In-Situ Synchrotron Characterization of Transformation Sequences in TiNi-Based Shape Memory Alloys during Thermal Cycling

Abstract

In-situ synchrotron radiation has been used to provide direct analysis of the transformation sequences in TiNi-based shape memory alloys during thermal cycling. The high resolution, narrow peak width Debye-Scherrer diffraction spectra enabled positive identification and quantification of the phase transformation sequences, which is not possible through normal laboratory studies. The results facilitate a clearer understanding of the development and influence of intermediate phases such as R or B19 on sequential martensitic transformations. Ti 50.2 Ni 49.8 transformed predominately via a single-step B2 M B19¢ transformation, although evidence of the R phase was found during cooling in every cycle. The martensitic start temperature was depressed by~0.6°C per cycle, while the R-phase start temperature was found to be unaffected. Ti 50 Ni 41 Cu 9 transformed through a two-step B2 M B19 M B19¢ sequence, with the B2 fi B19 transformation reaching completion prior to the formation of any B19¢. The transformation temperatures of Ti 50 Ni 41 Cu 9 were found to be insensitive to thermal cycling, remaining constant over the studied cycle range.

Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load

“…Heating into the austenitic region enabled the evaluation of the B2 phase and refined crystal parameters are given in Table 1. The Ti 2 Ni precipitates were present throughout the experiment, consistent with our previous findings [20].…”

“…The transformation temperatures were defined in a similar manner to Ref. [20], with the austenitic and martensitic start temperatures corresponding to 0.1 of the maximum peak intensity for that cycle, whilst the finish temperatures corresponded to 0.9 of the maximum intensity.…”

“…The R phase was also observed on cooling but always after the start of the B2 / B19 0 transformation and subsequently transformed to B19 0 by room temperature. Previous work on the same material found that the volume fraction of R produced is very small and thus unable to influence the transformation sequence [20]. Throughout each test, a constant load was maintained by allowing the actuator to move in response to thermal expansion and transformation strain.…”

Influence of the structure of the Ti-49.8 at % Ni alloy on its thermal expansion during the martensitic phase transformation

“…5, 6) and X ray diffraction analysis (Fig. 4) after various temperature modes of deformation show that the low temperature rolling at 470 and 570 K forms the strongly fragmented twin free structure in the mate rial, in which the martensitic structural transforma tions are hampered [2,3,15,[17][18][19][20][21][22]. As a result, the temperature range of the transition is broadened.…”

Pseudoelasticity response of aged Ti50Ni25Cu25 shape memory ribbon under nanoindentation tests

“…If a third element is added into TiNi binary SMAs, the phase transformation sequence, transformation temperatures and mechanical properties of the alloys can be adjusted [1]. Substituting Cu for Ni to form Ti 50 Ni 50Àx-Cu x ternary SMAs has attracted much attention due to their comparatively low temperature and stress hysteresis [3,4] and stable transformation temperatures [3,5,6]. In addition, Ti 50 Ni 50Àx Cu x ternary SMAs exhibit different transformation sequences according to different Cu contents.…”