This talk will discuss two experimental studies into a solid-to-solid state phase transformation in Nickel-Titanium that underlies its unique properties, including its ability to remember a previously defined shape and its superelasticity. The first, performed at the meso- and macro- scale, examines phase transformation as a function of crystallographic texture and mechanical cycling. This includes the discovery of a remarkable cyclic strain similarity in the formation of one of the phases at the microscopic length scale, and a discussion of the impact of crystallographic texture on this similarity. The second investigation utilizes a novel methodology to quantitatively examine full-field deformations (indicative of transformation) at the grain level. This is achieved by combining an optical technique known as digital image correlation with scanning electron microscopy, an approach termed here as SEM-DIC. The development of the methodology will be discussed, including new self-assembled nanoparticle patterning techniques for unprecedented spatial resolution, and approaches to correct micrographs for the complex spatial and temporal distortions inherent in SEM imaging. Using the DIC-calculated displacements across fields of view ranging from 10μm - 500μm horizontal field width, the progression of phase transformation and its relation to the underlying crystallography is examined at the grain level in mechanically loaded tensile samples. Relevant analysis will be discussed, including examination of locally active martensite variant pairs and the finding that similarly oriented grains do not necessarily transform similarly, in contrast to a common assumption in mean-field theories.
Samantha Daly is an Associate Professor in the Department of Mechanical Engineering and in the Department of Materials Science and Engineering at the University of Michigan, Ann Arbor. Her research interests include the mechanical behavior of materials, fatigue, fracture, creep, composites, multi-functional materials, and advanced experimental techniques with a focus on novel approaches for small-scale characterization. She received her Ph.D. and M.S. degrees from the Division of Engineering and Applied Science at Caltech in 2007 and 2002 respectively, and joined the faculty at the University of Michigan in 2008. She is a recipient of the NSF CAREER Award, the 2014 Journal of Strain Analysis Young Investigator Award, the 2014 International Journal of Solids and Structures Best Paper of the Year Award, the 2013 M. Hetényi Award (for the best paper published in the journal Experimental Mechanics in 2011), the DOE Early Career Research Program Award, the AFOSR YIP Award, the ASME Orr Award for early career research excellence in fatigue, creep, and fracture, the Robert M. Caddell Research Award in Materials and Manufacturing, the U-M 1938E and departmental faculty excellence awards, and the Everhart and Charles D. Babcock Awards from Caltech.