UCLA researchers create exceptionally strong and lightweight new metal

Magnesium infused with dense silicon carbide nanoparticles could be used for airplanes, cars, mobile electronics and more

A team led by researchers from the UCLA Henry Samueli School of Engineering and Applied Science has created a super-strong yet light structural metal with extremely high specific strength and modulus, or stiffness-to-weight ratio. The new metal is composed of magnesium infused with a dense and even dispersal of ceramic silicon carbide nanoparticles. It could be used to make lighter airplanes, spacecraft, and cars, helping to improve fuel efficiency, as well as in mobile electronics and biomedical devices.

To create the super-strong but lightweight metal, the team found a new way to disperse and stabilize nanoparticles in molten metals. They also developed a scalable manufacturing method that could pave the way for more high-performance lightweight metals. The research waspublished today in Nature. 

“It’s been proposed that nanoparticles could really enhance the strength of metals without damaging their plasticity, especially light metals like magnesium, but no groups have been able to disperse ceramic nanoparticles in molten metals until now,” said Xiaochun Li, the principal investigator on the research and Raytheon Chair in Manufacturing Engineering at UCLA. “With an infusion of physics and materials processing, our method paves a new way to enhance the performance of many different kinds of metals by evenly infusing dense nanoparticles to enhance the performance of metals to meet energy and sustainability challenges in today’s society.”

Structural metals are load-bearing metals; they are used in buildings and vehicles. Magnesium, at just two-thirds the density of aluminum, is the lightest structural metal. Silicon carbide is an ultra-hard ceramic commonly used in industrial cutting blades. The researchers’ technique of infusing a large number of silicon carbide particles smaller than 100 nanometers into magnesium added significant strength, stiffness, plasticity and durability under high temperatures.

The researchers’ new silicon carbide-infused magnesium demonstrated record levels of specific strength — how much weight a material can withstand before breaking — and specific modulus — the material’s stiffness-to-weight ratio. It also showed superior stability at high temperatures.

Ceramic particles have long been considered as a potential way to make metals stronger. However, with microscale ceramic particles, the infusion process results in a loss of plasticity.

Nanoscale particles, by contrast, can enhance strength while maintaining or even improving metals’ plasticity. But nanoscale ceramic particles tend to clump together rather than dispersing evenly, due to the tendency of small particles to attract one other.

To counteract this issue, researchers dispersed the particles into a molten magnesium zinc alloy. The newly discovered nanoparticle dispersion relies on the kinetic energy in the particles’ movement. This stabilizes the particles’ dispersion and prevents clumping.

To further enhance the new metal’s strength, the researchers used a technique called high-pressure torsion to compress it.

“The results we obtained so far are just scratching the surface of the hidden treasure for a new class of metals with revolutionary properties and functionalities,” Li said.

The new metal (more accurately called a metal nanocomposite) is about 14 percent silicon carbide nanoparticles and 86 percent magnesium. The researchers noted that magnesium is an abundant resource and that scaling up its use would not cause environmental damage.

The paper’s lead author is Lian-Yi Chen, who conducted the research as a postdoctoral scholar in Li’s Scifacturing Laboratory at UCLA. Chen is now an assistant professor of mechanical and aerospace engineering at Missouri University of Science and Technology.

The paper’s other authors from UCLA include Jia-Quan Xu, a graduate student in materials science and engineering; Marta Pozuelo, an assistant development engineer; and Jenn-Ming Yang, professor of materials science and engineering.

The other authors on the paper are Hongseok Choi, of Clemson University; Xiaolong Ma, of North Carolina State University; Sanjit Bhowmick of Hysitron, Inc. of Minneapolis; and Suveen Mathaudhu of UC Riverside.

UCLA

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