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Researchers discover polycrystalline materials can be derived from microscopic single crystal samples

June 29, 2018
Source: ASM International

Materials researchers at the Australian Nuclear Science and Technology Organization (ANSTO), Sydney, were able to derive the bulk properties of a polycrystalline material, in a way that is useful for engineering, by carrying out micro-mechanical testing on single crystals of nickel.

Tensile properties are usually extracted from computational or sophisticated mathematical models following indentation or micro-cantilever testing, when the available material volume is small—such as in the case of thin film multilayers, ion irradiated materials and surface coatings.

In a paper published in the International Journal of Plasticity, the investigators led by ANSTO senior scientist Dhriti Bhattacharyya, reported the results of in situ micro-tensile testing to evaluate the combined effects of strain rate and orientation on deformation behavior in single crystals of nickel.

Although different orientations were known to have different strengths, we wanted to determine the stress/strain behavior pulling along one direction of the crystal or another and if the strain rate, or deformation rate, had different effects in different directions,” said Bhattacharyya.

In addition to contributing to a fundamental understanding of mechanical deformation at the microscopic scale, the potential cost saving of extrapolating macro properties using micron-sized samples is significant when compared to millimeter sized standard samples, which are sometimes impossible to obtain in the case of thin films and surface modified materials.

They found that pulling along specific orientations of the crystal makes a difference to the ultimate strength, ductility and the way in which the crystal deformed.

Alan Xu pulled the samples using a state-of-the-art micromechanical testing machine along a direction perpendicular to the cubic face of the crystal unit cell (100) and along the face diagonal of the unit cell (110) and measured the response. The orientations were chosen because of the expectation of multiple slip on different numbers of slip systems.

The elongation of the <110> samples was nearly double that of the <100> samples. The stress/strain curve revealed that the <110> orientated sample exhibited an initial stress peak, followed by softening, a flat bottomed valley, a secondary hardening and peak and finally softening and failure.

“The extent the crystals deviate from their original orientation is an indicator of how much the crystal itself has rotated,” said Xu.

“Pulling along both orientations showed that the ductility increased by twofold along the face diagonal, which was interesting,” said Bhattacharyya.

“Importantly, the calculated strain rate sensitivity was found to be in the same order of magnitude as macroscopic samples of nickel,” said Bhattacharyya.


Image – The ANSTO research team in the Microscopy Facility (from left): Dr. Alan Xu, Michael Saleh, and Dr. Dhriti Bhattacharyya. Courtesy of ANSTO.

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Subject Classifications

Materials Properties and Performance | Mechanical Properties

Materials Testing and Evaluation | Materials Characterization

Materials Testing and Evaluation | Mechanical Testing

Metals and Alloys | Superalloys, Nickel, and Cobalt

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