Papers by Author: Donald W. Brown

Abstract: Time-of-flight neutron measurements have been made at 20, 400 and 650oC on δ-processed Inconel 718 in order to measure the load sharing between the γ-phase matrix and the orthorhombic δ-phase. The strain response parallel and perpendicular to the applied stress was measured for seven γ-phase reflections and five δ-phase reflections. The latter were about 50 times weaker than the former suggesting a 2.0% concentration of the δ-phase. At all temperatures the δ-phase strain becomes strongly tensile parallel to the loading direction but also exhibits plastic deformation. However, the nature of the three orthorhombic strains changes with temperature.

Abstract: The high-resolution neutron diffraction and acoustic emission (AE) techniques have been used for in-situ investigation of deformation twinning and microstructure evolution in cast polycrystalline magnesium. The combination of these two techniques results in obtaining complementary information about the twinning mechanism and evolution of the dislocation structure during the straining. The dependence of the mechanisms of the plastic deformation on loading mode is discussed in detail. The microscopy investigations revealed a difference in twin number and size after tension and compression, respectively.

Abstract: Neutron diffraction measurements of strain in austenitic stainless steel alloys are presented to examine whether there are contributions to the shifts from deformation induced stacking faults. Differences between successive orders of reflections are consistent with the presence of stacking faults for 316 stainless steel but not for a NiCrFe steel.

Abstract: After an overload was imposed during a constant amplitude fatigue experiment, a retardation period was observed. The deformation in the vicinity of a crack tip was studied using neutron and x-ray microbeam-diffraction techniques, which provide millimeter and submicrometer spatial resolutions, respectively. From the neutron-diffraction measurements, compressive lattice strains and higher dislocation densities at the macroscale were observed in front of the crack tip, which indicates a plasticity induced crack-closure phenomenon. Furthermore, Laue patterns obtained from the microbeam diffraction at different locations near the crack tip show alternating regions with high and low dislocation densities at the mesoscale.

Abstract: Residual strain profiles in friction-stir processed (FSP) AZ31B magnesium-alloy plates were measured using neutron diffraction. Two different specimens were prepared to investigate the influences of the tool shoulder and the tool pin on the residual-strain profiles: (Case 1) a plate processed with both the stirring pin and tool shoulder, i.e., a regular FSP plate subjected to both the plastic deformation and frictional heating, and (Case 2) a plate processed only with the tool shoulder, i.e., subjected mainly to the frictional heating. The results show that the strain profiles of both cases are qualitatively quite similar. The longitudinal strain is mainly tensile with its maximum near the bead of the FSP plate. On the other hand, the transverse and normal strains are mildly compressive in both Cases 1 and 2.

Abstract: Textured Mg alloys exhibit tension – compression strength asymmetry due to mechanical twinning. The distinction arises as the material deforms primarily by slip in one direction and by twinning in the other. In-situ neutron diffraction during cyclic loading in tension and compression of extruded bar allows study of the effect of twinning on subsequent load reversals. The diffraction data reveal the texture evolution and internal stress development as a function of deformation. De-twinning resulted in complete texture reversal during initial cycles, but eventually “fatigued” resulting in some residual twin component.

Abstract: ULTIMET® alloy, a cobalt-based superalloy with good corrosion and wear resistant properties, exhibits a deformation-induced phase transformation from the face-centered-cubic (FCC) phase to the hexagonal-close-packed (HCP) phase. The HCP phase formation during monotonic tensile loading was investigated using in-situ neutron diffraction. The HCP phase is first observed at a stress level of 810 MPa, which is well beyond macroscopic yielding. Strain analysis is performed on the FCC phase diffraction data in order to relate the lattice-strain development with the evolution of the new HCP phase. A method of calculating the effective macroscopic stress associated with the measured lattice strains is presented here. The effective stress can then be compared to the applied macroscopic stress in order to draw conclusions about the load-partitioning behavior of the material as a new phase develops.

Abstract: The martensitic phase transformation in an ultra fine grained (UFG) TRIP (transformation induced plasticity) steel with combination of high strength and high elongation was investigated during room temperature tensile test using in situ neutron diffraction. Two types of specimens, namely coarse grained (grain size of about 50 μm) and ultra-fine-grained (grain size of about 350 nm) specimens were examined. The lattice strain evolution of the austenite and martensite phases was observed and the load partitioning between the phases was identified.

Abstract: Plastic deformation in cubic metals is relatively simple due to the high crystallographic symmetry of the underlying structure. Typically, one unique slip mode can provide arbitrary deformation. This is not true in lower symmetry hexagonal metals, where prismatic and basal slip (the usual favored modes) are insufficient to provide arbitrary deformation. Often, either pyramidal slip and/or deformation twinning must be activated to accommodate imposed plastic deformation. The varied difficulty of activating each of these deformation mechanisms results in a highly anisotropic yield surface and subsequent mechanical properties. Further, the relative activity of each deformation mode may be manipulated through control of the initial crystallographic texture, opening new opportunities for the optimization of mechanical properties for a given application.