Effects of grain size on compressive behavior of NiTi polycrystalline superelastic macro- and micropillars

Polycrystalline NiTi pillars of 0.5 mu m diameter and 1.5 mu m height with average grain sizes from 10 to 421 nm are fabricated by focused ion beam and compressed by nanoindentation. It is found that stress-strain hysteresis loop area, transformation stress and transformation strain vary non-monotonically with grain size. Analysis of the results reveals that increasing the grain size from 10 nm enhances the transformation by promoting the nucleation and growth of martensite domains. When the grain size approaches the pillar size, the transformation is suppressed by an increase in granular constraints and heterogeneity of internal stress-strain state among large grains. (C) 2017 Elsevier B.V. All rights reserved.