Modelling Peak Shifts not Resulting from Lattice Strain in Anisotropic and Inhomogeneous Materials using Monte-Carlo Simulation of Neutron Diffraction

Neutron diffraction has been used by the nuclear industry for measuring residual stresses in structures for which integrity safety cases must be made. However measurement of stresses in materials containing large grains, or in anisotropic weld metal, or approaching air-metal interfaces, or metal-metal interfaces using neutron diffraction is particularly challenging. These types of measurements can give rise to errors (often termed “pseudo-strains”) that can be significantly larger than the residual stress actually present. Such errors arise when the gauge volume is partially filled (e.g. air to metal interface for measuring near-surface stresses), or when the gauge volume composition is inhomogeneous (metal to metal interface), or the gauge volume material is anisotropic (welds with bundles of elongated grains). To mitigate such errors several approaches have been proposed using numerical and analytical methods to calculate the magnitude of pseudo-strain. Whether these methods can be applied depends on the instrument used and the type of sample being measured. In this context a different approach based on the Monte-Carlo method, as embedded in the neutron ray tracing software package McStas is proposed. The aim is to validate by neutron measurements the ability of McStas to simulate pseudo strains associated with a gauge volume that is partially filled, or a gauge volume that samples inhomogeneous and anisotropic material. The instrument modelled for this research project is ENGINX a time-of-flight instrument located at ISIS (RAL). To understand and isolate the effects of traversing an interface made up of different materials a stepwise approach was taken. The initial phase involved building a new model of ENGINX in McStas and to validate its correctness by analysing the characteristics of the beam and gauge volume by simulating a steel pin scan. The subsequent phase focussed on virtual experiments to study the pseudo strain arising when measuring strain in an air-to-material interface, a material-to-material interface and when large grains or pores are present as in the vicinity of welds. The results presented are not intended to be used as a quantitative prediction of pseudo strain but to demonstrate how McStas can be used to model virtual experiments to study the pseudo strains occurring. Several virtual sample models have been built to demonstrate how this could be useful/interesting for beamline scientists and users of neutron diffraction. One such virtual sample model is used to demonstrate that the mitigation technique of rotating the sample 180° works when the detector is in transmission but not in reflection, and that it is strongly dependent on the attenuation of the investigated material. Another virtual sample model is used to demonstrate that when the ratio of a cavity (hole, pore, etc) to the gauge volume is more than 2%, then significant pseudo strains can arise. Moreover, this work delivers a new model for ENGINX

NASA Tech Briefs, December 1989

Topics include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Computer Programs, Mechanics, Machinery, Fabrication Technology, Mathematics and Information Sciences, and Life Sciences

Investigation of Powder Recyclability and Liquid-Solid-Gas Interactions during Powder Bed Selective Laser Melting of Stainless Steel 316L

This dissertation pertains to the fundamental understanding of powder degradation and the dissolution of gas, specifically interstitials such as oxygen, into metal alloy powders and parts during selective laser melting (SLM) additive manufacturing (AM) and their impact on defect generation, microstructure, and mechanical properties of parts built using this process.A powder recyclability experiment was conducted to investigate the effect that powder re-use has on bulk powder and build properties. Mechanical testing and analysis of parts produced during the experiment showed higher than normal yield strengths and provided evidence of powder re-use affecting the ductility of SLM parts.A heat affected zone experiment was conducted to investigate the local property changes in unmelted powder. Local oxidation in unmelted powder near melt zones was observed and characterized. The size of the melt zone adjacent to the unmelted powder bed was shown not to be a contributing factor to the extent of local oxidation.A variable oxygen environment experiment was conducted to investigate the effect of elevated levels of atmospheric oxygen on stainless steel 316L builds. Mechanical testing results indicate that the mechanical properties of additively manufactured 316L parts are insensitive to significant variation of cover gas O2 concentration in the build chamber during processing.A cooling rate study was conducted in an effort to use laser processing parameters to manipulate the microstructure/mechanical properties of as-built parts. High-speed infrared (IR) data was analyzed and representative cooling rates of the parts and the surrounding powder bed as a function of build height were obtained.Finally, stainless steel 316L spatter material was observed to solidify as single-crystal ferrite. This is unusual behavior considering that the composition of stainless steel 316L usually causes it to solidify as austenite. The single-crystal nature of the ferritic spatter particles is particularly interesting and suggests that the particles solidify at rapid velocities and that conditions exist that allow for the nucleation of ferrite to outcompete that of the austenite phase

Research Reports: 1997 NASA/ASEE Summer Faculty Fellowship Program

For the 33rd consecutive year, a NASA/ASEE Summer Faculty Fellowship Program was conducted at the Marshall Space Flight Center (MSFC). The program was conducted by the University of Alabama in Huntsville and MSFC during the period June 2, 1997 through August 8, 1997. Operated under the auspices of the American Society for Engineering Education, the MSFC program was sponsored by the Higher Education Branch, Education Division, NASA Headquarters, Washington, D.C. The basic objectives of the program, which are in the 34th year of operation nationally, are: (1) to further the professional knowledge of qualified engineering and science faculty members; (2) to stimulate an exchange of ideas between participants and NASA; (3) to enrich and refresh the research and teaching activities of the participants' institutions; and (4) to contribute to the research objectives of the NASA centers. The Faculty Fellows spent 10 weeks at MSFC engaged in a research project compatible with their interests and background and worked in collaboration with a NASA/MSFC colleague. This document is a compilation of Fellows' reports on their research during the summer of 1997. The University of Alabama in Huntsville presents the Co-Directors' report on the administrative operations of the program. Further information can be obtained by contacting any of the editors

Technology 2002: the Third National Technology Transfer Conference and Exposition, Volume 1

The proceedings from the conference are presented. The topics covered include the following: computer technology, advanced manufacturing, materials science, biotechnology, and electronics

NASA Tech Briefs, Fall 1977

Topics include: NASA TU Services: Technology Utilization services that can assist you in learning about and applying NASA technology; New Product Ideas: A summary of selected Innovations of value to manufacturers for the development of new products; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Life Sciences; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences

Cumulative index to NASA Tech Briefs, 1986-1990, volumes 10-14

Tech Briefs are short announcements of new technology derived from the R&D activities of the National Aeronautics and Space Administration. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This cumulative index of Tech Briefs contains abstracts and four indexes (subject, personal author, originating center, and Tech Brief number) and covers the period 1986 to 1990. The abstract section is organized by the following subject categories: electronic components and circuits, electronic systems, physical sciences, materials, computer programs, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Chemistry of airborne particles from metallurgical processing

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.Vita.Includes bibliographical references.Airborne particles fall into one of three size ranges. The nucleation range consists of nanoparticles created from vapor atom collisions. The decisive parameter for particle size and composition is the supercooling of the vapor. The accumulation range, which comprises particles less than 2 micrometers, consists of particles formed from the collision of smaller primary particles from the nucleation range. The composition of agglomerates and coalesced particles is the same as the bulk vapor composition. Coarse particles, the composition of which is determined by a liquid precursor, are greater than 1 micrometer and solidify from droplets whose sizes are controlled by surface, viscous, and inertial forces. The relationship between size and composition of airborne particles could be seen in welding fume, a typical metallurgical aerosol. This analysis was performed with a cascade impactor and energy dispersive spectrometry with both scanning electron microscopy (SEM-EDS) and scanning transmission electron microscopy (STEM-EDS). Other methods for properly characterizing particles were discussed. In the analysis, less than 10% of the mass of fume particles for various types of gas metal arc welding (GMAW) were coarse, while one-third of flux cored arc welding (FCAW) fume particles were coarse. Coarse particles had a composition closer to that of the welding electrode than did fine particles. Primary particles were not homogeneous. Particles larger than the mean free path of the carrier gas had the same composition as that of the vapor, but for particles 20 to 60 nanometers, smaller particles were more enriched in volatile metals than larger particles were. This was explained by the cooling path along the bubble point line of a binary phase diagram.(cont.) Particles were not necessarily homogenous internally. Because nanoparticles homogenize quickly, they may form in a metastable state, but will not remain in that state. In this analysis, the presence of multiple stable immiscible phases explains this internal heterogeneity. The knowledge contained herein is important for industries that depend on the properties of nanoparticles, and for manufacturing, where industrial hygiene is important because of respirable particle by-products, such as high-energy-density metallurgical processing.by Neil Travis Jenkins.Ph.D

NASA Tech Briefs, April 1992

Topics covered include: New Product Ideas; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences