Strain–temperature behavior of NiTiCu shape memory single crystals Original Research Article

Single crystal specimens of NiTi10Cu alloys were subjected to temperature cycling conditions under constant tensile and compressive stresses and the transformation strains were monitored. The [111] orientation exhibited the highest experimental transformation strains (6.64%) in tension while the [001] provides the highest transformation strains in compression (5.34%). These transformation strain levels are significantly higher than previously reported values on NiTiCu alloys. The theoretical treatment includes both the calculation of the CVP (correspondent variant pair) formation strain incorporating the growth of monoclinic phase from the most favorably oriented orthorhombic variant, and the concomitant detwinning of the monoclinic martensite. The experimental transformation strain values are consistently below the theoretical levels due to two main reasons: the slip deformation in the austenite domains as confirmed with TEM studies, and the incomplete transformation resulting in a mixture of orthorhombic and monoclinic phases as determined from diffraction patterns. The experimental transformation strains are higher in tension compared to compression for most single crystal orientations due to two factors: the additional strain associated with the detwinning of the B19′phase in the final microstructure (such as in [111] case), and the partial completion of the second step of the transformation limiting the compression strains.