MSE Seminar: Dan Coughlin, Microstructure and strengthening effects due to stoichiometry and aging of NiTiHf

PhD Candidate advised by Dr. Michael Mills, Department of Materials Science and Engineering, The Ohio State University

Abstract

Characterization of NiTiHf shape memory alloys have recently become important due to their useful mechanical properties, high temperature applications, and lower cost than other shape memory alloys with similar characteristics.  A certain microstructure and mechanical properties are obtained when Hf is added to NiTi and a proper aging cycle is used.  The alloys analyzed all had 20(at.)%  Hf with vary Ti concentrations (29(at.)%, 29.7(at.)%, 30(at.)%, and 30.5(at.)%).  Results from compression test executed at temperatures above the austenitic transformation temperature showed strengthening effects.  Tests were run at temperatures of Af+10C, Af+30C, Af+50C, Af+70C, Af+90C for each alloy. The strengthening effect is due to the yield stress and plasticity being related to stress induced martensite that is created during the compression test.   At room temperature the alloys are B19’ and transform to B2 at elevated temperatures.  When at the elevated temperature a stress is applied martensite forms. The microstructure and formation of martensite effects the mechanical properties.  Four different aging cycles were tested.

Bio

Daniel Coughlin obtained his BS in mechanical engineering from the University of Colorado in 2004.  While earning his degree he worked at the university’s earthquake simulation laboratory helping with mechanical testing that simulates structures subjected to earthquakes, wind, and other extreme loadings.  After graduating he was hired by Los Alamos National Laboratory as a graduate research assistant.  At Los Alamos he worked on DARPA’s TACTIC project researching and operating a laser based fluorescence detection system aimed at detecting biological warfare agents.  Also during this time he earned his MS in mechanical engineering from the University of New Mexico.  Then in 2008, Daniel joined the metallurgy group (MST-6) at Los Alamos and worked on various mechanical testing projects that include bend testing, through-thickness double-ligament tensile testing, and component testing. Since 2009 he has been working on his PhD at the Ohio State University characterizing NiTiHf high temperature shape memory alloys via several microscopy techniques and isothermal compression tests.