Production of Ni-35Ti-15Hf Alloy via the FFC Cambridge Process

Abstract

The NiTiHf system alloy is considered to be one of the most attractive shape memory alloy (SMA) at elevated temperatures. This paper outlines how the FFC Cambridge Process was applied to produce the Ni-35 atom % Ti-15 atom % Hf (henceforth referred to as NiTiHf) alloy from sintered precursors of NiO, TiO(2) and HfO(2). In order to illuminate the reduction pathway, a number of partial reductions were completed at different reduction times. The samples were characterised by SEM, X-EDS and XRD. It was found that the key stages of reduction involved: (1) the reduction of NiTiO(3) and NiO to Ni, (2) the reduction of CaTiO(3) to Ti and the simultaneous formation of Ni(3)Ti, (3) the reaction of Ni(3)Ti with CaTiO(3) to form NiTi, (4) the reduction of HfO(2) and CaHfO(3) to form NiTiHf alloy and finally (5) the deoxidation and the Ti/Hf homogenisation of NiTiHf alloy. The sintered oxides precursors were reduced to metal alloy after 9 h reduction. After twenty-four hours reduction, a homogeneous alloy was formed with an oxygen content of 1600 ppm. DSC analysis shows that the austenite transformation temperature of the produced NiTiHf alloy was close to that seen in literature. An electrochemical predominance diagram for the Hf-Ca-Cl-O system was constructed to help understand the reactions during reduction.