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[en] For this research, Split-Hopkinson pressure bar dynamic compression experiments were conducted to determine the defect/interface interaction dependence on interface type, bilayer thickness and interface orientation with respect to the loading direction in the Ag-Cu eutectic system. Specifically, the deformation microstructure in alloys with either a cube-on-cube orientation relationship with (111)Ag||(111)Cu interface habit planes or a twin orientation relationship with (3̄13)Ag||(1̄12)Cu interface habit planes and with bilayer thicknesses of 500 nm, 1.1 µm and 2.2 µm were probed using TEM. The deformation was carried by dislocation slip and in certain conditions, deformation twinning. The twinning response was dependent on loading orientation with respect to the interface plane, bilayer thickness, and interface type. Twinning was only observed when loading at orientations away from the growth direction and decreased in prevalence with decreasing bilayer thickness. Twinning in Cu was dependent on twinning partial dislocations being transmitted from Ag, which only occurred for cube-on-cube interfaces. Lastly, dislocation slip and deformation twin transfer across the interfaces is discussed in terms of the slip transfer conditions developed for grain boundaries in FCC alloys.
[en] Beamline 12.3.2 at the Advanced Light Source is a newly commissioned beamline dedicated to x-ray microdiffraction. It operates in both monochromatic and polychromatic radiation mode. The facility uses a superconducting bending magnet source to deliver an X-ray spectrum ranging from 5 to 22 keV. The beam is focused down to ∼ 1 um size at the sample position using a pair of elliptically bent Kirkpatrick-Baez mirrors enclosed in a vacuum box. The sample placed on high precision stages can be raster-scanned under the microbeam while a diffraction pattern is taken at each step. The arrays of diffraction patterns are then analyzed to derive distribution maps of phases, strain/stress and/or plastic deformation inside the sample.
[en] Low cycle fatigue tests on cast nickel-based superalloy IN-100 were conducted at various temperatures from 20 to 10000C in air under continuous-strain cycling at a constant total strain rate. The fatigue life was found to decrease with increasing temperature for a given total strain range. Fatigue cracks are partly crystallographic at low temperatures and strongly oxidized at high temperatures. The reduction in fatigue life is discussed using potential drop measurements and observations on interrupted test specimens. Testing at 10000C is shown to reduce drastically the crack initiation period and this behaviour is attributed to oxidation. (orig.)
[en] In-situ neutron diffraction measurements were performed in this paper during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650 °C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Finally, preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development.
[en] Low-cycle fatigue tests on cast nickel-based superalloy IN-100 were conducted at 10000C in air using sawtooth strain cycles over a wide range of frequencies (4x10-3-2 Hz) and tensile strain hold time tests. The influence of environment was investigated using tests under vacuum at the same temperature. The low cycle fatigue life was found to be strongly frequency dependent between 5x10-2 and 1 Hz. Testing in vacuum results in a much longer fatigue life than in air except for the higher test frequency. Crack growth rate data were measured using the potential drop technique and were used to show that the fatigue life in air is dominated by crack growth and to assess the influence of oxidation effects. (orig.)
[en] In this paper, a coating of the Zr-based thin-film metallic glass (TFMG) was deposited on the Zr50Cu30Al10Ni10 bulk metallic glass (BMG) to investigate shear-band evolution under four-point-bend fatigue testing. The fatigue endurance-limit of the TFMG-coated samples is ~ 33% higher than that of the BMG. The results of finite-element modeling (FEM) revealed a delay in the shear-band nucleation and propagation in TFMG-coated samples under applied cyclic-loading. The FEM study of spherical indentation showed that the redistribution of stress by the TFMG coating prevents localized shear-banding in the BMG substrate. Finally, the enhanced fatigue characteristics of the BMG substrates can be attributed to the TFMG coatings retarding shear-band initiation at defects on the surface of the BMG.
[en] Here, failure by fatigue is a common problem associated with cast aluminum alloys due to defects like shrinkage porosities, non-metallic inclusions, etc. Friction stir processing (FSP) has recently emerged as an effective technique for local modification of microstructure. This study investigates the fatigue crack initiation and growth mechanisms in cast and FSPed A356 aluminum alloy. Two sets of parameters were used to friction stir the cast alloy resulting in the complete modification the cast microstructure to a wrought microstructure. Both the FSPed microstructures exhibited severe abnormal grain growth (AGG) after heat treatment leading to a multimodal grain size distribution – the grain sizes ranging from a few microns to a few millimeters. One of the FSP conditions displayed an excellent improvement in fatigue life by an order of magnitude, while the other condition displayed an unexpectedly large scatter in fatigue lives. Detailed study of the fractured fatigue specimens by electron back scattered diffraction (EBSD) revealed that both, fatigue crack initiation and propagation, were intimately tied to the grain size as well as the grain misorientations in the microstructure.
[en] The nucleation and growth of intergranular cavities during the creep of type 321 stainless steel were investigated. Creep samples were solution heat treated at 11120C for 7.2 ks and then subjected to a stabilization treatment of 288 ks at 9100C. A clean microstructure was obtained with only a few coarse primary precipitates. Four samples were then deformed in creep at 49.6 MPa and 8120C for 83.5, 135, 163 and 238 ks. After a careful sample preparation procedure, a large number of cavities were measured in each sample. The cavity size distributions in the samples were obtained. The rate of nucleation of cavities was found to be approximately proportional to time in the interval studies. The cavity growth rate as a function of the cavity size was calculated using the cavity size distributions and was compared with theoretical models. (orig.)
[en] The defect microstructure in aluminized coatings on Rene 80, Inconel 713LC and MAR-M 247 was studied by transmission electron microscopy. Dislocations and subgrain boundaries were found within the β phase and interface dislocations between the β phase and the precipitates. The growth of β grains was inhibited by the polygonized σ lamellae and carbides in the diffusion zone and by α-Cr in the outer zone. Dislocation loops were found in the β phase throughout the coatings. The origins of the defects are also discussed. (orig.)
[en] The effect of hydrogen on mechanical behaviour was studied in V-10at.% Ti alloys undergoing cyclic deformation. These alloys have high hydrogen solubilities and do not show any evidence of hydride formation at room temperature. Smooth bar fatigued specimens were tested using a microcomputer-controlled servohydraulic machine under plastic strain control. In fatigue, these alloys show that hydrogen produces hardening associated with an increase in the yield stress as well as the saturation stress. For the non-hydrogenated specimens and those containing 0.5 at.% H, the fracture mode was ductile in the fatigued region and a mixture of intergranular, cleavage and ductile in the overload region. For higher amounts of hydrogen the fracture mode was cleavage in the fatigued region. The area fraction of cleavage fracture depended on the plastic strain amplitude and the hydrogen content. For 1 at.% H the fracture was 100% cleavage, independent of the plastic strain amplitude. The role of cyclic deformation in the hydrogen-induced hardening, embrittlement and fracture mode transitions of these alloys will be discussed. (orig.)