We acknowledge support from the National Natural Science Foundation of China (NSFC) under grant numbers 11621062, 11772294, U1530402, and 11811530001. This research was also partially supported by the China Postdoctoral Science Foundation (2021M690044). This research used the resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract number DE-AC02-05CH11231 and the Shanghai Synchrotron Radiation Facility. This research was partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 1606856.

1. Sample preparation
<ol>
	<li>Obtain 3 nm, 20 nm, 40 nm, 70 nm, 100 nm, 200 nm, and 500 nm nickel powder from commercial sources (see Table of Materials). The morphology characterization is shown in Figure 1.</li>
	<li>Prepare 8 nm nickel particles by heating 3 nm nickel particles using a reaction kettle (see Table of Materials).
	<ol>
		<li>Put ~20 mL of absolute ethanol and ~50 mg of 3 nm nickel powder into the reaction kettle. NOTE: The whole solution should not reach ~70% of the kettle volume.</li>
		<li>Heat the reaction kettle at 80 &#176;C for 24 h.</li>
		<li>Cool the solution to room temperature and drop a little solution to one copper mesh (TEM grid, see Table of Materials).</li>
		<li>Put the dried copper mesh into the Transmission Electron Microscopy (TEM) chamber and observe the sample morphology under 200 kV voltage electron beam. 
		NOTE: The copper mesh was air-dried for ~5 min or used a drying light of 5 min.</li>
		<li>Measure the particle size distribution from the TEM images manually. 
		NOTE: The particle size measurement can also be done using any freely available software such as Image J.</li>
		<li>Take out the solution and vaporize the ethanol at room temperature; then, the rest of the black solid is the pure nickel powder with an average particle size of 8 nm.</li>
	</ol>
	</li>
	<li>Prepare 12 nm nickel powder
	<ol>
		<li>Repeat step 1.2, but change the &#34;absolute ethanol&#34; and &#34;80 &#176;C for 24 h&#34; to &#34;absolute isopropanol&#34; and &#34;150 &#176;C for 12 h&#34; to obtain the pure nickel powder with the average particle of 12 nm.</li>
	</ol>
	</li>
</ol>
2. High-pressure radial DAC XRD measurements
<ol>
	<li>Make X-ray transparent boron-epoxy gasket utilizing a laser drilling machine (see Table of Materials).
	<ol>
		<li>Prepare Kapton (a kind of plastic) supporting gaskets 
		NOTE: Kapton is a polyimide film material (see Table of Materials).
		<ol>
			<li>Cut the inner circle with a laser drilling machine using the mentioned parameters: 35% laser power, three passes, 0.4 mm/s (cutting speed).</li>
			<li>Cut the outer rectangular using the parameters: 80% of laser power, two passes, 1.2 mm/s (cutting speed). The rectangular dimension is 8 x 1.4 mm.</li>
		</ol>
		</li>
		<li>Prepare boron-epoxy gaskets from a larger boron disc with a diameter of ~10 mm. 
		NOTE: The boron disc is made by compressing the mixture of amorphous boron powder and epoxy glue36.
		<ol>
			<li>Polish the raw discs to a thickness of 60-100 &#181;m with sandpaper manually. 
			NOTE: The sandpaper is from ~400 mesh to ~1000 mesh.</li>
			<li>Cut the inner circles with a laser drilling machine using the mentioned parameters: 35% laser power, three passes, 0.4 mm/s (cutting speed).</li>
			<li>Cut the outer circle with a laser drilling machine: 30% of laser power, one pass, 0.4 mm/s (cutting speed). Repeat and stop immediately when it comes off.</li>
		</ol>
		</li>
		<li>Assemble the gaskets
		<ol>
			<li>Place a Kapton supporting gasket (prepared in step 2.1.1) on a glass slide.</li>
			<li>Place a drilled boron gasket on the inner hole of the Kapton gasket. Ensure that the larger end of the boron gasket is at the top.</li>
			<li>Put another clean glass slide on the top. Hold it firmly and press it till the gasket is firmly inserted in the hole of Kapton gasket.</li>
			<li>Store the fabricated gasket assemblies between two clean glass slides and wrap them with glue tape for future use. 
			NOTE: The gasket diameter, &#216; = diamond culet size + 150 &#181;m. For better reproducibility, the same setups can be used (possibly with small adjustments if something is found wrong) for the laser drilling and cutting during the gasket preparation. For good size matching, the diameter of gaskets entered for laser cutting is &#216; + 23 &#181;m while the diameter of the inner hole of the Kapton supporting gaskets (entered for laser cutting) is &#216; - 23 &#181;m.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Radial DAC experiment loading
	<ol>
		<li>Mount the gasket assembly
		<ol>
			<li>On the viewing computer monitor (connected to the optical microscope), mark a dot to locate the center of the diamond (the piston diamond of the DAC).</li>
			<li>Mount the boron-epoxy gasket (prepared in step 2.1) and the mark at the center of the gasket hole.</li>
			<li>Use a glass slide to press down the gasket assembly such that the gasket firmly sets on the diamond of the piston. 
			NOTE: A DAC has two identical pieces of diamonds. Generally, the upper one is called cylinder diamond, and the lower one is called piston diamond.</li>
		</ol>
		</li>
		<li>Cleaning and compacting the gasket setup
		<ol>
			<li>Load samples with a chunk size smaller than the gasket hole such that there is no overflow of materials on the gasket surface. 
			NOTE: The samples here mean the candidate materials that we studied in our experiments. In this study, the samples are different-sized Ni powders and Pt chips.</li>
			<li>Close the cell after the loading of a new piece of sample to achieve compactness.</li>
		</ol>
		</li>
		<li>Loading of soft materials (such as gold)
		<ol>
			<li>Load only one piece of the soft sample (make the soft material as a small fraction of the loaded materials).</li>
			<li>Use hard amorphous materials to fill up the gasket hole for good compactness.</li>
		</ol>
		</li>
		<li>Loading of low atomic number materials (such as spinel, pyrope, serpentine)
		<ol>
			<li>Mix the sample with 10% Pt or Au. Fill up the gasket hole with the mixture but without overflow.</li>
			<li>If necessary, put hard amorphous materials on the top for good compactness.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>X-ray diffraction study
	<ol>
		<li>Mount the X-ray transparent boron-Kapton gasket (prepared in step 2.1) with a thickness of 100 &#181;m and a chamber hole of 60 &#181;m on the top of 300 &#181;m culet of DAC with the support of the clays.</li>
		<li>Place a small piece of Pt chip on top of the Ni sample as a pressure calibrant. 
		NOTE: No pressure medium was used to maximize the differential stress between the axial and radial.</li>
		<li>Use a monochromatic synchrotron X-ray (see Table of Materials) with an energy of 25 or 30 keV to conduct the x-ray diffraction experiments.</li>
		<li>Focus the X-ray beam to ~30 x 30 &#181;m2 surface area on the sample.</li>
		<li>Collect the X-ray diffraction patterns at pressure intervals of 1-2 GPa by a two-dimension image plate (see Table of Materials) with a resolution of 100 &#181;m/pixel. The setup used is shown in Figure 2 and Figure 3.</li>
	</ol>
	</li>
	<li>The experimental data analysis
	<ol>
		<li>Process each X-ray diffraction image into a file containing 72 spectra over 5&#176; azimuthal steps using Fit2d37,38,39,40,41,42. 
		NOTE: A two-dimensional diffraction image contains 360&#176; information. To analyze the stress and texture information, it is needed to separate into 72 files with 5&#176; azimuthal information contained in each one. Fit2d is the software used to analyze X-ray diffraction data37,38,39,40,41,42.</li>
		<li>Refine the diffraction pattern with the Rietveld method in the MAUD37 software. The lattice strain of each lattice plane was obtained by fitting the pattern37,40.</li>
		<li>Calculate the differential stress and yield strength according to the lattice strain theory38 and von Mises yield criterion38,39 following step 2.5.</li>
	</ol>
	</li>
	<li>The lattice strain theory for the experimental data analysis
	<ol>
		<li>Determine the differential stress (the difference between these maximum (&#963;22 = &#963;33) and minimum stress (&#963;11) components) that provides a lower-bound estimate of a material&#39;s yield strength38, &#963;y, based on the von Mises yield criterion following equation (1)38: 
		(1)&#160;t= &#963;11-&#963;33&#60;2&#964;= &#963;y.</li>
		<li>Obtain the direction-dependent deviatoric strain Qhkl by measuring the d-spacings from different diffraction directions by following equation (2)38: 
		(2) <img alt="Equation 1" src="/files/ftp_upload/61819/61819eq01.jpg" /> 
		where d0&#176; and d90&#176; are the d-spacings measured from &#936; = 0&#176; and &#936; = 90&#176; (the angle between the diffraction vector and load axis), respectively.</li>
		<li>Then, derive the value of t using equation (3)38: 
		<img alt="Equation 2" src="/files/ftp_upload/61819/61819eq02.jpg" /> 
		where GR(hkl) and Gv are the shear modulus of the aggregates under the Reuss (iso-stress) condition and Voigt (iso-strain) condition, respectively; &#945; is the factor to determine the relative weight of Reuss and Voigt conditions40. 
		NOTE: Considering the current experiments&#39; complicated stress/strain conditions, &#945; = 0.5 is used in this study.</li>
		<li>For a cubic system, calculate GR(hkl) and Gv as follows using equations 4-638,40,41: 
		(4) <img alt="Equation 3" src="/files/ftp_upload/61819/61819eq03.jpg" /> 
		(5) <img alt="Equation 4" src="/files/ftp_upload/61819/61819eq04.jpg" /> 
		(6) <img alt="Equation 5" src="/files/ftp_upload/61819/61819eq05.jpg" /> 
		where Sij are the single crystal elastic compliances and can be obtained from the elastic stiffness constants Cij of materials.</li>
	</ol>
	</li>
</ol>
3. TEM measurements
<ol>
	<li>Prepare thin pressurized Ni foils for TEM using a focused ion beam (FIB) system (see Table of Materials). To reduce possible artifacts during ion milling of the specimen, deposit a protective Pt layer using the Pt gun equipped in the SEM with a thickness of ~1 &#181;m on the candidate region.</li>
	<li>Perform TEM measurements on a 300 kV aberration-corrected transmission electron microscope equipped with high angle annular dark-field (HAADF) and bright-field (BF) detectors.</li>
	<li>Take high-resolution TEM images.</li>
</ol>

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<li>Singh, A. K., Balasingh, C., Mao, H. K., Hemley, R. J., Shu, J. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Analysis+of+lattice+strains+measured+under+nonhydrostatic+pressure%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">Analysis of lattice strains measured under nonhydrostatic pressure.</a> Journal of Applied Physics. 83 (12), 7567-7575 (1998).</li>
<li>Hemley, R. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((X-ray+imaging+of+stress+and+strain+of+diamond%2C+iron%2C+and+tungsten+at+megabar+pressures%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">X-ray imaging of stress and strain of diamond, iron, and tungsten at megabar pressures.</a> Science. 276 (5316), 1242-1245 (1997).</li>
<li>Merkel, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Deformation+of+polycrystalline+MgO+at+pressures+of+the+lower+mantle%5BTitle%5D)+AND+%22Journal+of+Geophysical+Research-Solid+Earth%22%5BJournal%5D)">Deformation of polycrystalline MgO at pressures of the lower mantle.</a> Journal of Geophysical Research-Solid Earth. 107, 2271(2002).</li>
<li>Singh, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+lattice+strains+in+a+specimen+%28cubic+system%29+compressed+nonhydrostatically+in+an+opposed+Anvil+device%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">The lattice strains in a specimen (cubic system) compressed nonhydrostatically in an opposed Anvil device.</a> Journal of Applied Physics. 73 (9), 4278-4286 (1993).</li>
<li>Van Swygenhoven, H., Derlet, P. M., Frøseth, A. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Stacking+fault+energies+and+slip+in+nanocrystalline+metals%5BTitle%5D)+AND+%22Nature+Materials%22%5BJournal%5D)">Stacking fault energies and slip in nanocrystalline metals.</a> Nature Materials. 3 (6), 399-403 (2004).</li>
<li>Chung, H. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Synthesis+of+ultra-incompressible+superhard+rhenium+diboride+at+ambient+pressure%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Synthesis of ultra-incompressible superhard rhenium diboride at ambient pressure.</a> Science. 316 (5823), 436-439 (2007).</li>
<li>Jo, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Theory+for+plasticity+of+face-centered+cubic+metals%5BTitle%5D)+AND+%22Proceedings+of+the+National+Academy+of+Sciences%22%5BJournal%5D)">Theory for plasticity of face-centered cubic metals.</a> Proceedings of the National Academy of Sciences. 111 (18), 6560(2014).</li>
<li>Klueh, R. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Miniature+tensile+test+specimens+for+fusion+reactor+irradiation+studies%5BTitle%5D)+AND+%22Nuclear+Engineering+and+Design%2C+Fusion%22%5BJournal%5D)">Miniature tensile test specimens for fusion reactor irradiation studies.</a> Nuclear Engineering and Design, Fusion. 2 (3), 407-416 (1985).</li>
<li>Konopík, P., Farahnak, P., Rund, M., Džugan, J., Rzepa, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Applicability+of+miniature+tensile+test+in+the+automotive+sector%5BTitle%5D)+AND+%22IOP+Conference+Series%3A+Materials+Science+and+Engineering%22%5BJournal%5D)">Applicability of miniature tensile test in the automotive sector.</a> IOP Conference Series: Materials Science and Engineering. 461, 012043(2018).</li>
<li>Yang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Strength+enhancement+of+nanocrystalline+tungsten+under+high+pressure%5BTitle%5D)+AND+%22Matter+and+Radiation+at+Extremes%22%5BJournal%5D)">Strength enhancement of nanocrystalline tungsten under high pressure.</a> Matter and Radiation at Extremes. 5 (5), 058401(2020).</li>
<li>Chen, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Exploring+nanomechanics+with+high-pressure+techniques%5BTitle%5D)+AND+%22Matter+and+Radiation+at+Extremes%22%5BJournal%5D)">Exploring nanomechanics with high-pressure techniques.</a> Matter and Radiation at Extremes. 5 (6), (2020).</li>
</ol>

The authors have nothing to disclose.

Computational simulations have been widely employed to study the grain size effect on the strength of nanometals5,6,16,17,27,42. Perfect dislocations, partial dislocations, and GB deformation have been proposed to play decisive roles in the deformation mechanisms of the nanomaterials. In a molecular dynamics simulation, Yamak...

The resistance to plastic deformation determines the materials&#39; strength. The strength of the metals usually increases with the decreasing grain sizes. This size strengthening phenomenon can be well illustrated by the traditional Hall-Petch relationship theory from the millimeter down to submicron regime1,2, which is based on the dislocation-mediated deformation mechanism of bulk-sized metals, i.e., dislocations pile up at grain boundaries (GBs) and hinder their motions, leading to the mechanical strengthening in metals3,4.
In contrast, mechanical softening, often referred to as the inverse Hall-Petch relationship, has been reported for fine nanometals in the last two decades5,6,7,8,9,10. Therefore, the strength of the nanometals is still puzzling as continuous hardening was detected for grain sizes down to ~10 nm11,12, while the cases of size softening below 10 nm regime were also reported7,8,9,10. The main difficulty or challenge for this debated topic is to make statistically reproducible measurements on the mechanical properties of ultrafine nanometals and establish a reliable correlation between the strength and grain size of the nanometals. Another part of the difficulty comes from the ambiguity in the plastic deformation mechanisms of the nanometals. Various defects or processes at nanoscale have been reported, including dislocations13,14, deformation twinning15,16,17, stacking faults15,18, GB migration19, GB sliding5,6,20,21, grain rotation22,23,24, atomic bond parameters25,26,27,28, etc. However, which one dominates the plastic deformation and thus determines the strength of nanometals is still unclear.
For these above issues, traditional approaches of mechanical strength examining, such as tensile test29, Vickers hardness test30,31, nano-indentation test32, micropillar compression33,34,35, etc. are less effective because the high quality of large pieces of nanostructured materials is so difficult to fabricate and conventional indenter is much larger than single nanoparticle of materials (for the single-particle mechanics). In this study, we introduce radial DAC XRD techniques36,37,38 to material science to in situ track the yield stress and deformation texturing of nano nickel of various grain sizes, which are used in the geoscience field in previous studies. It has been found that the mechanical strengthening can be extended down to 3 nm, much smaller than the previously reported most substantial sizes of nanometals, which enlarges the regime of conventional Hall-Petch relationship, implying the significance of rDAC XRD techniques to material science.

<table><tbody><tr><td>20 nm Ni</td><td>Nanomaterialstore</td><td>SN1601</td><td>Flammable</td></tr><tr><td>3 nm Ni</td><td>nanoComposix</td><td></td><td>Flammable</td></tr><tr><td>40, 70, 100, 200, 500 nm Ni</td><td>US nano</td><td>US1120</td><td>Flammable</td></tr><tr><td>Absolute ethanol</td><td></td><td></td><td>as the solution to make 8 nm Ni</td></tr><tr><td>Absolute isopropanol</td><td></td><td></td><td>as the solution to make 12 nm Ni</td></tr><tr><td>Amorphous boron powder</td><td>alfa asear</td><td></td><td></td></tr><tr><td>Copper mesh</td><td>Beijing Zhongjingkeyi Technology Co., Ltd.</td><td></td><td>TEM grid</td></tr><tr><td>Epoxy glue</td><td></td><td></td><td></td></tr><tr><td>Ethanol</td><td></td><td></td><td>clean experimental setup</td></tr><tr><td>Focused ion beam</td><td>FEI</td><td></td><td></td></tr><tr><td>Glass slide</td><td></td><td></td><td></td></tr><tr><td>Glue tape</td><td>Scotch</td><td></td><td></td></tr><tr><td>Kapton</td><td>DuPont</td><td></td><td>Polyimide film material</td></tr><tr><td>Laser drilling machine</td><td></td><td></td><td>located in high pressure lab of ALS</td></tr><tr><td>Monochromatic synchrotron X-ray</td><td>Beamline 12.2.2, Advanced Light Source (ALS), Lawrence Berkeley National Laboratory</td><td></td><td>X-ray energy: 25-30 keV</td></tr><tr><td>Optical microscope</td><td>Leica</td><td></td><td>to mount the gasket and load samples</td></tr><tr><td>Pt powder</td><td>thermofisher</td><td>38374</td><td></td></tr><tr><td>Reaction kettle</td><td>Xian Yichuang Co.,Ltd.</td><td></td><td>50 mL</td></tr><tr><td>Sand paper</td><td></td><td></td><td>from 400 mesh to 1000 mesh</td></tr><tr><td>Transmission Electron Microscopy</td><td>FEI</td><td></td><td>Titan G2 60-300</td></tr><tr><td>Two-dimension image plate</td><td>ALS, BL 12.2.2</td><td></td><td>mar 345</td></tr></tbody></table>

determining the mechanical strength of ultra-fine-grained metals

The mechanical strengthening of metals is the long-standing challenge and popular topic of materials science in industries and academia. The size dependence of the strength of the nanometals has been attracting a lot of interest. However, characterizing the strength of materials at the lower nanometer scale has been a big challenge because the traditional techniques become no longer effective and reliable, such as nano-indentation, micropillar compression, tensile, etc. The current protocol employs radial diamond-anvil cell (rDAC) X-ray diffraction (XRD) techniques to track differential stress changes and determine the strength of ultrafine metals. It is found that ultrafine nickel particles have more significant yield strength than coarser particles, and the size strengthening of nickel continues down to 3 nm. This vital finding immensely depends on effective and reliable characterizing techniques. The rDAC XRD method is expected to play a significant role in studying and exploring nanomaterial mechanics.

Under hydrostatic compression, unrolled X-ray diffraction lines should be straight, not curved. However, under non-hydrostatic pressure, the curvature (ellipticity of the XRD rings, which translates into the non-linearity of the lines plotted along the azimuth angle) significantly increases ultrafine-grained-nickel at similar pressures (Figure 4). At a similar pressure, the differential strain of the 3 nm sized nickel is the highest. The mechanical strength results (stress-strain curves) are...

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Determining the Mechanical Strength of Ultra-Fine-Grained Metals

The protocol presented here describes the high-pressure radial diamond-anvil-cell experiments and analyzing the related data, which are essential for obtaining the mechanical strength of the nanomaterials with a significant breakthrough to the traditional approach.

The mechanical strengthening of metals is the long-standing challenge and popular topic of materials science in industries and academia. The size ...

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2.2K Views.  Harbin Institute of Technology. The protocol presented here describes the high-pressure radial diamond-anvil-cell experiments and analyzing the related data, which are essential for obtaining the mechanical strength of the nanomaterials with a significant breakthrough to the traditional approach.

Video: Determining the Mechanical Strength of Ultra-Fine-Grained Metals

Quantitative Hardness Measurement by Instrumented AFM-indentation

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness measurements on an Au(111) surface with atomistic resolution of single plasticity events. A careful experimental procedure is described that includes the force sensor selection, its calibration, the calibration of the cantilever deflection detection system, and the minimization of instrumental drift for accurate and reproducible force-distance measurements. Also, a method for the data analysis is presented that allows the extraction of force-penetration curves from recorded force-distance curves. A typical curve displays a clear elastic deformation regime up to the first plasticity event, or pop-in, with a length in the range of one to two Burger's vectors. Later plasticity events exhibit the same magnitude. The work of plasticity is further extracted from the measurements. Finally, the hardness is determined in combination with the indentation curve using non-contact atomic force microscopy images of the remaining indents.

This experimental protocol describes how atomic force microscopy can be used to measure hardness at the true nanometer scale and to detect single atomistic plasticity events.

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness ...

Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and nanocrystalline materials. The traditional techniques associated with scanning electron microscopy (SEM), such as electron backscatter diffraction (EBSD), do not possess the required spatial resolution due to the large interaction volume between the electrons from the beam and the atoms of the material. Transmission electron microscopy (TEM) has the required spatial resolution. However, due to a lack of automation in the analysis system, the rate of data acquisition is slow which limits the area of the specimen that can be characterized. This paper presents a new characterization technique, Transmission Kikuchi Diffraction (TKD), which enables the analysis of the microstructure of UFG and nanocrystalline materials using an SEM equipped with a standard EBSD system. The spatial resolution of this technique can reach 2 nm. This technique can be applied to a large range of materials that would be difficult to analyze using traditional EBSD. After presenting the experimental set up and describing the different steps necessary to realize a TKD analysis, examples of its use on metal alloys and minerals are shown to illustrate the resolution of the technique and its flexibility in term of material to be characterized.

This paper provides a detailed method to characterize the microstructure of ultra-fine grained and nanocrystalline materials using a scanning electron microscope equipped with a standard electron backscatter diffraction system. Metal alloys and minerals presenting refined microstructures are analyzed using this technique, showing the diversity of its possible applications.

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Processing of Bulk Nanocrystalline Metals at the US Army Research Laboratory

Given their potential for significant property improvements relative to their large grained counterparts, much work has been devoted to the continued development of nanocrystalline metals. Despite these efforts, the transition of these materials from the lab bench to actual applications has been blocked by the inability to produce large scale parts that retain the desired nanocrystalline microstructures. Following the development of a method proven to stabilize the nanosized grain structure to temperatures approaching that of the melting point for the given metal, the US Army Research Laboratory (ARL) has progressed to the next stage in the development of these materials - namely the production of large scale parts suitable for testing and evaluation in a range of relevant test environments. This report provides a broad overview of the ongoing efforts in the processing, characterization, and consolidation of these materials at ARL. In particular, focus is placed on the methodology used for producing the nanocrystalline metal powders, in both small and large-scale amounts, that are at the center of ongoing research efforts.

This paper provides a brief overview of the ongoing efforts at the Army Research Laboratory on the processing of bulk nanocrystalline metals with an emphasis on the methodologies used for the production of the novel metal powders.

Given their potential for significant property improvements relative to their large grained counterparts, much work has been devoted to the continued ...

Fused Filament Fabrication (FFF) of Metal-Ceramic Components

Technical ceramics are widely used for industrial and research applications, as well as for consumer goods. Today, the demand for complex geometries with diverse customization options and favorable production methods is increasing continuously. With fused filament fabrication (FFF), it is possible to produce large and complex components quickly with high material efficiency. In FFF, a continuous thermoplastic filament is melted in a heated nozzle and deposited below. The computer-controlled print head is moved in order to build up the desired shape layer by layer. Investigations regarding printing of metals or ceramics are increasing more and more in research and industry. This study focuses on additive manufacturing (AM) with a multi-material approach to combine a metal (stainless steel) with a technical ceramic (zirconia: ZrO2). Combining these materials offers a broad variety of applications due to their different electrical and mechanical properties. The paper shows the main issues in preparation of the material and feedstock, device development, and printing of these composites.

Fused Filament Fabrication FFF of Metal-Ceramic Components

This study shows multi-material additive manufacturing (AM) using fused filament fabrication (FFF) of stainless steel and zirconia.

Technical ceramics are widely used for industrial and research applications, as well as for consumer goods. Today, the demand for complex geometries with ...

Fabrication of Mechanically Tunable and Bioactive Metal Scaffolds for Biomedical Applications

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness and low bioactivity of metals. In this study, we have developed a new method towards fabricating a new type of bioactive and mechanically reliable porous metal scaffolds-densified porous Ti scaffolds. The method consists of two fabrication processes, 1) the fabrication of porous Ti scaffolds by dynamic freeze casting, and 2) coating and densification of the porous scaffolds. The dynamic freeze casting method to fabricate porous Ti scaffolds allowed the densification of porous scaffolds by minimizing the chemical contamination and structural defects. The densification process is distinctive for three reasons. First, the densification process is simple, because it requires a control of only one parameter (degree of densification). Second, it is effective, as it achieves mechanical enhancement and sustainable release of biomolecules from porous scaffolds. Third, it has broad applications, as it is also applicable to the fabrication of functionally graded porous scaffolds by spatially varied strain during densification.

Bioactive and mechanically reliable metal scaffolds have been fabricated through a method which consists of two processes, dynamic freeze casting for the fabrication of porous Ti, and coating and densification of the Ti scaffolds. The densification process is simple, effective and applicable to the fabrication of functionally graded scaffolds.

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness ...

Bioengineering

Micromechanical Tension Testing of Additively Manufactured 17-4 PH Stainless Steel Specimens

This study presents a methodology for the rapid fabrication and micro-tensile testing of additively manufactured (AM) 17-4PH stainless steels by combining photolithography, wet-etching, focused ion beam (FIB) milling, and modified nanoindentation. Detailed procedures for proper sample surface preparation, photo-resist placement, etchant preparation, and FIB sequencing are described herein to allow for high throughput (rapid) specimen fabrication from bulk AM 17-4PH stainless steel volumes. Additionally, procedures for the nano-indenter tip modification to allow tensile testing are presented and a representative micro specimen is fabricated and tested to failure in tension. Tensile-grip-to-specimen alignment and sample engagement were the main challenges of the micro-tensile testing; however, by reducing the indenter tip dimensions, alignment and engagement between the tensile grip and specimen were improved. Results from the representative micro-scale in situ SEM tensile test indicate a single slip plane specimen fracture (typical of a ductile single crystal failure), differing from macro-scale AM 17-4PH post-yield tensile behavior.

Presented here is a procedure for measuring fundamental material properties through micromechanical tension testing. Described are the methods for micro-tensile specimen fabrication (allowing rapid micro-specimen fabrication from bulk material volumes by combining photolithography, chemical etching, and focused ion beam milling), indenter tip modification, and micromechanical tension testing (including an example).

This study presents a methodology for the rapid fabrication and micro-tensile testing of additively manufactured (AM) 17-4PH stainless steels by combining ...

Stress-Strain Characteristics of Steels

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
The importance of materials to human development is clearly captured by the early classifications of world history into periods such as the Stone Age, Iron Age, and the Bronze Age. The introduction of the Siemens and Bessemer processes to produce steels in the mid-1800s is arguably the single most important development in launching the Industrial Revolution that transformed much of Europe and the USA in the second half of the 19th century from agrarian societies into the urban and mechanized societies of today. Steel, in its almost infinite variations, is all around us, from our kitchen appliances to cars, to lifelines such as electrical transmission networks and water distribution systems. In this experiment we will look at the stress-strain behavior of two types of steel that bound the range usually seen in civil engineering applications - from a very mild, hot rolled steel to a hard, cold rolled one.

Uniaxial Tensile Test and Stress-Strain Characteristics of Steels

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
The importance of materials to human ...

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Structural Engineering

Stress-Strain Characteristics of Aluminum

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
Aluminum is one of the most abundant materials in our lives, as it is omnipresent in everything from soda cans to airplane components. Its widespread use is relatively recent (1900AD), primarily because aluminum does not occur in its free state, but rather in combination with oxygen and other elements, often in the form of Al2O3. Aluminum was originally obtained from bauxite mineral deposits in tropical countries, and its refinement requires very high-energy consumption. The high cost of producing quality aluminum is another reason why it is a very widely recycled material.
Aluminum, especially when alloyed with one or more of several common elements, has been increasingly used in architectural, transportation, chemical, and electrical applications. Today, aluminum is surpassed only by steel in its use as a structural material. Aluminum is available, like most other metals, as flat-rolled products, extrusions, forgings, and castings. Aluminum offers superior strength-to-weight ratio, corrosion resistance, ease of fabrication, non-magnetic properties, high thermal and electrical conductivity, as well as ease of alloying.

Uniaxial Tensile Strength and Stress-Strain Test of Aluminum

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
Aluminum is one of the most abundant materials ...

Charpy Impact Test of Cold Formed and Hot Rolled Steels Under Diverse Temperature Conditions

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
One of the more insidious types of failures that can occur in structures are brittle fractures,which are mostly due to either poor quality materials or poor material selection. Brittle fractures tend to occur suddenly and without much material inelasticity; think of a bone fracture, for example. These failures often occur in situations where there is little ability for the material to develop shear stresses due to three-dimensional loading conditions, where local strain concentrations are high, and where a logical and direct force path was not provided by the designer. Examples of this type of failure were observed in the aftermath of the 1994 Northridge earthquake in multi-story steel structures. In these buildings, a number of the key welds fractured without displaying any ductile behavior. Fractures tend to occur near connections, or at interfaces between pieces of base materials, as welding tends to introduce local discontinuities in both, materials and geometry, as well as three-dimensional stresses due to cooling.
When specifying materials for a structure that will see very low operating temperatures (i.e., the Alaska pipeline) many cycles of loading (a bridge on an interstate highway), or where welding is used extensively, it is necessary to have a simple test that characterizes the material&#39;s robustness, or resistance to fracture. In the civil engineering field that test is the Charpy V-notch test, which is described in this lab. The Charpy V-notch test is intended to provide a very simplistic measure of the material&#39;s ability to absorb energy when subjected to an impact load.

Charpy Impact Test of Cold Formed and Hot Rolled Steels

Source:&#160;Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
One of the more insidious types of failures ...

Determining the Mechanical Strength of Ultra-Fine-Grained Metals

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