A transmission electron microscopy procedure for in-situ straining of miniature pseudoelastic NiTi specimens

Abstract

The in-situ transmission electron microscopy technique allows direct observations of formation and growth of stress-induced martensite in pseudoelastic NiTi shape memory alloys. The present paper reports on the development of a miniature test procedure for in-situ straining experiments with specimens taken from small components. The deformation of an ultra-fine grained NiTi specimen is characterised by transmission electron microscopy (at early loading stages) and by optical microscopy (at larger strains). A complementary finite element analysis of the complex strain state in the specimen rationalises why the stress-induced martensitic transformation first occurs in the thin foil region of the specimen before spreading towards the outer rim of the specimen.

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

“…Understanding the mechanical behavior of pseudoelastic NiTi SMAs is important to accurately estimate the lifetime of these devices [9][10][11][12][13]. Pseudoelastic NiTi SMAs used in many applications involve processing such as wire-drawing [14][15][16][17], which results in the formation of ultrafine grains. In these ultrafine-grained materials, R phase, an intermediate martensitic phase, which involves a small ferroelastic distortion of the austenitic B2 lattice structure [18], is commonly observed [1,[19][20][21].…”

Proposition of R-phase transformation strip in the phase diagram of Ni-Ti shape memory alloy using electromechanical experiments

“…In tests with controlled strain, these fronts propagate at a relatively constant stress until the transformation front covers the entire length of the specimen leading to a maximum strain of around 8%. This phenomenon reverts back during unloading (Gollerthan et al, 2009; Olbricht et al, 2008; Schaefer et al, 2010). The stress–strain response in such a test reveals some characteristics that are correlated to the localized type of transformation: (1) for the time period in which the fronts move through the gauge length, a stress plateau is observed in the stress–strain curve during both loading and unloading cycles and (2) different subdivisions of the gauge length start to transform (and thus elongate) at distinctive instances.…”

Characteristics of the stress-induced formation of R-phase in ultrafine-grained NiTi shape memory wire

“…The stress-induced transformation to B19 0 in NiTi ribbons and wires is well known to occur in a localized (Lüders type) manner [19][20][21][22][23][24][25][26][27]. Typically, narrow transformation fronts emerge from the grips and move into the gauge length on loading.…”

“…Typically, narrow transformation fronts emerge from the grips and move into the gauge length on loading. In displacement controlled tests, these fronts propagate at constant stress until the whole gauge section of the specimen is covered by the transformation, involving a maximum strain of up to 8%, and the situation reverts back on unloading [19][20][21][22][23][24][25][26][27]. The stress-strain evolution of such a test exhibits characteristic features that are related to the localized type of transformation [19]: (i) a plateau-like constant stress interval is observed in the stress-strain path, both during loading and unloading, for the time period in which the fronts move through the gauge length, and (ii) different sections of the gauge length start to transform (and thus elongate) at distinct points of time.…”