A Nanoscale Look at Why a New Alloy is Amazingly Tough

Just in time for the icy grip of winter: A team of researchers led by scientists from the U.S. Department of Energy Lawrence Berkeley National Laboratory (Berkeley Lab) has identified several mechanisms that make a new, cold-loving material one of the toughest metallic alloys ever.

The alloy is made of chromium, manganese, iron, cobalt and nickel, so scientists call it CrMnFeCoNi. It’s exceptionally tough and strong at room temperature, which translates into excellent ductility, tensile strength, and resistance to fracture. And unlike most materials, the alloy becomes tougher and stronger the colder it gets, making it an intriguing possibility for use in cryogenic applications such as storage tanks for liquefied natural gas.

To learn its secrets, the Berkeley Lab-led team studied the alloy with transmission electron microscopy as it was subjected to strain. The images revealed several nanoscale mechanisms that activate in the alloy, one after another, which together resist the spread of damage. Among the mechanisms are bridges that form across cracks to inhibit their propagation. Such crack bridging is a common toughening mechanism in composites and ceramics but not often seen in unreinforced metals.

Their findings could guide future research aimed at designing metallic materials with unmatched damage tolerance. The research appears in the December 9, 2015, issue of the journal Nature Communications.

“We analyzed the alloy in earlier work and found spectacular properties: high toughness and strength, which are usually mutually exclusive in a material,” says Robert Ritchie, a scientist with Berkeley Lab’s Materials Sciences Division who led the research with Qian Yu of China’s Zhejiang University and several other scientists.

“So in this research, we used TEM to study the alloy at the nanoscale to see what’s going on,” says Ritchie.

In materials science, toughness is a material’s resistance to fracture, while strength is a material’s resistance to deformation. It’s very rare for a material to be both highly tough and strong, but CrMnFeCoNi isn’t a run-of-the-mill alloy. It’s a star member of a new class of alloys developed about a decade ago that contains five or more elements in roughly equal amounts. In contrast, most conventional alloys have one dominant element. These new multi-component alloys are called high-entropy alloys because they consist primarily of a simple solid solution phase, and therefore have a high entropy of mixing.

They’re a hot topic in materials research, and have only recently been available in a quality suitable for study. In 2014, Ritchie and colleagues found that at very cold temperatures, when CrMnFeCoNi deforms, a phenomenon called “twinning” occurs, in which adjacent crystalline regions form mirror arrangements of one another. Twinning likely plays a part in the alloy’s incredible toughness and strength. But twinning isn’t extensively found in the alloy at room temperature (except in the crack bridges), yet the alloy’s toughness and strength is still almost off the charts.

“If we don’t see twinning at room temperature, then what other mechanisms give the alloy these amazing properties?” asks Ritchie.

To find out, the scientists subjected the alloy to several straining experiments at room temperature, and used transmission electron microscopy to observe what happens.

Their time-lapse images revealed two phenomena related to shear stress: slow-moving perfect dislocations that give the material strength, and fast-moving partial dislocations that enhance ductility. They also saw a phenomenon involving partial dislocations called “three-dimensional stacking fault defects,” in which the 3-D arrangement of atoms in a region changes. These faults are big barriers to dislocation, like placing a stack of bricks in front of a growing fissure, and serve to harden the alloy.

The images also captured the nanoscale version of chewing a mouthful of toffee and having your teeth stick together: In some cases, tiny bridges deformed by twinning are generated across a crack, which help prevent the crack from growing wider.

“These bridges are common in reinforced ceramics and composites,” says Ritchie. “Our research found that all of these nanoscale mechanisms work together to give the alloy its toughness and strength.”

Berkeley Lab

Researchers Take First Steps to Create Biodegradable Displays for Electronics

Americans, on average, replace their mobile phones every 22 months, junking more than 150 million phones a year in the process. When it comes to recycling and processing all of this electronic waste, the World Health Organization reports that even low exposure to the electronic elements can cause significant health risks. Now,University of Missouri researchers are on the path to creating biodegradable electronics by using organic components in screen displays. The researchers’ advancements could one day help reduce electronic waste in the world’s landfills.

“Current mobile phones and electronics are not biodegradable and create significant waste when they’re disposed,” said Suchismita Guha, professor in the Department of Physics and Astronomy at the MU College of Arts and Science. “This discovery creates the first biodegradable active layer in organic electronics, meaning—in principle—we can eventually achieve full biodegradability.”

Guha, along with graduate student Soma Khanra, collaborated with a team from the Federal University of ABC (UFABC) in Brazil to develop organic structures that could be used to light handheld device screens. Using peptides, or proteins, researchers were able to demonstrate that these tiny structures, when combined with a blue light-emitting polymer, could successfully be used in displays.

“These peptides can self-assemble into beautiful nanostructures or nanotubes, and, for us, the main goal has been to use these nanotubes as templates for other materials,” Guha said. “By combining organic semiconductors with nanomaterials, we were able to create the blue light needed for a display. However, in order to make a workable screen for your mobile phone or other displays, we’ll need to show similar success with red and green light-emitting polymers.”

The scientists also discovered that by using peptide nanostructures they were able to use less of the polymer. Using less to create the same blue light means that the nanocomposites achieve almost 85 percent biodegradability.

“By using peptide nanostructures, which are 100 percent biodegradable, to create the template for the active layer for the polymers, we are able to understand how electronics themselves can be more biodegradable,” Guha said. “This research is the first step and the first demonstration of using such biology to improve electronics.”

The study “Self-Assembled Peptide-Polyfluorene Nanocomposites for Biodegradable Organic Electronics” recently was published as the inside cover article in Advanced Materials Interfaces.

The work was supported by the National Science Foundation (Grant IIA-1339011) and CNPq (400239/2014-0). The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agencies.

Tommi White, assistant research professor of biochemistry, and Thomas Lam, both with the Microscopy Core Research Facility at MU, contributed to the study. Other collaborators include Wendel A. Alves and Thiago Cipriano, professors of supramolecular chemistry at UFABC; Eudes E. Fileti, a professor of physics at the Federal University of São Paulo, Brazil.

University of Missouri

Duke engineers make strides toward artificial cartilage

Composite material closest yet to properties of the real thing

A Duke research team has developed a better recipe for synthetic replacement cartilage in joints.

Combining two innovative technologies they each helped develop, lead authors Farshid Guilak, a professor of orthopedic surgery and biomedical engineering, and Xuanhe Zhao, assistant professor of mechanical engineering and materials science, found a way to create artificial replacement tissue that mimics both the strength and suppleness of native cartilage. Their results appear Dec. 17 in the journal Advanced Functional Materials.

Tiny interwoven fibers make up the three-dimensional 

fabric "scaffold" into which a strong, pliable hydrogel is 

integrated and injected with stem cells, forming a 

framework for growing cartilage. This image appears 

on the cover of the Dec. 17, 2013, issue of Advanced 

Functional Materials.

Credit: courtesy of Frank Moutos and Farshid Guilak

Articular cartilage is the tissue on the ends of bones where they meet at joints in the body – including in the knees, shoulders and hips. It can erode over time or be damaged by injury or overuse, causing pain and lack of mobility. While replacing the tissue could bring relief to millions, replicating the properties of native cartilage -- which is strong and load-bearing, yet smooth and cushiony -- has proven a challenge.

In 2007 Guilak and his team developed a three-dimensional fabric "scaffold" into which stem cells could be injected and successfully "grown" into articular cartilage tissue. Constructed of minuscule woven fibers, each of the scaffold's seven layers is about as thick as a human hair. The finished product is about 1 millimeter thick.

Since then, the challenge has been to develop the right medium to fill the empty spaces of the scaffold -- one that can sustain compressive loads, provide a lubricating surface and potentially support the growth of stem cells on the scaffold. Materials supple enough to simulate native cartilage have been too squishy and fragile to grow in a joint and withstand loading. "Think Jell-O," says Guilak. Stronger substances, on the other hand, haven't been smooth and flexible enough.

That's where the partnership with Zhao comes in.

		This is a closer look at the scaffolding integrated with Xuanhe Zhao's hydrogel. The composite material, formed through a process comparable to pouring concrete over a steel framework, may be a serviceable synthetic replacement for the load-bearing cartilage found between bones. Credit: courtesy of I-Chien Liao, Frank Moutos, Brad Estes

Zhao proposed a theory for the design of durable hydrogels (water-based polymer gels) and in 2012 collaborated with a team from Harvard University to develop an exceptionally strong yet pliable interpenetrating-network hydrogel.
"It's extremely tough, flexible and formable, yet highly lubricating," Zhao says. "It has all the mechanical properties of native cartilage and can withstand wear and tear without fracturing."

He and Guilak began working together to integrate the hydrogel into the fabric of the 3-D woven scaffolds in a process Zhao compares to pouring concrete over a steel framework.

In their experiments, the researchers compared the resulting composite material to other combinations of Guilak's scaffolding embedded with previously studied hydrogels. The tests showed that Zhao's invention was tougher than the competition with a lower coefficient of friction. And though the resulting material did not quite meet the standards of natural cartilage, it easily outperformed all other known potential artificial replacements across the board, including the hydrogel and scaffolding by themselves.

"From a mechanical standpoint, this technology remedies the issues that other types of synthetic cartilage have had," says Zhao, founder of Duke's Soft Active Materials (SAMs) Laboratory. "It's a very promising candidate for artificial cartilage in the future."
The team's next step will likely be to implant small patches of the synthetic cartilage in animal models, according to Guilak and Zhao.

Their work was supported in part by National Institutes of Health grants AG15768, AR50245, AR48182, AR48852, the Arthritis Foundation, the Collaborative Research Center, AO Foundation, Davos, Switzerland and the NSF (CMMI-1253495, CMMI-1200515, and DMR-1121107).

"Composite Three-Dimensional Woven Scaffolds with Interpenetrating Network Hydrogels to Create Functional Synthetic Articular Cartilage," Liao, I.-C., Moutos, F. T., Estes, B. T., Zhao, X. and Guilak, F. Adv. Funct. Mater., 2013. doi: 10.1002/adfm.201300483

Source: http://today.duke.edu/2013/12/artcart

Mixing Nanoparticles to Make Multifunctional Materials

Standardized technique opens remarkable opportunities for 'mix and match' materials fabrication

DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials.

UPTON, NY—Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have developed a general approach for combining different types of nanoparticles to produce large-scale composite materials. The technique, described in a paper published online by Nature Nanotechnology on October 20, 2013, opens many opportunities for mixing and matching particles with different magnetic, optical, or chemical properties to form new, multifunctional materials or materials with enhanced performance for a wide range of potential applications. 

The approach takes advantage of the attractive pairing of complementary strands of synthetic DNA—based on the molecule that carries the genetic code in its sequence of matched bases known by the letters A, T, G, and C. After coating the nanoparticles with a chemically standardized "construction platform" and adding extender molecules to which DNA can easily bind, the scientists attach complementary lab-designed DNA strands to the two different kinds of nanoparticles they want to link up. The natural pairing of the matching strands then "self-assembles" the particles into a three-dimensional array consisting of billions of particles. Varying the length of the DNA linkers, their surface density on particles, and other factors gives scientists the ability to control and optimize different types of newly formed materials and their properties.

Future applications could include quantum dots whose glowing fluorescence can be controlled by an external magnetic field for new kinds of switches or sensors; gold nanoparticles that synergistically enhance the brightness of quantum dots' fluorescent glow; or catalytic nanomaterials that absorb the "poisons" that normally degrade their performance, Gang said."Our study demonstrates that DNA-driven assembly methods enable the by-design creation of large-scale 'superlattice' nanocomposites from a broad range of nanocomponents now available—including magnetic, catalytic, and fluorescent nanoparticles," said Brookhaven physicist Oleg Gang, who led the research at the Lab's Center for Functional Nanomaterials (CFN). "This advance builds on our previous work with simpler systems, where we demonstrated that pairing nanoparticles with different functions can affect the individual particles' performance, and it offers routes for the fabrication of new materials with combined, enhanced, or even brand new functions." 

"Modern nano-synthesis methods provide scientists with diverse types of nanoparticles from a wide range of atomic elements," said Yugang Zhang, first author of the paper. "With our approach, scientists can explore pairings of these particles in a rational way." 

Pairing up dissimilar particles presents many challenges the scientists investigated in the work leading to this paper. To understand the fundamental aspects of various newly formed materials they used a wide range of techniques, including x-ray scattering studies at Brookhaven's National Synchrotron Light Source (NSLS) and spectroscopy and electron microcopy at the CFN.

For example, the scientists explored the effect of particle shape. "In principle, differently shaped particles don't want to coexist in one lattice," said Gang. "They either tend to separate into different phases like oil and water refusing to mix or form disordered structures." The scientists discovered that DNA not only helps the particles mix, but it can also improve order for such systems when a thicker DNA shell around the particles is used. 

They also investigated how the DNA-pairing mechanism and other intrinsic physical forces, such as magnetic attraction among particles, might compete during the assembly process. For example, magnetic particles tend to clump to form aggregates that can hinder the binding of DNA from another type of particle. "We show that shorter DNA strands are more effective at competing against magnetic attraction," Gang said.

For the particular composite of gold and magnetic nanoparticles they created, the scientists discovered that applying an external magnetic field could "switch" the material's phase and affect the ordering of the particles. "This was just a demonstration that it can be done, but it could have an application—perhaps magnetic switches, or materials that might be able to change shape on demand," said Zhang. 

The third fundamental factor the scientists explored was how the particles were ordered in the superlattice arrays: Does one type of particle always occupy the same position relative to the other type—like boys and girls sitting in alternating seats in a movie theater—or are they interspersed more randomly? "This is what we call a compositional order, which is important for example for quantum dots because their optical properties—e.g., their ability to glow—depend on how many gold nanoparticles are in the surrounding environment," said Gang. "If you have compositional disorder, the optical properties would be different." In the experiments, increasing the thickness of the soft DNA shells around the particles increased compositional disorder.

These fundamental principles give scientists a framework for designing new materials. The specific conditions required for a particular application will be dependent on the particles being used, Zhang emphasized, but the general assembly approach would be the same. 

Said Gang, "We can vary the lengths of the DNA strands to change the distance between particles from about 10 nanometers to under 100 nanometers—which is important for applications because many optical, magnetic, and other properties of nanoparticles depend on the positioning at this scale. We are excited by the avenues this research opens up in terms of future directions for engineering novel classes of materials that exploit collective effects and multifunctionality." 

Source: http://www.bnl.gov/newsroom/news.php?a=11581