Structure evolution in coarse-grained nickel under ultrasonic treatment

Significance 

Properties of metals and alloys are generally achieved through structural modifications. This induces various effects in the microstructure of the materials as well as altering their mechanical, physical and chemical properties. Among the different methods for structural modifications in materials, ultrasonic treatment is widely used. Therefore, it is necessary to fully understand the effects of ultrasonic treatment on material structures and properties.

Generally, ultrasonic treatment results in several effects on the material structure. For instance, surface treatment leads to a decrease in the grain size and significant improvement in fatigue resistance and hardness. Recently, ultrasonic deformation effects on metal microstructure have attracted significant interest among researchers. This is due to its capability of producing ultrafine-grained microstructure, especially in bulk materials. Unfortunately, the effects of ultrasound on the properties of materials have not been fully explored.

Presently, it is difficult to use the experimental methods to observe the structural evolution in materials under ultrasonic treatment. Even though computer simulations have been widely used in the prediction of various dislocation behaviors such as relaxation of non-equilibrium grain boundaries, they are based on two-dimensional dislocation approaches which may, in turn, modify the predictions thus leading to errors. Thus, for an in-depth understanding of mechanisms of the ultrasonic effect on the structure of the material, researchers have identified experimental study of the characterized material areas under ultrasound before and after ultrasonic treatment while at the same time comparing the differences in the two.

Researchers at Institute for Metals Superplasticity Problems of the Russian Academy of Sciences Dr. Alexander Zhilyaev, Dr. Asiya Samigullina, Dr. Ayrat Nazarov and Elvina Shayakhmetova have recently investigated structural changes of coarse-grained nickel as a result of the ultrasonic treatment. Their main aim was to determine the effect of the amplitude of the ultrasound on the material structural parameters. Their research work is currently published in the journal, Materials Science and Engineering.

Briefly, the research team commenced their experimental work by annealing a pure nickel sample for one hour at a temperature of 1300°C so as to obtain coarse grained-structure exhibiting relatively low dislocation density. Furthermore, they utilized electron backscatter diffraction analysis and X-ray diffraction to characterize the selected areas of the sample as well as conducted microhardness measurements before and after ultrasonic treatment.

The authors observed a significant dislocation generation and evolution of the substructure under the ultrasonic treatment. Consequently, changes in the grain boundaries and corresponding increase in the low-angle boundaries fraction were noted. In particular, the density of statistically stored dislocations increased immensely in the region of a relatively low amplitude of the oscillating stress. An increase in the stress amplitude resulted in a leveling-off of the densitiy of statistically stored dislocations and a steady increase in the density of geometrically necessary dislocations thus leading to rearrangement of dislocations due to ultrasonic irradiation to form low-angle boundaries. With a further increase of the ultrasound intensity evolution of the newly formed boundaries to high angle ones occurs resulting in the grain refinement.

According to the scientists at the Russian Academy of Sciences, these structural transformations enhanced the microhardness. Along with the earlier studies of the authors, this study gives an evidence that ultrasonic treatment results in strengthening of well annealed, equilibriunm materials while enhancing the ductility of heavily deformed, nonequilibrium materials. Therefore, the study will advance the production of metals and alloys with desired combinations of properties including the strength and ductility which will, in turn, promote their use in various fields.

About the author

Alexander Zhilyaev is a Principal Research Scientist at the Institute for Metals Superplasticity Problems of the Russian Academy of Sciences (Ufa, Russia), with a joint appointment as a Director of the Laboratory for Mechanics of Gradient Nanomaterials at Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia). He graduated from the Bashkir State University in 1981, and received Ph.D. and Dr. Sci. degrees in Materials Science from the Institute for Metals Superplasticity Problems in 1993 and 2003, respectively. From 1996 to 2006, he had different appointments at McGill University (Montreal, Canada), UC Davis (Davis, CA), Naval Postgraduate School (Monterey, CA). Alexander Zhilyaev spent a quite bit period of his scientific carrier in Spain (UAB (2001-2003) and UPC (2016-2017) in Barcelona and at CENIM-CSIC (2006-2011) in Madrid, where he did research on a variety of topics in microstructure & properties of nanostructured materials processed by severe plastic deformation and held several positions as a Visiting Researcher and Professor.

His current research activities include gradient, bimodal and heterogeneous metallic nanomaterials of enhanced strength and ductility for advanced structural applications processed by asymmetric (cryo-) rolling; advanced manufacturing including metallic 3D printing and ultrasonic treatment; microstructure and properties of nanomaterials processed by high pressure torsion including diffusion bonding of different metals and alloys.

About the author

Dr. Sci. Ayrat A. Nazarov is the Deputy Director at the Institute for Metals Superplasticity Problems of the Russian Academy of Sciences (IMSP RAS), Ufa, Russia. He graduated from the Faculty of Physics of Bashkir State University, Ufa, USSR in 1981. He received the degree of Cand. Sci. from Tomsk State University in 1991 and Dr. Sci. degree from IMSP RAS in 1999. In 1986-2003 he worked at IMSP RAS at the positions of Junior Researcher, Researcher, Senior Researcher and Head of Sector. From 2003 to 2007 he was a Professor, Leading Researcher at Ufa State Aviation Technical University and in 2007 joined back IMSP RAS.

His research areas include grain boundaries in metals and covalent ceramics, bulk nanostructured materials, superplastic deformation of metals and alloys, atomistic computer simulation of materials, ultrasonic treatment and ultrasonic welding of metals.

About the author

Dr. Asiya A. Samugullina is a Researcher at the Institute for Metals Superplasticity Problems of the Russian Academy of Sciences (IMSP RAS), Ufa, Russia. She graduated from Bashkir State University, Ufa, Russia in 2008. From 2008 till 2012 she was a PhD student at IMSP RAS. She earned the Cand. Sci. degree from IMSP RAS in 2014.

Her research area includes the ultrasonic treatment of materials. She has demonstrated for the first time that using ultrasonic treatment it is possible to enhance simultaneously ductility, impact toughness and strength of ultrafine grained materials processed by severe plastic deformation. In 2013-2014 she was a PI of a grant from the Russian Foundation for Basic Research as a young researcher.

About the author

Elvina R. Shayakhmetova is a Master Student at Ufa State Aviation Technical University, Department of Materials Science and Physics of Metals. In 2018, she obtained a bachelor’s degree from the same department.

She is involved in the studies on the effect of ultrasound on the structure of nickel in coarse grained and nanostructured states.

Reference

Zhilyaev, A., Samigullina, A., Nazarov, A., & Shayakhmetova, E. (2018). Structure evolution in coarse-grained nickel under ultrasonic treatment. Materials Science and Engineering: A, 731, 231-238.

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