Localization Events and Microstructural Evolution in Ultra‐Fine Grained NiTi Shape Memory Alloys during Thermo‐Mechanical Loading

Abstract

Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra‐fine grained NiTi wires subjected to a broad range of thermo‐mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature‐induced martensite (formed during cooling at no or relatively small loads), stress‐induced martensite, and reoriented martensite (formed under load at low temperatures).

Temperature-induced transformations and martensitic reorientation processes in ultra-fine-grained Ni rich pseudoelastic NiTi wires studied by electrical resistance

“…2b where for the stress of 150 MPa, sudden resistance changes (steps) appear during cooling and heating. These abrupt variations were shown to be related to the localized transformation to martensite [21] as will be discussed later. A third difference is associated with the shapes of the E.R.-T curves.…”

“…In a previous work [21], it was shown that steps observed in the E.R.-T curves corresponded to the propagation of one or few transformation fronts during a so-called localized transformation to B19 0 martensite. In this way, the specimen locally changes from a mainly austenitic to a mainly martensitic state when the transformation front passes by, leading to a sudden alteration of the local resistive properties of the material.…”

On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys

“…This can be achieved by using more than one strain gage, [33,34] by thermal imaging, [35,36] by evaluating the strain fields using optical methods, [37,38] or by using appropriately positioned multiple electrical resistance probes. [39,40] This type of localized transformation to B19¢ is well established for pseudoelastic NiTi alloys subjected to tensile loading. [32][33][34][35][36][37][38][39][40] However, whether the stressinduced formation of R-phase also occurs in a localized manner still remains an open question.…”

“…[39,40] This type of localized transformation to B19¢ is well established for pseudoelastic NiTi alloys subjected to tensile loading. [32][33][34][35][36][37][38][39][40] However, whether the stressinduced formation of R-phase also occurs in a localized manner still remains an open question.…”

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.…”