Abstract

Shape memory alloys (SMAs) are a group of metallic alloys that can revert to a previously defined size or shape when deformed and then heated past a set transformation temperature. This “shape memory” behavior is due to a shear-dominated diffusionless transition between crystalline phases of different symmetries, termed austenite and martensite. SMAs display other unusual mechanical properties that make them highly useful, including superelasticity, high vibration damping, high yield stresses, and high power to weight ratios. Nickel-Titanium is a promising SMA used in a wide variety of applications, including advanced biocompatible and MEMs devices. 

This talk will discuss two experimental investigations into the propagation of stress-induced martensite through thin sheets of Nickel-Titanium. The first, performed at the meso- and macro- scale, investigates the phase transformation behavior as a function of material texture and cycling. This will include a discussion of the characteristics of the phase fronts as well as evidence of a remarkable cyclic memory in the phase transformation on the microscale. The second investigation utilizes a new methodology to examine phase 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 SEM-DIC patterning techniques for the reduced length scale and an approach to correct micrographs for the complex distortions inherent in SEM imaging. Using the DIC-calculated displacements, the progression of phase transformation and its relation to the underlying crystallography is examined at the grain level in mechanically loaded tensile samples.

Bio

Samantha Daly is an Assistant Professor in the Department of Mechanical Engineering and in the Department of Materials Science and Engineering at the University of Michigan, Ann Arbor. She received her Ph.D. and M.S. degrees in the Division of Engineering and Applied Science at the California Institute of Technology in 2007 and 2002 respectively, and joined the faculty at the University of Michigan in January of 2008. She is a recipient of the Department of Energy Office of Science Early Career Research Program Award, the Air Force Office of Scientific Research Young Investigator Award, the ASME Orr Award for early career research excellence in fatigue, creep, and fracture, the M. Hetényi Award from the Society of Experimental Mechanics, and the Everhart and Charles D. Babcock Awards from the California Institute of Technology. Her research interests include the mechanical behavior of materials, fatigue, fracture, creep, composites, smart materials, and advanced experimental techniques.

Please see the Colloquium Announcement for more details.

An informal coffee & cookie reception will be held prior to the lecture at 2:30 p.m. in the AAE/ARMS undergraduate lounge (directly in front of ARMS 3rd floor elevators).