Dale E. Wittmer

Determination of cations distribution in Mn3O4 by anomalous x-ray powder diffraction

The oxidation states of Mn at two different crystallographic sites in the distorted spinel structure of Mn3O4 were determined by the approach of anomalous synchrotron x-ray powder diffraction. The real parts of anomalous scattering factors for the two Mn sites were refined from a series of powder diffraction patterns collected at the vicinity of K absorption edge of Mn. The results of Rietveld refinements showed that the two distinct cation sites can obviously be distinguished from each other and that Mn2+ ions occupy the tetrahedral site and Mn3+ ions are located at the octahedral site.

Residual stresses in particle-reinforced ceramic composites using synchrotron radiation

Residual stresses were determined in particle-reinforced ceramic composites using synchrotron based x-ray diffraction. The baseline Si3N4 and the Si3N4-TiN composites were processed by turbomilling, pressure casting, and isopressing. They were then continuously sintered to full density, under a pressureless, flowing nitrogen atmosphere. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following quenching in water from 1000°C, 1100°C, and 1200°C. The residual stresses for both the baseline Si3N4 and the Si3N4-TiN composites were determined from the (441) and (531) reflections, by applying the 2Θ-sin2ψ method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the mechanical properties of each material. In both the baseline Si3N4 and Si3N4-TiN composites, after thermal shocking, the compressive residual stresses were developed in directions both parallel and perpendicular to the sample surface. The residual compressive stresses for the Si3N4-TiN composites were much higher than the baseline Si3N4. As a result, both fracture toughness and flexural strength of the Si3N4-TiN composites were improved. In addition, the addition of the TiN appears to improve both the strength and toughness of the baseline Si3N4.

Update in a Rietveld analysis program for x-ray powder spectro-diffractometry

A computer program for refining anomalous scattering factors using x-ray powder diffraction data was revised on the basis of the latest version of a versatile pattern-fitting system, RIETAN-2000 . The effectiveness of the resulting program was confirmed by applying it to simulated and measured powder-diffraction patterns of Mn 3 O 4 taken at a synchrotron light source.

Preliminary experiment for residual stress analysis at the advanced photon source

In order to evaluate and find the proper conditions for residual stress (RS) measurements, two kinds of preliminary x-ray diffraction experiments were conducted at the Advanced Photon Source: angle-dispersive x-ray diffraction and energy-dispersive x-ray diffraction. These two experimental methods and their precision were investigated for RS measurement. The results obtained and various factors that may influence the accuracy of these experiments are discussed.

Analysis of Residual Stress Gradients Below the Surface of a Material Using a Multi-Energy Method

The residual-stress-gradient distribution just below the surface of a material is an important factor to consider during the engineering and design of a component. With the availability of an intense energy-tunable synchrotron x-ray source, it becomes easier to analyze the stress gradient below the surface, using a multi-energy x-ray diffraction method. A program was developed to efficiently determine possible experimental parameters using a sample with a known stress gradient distribution. In addition, this program can also calculate the stress gradient distribution below the surface taking into account experimental results. It also includes a subroutine for calculating the x-ray absorption coefficients of all of the elements, generalizing it for use with any material. As an example, in the present study, the relationship between x-ray energy and the residual stress gradient is discussed according to the calculated result for a silicon nitride composition.

Residual Stresses in Silicon Nitride Based Composites Using Synchrotron Radiation

In order to measure the residual strain (stress) in the silicon nitride based composites, synchrotron based x-ray diffraction was employed on the BESSRC beamline at the APS. The residual stress for both the baseline Si 3 N 4 and the Si 3 N 4 -TiN composites were determined from the (441) and (531) reflections, obtained by applying the three-dimensional 2θ-sin 2 ψ method. In both the baseline Si 3 N 4 and the Si 3 N 4 -TiN composites, after thermal shocking, compressive residual stresses were developed in both directions parallel and perpendicular to the specimens surface. The average residual stresses in the direction parallel to the sample surface were much higher than in the perpendicular direction. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the strength and toughness of each composite. The results suggested that there should be a maximum thermal shock temperature, within the range of 1000 °C to 1100 °C, for improving the fracture toughness for both the baseline Si 3 N 4 and the Si 3 N 4 -TiN composites. Also, the addition of the TiN appeared to improve both the strength and toughness of the baseline composition.