Synchrotron X-Ray Study of Texture in Cold-Worked Shape-Memory NiTi-Wires
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Synchrotron X-Ray Study of Texture in Cold-Worked Shape-Memory NiTi-Wires Andreas Schuster1, Heinz Voggenreiter1, Dorian K. Balch and David C. Dunand, Northwestern University, Dept. of Materials Science and Engineering, Evanston IL 60208, USA 1 EADS, Corporate Research Center, 81663 Munich, Germany ABSTRACT A series of martensitic, near-equiatomic NiTi shape-memory alloy wires was deformed to strains ranging from 1 to 40% up to stresses of 920 MPa. After deformation, the wires were exposed to a monochromatic, parallel beam of high energy x-rays oriented perpendicular to the wire axis. The transmitted low index diffraction rings show that martensitic texture is increasing with prestrain up to ε=15% after twinning is complete. Further prestraining in the plastic range lowers the texture again indicating that twinning- and plasticity-textures cancel partially each other. Also, deformed NiTi-wires were heated and cooled from 20°C to 200°C under a small constant stress of 6 MPa. The strain change due to the Two-Way Shape-Memory Effect was measured and correlated to the diffraction results. INTRODUCTION Some of the best-studied shape-memory alloys have near-equiatomic NiTi composition. The shape-memory effect (SME) relies on a reversible, diffusionless, thermoelastic transformation from a high-temperature austenite phase to a low-temperature martensite phase [1]. When the martensite is mechanically loaded, these martensitic variants can reorient in a twinning process, leading to up to about 6% macroscopic tensile strain. After unloading, the twinned structure is stable so no deformation is recovered, except for a small elastic component. Subsequent heating triggers the phase transformation and a concomitant recovery of the twinning strain (i.e., SME). Mechanical loading beyond the twinning stress results in elastic deformation, and eventually plastic deformation, of the twinned martensite. High-stress deformation can significantly alter the phase transformation temperatures and the amount of recoverable shape memory strain. It can also produce the two-way shape-memory effect (TWSME), whereby reversible strains are produced upon both heating and cooling transformation. Therefore it is interesting to study the phase and texture evolution in the bulk of these materials as a function of deformation. This can be achieved by synchrotron x-ray or neutron diffraction, as recently studied in superelastic and shape-memory NiTi deformed in-situ [2-4]. In the present study, we use high-intensity x-ray diffraction to shed light on the texture developed after uniaxial deformation and its link to the TWSME in NiTi wires. EXPERIMENTAL PROCEDURES The NiTi wires used were near-equiatomic (Ni-49.7 at%Ti) in composition and 0.8 mm in diameter. The wires were procured from Memory-Metalle GmbH (Weil am Rhein, Germany) in the “straight anneal” condition (600°C treatment) and were martensitic at room temperature. The wires exhibit the following transformation temperatures: Ms≈67°C and Mf≈54Û&RQFRROLQJ As≈93°C and Af≈116Û&RQKHDWLQJ8QLD[LD
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