Unusual Functionalities in Martensitically Transforming Materials

Reversible martensite phase transformation, which readily occurs in shape memory alloys (SMAs), enables easily-controlled switching between significantly differing crystal structures due to the small energy differences between the transforming phases. We have achieved a number of new and unique functionalities in these materials near phase stability limits by engineering the microstructure, texture, and level of structural disorder, and their interactions with the reversible martensitic phase transformation. For example, alloys with tunable thermal expansion coefficients were created from polycrystalline SMAs through texturing and selection of appropriate stress-induced martensite variants by thermo-mechanical processing. Strain glass and spin glass behaviors were observed in the same magnetic SMA by controlling the martensitic transformation and its frustration through changing structural disorder. Grain refinement could be easily achieved by activating stress-induced martensitic transformation and creating reversible deformation twins in martensite. Grain boundary engineering is possible through the reversible transformation / deformation twins, which help to obtain low energy grain boundaries in nanostructured austenite. Finally, the effective elastic modulus of SMAs could be easily manipulated by varying the stress level at which stress-induced transformation occurs through controlling the dislocation density, grain size, and precipitation size and volume fraction of the alloy. These examples show that reversible martensitic phase transformation can be used as an engineering tool to precisely tailor the properties of materials, and functions as a versatile and powerful method to create “designer” materials for various applications.

Biography
Ibrahim Karaman is the Chevron Professor and Department Head of Materials Science and Engineering at Texas A&M University. He received his Ph.D. from University of Illinois at Urbana-Champaign in Mechanical Engineering in 2000. He joined the faculty of Department of Mechanical Engineering at Texas A&M University in 2000. He was promoted to the rank of Professor in 2011. He has served as the Chair of the Interdisciplinary Graduate Program in Materials Science and Engineering (MSEN) from 2010 to 2013. The MSEN program became a new department in 2013, where Dr. Karaman serves as the head. His main research interests are processing-microstructure-mechanical/functional property relationships in metallic materials and composites including 1) ultrafine and nanocrystalline materials, and 2) conventional, high temperature and magnetic shape memory alloys; micro-mechanical constitutive modeling of crystal plasticity; twinning and martensitic phase transformation. Dr. Karaman received several national and international awards including the NSF CAREER Award, ONR Young Investigator Award, The Robert Lansing Hardy Award from The Minerals, Metals and Materials Society (TMS), an Honorable Mention for the Early Career Faculty Fellow Award from TMS, and Gary Anderson Early Achievement Award from ASME and AIAA. He is an author or co-author over 200 refereed journal articles.