The relationship between microstructure, dislocation motion and mechanical response of metallic multilayered nanomaterials is investigated. Several competing theories for the dependence of hardness on layer thickness, namely Confined Layer Slip (CLS) and Hall-Petch (H-P) theories are discussed. Analysis of homophase and heterophase experimental data suggests that Hall-Petch with modified coefficients provides a good fit down to layer thicknesses of about 5 nm, below which experimental data starts to deviate. We suggest that at this layer thickness, dislocations accumulate in the interface, and assuming there is a constant dislocation density in each interface, the strength varies as h(-1/2). (C) 2017 Published by Elsevier Ltd.

A validation is reported for micromechanical simulation using a reimplementation of an elastoviscoplastic FFT-based (EVPFFT) formulation, i.e., the Micromechanical Analysis of Stress-strain In homogeneities with fast Fourier transform (MASSIF) code, against experimental data obtained from synchrotron x-ray diffraction. The experimental data was collected during in-situ deformation of a titanium alloy specimen by High Energy Diffraction Microscopy (HEDM), which provided the average elastic strain tensor and orientation of each grain in a polycrystalline sample. MASSIF was used to calculate the local micromechanical fields in a Ti-7Al polycrystalline sample at different load levels. The initially attempted simulation showed that, although the effective response was calibrated to reproduce the experiment, MASSIF was not able to reproduce the micromechanical fields at the scale of individual grains. The differences between calculated and measured averages at the grain scale were related to initial residual strains resulting from the prior processing of the material, which had not been incorporated in the original calculation. Accordingly, a new simulation was instantiated using information on the measured residual strains to define a set of eigenstrains, calculated via an Eshelby approximation. This initialization significantly improved the correlation between calculated and simulated fields for all strain and stress components, for measurements performed within the elastic regime. For the measurements at the highest load, which was past plastic yield, the correlations deteriorated because of plastic deformation at the grain level and the lack of an accurate enough constitutive description in this deformation regime. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

As-processed metals and alloys by selective laser melting (SLM), or laser powder bed fusion (L-PBF), are often full of defects and flaws such as dislocations, twins, elemental segregations, impurities and porosities, which can positively or negatively impact mechanical properties. Here, we systematically characterize the tensile behavior of L-PBF Ti-6Al-4V at quasi-static strain rate and room temperature, including state-of-the-art in situ synchrotron X-ray diffraction (SXRD) and computed tomography (SXCT). These studies reveal that the tensile yield strength and uniform elongation are mainly dictated by the as-built microstructure, while the strain-to-failure is sensitive to the porosity, even in very high-density samples (>99.5\%). in situ SXRD reveals that the micro-plasticity in as-built Ti64 initiates at a stress level well below its macroscopic yield strength, signified by the early lattice strain deviation behavior of \0002\ and \11 (2) over bar0\ reflections. SXCT reveals pore growth mechanisms when the tensile axis is perpendicular to the build direction, whereas no such behavior is observed as the tensile axis is along the build direction. These anisotropic pore growth mechanisms result in vast differences in the strain-to-failure of L-PBF materials. Our melt-pool dynamics modeling with similar laser conditions to the experiments identifies a previously unknown pore source; i.e., edge-of-track pores. We present a normalized energy diagram to identify the optimized processing window for high quality samples. (C) 2018 Elsevier Ltd. All rights reserved.

The present study is devoted to the creation of a process-structure-property database for dual phase titanium alloys, through a synthetic microstructure generation method and a mesh-free fast Fourier transform based micromechanical model that operates on a discretized image of the microstructure. A sensitivity analysis is performed as a precursor to determine the statistically representative volume element size for creating 3D synthetic microstructures based on additively manufactured Ti-6Al-4V characteristics, which are further modified to expand the database for features of interest, e.g., lath thickness. Sets of titanium hardening parameters are extracted from literature, and The relative effect of the chosen microstructural features is quantified through comparisons of average and local field distributions.

The porosity observed in additively manufactured (AM) parts is a concern for components intended to undergo high-cycle fatigue. The morphology and location of pores can help identify their cause; irregularly shaped lack of fusion or key-holing pores can usually be linked to incorrect processing parameters, while spherical pores suggest trapped gas. Synchrotron-based X-ray microtomography was performed on laser powder-bed AM Ti-6Al-4V samples over a range of processing conditions to investigate the effects of processing parameters on porosity. The process mapping technique was used to control melt pool size. Tomography was also performed on the powder to measure porosity within the powder that may transfer to the parts. This work builds off the authors previous experiments with the Electron Beam Melting technology, which displayed significant variations in porosity with changes to processing parameters. A clear connection between processing parameters and resulting porosity formation mechanism was observed.

Abstract The aging behavior and orientation relationships in Fe—31.4Mn—11.4Al—0.89C low-density steel were investigated with respect to constituent phases and precipitates, including γ-austenite matrix, β-Mn, and α-precipitate. After aging treatment at 550 $\,^\circ$C for various periods of time, the microstructural changes and corresponding mechanical response were characterized by Vickers hardness measurement combined with EBSD and TEM observations. The precipitation sequence during the aging treatment showed that nano-sized κ-carbides firstly precipitated within the γ-austenite matrix at the incipient stage of aging, and induced the primary age hardening. After aging for 300 min, the lath-type β-Mn phase was formed, leading to the dramatic secondary hardening response. The α-precipitates with partial D03 order were subsequently produced at the β-Mn interior, grain/phase boundary region, and the γ-austenite interior after further aging over 10,000 min. The misorientation-angle distribution, Rodrigues—Frank vector space, and orientation relationship stereogram (OR stereogram) from EBSD measurements were employed for analyzing γ-matrix/β-Mn and β-Mn/α-precipitate interphase boundaries, respectively. The OR stereograms showed that the preferred orientation relationships were represented as (111)γ//(221)β−Mn, (011¯)γ//(012¯)β−Mn, (2¯11)γ//(5¯42)β−Mn for γ-matrix/β-Mn interface, and (012)β−Mn//(001)α, (021¯)β−Mn//(010)α, (100)β−Mn//(100)α for β-Mn/α-precipitate interface, respectively. The orientation relationships obtained from the OR stereograms were clarified by checking the deviation angle distributions of interface segments from the ideal orientation relationships, as well as the TEM diffraction patterns at the interface boundaries. In addition, the misorientation distribution between γ-matrix and α-precipitate was examined and compared to conventional fcc/bcc orientation relationships.

Abstract The finite-element (FE) model and the Rosenthal equation are used to study the thermal and microstructural phenomena in the laser powder-bed fusion of Inconel 718. A primary aim is to comprehend the advantages and disadvantages of the Rosenthal equation (which provides an analytical alternative to \FE\ analysis), and to investigate the influence of underlying assumptions on estimated results. Various physical characteristics are compared among the \FE\ model, Rosenthal equation, and experiments. The predicted melt pool shapes compared with reported experimental results from the literature show that both the \FE\ model and the analytical (Rosenthal) equation provide a reasonably accurate estimation. At high heat input, under conditions leading to keyholing, the reported melt width is narrower than predicted by the analytical equation. Moreover, a sensitivity analysis based on choices of the absorptivity is performed, which shows that the Rosenthal approach is more sensitive to absorptivity, compared with the \FE\ approach. The primary reason could be the effect of radiative and convective losses, which are assumed to be negligible in the Rosenthal equation. In addition, both methods predict a columnar solidification microstructure, which agrees well with experimental reports, and the primary dendrite arm spacing (PDAS) predicted with the two approaches is comparable with measurements.