Tension-compression testing is commonly conducted to understand and predict springback during a stamping process. However, large strains are generally difficult to achieve during the in-plane compression portion of the test. Proper specimen design and control of frictional forces are necessary for obtaining large strains. This paper describes extensive finite element analyses (FEA) and optimization studies (Phase 1) that were conducted to calibrate the model test assembly for three different buckling modes obtained in uniaxial compression tests of aluminum alloy 2024 and American Iron and Steel Institute (AISI) 1008 steel specimens. In addition to obtaining these three buckling modes correctly, calibrated FEA model predicted forces matched measured forces reasonably well. Also, a good agreement between computed and measured stress-strain data was demonstrated for one compression experiment. In the Phase 2 optimization study, optimum specimen geometries will be developed by using these verified, optimum FEA model test assemblies in three types of compression buckling experiments.

In-situ strain neutron diffraction measurements were conducted at temperature on specimens coming from a clock-rolled alpha-uranium plate, and Elasto-Plastic Self-Consistent (EPSC) modeling was employed to interpret the findings. The modeling revealed that the active slip systems exhibit a thermally activated response, while deformation twinning remains athermal over the temperature ranges explored (25-150 degrees C). The modeling also allowed assessment of the effects of thermal residual stresses on the mechanical response during compression. These results are consistent with those from a prior study of room-temperature deformation, indicating that the thermal residual stresses strongly influence the internal strain evolution of grain families, as monitored with neutron diffraction, even though accounting for these residual stresses has little effect on the macroscopic flow curve, except in the elasto-plastic transition. (C) 2018 Elsevier B.V. All rights reserved.

Many advanced high-strength steels rely on a metastable austenite phase for improvements in strength and formability. To date, no method has demonstrated the ability to provide accurate austenite phase fraction measurements in textured steels. Several techniques have been proposed, such as averaging the intensity of several peaks and/or summation of intensity from several sample orientations. The series of numerical experiments performed in this work sought to quantify the effects of texture on the measurement of the austenite phase fraction, with an emphasis on techniques suitable for laboratory X-ray diffraction. Simulated diffraction profiles were created with the following variables: texture components for the ferrite and austenite phases, the sharpness of each of the texture components, the number of peaks used for averaging in the phase fraction calculation, and the sampling scheme used for sample orientation summation in the phase fraction calculation. The resulting phase fraction calculations showed that texture, the number of peak pairs and the sampling method have a drastic effect on phase fraction measurements, causing significant bias errors. Hexagonal grids produced minimal bias errors and demonstrated a robust method of measuring phase fractions in textured materials.

Due to the excellent balance of strength and ductility exhibited by some Mg-Zn-RE (Y subgroup rare earth element) alloys, which contain icosahedral quasicrystalline precipitates, it is of interest to examine their deformation mechanisms. The internal strain evolution Mg-3at%Zn-0.5 at%Y with 4 vol% i-phase was measured using in-situ neutron diffraction. The extruded samples exhibit an initially weak <10.0> || extrusion direction “rod texture,” distinct from the normally strong texture of extruded Mg alloys, but the grain size is unexceptional (16.7 ± 2.1 μm). The initially weak texture contributes to a nearly symmetric yielding response between tension and compression. The hardening responses are asymmetric, however, since {10.2} extension twinning is significantly more active during compressive straining, despite the initially weak texture. In-situ neutron diffraction tension and compression experiments parallel to the extrusion direction, together with elasto-plastic self-consistent (EPSC) crystal plasticity modeling, reveal the strength and hardening behavior of individual slip and twinning modes. The previously published twinning-detwinning (TDT) model is implemented within the EPSC framework, and it is proven effective for describing the observed, mild tension-compression asymmetry. This is not possible with previous EPSC-based models of twinning. Finally, the description of hardening within the TDT model is modified, in order to accurately describe the evolution of internal strains within the twins.

Finite element (FE) analysis was used to simulate the strain history of an α-uranium foil during cold straight-rolling, with the sheet modeled as an isotropic elastoplastic continuum. The resulting strain history was then used as input for a viscoplastic self-consistent (VPSC) polycrystal plasticity model to simulate crystallographic texture evolution. Mid-plane textures predicted via the combined FE→VPSC approach show alignment of the (010) poles along the rolling direction (RD), and the (001) poles along the normal direction (ND) with a symmetric splitting along RD. The surface texture is similar to that of the mid-plane, but with a shear-induced asymmetry that favors one of the RD split features of the (001) pole figure. Both the mid-plane and surface textures predicted by the FE→VPSC approach agree with published experimental results for cold straight-rolled α-uranium plates, as well as predictions made by a more computationally intensive full-field crystal plasticity based finite element model. α-uranium foils produced by cold-rolling must typically undergo a recrystallization anneal to restore ductility prior to their final application, resulting in significant texture evolution from the cold-rolled plate deformation texture. Using the texture measured from a foil in the final recrystallized state, coefficients of thermal expansion and the elastic stiffness tensors were calculated using a thermo-elastic self-consistent model, and the anisotropic yield loci and flow curves along the RD, TD, and ND were predicted using the VPSC code.

The a-phase transformation kinetics of as-cast U - 8 wt% Mo below the eutectoid temperature have been established by in situ neutron diffraction. alpha-phase weight fraction data acquired through Rietveld refinement at five different isothermal hold temperatures can be modeled accurately utilizing a simple Johnson-Mehl-Avrami-Kolmogorov impingement-based theory, and the results are validated by a corresponding evolution in the g-phase lattice parameter during transformation that follows Vegard's law. Neutron diffraction data is used to produce a detailed Time-Temperature-Transformation diagram that improves upon inconsistencies in the current literature, exhibiting a minimum transformation start time of 40 min at temperatures between 500 degrees C and 510 degrees C. The transformation kinetics of U - 8 wt% Mo can vary significantly from as-cast conditions after extensive heat treatments, due to homogenization of the typical dendritic microstructure which possesses non-negligible solute segregation. (C) 2016 Elsevier B.V. All rights reserved.

A yield plateau occurs in some Mg alloys during compressive deformation that has been ascribed to the localization of twinning. In fine-grained AZ31, this plateau was explained by a region of large twin volume fraction nucleating in a small band and propagating, similar to a "Luders band" like phenomena. Once the band traverses the entire gauge length, the sample begins to strain harden. Similarly, ZK60 samples exhibit the same Luders like phenomena during extrusion direction compression as confirmed by digital image correlation. However, the hand is not sufficient to fully explain the plateau in the stress-strain curve. Postmortem electron backscattered diffraction (EBSD) reveals the twin structure evolution through this plateau. It is found that twinning occurs in large elongated grains before spreading to the finer grains.

Recently published experimental results have suggested a connection between the I1 basal stacking fault and the non-basal 13〈112¯3〉 (i.e. 〈c+a〉) dislocation, particularly in Mg–Y and Mg–Y–Zn alloys. Deformed Mg–Y alloys contain more 〈c+a〉 dislocations and I1 faults than pure Mg, and the I1 fault energy has been shown to be significantly reduced by the addition of Y solute atoms using ab initio modeling. The TEM evidence of a possible connection between the two crystal defects is reanalyzed to reveal: (i) a non-planar dissociation of the pyramidal 〈c+a〉 edge dislocation into the basal plane, resulting in an I1 fault bounded by 16〈202¯3〉 partial dislocations; and/or (ii) a new source mechanism involving nucleation of the pyramidal 〈c+a〉 dislocation from a pre-existing I1 fault. The former reaction is energetically favorable, but non-conservative. The latter is shown to be energetically conceivable for a wide range of fault geometries. The concepts hypothesized in this paper provide explanations for the frequent TEM observation of rectilinear, edge 〈c+a〉 dislocations in Mg alloys, as well as a possible explanation for the yield strength anomaly observed in Mg and Mg alloy single crystals. Implications for radiation damage in hexagonal close packed metals are also suggested.

The presence of hydrogen is known to embrittle and drastically reduce the ductility of α-uranium. However, it has not been established whether aging amounts of hydrogen alter the mechanisms of plastic deformation (i.e. slip and twinning). In-situ neutron diffraction experiments and polycrystalline plasticity modeling were employed to address this issue. Samples were thermally charged to 0.3 and 1.8 wppm H then tested in-situ. It was seen that hydrogen charging decreased the ductility drastically. However, it did not affect the bulk plastic deformation processes. Specifically, the grain-level onset of dislocation glide and deformation twinning as well as the hardening behaviors of the individual mechanisms remained consistent between samples. The texture evolution did not change with hydrogen content either. What did clearly change with increasing hydrogen content was the fracture mechanism, which transitioned from transgranular ductile rupture to brittle intergranular decohesion.

The efficiency of the critical plane model of Smith, Watson, and Topper is demonstrated for the actuation lifetime prediction of shape memory alloys undergoing thermal cycling while under constant tensile stress. The model can be in principle extended to more complex actuation loading paths and predict the critical plane for failure in structural components. It is moreover important to note the correlation of the model to the actuation energy density for the data used for its experimental verification. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The strong thermoelastic–plastic anisotropy of single crystal, orthorhombic α-uranium leads to the generation of significant thermal residual stresses (TRSs) after cooling from the processing temperatures of polycrystals. The present study shows the effect that these TRSs have upon subsequent room temperature deformation after aging. In situ neutron diffraction strain experiments performed on specimens machined from a clock-rolled plate, combined with ex situ texture measurements and elastoplastic self-consistent polycrystalline plasticity modeling, provides the necessary information to quantify the relative strengths and activities of the various deformation mechanisms known to operate within α-uranium. The results demonstrate that the activation of hard slip modes is necessary in order to generate macroscopic flow at the stress levels observed experimentally. Although the thermal residual stresses do not drastically affect the “bulk” response, inclusion of the TRSs drastically improve the agreement between predicted and experimentally measured internal strain evolution and strongly alter the predicted slip and twinning mode activities. As such, modeling efforts to discern the roles of the various mechanisms must account for the presence of thermal residual stress. In agreement with prior crystal plasticity modeling efforts, the crystallographic texture and polarity of twinning mechanisms explain the tension–compression strength asymmetry observed in the material (i.e. significantly more of the soft {130} twinning mode is observed to occur during compression along the prior plate RD than tension along the same direction).

Lattice strains induced by cooling textured, depleted U, from 500 degrees C to ambient temperature are measured using in situ neutron diffraction and simulated using an elastoplastic self-consistent polycrystal model. The results show that the high anisotropy of the single-crystal thermoelastic response induces thermal stresses sufficient to cause plastic relaxation. Incorporation of 1/2 < 1 (1) over bar 0 >{1 1 0} slip enables modeling of the observed internal strain evolution, although diffusional effects may also contribute to the observed relaxation. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In situ neutron diffraction compression tests were performed on Mg–Y–Nd–Zr alloy WE43, in the solution heat-treated, peak- and over-aged conditions. The flow curves and internal strain evolutions were modeled using polycrystal plasticity simulation, with the inclusion of an elastic phase to account for the presence of precipitates. The results reveal that prismatic plate-shaped precipitates strongly impede basal slip; the critical resolved shear strength (CRSS) of basal slip increases from 12 to 37MPa, an increase of over 200%. However, hard deformation modes such as non-basal slip of 〈a〉 dislocations are required for macroscopic yielding. These hard modes are not as strongly affected by aging, with CRSS values which increase from 78 to 92MPa, an increase of only 18%. The results of the study are consistent with recent modeling of the relative Orowan strengthening of individual deformation modes and the superposition of various strengthening effects (solid solution and precipitation). This finding helps to explain why the age-hardening response of Mg–Y–Nd–Zr alloys is not exceptional. It is concluded that future precipitation-strengthened alloy and process design strategies should focus on promoting high number densities of particles. The effect of aging upon twinning is surprising. The most age-hardened material exhibits more twinning than the solutionized material. To model this behavior using polycrystal plasticity, the critical stress to activate twinning (especially the strain hardening thereof) must be decreased.

ABSTRACTMean stress effects in finite‐life fatigue are studied for a number of sets of experimental data for steels, aluminium alloys and one titanium alloy. Specifically, the agreement with these data is examined for the Goodman, Morrow, Smith–Watson–Topper and Walker equations. The Goodman relationship is found to be highly inaccurate. Reasonable accuracy is provided by the Morrow and by the Smith–Watson–Topper equations. But the Morrow method should not be used for aluminium alloys unless the true fracture strength is employed, instead of the more usual use of the stress‐life intercept constant. The Walker equation with its adjustable fitting parameter γ gives superior results. For steels, γ is found to correlate with the ultimate tensile strength, and a linear relationship permits γ to be estimated for cases where non‐zero mean stress data are not available. Relatively high‐strength aluminium alloys have γ≈ 0.5, which corresponds with the SWT method, but higher values of γ apply for relatively low‐strength aluminium alloys. For both steels and aluminium alloys, there is a trend of decreasing γ with increasing strength, indicating an increasing sensitivity to mean stress.

A study is reported of the cyclic deformation, fatigue, and mean stress relaxation behavior of extruded material of aluminum alloys 7075-T6511 and 7249-T76511. Experimental data and curve fitting constants are given for the cyclic stress-strain and strain-life curves. The stress-life (elastic strain) component of the strain-life curve shows a transition to a shallow slope at short life, necessitating a two-segment fit. Mean stress effects are included in the stress-life fit by applying the Walker mean stress method, and this is incorporated into the strain-life curve. Other mean stress relationships are also compared to the test data, specifically those of Goodman, Morrow, and Smith-Watson-Topper. Mean stress relaxation was observed even at quite low strain amplitudes where there is no measurable plastic deformation, with a transition in behavior to a strong relaxation effect at relatively large strains. Hence, the data are partitioned around 006 . 0 = ε a and different sets of relaxation constants