Precipitation Strengthenable NiTiPd High Temperature Shape Memory Alloys

In binary NiTi alloys, it has long been known that Ni-rich alloys can be heat treated to produce precipitates which both strengthen the matrix against dislocations and improve the behavior of the material under thermal and mechanical cycling.  Within recent years, the same effect has been observed in Ni-rich NiTiHf high temperature shape memory alloys and heat treatment regimens have been defined which will reliably produce improved properties.  In NiTiPd alloys, precipitation has also been observed, but studies are still underway to define reliable heat treatments and compositions which will provide a balance of strengthening and good thermomechanical properties.  For this study, a series of NiTi-32 at.%Pd alloys was produced to determine the effect of changing nickel/titanium content on the transformation behavior and heat treatability of the material.  Samples were aged at temperatures between 350ºC and 450ºC for times up to 100 hours.  Actuation type behavior was evaluated using uniaxial constant force thermal cycling (UCFTC) to determine the effect of composition and aging on the material behavior.  TEM/SEM was used to evaluate the microstructure and determine the types of precipitates formed.  The correlation between composition, heat treat, microstructure, and thermomechanical behavior will be addressed and discussed.