The effect of thermal cycling on the martensitic transformation in equiatomic CuZr shape memory alloy
Graphical abstract
Introduction
Because of their great potential in developing new smart actuators with enhanced functional properties, nowadays the investigations of high temperature shape memory alloys (HTSMAs) are gathering a worldwide interest [1]. Among HTSMAs, the intermetallic CuZr is very promising since it exhibits a reversible martensitic transformation above 100 °C. Some studies on microstructural aspects and functional properties of both CuZr [2], [3], [4], [5], [6] and CuZr-based alloys are available in open literature [7], [8], [9], [10], [11], [12]. Firstov et al. [2], [3] showed that CuZr intermetallic undergoes a martensitic transformation (MT) below 140 °C from the parent, austenite (A) phase (cubic B2, space group Pm-3m) stable at high temperature, into two martensitic phases (M) with monoclinic structure (space groups Cm and P21/m). The same authors also discussed the peak associated to the forward MT in terms of a double step transformation, each one related to a single martensitic phase [3]. In addition, electrical resistivity measurements showed a large shift of the characteristic temperatures, followed, after some tenths of thermal cycles, by a new stage of the MT at approximately −10 °C, with thermal hysteresis limited to few degrees [2]. Biffi et al. showed that the characteristic temperatures of the MT at the first thermal cycle and the Cu/Zr ratio in proximity of the equiatomic composition are strongly correlated [4]. It was reported that the equiatomic composition appears to be the best compromise between the largest amount of material involved in the MT and the highest transformation temperatures for considering CuZr within HTSMAs. For improving functional and workability properties, alloying of CuZr with metallic elements (Co, Cr, Ni, Al and Ti) were also reported [7], [8], [9], [10], [11], [12]. It seems that Ni stabilizes the MT at high temperatures and Co can make the MT more stable during thermal cycling [7], [8]. Hot workability can also be improved by addition of Ni and Co while Ti alloying does not support evident benefits in CuZr [11].
In this work we report on the thermal cycling behavior of an equiatomic CuZr alloy by employing calorimetric, X-ray diffraction and Scanning Electron Microscopy (SEM) observations in a wide temperature range.
Section snippets
Experimental
Pure Zr and OFHC grade Cu were melted by means of a non-consumable tungsten electrode vacuum arc furnace (Leybold mod. LK 6/45). Buttons of equiatomic CuZr (1:1 at.%) were produced into a water-cooled copper crucible under pure argon atmosphere. Each button was re-melted six times for reaching a high homogeneity degree. Later, the produced CuZr material was fully annealed at 850 °C for 3 h and water quenched (hereafter named fully annealed). The calorimetric analysis was carried out on small
Analysis of results and discussion
The DSC curves across the thermal cycling of a fully annealed sample are reported in Fig. 1. At the first cycle, direct and inverse single step transformations occur at As = 265 °C, Af = 315 °C upon heating and at Ms = 150 °C and Mf = 90 °C upon cooling.
From the 4th cycle, a further low temperature peak was detected on both cooling and heating DSC curves. These peaks, associated to a new MT (A* ↔ M*), appeared below 0 °C. The increasing of the A* ↔ M* transformation peak area is concomitant
Conclusions
The present work highlighted the lack of MT stability during thermal cycling of equiatomic CuZr. The MT moves forward low temperatures so this compound could not be classified among the HTSMAs. Within 10 thermal cycles the initial MT is inhibited whilst a new transformation, from austenite to a lower symmetry system, was observed at around – 20 °C. The derivation in MT temperature, as well as the inset of MTA∗↔M∗ were due to thermal cycling.
The reversible character of the observed
Acknowledgments
The work was developed within the framework of 2°Accordo Quadro CNR/Regione Lombardia. The authors would like to thank Marco Pini, Nicola Bennato and Giordano Carcano of CNR IENI Lecco Unit for technical assistance.
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