Combustion synthesis of Niti-X (X=FE, AL, PD) shape memory intermetallic compounds

Abstract

The NiTi, NiTi-X (X=Fe, Al, Pd) SMA's have been successfully produced by the thermal explosion mode of SHS technique. It was demonstrated that most intermetallic compounds can be produced by the thermal explosion mode which is superior to the combustion mode in producing such compounds in terms of obtaining a cast product. The combustion parameters (Tig and Tc) were measured for all of the combustions. From this, the heat of fusion and heat capacity of the liquid NiTi compound were estimated, and provides an alternative technique for obtaining such thermidynamic values providing the measurements are accurate and reliable. Cast products have been obtained for all of the above mentioned alloys. They were fabricated into plates form exhibiting SME. Their transition temperatures ranged from -78�C (TiNiFe) to 459�C (TiNiPd), thereby provided the potential for increasing their applications. The microstructures of these alloys were investigated systematically by means of optical and scaning electron microscopy, and X-ray diffraction techniques. It was found that the SHS technique synthesized these alloys with microstructures composed of parent and second phases. The stochiometry of the second phase is (Ni,X)Ti2, regardless of X. These second phases were found to be responsible for deteriorated mechanical properties and lowered transition temperatures. A trigger reaction theory was proposed to explain the combustion phenomenon in air. It is assumed that a second, more highly exothermic reaction can supply extra heat to the system which initiates the main SHS reaction at a lower ignition temperature at which the SHS reaction can not be initiated under an inert or protected atmosphere. This theory may be important both academically and commercially. A solid-solid hypothetical reaction model was also presented. It is proposed that for the thermal explosion mode, the combustion can be divided into two stages, i.e, pre-combustion and combustion stage. In the former, the reaction is a rate-controlling process.