Jae-Hwang Lee of the Mechanical and Industrial Engineering (MIE) Department at the University of Massachusetts Amherst is a member of the team of researchers from UMass and Rice University that is publishing an article in the prestigious journal Science about a dynamic new process for producing high-strength metals. The title of the article is “Dynamic Creation and Evolution of Extreme Gradient Nanostructure in Single-crystal Metallic Micro-cubes.”
As the Science Article says, “The nanostructural transformations produced in our experiments show promising pathways to developing gradient-nano-grained (GNG) metals for engineering applications requiring both high-strength and -toughness—for example, in structural components of aircraft and spacecraft.”
The Science article was authored by Ramathasan Thevamaran, Olawale Lawal, Sadegh Yasde, and Edwin L. Thomas from the Department of Materials Science and NanoEngineering at Rice University, by MIE’s Lee, and by Seog-Jin Jeon of the Department of Polymer Science and Engineering at UMass
“I carried out the initial study of this project when I was at Rice University and have been working on this project since I moved to UMass in 2014,” says Lee, who is an assistant professor in the MIE department and the head of the Nano-Engineering Laboratory at UMass Amherst.
As Lee says about his Nano-Engineering Laboratory, “Our group has a unique experimental method for investigating the high-strain-rate and high-strain characteristics of nano-materials, known as Laser Induced Projectile Impact Test (LIPIT). In the LIPIT, a micrometer-scale projectile is accelerated up to ~3 km/s by the use of laser ablation and impacts a very localized area of a nano-material to induce a HSR deformation. Using the LIPIT, we are quantitatively studying the dynamic responses of nano-structured materials.”
In their Science article, the six co-authors explain that “We demonstrate the dynamic creation and subsequent static evolution of extreme GNG structures in initially near-defect-free single-crystal silver micro-cubes. Extreme nanostructural transformations are imposed by high strain rates, strain gradients, and recrystallization in high-velocity impacts of the micro-cubes against an impenetrable substrate.”
Jeon synthesized the single-crystal silver micro-cubes in a bottom-up seed-growth process and then used LIPIT to launch them selectively at supersonic velocities.
“Our study provides new insights into the fundamental deformation mechanisms, and the effects of crystal and sample-shape symmetries resulting from high-velocity impacts,” the article says.
As the Science article explains, a clear understanding of the fundamental deformation mechanisms in high-velocity impacts and shock compressions is critical to developing advanced protective technologies for applications in automobile and aircraft crashes, sport-related collisions, and body and vehicle armors. This knowledge is also crucial for developing advanced material processing techniques such as shot peening, laser shock peening, and explosive welding. Supersonic velocity micro-particle impacts are especially relevant in cold spray technologies, collision of undetectable small-sized space debris and micrometeorites with spacecraft, and micro-particle impacts on turbine blades.
According to the Science article, numerous studies over the last decade have demonstrated the increase in strength with decreasing grain sizes in metals. However, this improvement comes at the cost of increased susceptibility to catastrophic brittle failure due to strain localization and crack formation in tension. It has recently been shown both experimentally and numerically that the creation of spatial gradients in grain structure can potentially alleviate the catastrophic failure through progressive ductile behavior under applied uniform tensile stresses.
Multi-step surface mechanical grinding has been used to reduce the micron-sized grains at the surface of the material down to nanoscale, thereby creating a GNG structure from the surface to the bulk. But the powerful new method described in the Science article offers a significant improvement over this multi-step process.
“We demonstrate effective creation of an extreme GNG structure with a single-step, high-velocity-impact process,” the Science article notes. “The GNG structure is created by high strain rates, pressure, and strain gradients during impact. This nanostructure evolves through continuous (static) recrystallization over the course of weeks at room temperature.”
As the Science article concludes, “This technique allows the sample to deform in an unconstrained manner and enables us to investigate the role that the intrinsic crystal symmetries and the extrinsic micro-cube symmetries play on resulting deformations, when the micro-cube impacts along specific crystal-symmetry directions.” (October 2016)