Nanoindentation of pseudoelastic NiTi shape memory alloys: Thermomechanical and microstructural aspects

Abstract

Nanoindentation allows local characterization of the microstructural processes associated with pseudoelastic recovery in NiTi shape memory alloys. We discuss nanoindentation and tensile testing results, both carried out at different temperatures on a NiTi alloy with three different microstructures (containing no, small and large Ni-rich precipitates). Maximum shape recovery in indentation experiments and during tensile testing occurs in microstructures with fine precipitates. Thermomechanical constraints affect pseudoelasticity differently in micro- and macro-scale testing: Indentation at temperatures as close as possible to the transformation start temperatures is associated with better shape recovery than testing at higher temperatures, whereas maximum shape recovery is only observed at slightly higher temperatures in macroscopic testing. These results are rationalized by considering the effect of temperature on stress-induced martensitic transformation and dislocation plasticity.

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

“…However, it should be noted that this apparent elastic modulus results from various deformation mechanisms which act simultaneously, such as elasto-plastic deformation, variant reorientation and detwinning of martensite [27,62,63,65,66]. Understanding the complex interactions between these deformation mechanisms and the stress-induced martensitic transformation is a key objective of current research activities [63,67,68].…”

Influence of fatigue on the nanohardness of NiTiCr-wires

“…The recovery of wires fatigued at 2 % strain is somewhere in between that of the asreceived and the solution annealed materials (RDR = 0.57). It is not surprising that the wires examined in this study have a remnant depth ratio larger than zero, because full pseudoelastic recovery is rarely reported in the literature for indentation studies [9,11]. Although nanoindentation was performed as close to the fracture surface as possible, no clear dependence of the nanohardness or RDR on the distance from the fracture surface of the fatigued wires is observed.…”

“…Full pseudoelastic recovery is associated with a RDR close to zero [9]. The dependence of the nanohardness and RDR on the maximum indentation depth was examined by indenting as-received specimens to depths of 50 to 1,000 nm.…”

Nanoindentation of a Pseudoelastic NiTiFe Shape Memory Alloy

“…[17] To compare nanoindentation results obtained with various experimental settings, and to quantify shape memory and pseudoelastic recovery of different SMA specimens, it is useful to study the characteristic ratio of remnant indentation depth (D rem ) and maximum indentation depth (D max ). [18][19][20][21] This so-called remnant depth ratio (RDR) is defined asRDR-values near zero (RDR 10%) are expected for perfect pseudoelastic recovery.Nanoindentation can be performed with different indenter tips, for example, pyramidal tips or spherical tips, and the corresponding stress and strain states below these indenter tips are quite different. [17] As shape memory and pseudoelastic phase transformations can only accommodate strains up to 6-8%, it has to be clarified which tip geometries are suitable for investigating pseudoelastic recovery.…”

“…[17] To compare nanoindentation results obtained with various experimental settings, and to quantify shape memory and pseudoelastic recovery of different SMA specimens, it is useful to study the characteristic ratio of remnant indentation depth (D rem ) and maximum indentation depth (D max ). [18][19][20][21] This so-called remnant depth ratio (RDR) is defined as…”