Papers by Author: Thomas L. Christiansen

Abstract: The present paper addresses the influence of chemical induced stresses on diffusion in interstitial systems. This is exemplified by simulations of carbon diffusion in austenite at high temperatures and it is shown that old well established literature data is flawed by the occurrence of composition induced stress. For the technological relevant system of expanded austenite the diffusion can be dramatically affected by composition induced stress.

Abstract: In the present paper various experimental procedures to experimentally determine the concentration dependent diffusion coefficient of carbon in expanded austenite are evaluated. To this end thermogravimetric carburization was simulated for various experimental conditions and the evaluated composition dependent diffusivity of carbon derived from the simulated experiments was compared with the input data. The most promising procedure for an accurate determination is shown to be stepwise gaseous carburizing of thin foils in a gaseous atmosphere; the finer the stepsize, the more accurate the approximation of the diffusivity. Thermogravimetry was applied to continuously monitor the weight change of thin foils of AISI 316 during carburizing in CO-H2 gas mixtures for one of the simulated experimental procedures.

Abstract: The applicability of the Boltzmann-Matano method for evaluation of a diffusion coefficient and its concentration dependency by line profile analysis is tested on three different (model) systems. All systems involve interstitial diffusion. It is shown that the occurrence of trapping corrupts the applicability of the Boltzmann-Matano method.

Abstract: On evaluating lattice strain-depth or stress-depth profiles with X-ray diffraction, the variation of the information depth while combining various tilt angles, in combination with lattice spacing gradients leads to artefacts, so-called ghost or fictitious stresses. X-ray diffraction lattice-strain analysis was simulated for a model stress-depth profile combined with a composition-depth profile. Two principally different methods were investigated for the reconstruction of the actual stress and composition profiles from the simulated data: - considering the stress/strain determined at a specific depth as a weighted average over the actual stress/strain depth profile - considering the lattice spacing determined at a specific depth, for a specific value for as a weighted average over the actual lattice spacing profile for this direction. On the basis of the results it is possible to propose a preferred method for the evaluation of stress/strain and composition profiles, while minimising the risk for ghost stresses.