Judith A. Todd

The high temperature creep deformation of Si3N4-6Y2O3-2Al2O3

AbstractThe creep properties of silicon nitride containing 6 wt % yttria and 2 wt% alumina have been determined in the temperature range 1573 to 1673 K. The stress exponent, n, in the equation

Absorption of laser irradiation in a porous powder layer

\dot \varepsilon

Microstructure-mechanical property relationships in isothermally transformed vanadium steels

∝ σn was determined to be 2.00±0.15 and the true activation energy was found to be 692±25 kJ mol−1. Transmission electron microscopy studies showed that deformation occurred in the grain boundary glassy phase accompanied by microcrack formation and cavitation. The steady state creep results are consistent with a diffusion controlled creep mechanism involving nitrogen diffusion through the grain boundary glassy phase.

Investigations of laser-sustained plasma and its role in laser nitriding of titanium

A new model has been applied to the precipitation of vanadium carbide (VC) particles in sheets at austenite/ferrite interphase boundaries during the isothermal transformation of Fe-C-V steels. Linear relationships between the intersheet spacings, the VC particle sizes, and the square root of the vana-dium diffusivity in ferrite have been identified. The model establishes predictive correlations be-tween the interphase precipitate sheet widths, intersheet spacings, and boundary migration speeds as a function of isothermal transformation temperature, and shows that the time associated with the in-terphase precipitate repeat period is a constant for each alloy over the temperature ranges studied. From a single measurement of interphase boundary velocity in a volume where the intersheet spac-ing is known, it is possible to predict the intersheet spacing and width as a function of temperature.

A new model for precipitation at moving interphase boundaries

An analytical relationship was derived to describe the amount of energy absorbed within preplaced powder during the laser deposition process. The relationship, which reflects an exponential decay of Beer–Lambert, may be used to define internal absorption due to scattering within the powder layer regardless of the beam shape and energy distribution if the attenuation coefficient and bulk absorption are known. Experiments were conducted to estimate the attenuation coefficients for pure iron and pure copper powders, representing three powder size distributions, during CO2 and Nd:yttrium-aluminum-garnet (YAG) laser irradiation. Qualitative observations and trends of the experimental data indicated that greater beam penetration, accompanied by a decrease in the estimated attenuation coefficients, was associated with the larger powder particles, the lower wavelength of the Nd:YAG laser, and the copper powder. Attenuation coefficients were determined for the original powder size distributions and a larger size d...

A NEW MECHANISM OF CRACK CLOSURE IN CATHODICALLY PROTECTED ASTM A710 STEEL

Abstract The precipitation kinetics of an advanced austenitic steel, modified with stabilizing additions of titanium, niobium and vanadium, has been studied in both the solution-annealed and the solution-annealed plus cold-worked conditions, for times up to 3000 h at 800°C. Matrix phosphide precipitates and a very few MC precipitates evolved in the solution-annealed material. In contrast, stable MC matrix precipitate distributions were observed after only one hour at 800°C and persisted for 3000 h in the samples which were solution annealed and cold worked. Observations using transmission electron microscopy provide an explanation for the significant improvements in creep rupture strength observed in the advanced alloy compared with 316 stainless steel.

Development of a Prototype Laser Processing System for Shaping Advanced Ceramic Materials

The relationships between the interphase precipitation reaction and the mechanical properties of an Fe-0.2C-l.0V-0.5Mn steel were studied after isothermal transformation in the temperature range 600 °C to 750 °C. The strength and room temperature toughness of the transformed steel are found to be determined by the austenitization temperature, vanadium carbide solubility, volume fraction of VC available for precipitation, size of the precipitates, and ferrite grain size. Yield strength increments due to precipitation are predicted by Melander’s model for critical resolved shear stress, when all the available carbide precipitated as interphase VC. For lower austenitization temperatures, yield strength increments are modeled by a bimodal distribution of undissolved and interphase (or matrix) precipitates. Six classifications of VC morphologies are identified in the transformed microstructures, but one of these, the “fibrous” VC morphology, could not be associated with degradation of toughness as suggested by Mishima. The impact transition temperatures are approximated by regression analyses for bainitic steels. The results show that both strength and toughness can be simultaneously optimized in this steel and suggest that microstructures with strength and toughness levels equivalent to those of quenched and tempered steels can be produced in vanadium steels by thedirect decomposition of austenite.

A Comparison of the Near-Threshold Corrosion Fatigue Crack Propagation Rates in Mil S-24645 HSLA Steel and Its Weld Metal

Laser-sustained plasma (LSP) and CCD imaging of reactant species were employed to investigate the role of near-surface plasma in CO2 laser nitriding of titanium in open atmosphere. Insights were gained regarding the role of plasma processes and the role of reactive nitriding species in the nitriding process. Studies of single nitrided trails have identified the following regimes, as a function of LSP off-focal distance and beam translation speed, characterized by (1) the formation of heavily oxidized surfaces, (2) the formation of titanium nitride (TiN) nanoparticulate, (3) nitride formation in the absence of a surface-struck or LSP and (4) the formation of near-stoichiometric, oxide-free TiN surfaces with a LSP. For the first time it will be shown that the LSP can access nitriding conditions beyond those achieved with surface-struck plasma (or in the absence of plasma) to produce uniform, near-stoichiometric, titanium nitride coatings.