Microstructure and Martensitic Transformation Behavior in Thermal Cycled Equiatomic CuZr Shape Memory Alloy

Abstract

Equiatomic CuZr alloy undergoes a martensitic transformation from the B2 parent phase to martensitic phases (P21/m and Cm) below 150 °C. We clarified the effect of the thermal cycling on the morphology and crystallography of martensite in equiatomic CuZr alloy using a transmission electron microscopy. The 10th cycled specimens consisted of different multiple structures at the maximum temperature of differential scanning calorimetry (DSC) measurement −400 °C and 500 °C, respectively. At the maximum temperature 400 °C of DSC measurement, it is composed of the fine plate-like variants, and a lamellar eutectoid structure consisting of Cu10Zr7 and CuZr2 phases on the martensitic variant. Concerning the maximum temperature of 500 °C of DSC measurement, it is observed the martensitic structure and the lamellar structure in which the martensitic phase was completely eutectoid transformed. The formation of this lamellar eutectoid structure, due to thermal cycling leads to the shift of forward and reverse transformation peaks to low and high temperature side. In addition, new forward and reverse transformation peaks indicating a new transformation appeared by thermal cycling, and the peaks remained around −20 °C. This new martensitic transformation behavior is also discussed.

Effect of Al Addition on Martensitic Transformation Stability and Microstructural and Mechanical Properties of CuZr Based Shape Memory Alloys

“…It was reported that the equiatomic composition appears to be the best compromise between the greatest amount of material involved in the MT and the highest transformation temperatures for considering CuZr within high temperature shape memory alloys (HTSMAs). In other studies, it was found that upon thermal cycling the initial MT, present at the first thermal cycle, is suppressed, and a new one, stable at temperatures below 0 • C and with limited thermal hysteresis, takes its place [8][9][10]. Electrical resistivity measurements showed a large shift of the characteristic temperatures, followed, after tens of thermal cycles, by a new type of MT at approximately −10 • C [8].…”

“…Electrical resistivity measurements showed a large shift of the characteristic temperatures, followed, after tens of thermal cycles, by a new type of MT at approximately −10 • C [8]. Moreover, thermal cycling through differential scanning calorimetry evidenced this new MT, from austenite to a new martensitic structure, which was different than the original one present in the first few cycles [9,10].…”

“…Among these, the intermetallic CuZr has been studied extensively, since it exhibits a reversible, non-thermoelastic martensitic transformation (MT) above 100 • C when subjected to thermal cycles. The literature offers some studies reporting investigations into the microstructure and functional properties of both CuZr [3][4][5][6][7][8][9] and CuZr based shape memory alloys [10][11][12][13][14][15][16][17][18].…”

“…Due to this lack of stability during phase transformations, as well as to improve the alloys' workability, efforts have been made to study the addition of several alloying elements, like Co, Cr, Ni, Al, and Ti [10][11][12][13][14][15][16].…”

Intermetallic Alloys

“…The equiatomic CuZr alloy is expected to fulfill this demand. The effect of thermal cycling on the morphology and crystallography of martensites in an equiatomic CuZr alloy was investigated using a transmission electron microscope (TEM) by Hisada et al [1]. A new transformation scheme and martensite phase (structure) were found, depending on thermal cyclic range, −100 • C to 400 • C or 500 • C. It was demonstrated that the new martensitic transformation is closely related to the strain and stress caused by thermal cycling.…”