PSI - Issue 69

Alberto Coda et al. / Procedia Structural Integrity 69 (2025) 26–34

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Introduction Shape memory alloys are used in many areas, like medical, automotive, sensors, actuation, robotic, aerospace, consumer goods, transport, building, energy and more [1]. However, except of few cases, this class of material’s applications is still limited to operating temperatures below 130°C as alloys that transform at higher temperatures are either brittle, show poor performance or are highly expensive [2-5]. This is a significant drawback, as higher transformation temperatures would open the possibility to use these alloys in several new applications [6]. In particular, they could be applied in the waste heat management in industrial plants, contributing to the reduction of CO 2 gases with benefits for a green transition in energy and manufacturing sector [7]. One approach to overcome the shortcomings of high temperature shape memory alloys is alloying with the scope to improve and fine-tune the transformation temperature, workability and functional properties [8]. Many systems have been investigated to reach this goal, among these NiTiHf alloys [9], Cu-based alloys [10], Ti-Ni-Pd [11], Ti-Ta alloy [12], Ni-Mn-Ti [13] or Ru-Nb [14]. In particular, in case of the most prominent high temperature shape memory alloy system, Ni-Ti-Hf, much effort was recently dedicated to improve the workability by adding a fourth element or even more [15 – 17]. But none of them could find widespread application by providing a good combination of costs, workability and sufficient functional behavior. In this regard, Ni 3 Ta is a potentially interesting shape memory alloy, as it transforms at significantly higher temperatures as binary NiTi [18-21]. On the other hand, this intermetallic is much more brittle than NiTi and its deformation is limited to hot processing [19]. Improving the alloy properties by adding a third element was studied rarely so far, previous approaches explored Zr and Hf [22] or B [23] additions. In order to reduce its hardness and improve the workability without highly affecting the transformation temperatures, a similar alloying approach was followed in this study. Based on the possible impact on the Ni-Ta phase diagram, Cu and Co were identified as suitable element additions [ 24] . Vacuum arc (VAR) melted ingots were studied in the as-cast and heat-treated state to explore the microstructural and shape memory transformation stability. 1. Experimental 2.1 Alloys preparation The intermetallic compound Ni 3 Ta shows a solid-solid phase transformation between a tetragonal (austenite) and a monoclinic (martensite) structure (see Figure 1). Additions of Cu and Co were designed to substitute 1 atom of Ni or Ta, respectively, according to the chemistries reported in Table 1.

Figure 1 Ni-Ta Phase diagram. The area of interest of designed alloys is highlighted by a red square [24].