Designing NiTiAg shape memory alloys by vacuum arc remelting : first practical insights on melting and casting.

Abstract

NiTi-based shape memory alloys are successful owing to its capacity to cover specific applications unreachable by binary NiTi. The additions of ternary, and even quaternary, elements are intended to change specific properties. Known for its antibacterial activity, Ag became an alloying element in a search for a functional biomaterial; however, the melting appears to hampering the system exploration. A special melting procedure by vacuum arc remelting was developed based on chemical and thermal analysis, via EDS, XRF, and DSC, assessing the element loss and ingot homogeneity, respectively. By alloy design, different Ag content NiTiAg SMA were produced and analyzed on as-cast condition. The melting procedure developed involves specific feedstock cares and preparation, melting, and some remelting steps. The measured chemical composition slightly differs from the nominal due to alloying element loss and the melting reaction thermodynamics. Being the lower the possible, the remelting steps were optimized to maintain the compromise between chemical composition and compositional homogeneity through the ingot, since the Ag content stabilizes along them, also indicating a limited content possible to be alloyed. Ag-yields are content-dependent, while the Ni:Ti relation is stable, being therefore the melting of NiTiAg SMA better performed by VAR than other melting routes under high vacuum conditions.