"Pulling along both orientations showed that the ductility increased by twofold along the face diagonal, which was interesting," said Bhattacharyya. "The extent the crystals deviate from their original orientation is an indicator of how much the crystal itself has rotated," said Xu. Progressively more slip steps formed on multiple slip planes around the necked region to accommodate stress and the sample fractured in a knife edge-like manner.Įlectron beam backscatter diffraction was used to identify the orientation of the crystals.Ĭredit: Australian Nuclear Science and Technology Organisation (ANSTO) The oriented sample slips on at least two different planes initially (as depicted in video above) followed by four possible slip systems. However, once the flow stress was increased due to strain hardening on this plane, accompanied by a rotation of the crystal, a second slip system on a different plane was activated. The investigators attributed this to an initial slip by a series of parallel slip steps along the whole gauge length. The double peak in a stress/strain curve is believed to be a unique phenomenon. "In this sample, the slipping occurs like the fanning out of a stack of cards," said Bhattacharyya. The stress/strain curve revealed that the 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 elongation of the samples (as depicted above in video) was nearly double that of the samples. "We found different behaviours both in terms of the displacement, how the shape is deforming, but also in terms of the stress/strain curve," said Xu. The orientations were chosen because of the expectation of multiple slip on different numbers of slip systems. In these experiments, the effect of strain rate and crystal orientation were measured in 10 samples with 12 micron size. In polycrystalline materials, because each grain has a different orientation, some kind of averaging is used to determine the properties of the grain 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. "We chose tensile testing because, for almost all materials you want to know how the material performs under tension," 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 millimetre sized standard samples, which are sometimes impossible to obtain in the case of thin films and surface modified materials. Strain rate sensitivity is a well-known phenomenon in macroscopic tests but it is not well understood at the microscopic level," he said. "Although different orientations were known to have different strengths, we wanted to determine the stress/strain behaviour 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. Dhriti Bhattacharyya reported the results of in situ micro-tensile testing to evaluate the combined effects of strain rate and orientation on deformation behaviour in single crystals of nickel. In a paper published in the International Journal of Plasticity, the investigators led by ANSTO senior scientist Dr. 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.
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