Papers by Keyword: Neutron Diffraction

Abstract: A great variety of techniques are nowadays used to spray coatings with different functionality and properties for the purpose of surface enhancement. Depending on application and design, these can be thermal (plasma or high-velocity oxy-fuel are the most widely used) and warm or cold spraying, which are known to generate considerable residual stresses. This stress is a function of the spaying process as well as the material and thickness of the coating-substrate system. The mechanical integrity of coatings is critical for certain applications, e.g. wear resistant and thermal-barrier coatings, hence residual stress control and mitigation are essential in preventing the coating’s mechanical failure, improving the coating’s performance and the its operational lifetime. Although hole drilling technique or x-ray diffraction combined with layer removal method can be applicable for stress measurements in coatings, the neutron diffraction stress analysis also provides an effective and efficient tool for non-destructive through-thickness stress measurements with a commensurately high resolution, down to 0.1-0.2 mm. The most recent results of neutron diffraction stress measurements in coating systems are presented herein.

Abstract: Suspension-bridge cables are constructed from strands of galvanized steel wire. They are failure-critical structural members, so a fundamental understanding of their mechanics is imminently important in quantifying suspension bridge safety. The load-carrying capabilities of such strands after local wire failures have been the subject of many theoretical studies utilizing analytical equations and finite-element analysis. Little experimental data, however, exists to validate these models.Over the past five years we have developed a methodology for measuring stress/strain transfer within parallel wire strands of suspension bridge cables using neutron diffraction [1,2]. In this paper we describe the design and verification of parallel cable strands used in our studies. We describe the neutron diffraction strain measurements performed on standard 7-wire and expanded 19-wire models in various configurations at both the Los Alamos National Laboratory Spectrometer for Materials Research at Temperature and Stress (LANL SMARTS) and at the Oak Ridge National Laboratory VULCAN Engineering Materials Diffractometer (ORNL VULCAN). Particular attention is placed on the challenges of aligning and measuring multibody systems with high strain gradients at body-to-body contact points.

Abstract: The high flux neutron diffractometer STRESSSPEC at FRM II, Garching Germany, offers a flexible instrument setup suitable for fast and surface residual strain measurements. Likewise bulk, local or gradient texture analyses are feasible. Here improvements of the hardware (detector, slits) of the instrument as well as developments on methods for residual stress analysis are presented. A new detector system developed inhouse was recently installed and successfully commissioned. Compared to the original delay line detector the new detector provides much higher resolution and allows event mode type measurements. Results of the commissioning measurements show a performance increase of nearly a factor of 2 compared to the former detector. Moreover the new analytical model, recently developed for surface spurious strain corrections, was successfully applied at a welded austenitic steel sample. Thus nondestructive measurements from the surface (200 μm) into the bulk (several millimeters) are possible without any extra time consuming experiments for spurious strains corrections.

Abstract: The accurate determination of strain during measurement using neutron diffraction depends on many factors. The statistical uncertainty of the diffraction data is not always the most important contributor to the total uncertainty in the measured strain. Other contributors, such as sample positioning, size and shape of the sampling (gauge) volume and the size and distribution of grains within the sampling volume, often play an important role as well. Grain size issues have been the least studied and their impact is often ignored even though the potential uncertainty contribution can be large. Certain methods such as oscillating the sample during measurement can help in reducing the magnitude of the grain size effect and hence also that of the related uncertainty contribution. A thorough characterization of uncertainties due to grain size effects however, in terms of absolute values that should be added to the statistical peak fitting uncertainties has not yet been implemented. This paper will present an improved method to characterize and estimate absolute uncertainty values due to grain size effects.

Abstract: To understand the strengthening mechanism of a metallic material with high dislocation density, the plastic deformation behavior of lath martensite was studied by means of in situ neutron diffraction measurements during tensile deformations using a 22SiMn2TiB steel and a Fe-18Ni alloy. The characteristics of dislocation were analyzed and were discussed with the relation of stress-strain curves. The dislocation densities (ρ) induced by martensitic transformation during heat-treatment in both materials were found to be originally as high as 10¹⁵ m^-2 order, and subsequently to increase slightly by the tensile deformation. The parameter M value which displays the dislocation arrangement dropped drastically at the beginning of plastic deformation in both materials, indicating that the random arrangement became more like a dipole arrangement.

Abstract: The evolution of deformation mechanisms in randomly textured magnesium alloy during uniaxial and biaxial mechanical tests has been monitored using concurrent application of acoustic emission and neutron diffraction methods. The influence of the loading path on both twinning and dislocation slip is discussed in detail. It is shown that both the twinning and non-basal slip are sensitive to the loading direction.

Abstract: Measurement of residual strains by neutron diffraction of linear friction welded Inconel^® 718 (IN 718) superalloy acquired from a mid-service aero-engine disk was undertaken in this study. Residual strain and stress throughout the various weld regions including the heat affected zone (HAZ), thermomechanical affected zone (TMAZ) and dynamically recrystallized zone (DRX) were characterized. The residual stresses were observed to increase from the base material to the weld interface, with a peak stress at the weld interface in all orthogonal directions. The trends for residual stress across the weld are in agreement with other work published in literature for solid state welding of aerospace alloys, where high residual stresses were commonly reported at the weld interface.

Abstract: In this study local texture of process vessels made of carbon steel cladded by protective layers of stainless steel by submerged arc welding (SAW) were investigated by neutron diffraction using the diffractometer STRESSSPEC at FRM 2 (Garching, Germany). Different samples were prepared: as welded and as welded plus relevant industrial heat treatment. Local texture measurements with a gauge volume of 3 x 3 x 2 mm³ of the three cladding layers (at depths of 2 mm, 5 mm and 7.5 mm) for each sample were determined. Texture results indicated that there exists an annealed cube component in all the studied samples. Based on the measured pole figures at each depth and sample, the calculated orientation distribution functions data were used to calculate the Young's modulus with respect to the main welding directions. The calculated local and bulk anisotropic Young’s modulus in depth is presented and discussed.

Abstract: The development and application of low density alloys, such as Al and Mg alloys, has rapidly increased in the automotive sector in recent years. This necessitates advanced characterization techniques to assess the evolution of microstructure and phases during casting and processing. Further, understanding the mechanism of evolution of the defects is important in ensuring their minimization. Neutron diffraction has provided a method to determine the factors that trigger hot tearing in Al and Mg alloys as well as determining factors compromising integrity of powertrain components. In addition, neutron diffraction has been applied to examine the phase evolution during solidification of Al and Mg alloys enabling a better understanding of the effect of inoculants and solute additions on the solidification characteristics, resulting in improved castability. This paper highlights the frontiers of neutron diffraction analysis undertaken by the Centre for Near-Net-Shape Processing of Materials, Ryerson University and the CNL-Canadian Neutron Beam Centre.

Abstract: The authors have developed the texture measurement system at iMATERIA, which is the neutron diffractometers built in Materials and Life Science Experimental Facility (MLF) at J-PARC, Japan. The high flux of the incident beam and Time-Of-Flight method enabled the complete texture measurement within several minutes in case of steels. Since the neutron beam can transmit most of the materials, the measured texture represents the state of whole exposed volume. The multi-histogram analysis also enables to determine phase fractions in a multiphase material as well as the texture of each phase.