Wanchuck Woo

Effect of wavelength-dependent attenuation on neutron diffraction stress measurements at depth in steels

The wavelength dependence of the maximum feasible penetration depth was studied for neutron diffraction stress measurements in ferritic and austenitic steels. This property was examined with wavelengths from the close vicinity of the Bragg edges, where the neutron total cross section has its local minimum and for which the scattering angles are convenient for stress measurements. These wavelengths (e.g. 2.39 and 2.19 A) are longer than those commonly used in stress measurements (∼1.6 A). By using such wavelengths, configured by a focusing bent perfect crystal Si(111) monochromator, it was observed that the available total beam path length is about 85 mm in both ferritic and austenitic steels. This study provides specific information for choosing the instrument configuration suitable for most strain-scanning experimental tasks.

Deconvoluting the influences of heat and plastic deformation on internal strains generated by friction stir processing

Internal-strain profiles in friction-stir processed aluminum-alloy plates were investigated using neutron diffraction. Three different specimens were prepared with a purpose of separating the effects of frictional heating and severe plastic deformation on the internal-strain distribution: (Case 1) a plate processed with both stirring pin and tool shoulder, (Case 2) a plate processed only with the tool shoulder, and (Case 3) a plate processed only with the pin. The comparison between Cases 1 and 2 shows distinctly different strain profiles revealing deconvoluted effects of the different sources (i.e., heat, deformation, or the combination) on the internal strains generated during the friction-stir processing.

Softening behaviour of friction stir welded Al 6061-T6 and Mg AZ31B alloys

Abstract The influence of friction stir welding (FSW) on changes in microstructures has been studied extensively. In particular, the microstructural softening, which is manifested as a significant reduction in the hardness, tensile strength and residual stress, is one of the most significant issues. Here, we provide a brief review of the dominant softening mechanisms in FSW aluminium and magnesium alloys available in the literature. In particular, a direct comparison between a heat treatable Al 6061-T6 alloy and an AZ31B Mg alloy is presented to highlight key differences in the underlying mechanisms responsible for the apparently similar softening behaviour. The present study shows that the softening occurs in the wide region of the FSW Al 6061-T6 alloy due to the dissolution of the strengthening precipitates, while it happens mostly within the stir zone due to the localised texture variations in the FSW Mg AZ31B alloy. These softening behaviours are clearly represented in the residual stress profiles.

Effect of interlayer sliding on the estimation of elastic modulus of multilayer graphene in nanoindentation simulation

Nanoindentation experiments and simulations are carried out to estimate the elastic modulus of freely-suspended-multilayer graphene. However, due to the difficulty of clamping all layers of multilayer graphene in experiments, and to the ambiguity of imposing the clamped boundary conditions in numerical simulations, the estimated values of elastic modulus exhibit large variation. In particular, interlayer sliding can affect the estimation of elastic modulus. From a series of molecular dynamics simulations, we demonstrate that the estimated elastic modulus of multilayer graphene depends on the level of interlayer sliding involved in boundary conditions. Under fully clamped boundary conditions that prevent interlayer sliding, the elastic modulus is constant regardless of the number of layers. In contrast, under weakly clamped boundary conditions that involve interlayer sliding, the elastic modulus decreases with increasing number of layers. In the case of weakly clamped conditions, a few wrinkles form in the interlayer and then coalesce into a single large wrinkle due to interlayer sliding. Our findings provide an understanding of the variation of elastic modulus observed in other experimental and numerical studies.

Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.

Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction

Quantitative determination of the stress fields around the crack tip is a challenging and important subject to understand the fatigue crack-growth mechanism. In the current study, we measured the distribution of residual stresses and the evolution of the stress fields around a fatigue crack tip subjected to the constant-amplitude cyclic loading in a 304L stainless steel compact-tension (CT) specimen. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the CT specimen were determined as a function of distance from the crack tip with 1-mm spatial resolution along the crack-propagation direction. In-situ neutron-diffraction results show that the enlarged tensile stresses were developed during loading along the through-thickness direction at a localized volume close to the crack tip, resulting in the lattice expansion in all three orthogonal directions during Pmax. The current study suggests that the atypical plane strainlike behavior observed at the midthickness position might be the reason for the mechanism of the faster crack-growth rate inside the interior than that near the surface.

Double bent crystal dispersive arrangement for high resolution diffractometry

In this paper some properties of the dispersive double-bent-crystal setting are described. Thanks to an easy manipulation with focusing of the monochromatic beam, high reflection probability of both bent crystals and very narrow (1-3 mm) obtained monochromatic beam such an arrangement could be attractive for an employment. We tested the properties of the double-bent-crystal setting of the bent Si(111) + bent Si(220) slabs on the diffractometer at the neutron wavelength of 0.163 nm for various curvatures of the Si(220) slabs. By using a standard polycrystalline sample of Fe it has been found that besides an excellent resolution the neutron current is sufficient even for powder diffraction experiments.

Influence of multiple small-angle neutron scattering on diffraction peak broadening in ferritic steel

Peak broadening of neutron diffraction was studied at various neutron wavelengths (1.24–2.61 A). As the neutron beam path through a specimen increased, significant peak broadening was observed in ferritic steel, but not in austenitic steel. The peak broadening was reduced under a magnetic field applied perpendicular to the beam direction. Small-angle neutron scattering results showed significant reduction in scattering intensities under a magnetic field of 1.2 T. It is suggested that the peak broadening can be attributed to multiple small-angle neutron scattering by magnetic domains. Thus, a sufficiently strong magnetic field could enhance the deep penetration capability of neutron diffraction by reducing the peak broadening.

Residual stress determination in thick welded steel plates

Through thickness strain distribution of 50 mm thick welded ferritic steel (bcc) plate was studied using diffraction reflections 211 and 110 and neutron wavelengths of 1.55 and 2.39A, respectively. Experimental results showed that for stress measurements in a possibly maximum thick weld, the different strain components should be measured with different reflections 211 and 110. The strains measured with these reflections for the same component are close. Since planes (211) and (110) of bcc ferrite have the same diffraction elastic constants the appropriate values of stresses could be derived from strains measured with reflections 211 and 110.

Neutron diffraction studies of a high resolution double crystal (+n,-m) setting containing Si(220) and Si(311) bent perfect crystals in symmetric and fully asymmetric diffraction geometry, respectively

In this paper studies of neutron diffraction properties of the double crystal (+n,-m) setting containing Si(220) and Si(311) bent perfect crystals (BPC) in symmetric and fully asymmetric diffraction (FAD) geometry with the output beam expansion (OBE), respectively, are presented. Namely, our attention was focused on the properties of the FAD geometry of the BPC Si(311) crystal slab. It has been found that after a beam expansion this FAD geometry can provide a monochromatic beam of a rather large cross-section and of very small divergence with some possible application use.