New Discovery Could Help in Engineering Stronger and More Ductile Alloyed Materials

In a recent announcement, Scientists in the United States and China mentioned about a discovery that they assume could help in the engineering of alloyed materials that are more ductile and stronger and possess superior magnetic, physical and electrical properties.

The discovery announced in Science by Harmer’s group sheds light on the atomic structures that form when atoms segregate at general grain boundaries, which are important in controlling many properties of polycrystalline alloys. (Image by Zhiyang Yu, Patrick R. Cantwell and Jian Luo)

Writing in Science, the leading scientific journal in the nation, the Researchers explained they had discovered a surprising degree of order in the interior grain boundaries — the interfaces between crystalline grains — that govern the properties of polycrystalline materials such as ceramics and metals.

Traditionally, Scientists have identified two categories of grain boundaries in polycrystalline materials, states Martin P. Harmer, the Alcoa Foundation Professor of Materials Science and Engineering at Lehigh.

Special grain boundaries happen at a comparatively small fraction of the internal interfaces of a material when adjoining lattices (the 3D arrangement of atoms inside a crystal) of individual grains join together with minimal mismatch and develop a patterned or periodic superstructure.

General grain boundaries are considered to be more common and occur in the inner side of a material when there is a huge misfit between adjoining grains and no matching of the adjacent crystal lattices.

General grain boundaries tend to be the weak regions of an engineering material. But while special grain boundaries typically have much better properties, general grain boundaries are more prevalent, and therefore more important, because they determine the bulk properties of a material.

Martin P. Harmer, the Alcoa Foundation Professor of Materials Science and Engineering, Lehigh University

General grain boundaries have been problematic for Scientists to study, says Harmer, since they are harder to access than special grain boundaries and also because they can develop a wide range of possible configurations.

By employing numerical calculations and atomic-resolution electron microscopy, Harmer and his colleagues have succeeded in characterizing 12 randomly selected general grain boundaries in polycrystalline nickel doped with bismuth. The Office of Naval Research funded the six-year project through its Multidisciplinary University Research Initiative (MURI).

The Researchers discovered that bismuth atoms segregating, or adsorbing, at the general grain boundaries of nickel developed superstructures whose weak bismuth-bismuth bonds at critical junctures resulted in the alloy becoming brittle.

“This discovery,” they wrote in Science, “shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.”

“We believe that, for the first time, we have discovered superstructures at general grain boundaries in a metal alloy,” says Harmer. “Until now, this had been a very hidden phenomenon in metals.

This breakthrough helps us understand why, in the case of nickel-bismuth, the alloy embrittles. But beyond that, there will potentially be superstructures in general grain boundaries which enhance a material’s performance, strength and ductility. The question is, to what extent can we engineer these superstructures in general grain boundaries and make new materials with desired properties?

Martin P. Harmer, the Alcoa Foundation Professor of Materials Science and Engineering, Lehigh University

The Science paper is titled “Segregation-induced ordered superstructures at general grain boundaries in a Ni-Bi alloy.” Zhiyang Yu, the Key Author, is a Faculty Member at Xiamen University of Technology in China and a Former Postdoctoral Researcher at Lehigh.

The other Authors include Patrick R. Cantwell, Assistant Professor of Mechanical Engineering at the Rose-Hulman Institute of Technology in Indiana and a former Research Associate at Lehigh; Denise Yin ’11, a former Ph.D. Graduate Student at Lehigh; Qin Gao and Michael Widom of the department of Physics at Carnegie Mellon University; Jian Luo, Yuanyao Zhang and Naixie Zhou of the department of Nanoengineering at the University of California at San Diego; and Gregory S. Rohrer of the department of Materials Science and Engineering at Carnegie Mellon.

The Concept of Complexions

Harmer has been reviewing grain boundaries for a huge part of his career. In 2006, he along with  Shen J. Dillon ’02 ’07 Ph.D. identified six interphase structures, or distinct grain-boundary “complexions,” in the ceramic alumina.

According to Harmer, the concept of complexions proposes that grain boundaries transition from one structure, or state, to another as a function of composition and temperature and sometimes with dramatic changes in property. The concept is analogous to the manner in which bulk structures modify phases from gas to liquid to solid. Dillon, presently an Associate Professor of Materials Science at the University of Illinois at Urbana-Champaign, employed Lehigh’s aberration-corrected scanning transmission electron microscope (STEM) and for the first time directly studied the phenomenon.

The concept of grain boundary complexions has helped explain phenomena that have confused Scientists for decades, says Harmer. These include the embrittlement of ductile alloys when exposed to a specific element, such as bismuth and nickel-copper, and the abnormal growth of a small number of grains in ceramics. In 2011, Harmer and Jian Luo co-authored an article titled “The Role of a Bilayer Interfacial Phase on Liquid Metal Embrittlement” for Science.

Harmer’s group, in the past decade, published several influential articles on complexions. Dillon was the Key Author of “Complexion: A New Concept for Kinetic Engineering in Materials Science,” which was published in 2007 in Acta Materialia, the prominent journal for Engineers and Materials Scientists. The article was coauthored with two greatly regarded complexions theorists — W. Craig Carter of the Massachusetts Institute of Technology and Ming Tang of Rice University.

Harmer published a perspective titled “The Phase Behavior of Interfaces” in Science in 2011 and in 2014, Cantwell was the First Author of a highly cited overview titled “Grain Boundary Complexions,” which was published in Acta Materialia. Tang, Dillon, Luo, Rohrer and Harmer coauthored that article.

“Amazing periodicity” in Ni-Bi

A continuation of Harmer’s study of the nickel-bismuth alloy has been represented in the present ONR-funded project.

When we looked at the liquid metal embrittlement of nickel-bismuth alloys under the aberration-corrected STEM, we were curious when we saw an amazing amount of periodicity to the structure of the bismuth atoms at general grain boundaries that we randomly selected. Zhiyang Yu, who is a very skilled electron microscopist, worked five years on this project and did a phenomenal job characterizing the structures of these periodic arrays of bismuth at the general grain boundaries.

Martin P. Harmer, the Alcoa Foundation Professor of Materials Science and Engineering, Lehigh University

According to Harmer, one of the challenges facing Yu was to recreate the three-dimensionality of the specimen being observed since the electron microscope produces a very thin two-dimensional projection of a three-dimensional structure. For this, Yu had to take thousands of images and then piece them together in order to simulate the original 3D image. The group also depended on computational modeling carried out Widom and Gao of Carnegie Mellon University.

“Michael Widom and Qin Gao did calculations of the conditions under which the atomic structures [of the grain boundaries] become stable,” said Harmer, “and predicted the structures we would see. We matched their predictions with the experimental results. It turned out to be a great match... The most surprising result that came out of all this work was the discovery that these general random grain boundaries had a variety of superstructures. We’ve known for a long time that these superstructures in metal alloys develop on the free surfaces of crystals and that they have very interesting physical properties.”

Harmer added, “But we’ve never seen superstructures inside engineering materials at general grain boundaries. It’s something that the textbooks would tell you not to expect because we’ve considered that the arrangement of atoms in general grain boundaries was disordered and ill-defined. But we found these superstructures in nickel-bismuth alloys.”

“What’s more, we found several different types of superstructures buried inside the material that are very close to the type of superstructures that we predicted for the surfaces of a crystal.” Harmer concluded.

In Science, Harmer and his colleagues wrote that the superstructures they discovered in general grain boundaries are capable of playing a role in the magnetic, electronic and other properties of materials.

“The discovery of bismuth segregation-induced superstructures at general grain boundaries greatly enriches our limited knowledge of the atomic structure of complexions,” the group wrote, “and may offer new insights into a spectrum of structure-related grain boundary properties such as plasticity, diffusivity and conductivity... We suggest that ordered grain boundary superstructures may indeed be a general, although not necessarily universal, feature of polycrystalline materials.”

Harmer compared the project to “a jigsaw puzzle with a million pieces.”

The payoff, he added, “will be huge.”

“Grain boundaries, whether special or general, dictate to a large degree the properties of the materials we use in the world,” said Harmer. “We’ve been studying grain boundaries for many years. Now, very sophisticated models and new tools are making it possible to see things in general grain boundaries that we couldn’t see 10 years ago... This is changing the way we think about materials and the way we engineer them.”

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