Loss of pseudoelasticity in nickel-titanium sub-micron compression pillars

Nickel-titanium (NiTi) is capable of undergoing pseudoelastic deformation wherein relatively large amounts of inelastic deformation are recovered upon load removal due to a martensitic phase transformation. This study investigates pseudoelastic as well as plastic deformation in sub-micron diameter NiTi compression pillars. Pillars ranging in diameter from approximately 2 mu m to 200 nm were prepared using focused ion beam micro-machining of aged [111] single crystal NiTi. Results reveal pseudoelasticity in all samples tested with diameters between 2 mu m, and 400 nm, although permanent strain was introduced at relatively low strains compared to bulk. Decreased sample size generally showed a smaller stress-strain hysteresis, with a full loss of recoverable pseudoelastic strain for samples with a diameter smaller than 200 nm. In addition, plastic flow stress of the martensite was shown to be independent of sample diameter for the aged NiTi material. Lastly, it is observed that crystallographic orientation has a stronger influence on martensite plastic flow strength than pillar size. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.