When 3:30 PM - 5:00 PM Sep 17, 2010
Where 1670 CSE
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Experimental Studies of Phase Transformation in Shape Memory Alloys

Samantha Daly, Mechanical Engineering, University of Michigan

Shape memory alloys (SMAs) are a group of metallic alloys that are able to 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. 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. In addition, the alloys are durable and corrosion resistant. Nickel-Titanium is a promising SMA used in a wide variety of applications, including advanced biocompatible and MEMs materials.  

This talk presents an experimental investigation into the propagation of stress-induced martensite through thin sheets of Nickel-Titanium. The martensitic phase boundary is observed using three-dimensional stereo-imaged digital image correlation (DIC) to map local strain fields, and simultaneous infrared (IR) imaging to map corresponding local temperature variations. Although other methods have been used to explore phase transformation in shape memory alloys, this methodology is unique in providing a quantitative estimate of the strain inside the area of martensitic transformation, as well as direct correlations of local strain and temperature fields. Using this combined methodology, we can quantify the complex local interactions between released/absorbed latent heat and the extent of transformation. The characteristics of the phase fronts and evolution of martensitic volume fraction will be discussed, as well as evidence of a remarkable amount of cyclic strain memory on the microscale. 

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