The Impact of Martensite Deformation on Shape Memory Effect Recovery Strain Evolution
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SHAPE memory alloys (SMAs) are a class of multifunctional materials that recover deformation as a result of an underlying crystallographically reversible martensitic phase transformation (MT). The MT is between a high-temperature solid austenitic microstructure (A) and a low-temperature martensitic microstructure (M). Several review textbooks explain the MT morphology.[1–5] The austenite atomic crystal structure is of higher symmetry than the martensite structure and thus the MT gives rise to multiple martensite variants.[4,6,7] Its crystallographic planes and directions unique correspondence to specific crystallographic planes and directions in austenite defines each variant and each is oriented differently with respect to the austenite.[4,6,7] For the MT of NiTi, a B2 austenite structure transforms to a B19¢ martensite structure and 24 martensite variants are possible.[3,6,8,9] During the forward A fi M phase transformation, the variants arrange naturally into twinrelated correspondent variant pairs (CVPs) that can grow as plates within the microstructure.[2,3] Multiple differently oriented plates arrange in a manner that minimizes the elastic strain energy associated shape and volume ASHEESH LANBA, Ph.D. Student, and REGINALD F. HAMILTON, Assistant Professor, are with the Department of Engineering Science and Mechanics, The Pennsylvania State University, 212 EarthEngineering Sciences Bldg., University Park, PA 16802-6812. Contact e-mail: [email protected] Manuscript submitted December 29, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
change; the arrangement is referred to as self-accommodated martensite (SAM).[7,8] The volume fraction of SAM material grows during A fi M as the cooling temperature decreases and the SAM shrinks during heating and follows the reverse of the cooling morphology.[2] The austenite exhibits the original crystal structure and orientation, and the MT is referred to as crystallographically reversible.[3,10] SMAs can exhibit two types of shape memory responses: the shape memory effect (SME) and the pseudoelastic effect (PE) (also referred to as superelasticity). For the one-way SME, the material can be deformed in the SAM state and the MT morphology will depend on the stress level.[5,9,11] When a completely SAM microstructure is stressed, a preferentially oriented CVP grows at the expense of others.[12–15] At a critical stress level, the CVP will begin to detwin until it finally converts to a single crystal variant at even higher stress levels. In the PE case, the material is deformed at a constant temperature at which austenite is the stable structure. Deformation ensues at a high enough stress level to grow preferentially oriented CVP and facilitate detwinning. Austenite, CVP, and/or detwinned martensite coexist during deformation. For the SME, SAM may also be present. The coexistence gives rise to strain localization during deformation. Fullfield deformation measurements from digital image correlation (DIC) strain analysis technique provide a means to visualize the evolution of strains
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