ORIENTATION DEPENDENCE OF THE PSEUDOELASTIC BEHAVIOR OF SINGLE-CRYSTALS OF CU-AL-NI IN TENSION

Uniaxial tension experiments were performed on single crystals of Cu-13.95 wt% Al-3.93 wt% Ni. Three specimens were prepared with tension axes in directions that were chosen based on Schmid law calculations using the 96 possible Austenite-Martensite (A-M) interface orientations in this alloy. Specimen number one was chosen to have a tensile axis of [2,43,1,0] which results in a very near minimum value for its predicted tension transformation stress. Specimen number two was oriented 15 degrees from [111] direction and has a [1,1,1.73] tensile axis direction. The third specimen has the [111] direction as its tensile axis, which is the direction of maximum tensile transformation stress. A strong relationship is found between the mechanical behavior of the specimens in tension and their observed microstructure. Specimen one exhibits an extremely flat stress plateau during transformation and almost no hysteresis. The microstructure observed in this specimen consists of two nearly perpendicular A-M interfaces that interact to form an X-structure that results in a purely uniaxial deformation. This microstructure is completely reversible and seems to present no restriction on the motion of either interface. Specimen two was observed to have only a single A-M interface after transformation. This interface appears to preclude the formation of any other interfaces. Specimen three required five times the normal stress of that needed to transform specimen one. This specimen also exhibited a large amount of hysteresis. The microstructure observed consisted of two A-M interface systems that meet to form wedges. Because the interfaces must end at the wedge apex, the formation of the wedges resulted in a kinematic coupling between the two A-M interface systems. The amount of coupling between the interfaces in the microstructure correlates to the amount of hysteresis observed.