New methods for controlling twin configurations and characterizing twin boundaries in 5M Ni-Mn-Ga for the development of applications

摘要

Traditional methods for studying the magnetic shape memory (MSM) alloys Ni-Mn-Gainclude subjecting the entire sample to a uniform magnetic field or completely actuatingthe sample mechanically. These methods have produced significant results incharacterizing the MSM effect, the properties of Ni-Mn-Ga and have pioneered thedevelopment of applications from this material.Twin boundaries and their configuration within a Ni-Mn-Ga sample are a keycomponent in the magnetic shape memory effect. Applications that are developedrequire an understanding of twin boundary characteristics and, more importantly, theability to predictably control them. Twins have such a critical role that the twinningstress of a Ni-Mn-Ga crystal is the defining characteristic that indicates its quality andsignificant research has been conducted to minimize this property.This dissertation reports a decrease in the twinning stress, predictably controlling thetwin configuration and characterizing the dynamics of twin boundaries. A reduction ofthe twinning stress is demonstrated by the discovery of Type II twins within Ni-Mn-Gawhich have as little as 10% of the twinning stress of traditional Type I twins.Furthermore, new methods of actuating a Ni-Mn-Ga element using localizedunidirectional or bidirectional magnetic fields were developed that can predictablycontrol the twin configuration in a localized area of a Ni-Mn-Ga element.This method of controlling the local twin configuration was used in the characterizationof twin boundary dynamics. Using a localized magnetic pulse, the velocity andacceleration of a single twin boundary were measured to be 82.5 m/s and 2.9 × 107 m/s2,and the time needed for the twin boundary to nucleate and begin moving was less than2.8 μs. Using a bidirectional magnetic field from a diametrically magnetized cylindricalmagnet, a highly reproducible and controllable local twin configuration was created in aNi-Mn-Ga element which is the fundamental pumping mechanism in the MSMmicropump that has been co-invented and extensively characterized by the author.