Third element additions to TiNi provide a wide range of modifications of its shape memory properties. The advantages of Cu additions are to provide a more narrow hysteresis, less sensitivity to the Ti::Ni(+Cu) ratio of the temperature at which martensite starts to form (Ms), a larger strength differential between the austenite and martensite phases, and superior fatigue resistance. The substitution of a few atomic percent Cu for Ni does not significantly alter the crystal structure of either the cubic B2 austenite nor the monoclinic B19ʹ martensite phases; however, the addition of greater than 10% Cu results in an orthorhombic B19 martensite phase. For the case of 10% Cu, a two-step martensitic transformation occurs upon cooling, with the cubic austenite transforming to the orthorhombic B19 martensite and subsequently to the monoclinic B19ʹ martensite. As a result of this two-step crystallographic transformation, material properties such as resistivity and shape change also exhibit a two-step transformation.
In situ transmission electron microscopy heating and cooling experiments are used to observe the two-step martensitic transformation and to establish an orientation relationship between the B19 orthorhombic and the B19ʹ monoclinic structures. Strain vs temperature Ms tests establish the relative shape changes associated with both the cubic-to-orthorhombic transformation and the orthorhombic-to-monoclinic transformation. Similar Ms tests, where an applied load is removed during the transformation, establishes a crystallographic dependence between the two shape changes. Whereas binary TiNi is "trained" to undergo a specific shape change, this ternary TiNiCu alloy has a "natural" direction associated with the second step of its shape change.