Effect of temperature on the transition in deformation modes in Mg single crystals

Sim, Gi-Dong,Xie, Kelvin Y.,Hemker, Kevin J.,El-Awady, Jaafar A. Elsevier 2019 ACTA MATERIALIA Vol.178 No.-

<P><B>Abstract</B></P> <P>Here, an experimental study utilizing <I>in-situ</I> scanning electron microscopy (SEM) micro-compression testing and post-mortem transmission electron microscopy (TEM) imaging is presented to quantify the effect of temperature on the transition in deformation modes in twin-oriented Mg single crystals. Single crystal micropillars were fabricated using FIB milling, then tested by <I>in-situ</I> SEM micro-compression from 20 °C to 225 °C. It is observed that plasticity in the deformed Mg microcrystals at temperatures at and below 100 °C is governed by { 10 1 ¯ 2 } extension twinning. However, an anomalous increase of the flow stresses is observed at 100 °C, which is likely due to paucity of dislocation sources that are required to promote twin boundary migration. At 150 °C and above, extension twinning is suppressed and a continuous plastic flow and strain softening induced by prismatic dislocation mediated plasticity is observed. By comparing the current results with those from bulk scale studies for other hexagonal-closed-pack single crystals (e.g. titanium (Ti) and zirconium (Zr)), a general trend for the effect of temperature on the transition in deformation modes in HCP materials is proposed.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Fabrication and mechanical characterization of 3D woven Cu lattice materials

Zhang, Y.,Ha, S.,Sharp, K.,Guest, J.K.,Weihs, T.P.,Hemker, K.J. Elsevier Ltd 2015 Materials & Design Vol.85 No.-

3D metallic lattices designed to have two distinctly different material architectures have been woven with metallic Cu wires. A vacuum soldering technique was employed to metallurgically bond the wire nodes and form stiff 3D lattice materials. The structures and mechanical properties of the as-woven and soldered lattices were characterized by optical microscopy and micro-scale mechanical property experiments. The measured in-plane shear stiffness shows good agreement with predictions from finite element (FE) models that account for variations in the manufacturing and solder bonding. The study indicates that stiffness is influenced by the percentage of bonded nodes and the location of bonding. The 3D woven lattice materials manufactured in this study exhibited a very high percentage (80%) of bonded nodes and a unique combination of stiffness and density as compared to that typically reported for ultra lightweight lattice materials.

Tailoring the mechanical properties of sputter deposited nanotwinned nickel-molybdenum-tungsten films

Sim, Gi-Dong,Krogstad, Jessica A.,Xie, Kelvin Y.,Dasgupta, Suman,Valentino, Gianna M.,Weihs, Timothy P.,Hemker, Kevin J. Elsevier 2018 Acta materialia Vol.144 No.-

<P><B>Abstract</B></P> <P>Advanced metallic alloys are attractive in microelectromechanical systems (MEMS) applications that require high density, electrical and thermal conductivity, strength, and dimensional stability. Here we report the mechanical behavior of direct current (DC) magnetron sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) films annealed at various temperatures. The films deposit as single-phase nanotwinned solid solutions and possess ultra-high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and nucleation of Mo-rich precipitates. While films annealed at 600 °C or 800 °C for 1 h still showed brittle behavior, films annealed at 1,000 °C for 1 h were found to exhibit strength greater than 1.2 GPa and near 10% tensile ductility. In addition to the excellent mechanical properties, alloy films further exhibit remarkably improved dimensional stability – a lower coefficient of thermal expansion and greater microstructural stability. An excellent balance between mechanical properties and dimensional stability make sputter deposited Ni-Mo-W alloys promising structural materials for MEMS applications.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Combining a distributed flow manifold and 3D woven metallic lattices to enhance fluidic and thermal properties for heat transfer applications

Zhao, Longyu,Ryan, Stephen M.,Lin, Sen,Xue, Ju,Ha, Seunghyun,Igusa, Takeru,Sharp, Keith W.,Guest, James K.,Hemker, Kevin J.,Weihs, Timothy P. Elsevier 2017 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.108 No.2

<P><B>Abstract</B></P> <P>The fluidic and heat transfer capabilities of 3D woven lattice materials were reported recently under axial and bifurcated flow patterns, but three critical performance indices – pressure drop, average surface temperature and temperature uniformity – could not be optimized simultaneously using these flow patterns. Here we combine the 3D weaves with manifolds to create a novel 3D flow pattern that enhances temperature uniformity, while also maintaining low pressure drops and surface temperatures. These three properties were characterized at room temperature for a range of flow rates using water as the working fluid. Three different weaves thicknesses were investigated: 12.7mm, 6.4mm, and 3.2mm, with manifold thicknesses of 12.7mm, 19.0mm, and 22.2mm, respectively, to provide a constant, combined weave-manifold thickness of 25.4mm. The properties of this new weave/manifold system are compared to those obtained using just the manifold (with no weave) and just the weave (with no manifold). Comparisons show that the addition of the weave lowers the average substrate temperature and temperature variations significantly, although pressure drop is increased. They also show that the addition of the manifold improves temperature uniformity significantly, and also lowers the average substrate temperature and the pressure drop. No specific ratio of weave to manifold thickness was found to be superior in all of the performance indices. The thermal performances are then evaluated at different pumping powers: the weave/manifold system and its distributed array flow pattern prevail. Finite element simulations were performed on a reduced and simplified model to explain the observed experimental trends, and manifold opening patterns were manipulated to demonstrate further potential property enhancements. The multiple benefits of this manifold system can be extended to common heat exchanger media beyond weaves.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Propose and design novel flow manifold that is combined with 3D woven lattices. </LI> <LI> Characterization of pressure drop, average temperature and temperature uniformity. </LI> <LI> Much superior thermal performance with 3D woven lattices than open spaces. </LI> <LI> Comparison of the new distributive flow pattern with axial and bifurcated flow cases. </LI> <LI> Trends between experiments and simulations are in good agreement. </LI> </UL> </P>

Tensile behavior of Al_1-xMo_x crystalline and amorphous thin films

Gianola, D.S.,Lee, Z.,Ophus, C.,Luber, E.J.,Mitlin, D.,Dahmen, U.,Hemker, K.J.,Radmilovic, V.R. Elsevier Science 2013 ACTA MATERIALIA Vol.61 No.5

The exceptional strength and distinct deformation physics exhibited by pure ultrafine-grained and nanocrystalline metals in comparison to their microcrystalline counterparts have been ascribed to the dominant influence of grain boundaries in accommodating plastic flow. Such grain-boundary-mediated mechanisms can be augmented by additional strengthening in nanocrystalline alloys via solute and precipitate interactions with dislocations, although its potency is a function of the changes in the elastic properties of the alloyed material. In this study, we investigate the elastic and plastic properties of Al<SUB>1-x</SUB>Mo<SUB>x</SUB> alloys (0≤x≤0.32) by tensile testing of sputter-deposited freestanding thin films. Isotropic elastic constants and strength are measured over the composition range for which three microstructural regimes are identified, including solid solutions, face-centered cubic and amorphous phase mixtures and body-centered cubic (bcc)/amorphous mixtures. Whereas the bulk modulus is measured to follow the rule of mixtures over the Mo composition range, the Young's and shear moduli do not. Poisson's ratio is non-monotonic with increasing Mo content, showing a discontinuous change at the onset of the bcc/amorphous two-phase region. The strengthening measured in alloyed thin films can be adequately predicted in the solid solution regime only by combining solute strengthening with a grain boundary pinning model. The single-step co-sputtering procedure presented here results in diversity of alloy compositions and microstructures, offering a promising avenue for tailoring the mechanical behavior of thin films.