Two-step Martensitic Transformation Research Articles

Influence of electron irradiation on the martensitic transformation of a binary TiNi shape memory alloy

TiNi shape memory alloy samples annealed at 673 K for 1 h were irradiated within R-phase by 1.7-MeV electrons with different doses. The two-step martensitic transformation temperatures were measured by differential scanning calorimeter. The results indicated that the temperature M s of the onset of R-phase-to-martensite transformation decreased with increasing electron dose. The electron irradiation had a slight effect on the other transformation temperatures. The second lifetime of positrons determined by positron annihilation technology were lowered with increment of the irradiated dose. Relaxation of the elastic stress fields around the Ti 3Ni 4 precipitates was the cause of the observed change of the transformation characteristics because of the migration and accumulation of irradiation-induced point defects.

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

AbstractThird 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.

AEM of Ti50Ni40Cu10 Shape Memory Alloys

Equiatomic Ni-Ti alloys exhibit shape-memory properties, whereby an article deformed at low temperature regains its original shape at higher temperatures. This effect is due to a reversible martensitic transformation; in binary alloys the parent phase has an ordered cubic (B2) structure which transforms to a monoclinic B19' structure. Substitution of approximately 10% Cu for Ni results in an improvement of shape memory properties which has been attributed to a two-step martensitic transformation from B2 to untwinned B19 and then to twinned B19'. There is, however, some controversy in the literature concerning which phases are in equilibrium and which type of martensite is stable after various thermomechanical treatments. The purpose of this paper is to report on the preliminary findings from an AEM study in order to illuminate the microstructural origins of these controversies.Ti50Ni40Cu10 alloys were cast in a plasma-arc furnace, sectioned and prepared for scanning and transmission electron microscopy.