Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Abstract

Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B199 martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.

Size effects in NiTi from density functional theory calculations

“…Experimental studies on nanocrystalline NiTi revealed strong size effects for grain sizes in the range of 50 nm to 350 nm. 9 For samples with mean grain diameter of approximately 60 nm and with 90% of the grains smaller than 100 nm, the martensite start (M s ) and martensite finish (M f ) transformation temperatures decrease from 330 K to 319 K and from 302 K to less than 197 K respectively, when compared to a coarse grained polycrystalline sample. TEM studies show that grains with diameters less than 50 nm fail to transform to martensite even after quenching to 197 K. Experimental studies by Glezer and collaborators 10 on Ni 50 Ti 25 Cu 25 nano-particles embedded in an amorphous matrix show similar trends for the B2 -B19 martensite phase transformation.…”

Simulation of the shape memory effect in a NiTi nano model system

“…The question, whether and how this behavior takes place in systems with very small lengths scales, where a strong influence of the free surfaces comes into play, is of increasing interest in research and technology, for example in order to construct functional nanoscale actuators [2,3]. A lot of effort has been done in studying the martensitic and reverse ("austenitic") transformation in nanosized systems (an overview is given in [4]), where for example new types of domain configurations emerge upon cooling of nickel-titanium nanocrystals below strongly size dependent transition temperatures [5]. Experimental studies of SE and SME of NiTi [6] and CuAlNi nanopillars [7] show, that these effects take place at least down to length scales of several hundred nanometers, but, in case of CuAlNi, there is a considerable size dependence of the stress-strain curves if compared to bulk systems [8].…”

Structural evolution in Ti-Cu-Ni metallic glasses during heating

“…5 The size of the shape memory crystals also influences the MT since grain boundaries hinder the growth of the martensite and impede its autocatalytic nucleation, which is triggered by stresses that arise during its growth. 7 The transformation can be even suppressed for small grain sizes (around 50 nm for TiNi-based alloys) 8 because the stresses required increase with decreasing crystal size. 8 a Author to whom correspondence should be addressed.…”

“…The structural evolution of Ti 50 Cu 43 Ni 7 and Ti 55 Cu 35 Ni 10 metallic glasses during heating was investigated by in-situ synchrotron X-ray diffraction. The width of the most intense diffraction maximum of the glassy phase decreases slightly during relaxation below the glass transition temperature.…”

“…Ingots of the alloys Ti 50 Cu 43 Ni 7 and Ti 55 Cu 35 Ni 10 were produced by arc-melting elements with high purity (more than 99.5%). They were repeatedly arc-melted in a Ti-gettered argon atmosphere to ensure complete melting and compositional homogeneity.…”