Influence of Structure and Microstructure on Deformation Localization and Crack Growth in NiTi Shape Memory Alloys
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Porous NiTi shape memory alloys have applications in the biomedical and aerospace fields. Recent developments in metal additive manufacturing have made fabrication of near-net-shape porous products with complicated geometries feasible. There have also been developments in tailoring site-specific microstructures in metals using additive manufacturing. Inspired by these developments, we explore two related mechanistic phenomena in a simplified representation of porous shape memory alloys. First, we computationally elucidate the connection between pore geometry, stress concentration around pores, grain orientation, and strain-band formation during tensile loading of NiTi. Using this, we present a method to engineer local crystal orientations to mitigate the stress concentrations around the pores. Second, we experimentally document the growth of cracks around pores in a cyclically loaded superelastic NiTi specimen. In the areas of stress concentration around holes, cracks are seen to grow in large grains with [1 1 0] oriented along the tensile axis. This combined work shows the potential of local microstructural engineering in reducing stress concentration and increasing resistance to propagation of cracks in porous SMAs, potentially increasing the fatigue life of porous SMA components.
KeywordsNickel titanium Shape memory alloys Electron back scattering diffraction Crystal plasticity Additive manufacturing
The authors would like to acknowledge the use of NUANCE (NSF ECCS-1542205), MatCI and CLAMMP (NSF DMR-1121262) facilities at Northwestern University for electron microscopy and mechanical testing and the computing facilities at XSEDE (Extreme Science and Engineering Discovery Environment, NSF ACI-1053575). Financial support for this work was provided by the Department of Energy, Office of Basic Energy Science (Grant No. DE-SC0010594). The authors thank Dr. Sivom Manchiraju (formerly at The Ohio State University) for making available the Abaqus UMAT code used in this work, and Ms. Amy Brice (Colorado School of Mines) for helping with proof-reading and formatting of the manuscript.
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