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Acta Mater 2018 ;152

Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD, 20899, USA.

Elemental segregation is a ubiquitous phenomenon in additive-manufactured (AM) parts due to solute rejection and redistribution during the solidification process. Using electron microscopy, synchrotron X-ray scattering and diffraction, and thermodynamic modeling, we reveal that in an AM nickel-based superalloy, Inconel 625, stress-relief heat treatment leads to the growth of unwanted δ-phase precipitates on a time scale much faster than that in wrought alloys (minutes versus tens to hundreds of hours). The root cause for this behavior is the elemental segregation that results in local compositions of AM alloys outside the bounds of the allowable range set for wrought alloys. Read More

Acta Mater 2017 Oct 4;139:244-253. Epub 2017 May 4.

Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, U. S. A.

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Read More

Acta Mater 2016 Jun 18;112:84-93. Epub 2016 Apr 18.

Center for Automotive Lightweighting, National Institute of Standards and Technology, USA.

Constitutive behaviors of an interstitial-free steel sample were measured using an augmented Marciniak experiment. In these tests, multiaxial strain field data of the flat specimens were measured by the digital image correlation technique. In addition, the flow stress was measured using an X-ray diffractometer. Read More

Acta Mater 2016 Mar 21;106:272-282. Epub 2016 Jan 21.

Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A.

The microstructurally-induced heterogeneous stress fields arising in a series of Cr-doped polycrystalline alumina materials are mapped with sub-micrometer sub-grain size resolution using fluorescence microscopy. Analysis of the hyperspectral data sets generated during imaging enabled both the amplitude and position of the characteristic Cr R1 fluorescence peak to be determined at every pixel in an image. The peak amplitude information was used to segment the images into individual grains and grain boundary regions. Read More

Acta Mater 2016 Jun 13;111:385-398. Epub 2016 Apr 13.

X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA.

The precipitate structure and precipitation kinetics in an Al-Cu-Mg alloy (AA2024) aged at 190 °C, 208 °C, and 226 °C have been studied using Transmission Electron Microscopy (TEM) and synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates while SAXS and WAXS provide qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence. Within the experimental time resolution, precipitation at these ageing temperatures involves dissolution of nanometer-sized small clusters and formation of the planar S phase precipitates. Read More

Acta Mater 2015 Sep;97:245-256

New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA.

Currently, there is significant interest in magnetocaloric materials for solid state refrigeration. In this work, polycrystalline Heusler alloys belonging to the NiMnGa family, with x between 0.08 and 0. Read More

Acta Mater 2015 Sep 12;96:66-71. Epub 2015 Jun 12.

Center for Superconducting and Magnetic Materials (CSMM), Department of Materials Science and Engineering, The Ohio State University Columbus, OH 43210 USA.

Significantly enhanced critical current density () for MgB superconducting wires can be obtained following the advanced internal Mg infiltration (AIMI) route. But unless suitable precautions are taken, the AIMI-processed MgB wires will exhibit incomplete MgB layer formation, i.e. Read More

Acta Mater 2015 Nov 30;100:126-134. Epub 2015 Aug 30.

Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic.

Nucleation and growth kinetics of nanoparticles of hexagonal phase in a body-centered cubic titanium matrix in single crystals of -Ti alloys were investigated by small-angle x-ray scattering measured during ageing at various temperatures up to 450 °C. The experimental data were compared with numerical simulations based on a three-dimensional short-range order model of nanoparticle self-ordering. The x-ray contrast of the particles is caused by an inhomogeneous distribution of impurity atoms (Mo, Fe and Al), whose density profile around growing nanoparticles was simulated by solving the corresponding diffusion equation with moving boundary conditions. Read More

Acta Mater 2015 May;89:278-289

Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany.

Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. Read More

Acta Mater 2015 Jan;83:276-284

Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, Vienna University of Technology, A-1040 Vienna, Austria ; Institute of Materials Science and Technology, Vienna University of Technology, A-1040 Vienna, Austria.

The complex structure of TaO led to the development of various structural models. Among them, superstructures represent the most stable configurations. However, their formation requires kinetic activity and long-range ordering processes, which are hardly present during physical vapor deposition. Read More

Acta Mater 2014 Sep;77(100):401-410

Erich Schmid Institute of Materials Science, Jahnstrasse 12, 8700 Leoben, Austria.

Ultra-fine-grained high-purity copper (99.99%) deformed by means of high-pressure torsion into the steady-state regime was subjected to additional rolling deformation. The microstructural changes as a function of the applied strain were analysed by means of orientation imaging microscopy. Read More

Acta Mater 2014 Apr;68(100):189-195

Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.

The release of excess volume upon recrystallization of ultrafine-grained Cu deformed by high-pressure torsion (HPT) was studied by means of the direct technique of high-precision difference dilatometry in combination with differential scanning calorimetry (DSC) and scanning electron microscopy. From the length change associated with the removal of grain boundaries in the wake of crystallite growth, a structural key quantity of grain boundaries, the grain boundary excess volume or expansion [Formula: see text] m was directly determined. The value is quite similar to that measured by dilatometry for grain boundaries in HPT-deformed Ni. Read More

Acta Mater 2013 Feb;61(3):931-944

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, USA.

Advances in hydrogel design have revolutionized the way biomaterials are applied to address biomedical needs. Hydrogels were introduced in medicine over 50 years ago and have evolved from static, bioinert materials to dynamic, bioactive microenvironments, which can be used to direct specific biological responses such as cellular ingrowth in wound healing or on-demand delivery of therapeutics. Two general classes of mechanisms, those defined by the user and those dictated by the endogenous cells and tissues, can control dynamic hydrogel microenvironments. Read More

Acta Mater 2012 Sep;60(15):5481-5493

Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria.

Phase separation in a three-component system that results from the uphill diffusion of chemical components is considered. The binary decomposition model of Cahn and Hilliard is generalized to account for the interdiffusion of several chemical components with considerably different diffusion constants. Thereafter the decomposition dynamics and the phase relations of the final system state are investigated by means of finite-element modeling. Read More

Acta Mater 2013 May;61(8):2973-2983

Department of Materials Physics, Montanuniversität Leoben, Jahnstr. 12, A-8700 Leoben, Austria.

The fracture behaviour of pure iron deformed by equal-channel angular pressing via route A was examined. The fracture toughness was determined for different specimen orientations and measured in terms of the critical plane strain fracture toughness, , the critical integral, , and the crack opening displacement for crack initiation, . The results demonstrate that the crack plane orientation has a pronounced effect on the fracture toughness. Read More

Acta Mater 2013 Apr;61(7):2425-2433

Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria.

Tensile specimens of metal films on compliant substrates are widely used for determining interfacial properties. These properties are identified by the comparison of experimentally observed delamination buckling and a mathematical model which contains the interface properties as parameters. The current two-dimensional models for delamination buckling are not able to capture the complex stress and deformation states arising in the considered uniaxial tension test in a satisfying way. Read More

Acta Mater 2012 Mar;60(5):2091-2096

Department Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria ; Christian Doppler Laboratory for Application Oriented Coating Development, Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria.

Monolithic single phase cubic (c) Ti Al N thin films are used in various industrial applications due to their high thermal stability, which beneficially effects lifetime and performance of cutting and milling tools, but also find increasing utilization in electronic and optical devices. The present study elucidates the temperature-driven evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and links them to structural and chemical changes in Ti Al N coatings. It is shown that various decomposition phenomena, involving recovery and spinodal decomposition (known to account for the age hardening phenomenon in c-Ti Al N), as well as the cubic to wurtzite phase transformation of spinodally formed AlN-enriched domains, effectively increase the thermal conductivity of the coatings from ∼3. Read More

Acta Mater 2012 Apr;60(6-7):2568-2577

Institute of Materials Science and Technology, Vienna University of Technology, Karlsplatz 13/308, A-1040 Vienna, Austria ; Eötvös Loránd University, Department of Materials Physics, POB 32, H-1518 Budapest, Hungary.

The solidification sequence of an AlMg4.7Si8 alloy is imaged in situ by synchrotron microtomography. Tomograms with (1. Read More

Acta Mater 2013 Feb;61(3):912-930

Institute for BioNanotechnology in Medicine, Feinberg School of Medicine, Northwestern University, 303 E. Superior St., Suite 11-131, Chicago, IL 60611, USA.

The use of self-assembly for the construction of functional biomaterials is a highly promising and exciting area of research, with great potential for the treatment of injury or disease. By using multiple noncovalent interactions, coded into the molecular design of the constituent components, self-assembly allows for the construction of complex, adaptable, and highly tunable materials with potent biological effects. This review describes some of the seminal advances in the use of self-assembly to make novel systems for regenerative medicine and biology. Read More

Acta Mater 2012 Feb;60(3):860-871

Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstr. 12, A-8700 Leoben, Austria.

Fully dense bulk nanocomposites have been obtained by a novel two-step severe plastic deformation process in the immiscible Fe-Cu system. Elemental micrometer-sized Cu and Fe powders were first mixed in different compositions and subsequently high-pressure-torsion-consolidated and deformed in a two-step deformation process. Scanning electron microscopy, X-ray diffraction and atom probe investigations were performed to study the evolving far-from-equilibrium nanostructures which were observed at all compositions. Read More

Acta Mater 2011 Nov;59(19):7228-7240

Erich Schmid Institute of Materials Science - Austrian Academy of Sciences, Jahnstr. 12, A-8700 Leoben, Austria.

Ni with different purities between 99.69 and 99.99 wt. Read More

Acta Mater 2011 Sep;59(16):6420-6432

Vienna University of Technology, Institute of Materials Science and Technology, Karlsplatz 13/308, A-1040 Vienna, Austria.

The three-dimensional (3-D) architecture of rigid multiphase networks present in AlSi10Cu5Ni1 and AlSi10Cu5Ni2 piston alloys in as-cast condition and after 4 h spheroidization treatment is characterized by synchrotron tomography in terms of the volume fraction of rigid phases, interconnectivity, contiguity and morphology. The architecture of both alloys consists of α-Al matrix and a rigid long-range 3-D network of Al(7)Cu(4)Ni, Al(4)Cu(2)Mg(8)Si(7), Al(2)Cu, Al(15)Si(2)(FeMn)(3) and AlSiFeNiCu aluminides and Si. The investigated architectural parameters of both alloys studied are correlated with room-temperature and high-temperature (300 °C) strengths as a function of solution treatment time. Read More

Acta Mater 2010 Oct;58(17):5631-5638

Physics of Nanostructured Materials, University of Vienna, Boltzmanngasse 5, A-1090 Wien, Austria.

The deformation-induced nanostructure developed during high-pressure torsion of B2 long-range ordered FeAl is shown to be unstable upon heating. The structural changes were analyzed using transmission electron microscopy, differential scanning calorimetry and microhardness measurements. Heating up to 220 °C leads to the recurrence of the chemical long-range order that is destroyed during deformation. Read More

Acta Mater 2010 Jun;58(10):3773-3780

Materials Research Institute, Pennsylvania State University, University Park, PA, 16802.

Electric fatigue tests have been conducted on pure and manganese modified Pb(In(0.5)Nb(0.5))O(3)-Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PIN-PMN-PT) single crystals along different crystallographic directions. Read More

Acta Mater 2009 May;57(9):2721-2729

Department of Biomaterials and Biomimetics, New York University College of Dentistry, 345 East 24th Street, New York, NY 10010, USA.

We present a new concept for strengthening ceamics by utilizing a graded structure with a low elastic modulus at both top and bottom surfaces sandwiching a high-modulus interior. Closed-form equations have been developed for stress analysis of simply supported graded sandwich beams subject to transverse center loads. Theory predicts that suitable modulus gradients at the ceramic surface can effectively reduce and spread the maximum bending stress from the surface into the interior. Read More

Acta Mater 2007 Oct;55(18):6365-6371

Materials Department, University of California, Santa Barbara, CA 93106, USA.

Using mechanics models of contacting surfaces under both normal and tangential loads, a mechanistic framework has been developed for assessing material resistance to initiation of abrasive damage. Solutions are presented for the critical loads to initiate yielding and cracking at blunt contacts, as well as those to attain a prescribed plastic penetration depth and cause cracking from within the plastic zone at sharp contacts. Material property groups that characterize abrasion resistance emerge from the models. Read More

Acta Mater 2007 Jan;55(1):131-139

Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294, USA.

The orientation texture of pulsed laser deposited hydroxyapatite coatings was studied by X-ray diffraction techniques. Increasing the laser energy density of the KrF excimer laser used in the deposition process from 5 to 7 J/cm(2) increases the tendency for the c-axes of the hydroxyapatite grains to be aligned perpendicular to the substrate. This preferred orientation is most pronounced when the incidence direction of the plume is normal to the substrate. Read More

Acta Mater 2007 Apr;55(7):2479-2488

Ceramics Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8520, USA.

A study is made of the competition between failure modes in ceramic-based bilayer structures joined to polymer-based substrates, in simulation of dental crown-like structures with a functional but weak "veneer" layer bonded onto a strong "core" layer. Cyclic contact fatigue tests are conducted in water on model flat systems consisting of glass plates joined to glass, sapphire, alumina or zirconia support layers glued onto polycarbonate bases. Critical numbers of cycles to take each crack mode to failure are plotted as a function of peak contact load on failure maps showing regions in which each fracture mode dominates. Read More