Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys
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INTRODUCTION
NITI shape memory alloys (SMAs) have been extensively applied in civil,[1–3] medical,[4–8] and aerospace[9–11] devices for their unique shape memory effect, pseudoelasticity, bio-compatibility, and corrosion resistance.[12–18] The characteristic transformation temperatures for the austenite-to-martensite phase transformation as well as the mechanical response can be modified to meet application requirements through (i) thermo-mechanical processing,[19–21] (ii) slight variation from the equi-atomic NiTi chemical composition,[22,23] or (iii) addition of alloying elements.[24–28] This phase transformation generally involves an austenitic cubic B2 structure transforming to and from either a martensitic monoclinic B19 or orthorhombic B19¢ structure. Depending on the thermo-mechanical processing, intermediate phase transformations may occur as well; one of the particular notes is the commonly observed R-phase transformation, which involves a slight distortion of the austenitic cubic B2 structure.[13] Precipitates, such as Ni4Ti3 in austenitic (Ni-rich) NiTi alloys and Ti2Ni in martensitic (Ti-rich) NiTi alloys, play a significant role in the concentration of Ni in small localized regions, which change the phase
YING QIU, Ph.D. Graduate Student, is with the Materials Science and Engineering, University of North Texas, Denton, TX, and also with the Mechanical and Energy Engineering, University of North Texas, Denton, TX. MARCUS L. YOUNG, Assistant Professor, is with the Materials Science and Engineering, University of North Texas. Contact e-mail: [email protected] XU NIE, Assistant Professor, is with the Mechanical and Energy Engineering, University of North Texas. Manuscript submitted November 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation temperatures and mechanical properties. Although most dynamic investigations have been focused on austenitic NiTi alloys,[29–39] martensitic NiTi alloys also exhibit thermoelastic behavior and even better damping capacity,[40,41] due to the movement of twin interfaces. NiTi SMAs, which are fully martensitic at room temperature,[42] often have coarse brittle Ti2Ni precipitates. These precipitates can severely reduce the mechanical properties, lead to significant internal stresses at precipitate-matrix interfaces, and aid in crack propagation during loading. Mechanical properties of martensitic NiTi SMAs under quasi-static loading have been extensively studied.[12,13] The martensite reorientation deformation proceeds in four stages: (I) elastic and homogeneous deformation of martensite, (II) propagation of localized reorientation bands (e.g., twinning/detwinning), (III) elastic deformation of reoriented martensite, and (IV) plastic deformation (e.g., slip). However, these distinct stages become difficult to observe especially during dynamic loading since two or more stages may be present locally at any given point in loading. After quasi-static compression loading, deformation bands and dislocations are generated in the martensite variant regions. Dis
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