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.
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.Go To Materials Science and Engineering: A