Deformation Behavior of a Shape Memory Alloy, Nitinol

摘要

Nickel-Titanium, commonly referred to as Nitinol, is a shape-memory alloy with numerousudapplications due to its superelastic nature and its ability to revert to a previously defined shapeudwhen deformed and then heated past a set transformation temperature. While theudcrystallography and the overall phenomenology are reasonably well understood, much remainsudunknown about the deformation and failure mechanisms of these materials. These latter issuesudare becoming critically important as Nitinol is being increasingly used in medical devices andudspace applications. The talk will describe the investigation of the deformation and failure ofudNitinol using an in-situ optical technique called Digital Image Correlation (DIC). With thisudtechnique, full-field quantitative maps of strain localization are obtained for the first time inudthin sheets of Nitinol under tension. These experiments provide new information connectingudprevious observations on the micro- and macro- scale. They show that martensiticudtransformation initiates before the formation of localized bands, and that the strain inside theudbands does not saturate when the bands nucleate. The effect of rolling texture, the validity ofudthe widely used resolved stress transformation criterion, and the role of geometric defects areudexamined.