boundaries disappear. Mobility of edge dislocations is correlated with rapid diffusion of carbon, which apparently pins dislocations up to temperatures that allow diffusion of carbon from dislocations into the bulk. The main contaminants of the Mo surface are carbon, oxygen and carbon monoxide. The most stable impurities are carbides that persist to 1700 K. Oxygen promotes bunching of monatomic steps into groups of two, three and four. Electron beams dissociate CO with energy less than 1 eV and deposit residues of carbon. Fairly ideal single crystal films of Mo produced by annealing exhibit monatomic surface step and terrace structure, and a minimum of dislocations. The quality of surfaces on these films exceeds that of typical single crystal bulk samples and is well suited for fundamental studies in surface science.