Criteria of Transformation Sequences in NiTi Shape Memory Alloys

Metallurgical origin of the effect of Fe doping on the martensitic and magnetic transformation behaviours of Ni50Mn40-xSn10Fex magnetic shape memory alloys

“…It is also evident that the transformation hysteresis increased significantly with decreasing e/a ratio. This is consistent with expectation for a decreased transformation entropy change DS with transformations at lower temperatures according to thermodynamic principles [27,28]. It is seen that both DH and DS decreased continuously with decreasing T O and with e/a ratio, caused by Fe addition.…”

Size effects on the martensitic phase transformation of NiTi nanograins

“…Compared to this linear change, in the case of a large undercooling the chemical driving force is expected to increase less with decreasing temperature (by the third law of thermodynamics dDj chem =dT decreases, finally reaching zero at 0 K). Ds ¼ À5:0 Â 10 5 J=ðm 3 KÞ is the difference of the specific entropies of B2 and B19 0 (Liu and McCormick, 1996) and T 0 is the temperature of chemical equilibrium (Dj chem ðT 0 Þ ¼ 0). As outlined by Tong and Wayman (1974) the temperature T 0 of chemical equilibrium can be approximated by 1 2 ðM s þ A f Þ yielding a value of 370 K (A f ¼ 415 and M s ¼ 325 K are the austenite finish and martensite start temperatures, respectively, measured by differential scanning calorimetry in the coarse grained alloy).…”

Analysis of multistep transformations in single-crystal NiTi

“…The additional step is associated with the internal elastic stresses, which facilitate the transformation to the intermediate R-phase with a lower transformation shear. [2,4,5,8,10,[13][14][15] The transformation sequences that are not as well described are those that include greater than two steps on cooling or heating. They are referred to here as multiple-step transformations (MSTs) and include sequences such as three-step/two-step and threestep/three-step transformations, for example.…”

“…The single-step forward (cooling) and reverse (heating) transformation sequence, austenite (A) ↔ martensite (M), is typically observed in solutionized NiTi with little or no plastic deformation [6][7][8][9][10][11] or deformed then recrystallized NiTi. [1,12,13] Another sequence frequently reported is the twostep/two-step A ↔ R ↔ M transformation. The additional step is associated with the internal elastic stresses, which facilitate the transformation to the intermediate R-phase with a lower transformation shear.…”