We studied the compressive behavior of single crystalline NiAl micropillars at room temperature in to determine the impact of sample size on the plastic behavior of such a semi-brittle material. Overall, more than 200 pillars with different crystal orientations and aspect ratios were tested at different compression rates. The following was found: The -, - and -oriented micropillars exhibited very different plastic response as a consequence of the activation of different slip systems: for -oriented micropillars and - and -oriented micropillars. In contrast to larger pillars, 200 nm diameter pillars deformed in and slip steps irrespective of crystal orientation. This might be caused by surface dislocation source activation promoted at this small size scale. Constant strain rate tests on pillars with different aspect ratios compressed along the and axes showed that kinking also takes place at the microscale as a result of crystal misorientation; -oriented micropillars deformed by slip in while micropillars deformed by kink banding. Very high ductility was obtained for the compressed micropillars in contrast to the semi-brittle bulk specimens. This might be due to surface effects such as Ga ion damage as well as the fact that, at the (sub-) micron scale, plasticity is dominated by dislocation source activation rather than dislocation-dislocation interaction (thus minimizing deformation induced embrittlement effects). The strain-stress response of the pillars evidenced the plastic behavior of highly anisotropic materials, showing the effect that crystal orientation has in the overall mechanical response. More importantly, plastic anisotropy, defined as the ratio of the critical resolved shear stresses between hard and soft crystals, strongly decreased with decreasing sample size. The reduced plastic anisotropy might be attributed to a decrease in the intrinsic resistance of the specimens at the sub-micron scale (surface effects). Very dissimilar size effects were observed as a function of crystal orientation. Soft crystals exhibited a strong size effect, similar to that observed in FCC metals. By contrast, hard crystals exhibited a diminished size effect, similar to
We studied the compressive behavior of single crystalline NiAl micropillars at room temperature in to determine the impact of sample size on the plastic behavior of such a semi-brittle material. Overall, more than 200 pillars with different crystal orientations and aspect ratios were tested at different compression rates. The following was found: The -, - and -oriented micropillars exhibited very different plastic response as a consequence of the activation of different slip systems: for -oriented micropillars and - and -oriented micropillars. In contrast to larger pillars, 200 nm diameter pillars deformed in and slip steps irrespective of crystal orientation. This might be caused by surface dislocation source activation promoted at this small size scale. Constant strain rate tests on pillars with different aspect ratios compressed along the and axes showed that kinking also takes place at the microscale as a result of crystal misorientation; -oriented micropillars deformed by slip in while micropillars deformed by kink banding. Very high ductility was obtained for the compressed micropillars in contrast to the semi-brittle bulk specimens. This might be due to surface effects such as Ga ion damage as well as the fact that, at the (sub-) micron scale, plasticity is dominated by dislocation source activation rather than dislocation-dislocation interaction (thus minimizing deformation induced embrittlement effects). The strain-stress response of the pillars evidenced the plastic behavior of highly anisotropic materials, showing the effect that crystal orientation has in the overall mechanical response. More importantly, plastic anisotropy, defined as the ratio of the critical resolved shear stresses between hard and soft crystals, strongly decreased with decreasing sample size. The reduced plastic anisotropy might be attributed to a decrease in the intrinsic resistance of the specimens at the sub-micron scale (surface effects). Very dissimilar size effects were observed as a function of crystal orientation. Soft crystals exhibited a strong size effect, similar to that observed in FCC metals. By contrast, hard crystals exhibited a diminished size effect, similar to
original_Thesis_OscarTorrents.docx