Composition Dependences of Entropy Change and Transformation Temperatures in Ni-rich Ti–Ni System

Abstract

For Ni-rich Ti-Ni alloys, physical properties such as specific heat and electric resistance were systematically investigated. The B2/B19 0 martensitic transformation temperatures ranging from 180 to 373 K were determined for Ni contents of 49.98-51.09 %, and a sudden disappearance of martensitic transformation was confirmed for Ni contents greater than 51.23 %, which has also been well reported in the literatures. The entropy change was also evaluated from differential scanning calorimeter measurement, and it was clarified that the entropy change plotted to T 0 temperature shows an S-shaped curve, starting to drastically decrease at about 300 K. Thermodynamic approaches were then carried out attempting to determine the reason for the disappearance of transformation. The entropy change estimated from direct measurements of specific heats for 51.75 Ni (B2) and 50.92 Ni (B19 0 ) was found to be more consistent with the experimental data, rather than the calculated curve based on the Debye model for vibration specific heat. It was proposed that the equilibrium between the parent and martensite phases obeys the Clausius-Clapeyron relationship in the composition-temperature system. Using the constructed composition-temperature diagram, the disappearance of martensitic transformation in the Ti-Ni system can be well understood as being due to the drastic increase of hysteresis at low temperature.

Martensitic Transformation and Superelasticity in Fe–Mn–Al-Based Shape Memory Alloys

“…7b) and such microstructural change is a typical feature of thermoelastic MT. The required driving force for the MT DG is given by DG & DSÁDT (DT: supercooling from T 0 ) and DG for Fe-Mn-Al-Ni is estimated to be 32 J mol -1 (DS = -0.43 J mol -1 K -1 , DT & DT hys /2 = 75 K), which is much smaller than that of roughly 1000 J mol -1 in non-thermoelastic transformation in Fe-based alloys [41] and even smaller than that of about 80 J mol -1 in thermoelastic Ni-Ti alloys [42]. As discussed later, the DS is smaller in the Fe-Mn-Al-Ni alloy than that in other SMAs, and the large hysteresis is not due to the large driving force but caused by this thermodynamic property.…”

“…11 Tensile superelasticity with a superelastic strain of 9.7% in a Fe-34Mn-15Al-7.5 Ni single crystal aged at 200°C for 3 h [24]. The crystal orientation for the tensile direction is shown in the inset From a comparison between the calculations and experiments for a few orientations [24,42,49,56], it seems that the suitable theory should be selected depending on the crystallographic tensile or compressive orientation, but further investigations are required for better prediction of the maximum superelastic strain.…”

Shape memory strains and temperatures in the extreme

“…The entropy change correlates with the slope of the transformation stresstemperature curve which is unusually high for NiTiHf 13.3 alloy opening new possibilities [77]. Among SMAs the NiTi alloys possess some of the largest entropy changes [85]. These other considerations are left aside for future research reviews.…”

Martensitic Transformations and Superelastic Behavior at Low Temperatures in Ti_50&minus;<i>_x</i>Ni₄₀₊<i>_x</i>Cu₁₀ Shape Memory Alloys

“…A possible discontinuity of entropy change was found at about 250 K. Results for Ti-Ni binary alloys 7,17) are also plotted. Solid lines are fitting curves by use of eq.…”