Tailoring martensitic transformation behavior in TiCu-based super-elastic alloys and their composites

Abstract

Shape memory alloys (SMAs) have opened up an area of new material called multifunctional materials representing sensing and actuation capabilities. SMAs can be utilized for various applications according to temperature, stress field and magnetic field change by utilizing reversible martensitic phase transformation. One of the modes, superelasticity, that utilize the martensitic phase transformation is to lower the phase transformation temperature than the operating environment, so that the forward martensitic phase transformation occurs when the stress is applied and the reverse martensitic phase transformation occurs when the stress is relaxed. Recently, It have attracted great interest in research that inducing work hardening behavior and improvement of toughness into bulk metallic glass by precipitating a superelastic secondary phase, and utilizing the elastic calorie effect through the release and absorption of latent heat generated during the martensitic phase transformation. However, there are few studies on the development of new superelastic alloys with optimized characteristics. Most of the reports are mainly focused on the application of conventional alloys. Since an alloy system exhibiting reversible martensitic phase transformation mainly forms an intermetallic compound phase, superelastic alloys can be manufactured only in a few specific alloy systems such as NiTi, CuZn, and NiMgGa, and the solubility of the third and fourth alloying elements is very limited. In this study, in order to overcome the limitation of the alloy design, we devised a method to expand the solid solution limit of the superelastic phase (B2, BCC, etc.) reported on the known phase diagram. The phase showing reversible martensitic phase transformation is mostly stable at high temperatures, and precipitates tend to occur as the alloy system becomes complicated. When precipitates are suppressed, and primary phase is stabilized, the superelastic alloy can be produced in an extended composition range. In the NiTiCu alloy system, which is known to be possibly used as an elastocaloric material and amorphous matrix composite, it has been found that the addition of a small amount of Si can effectively inhibit the precipitation of superfine B2 phase and other intermediate compound phases. In particular, as the Cu content increases, the amount of energy dissipation due to the internal friction occurring during the phase transformation of martensite is reduced, which is advantageous for the superelastic characteristics. We aimed to maximize the Cu content in B2 phase. As a result, it was confirmed that superelastic B2 alloys can be manufactured by employing Cu up to "43 at. %", and shows very low hysteresis and mechanical energy dissipation during the phase transformation. In order to utilize the TiCuNiSi superelastic alloy, it is essential to optimize the phase transformation temperature and mechanical properties. In this study, we tried to control the phase transformation temperature by adding Sn element which is known to decrease the phase transformation temperature of Ti alloys. The Sn element was employed up to 5 at. %, which showed a sharp increase in strength, unlike the general solid solution hardening phenomenon. First principle calculations have confirmed that the mechanism is hardening effect by antisite defect, and compared with general solid solution hardening theory. The change of phase transformation temperature due to solid solution hardening is closely related to the change of yield strength. Especially, the correlation between critical stress for martensitic transformation and yield strength was investigated. It is suggested that the temperature and strength of TiCuNiSiSn superalloy can be easily controlled through the correlation between alloy composition and antisite defect hardening. The alloying composition of TiCuNiSiSn alloy system was designed to show the excellent elastocaloric effect through the process of controlling the temperature of the phase transformation. The coefficient of performance and fatigue characteristics were analyzed. As a result, the alloys developed through this study showed the most efficient elastocaloric effect than all the superelastic alloy systems reported so far. The TiCuNiSiSn alloys with high Cu contents are capable of producing bulk metallic glass and confirmed that bulk metallic glass with a maximum diameter of 4 mm or more can be produced by adding elements such as Zr and Nb. We also developed a composite material with a bulk metallic glass as a matrix and a superelastic alloy as a second phase, and modulated the volume fraction of the second phase by controlling quenching rate. In addition, the enhancement of the mechanical properties of the amorphous matrix composites was found to be due to the increase of the work hardening ability of the second phase by applying the solid solution hardening by antisite defect caused by Sn addition. As a result, bulk metallic glass matrix composite materials having high yield strength and excellent work hardening ability of about 1 GPa or more were developed. In this paper, we present a unique alloy design method and theoretical considerations that have not been attempted. We have extended the maximum solid solubility limit of alloying element in B2 phase, and proposed a method to control the characteristic temperature of martensitic transformation and mechanical properties of SMAs. In addition, it is considered that this work has an important meaning not only in the academic aspect but also in the industry as well, by developing a material having excellent elastocaloric performance and an excellent mechanical property. Especially, since the B2 phase stability can be enhanced by suppressing precipitations, it will be the core technology that will lead the fourth industrial revolution by utilizing sputtering and 3-D printing. The same concept of alloy design can be applied to other alloy systems as well as the TiCuNi alloy system proposed in this paper, which can be used as a guideline for further study.