Two - Step Martensitic Transformations in TiNi(10% Cu) Shape Memory Alloys

Abstract

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 (M_s), 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 B19′ 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 B19′ 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 B19′ monoclinic structures. Strain vs temperature M_s tests establish the relative shape changes associated with both the cubic-to-orthorhombic transformation and the orthorhombic-to-monoclinic transformation. Similar M_s 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.