Analysis of multistep transformations in single-crystal NiTi
- PDF / 1,361,096 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 34 Downloads / 171 Views
I. INTRODUCTION
THE phase-transformation temperatures and sequence in near-equiatomic NiTi alloys are significantly influenced by alloy or matrix composition[1] and internal stresses in the matrix,[2] both of which can be altered by the precipitation of coherent Ti3Ni4 particles introduced under certain aging conditions.[3] Internal stresses can also be altered by the presence of defects such as dislocations introduced through mechanical deformation[2,4] or stress-free thermal cycling.[5] Extensive information on transformation sequences in polycrystalline NiTi is reported in the literature using differential scanning calorimetry (DSC), but few studies have considered the phase-transformation behavior of single-crystal NiTi. The single-step forward (cooling) and reverse (heating) transformation sequence, austenite (A) ↔ martensite (M), is typically observed in solutionized NiTi with little or no plastic deformation[6–11] or deformed then recrystallized NiTi.[1,12,13] Another sequence frequently reported is the twostep/two-step A ↔ R ↔ M transformation. The additional step is associated with the internal elastic stresses, which facilitate the transformation to the intermediate R-phase with a lower transformation shear.[2,4,5,8,10,13–15] The transformation sequences that are not as well described are those that include greater than two steps on cooling or heating. They are
A.J. WAGONER JOHNSON, Research Assistant Professor, H. SEHITOGLU, Professor, C.J. Gauthier Professor and Interim Head, and R.F. HAMILTON, Graduate Research Assistant, are with the Department of Mechanical Engineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801. Contact e-mail: [email protected] G. BIALLAS, Postdoctoral Associate, and H.J. MAIER, Professor, are with the Lehrstuhl für Werkstoffkunde (Materials Science), University of Paderborn, 33095 Paderborn, Germany. Y.I. CHUMLYAKOV, Professor, is with the Siberian Physical and Technical Institute, 634050 Tomsk, Russia. H.S. WOO, Professor, is with the Safety Engineering Department, Dongguk University, Sukjang-dong Kyongju Kyongbuk, South Korea. Manuscript submitted February 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
referred to here as multiple-step transformations (MSTs) and include sequences such as three-step/two-step and threestep/three-step transformations, for example.[15,16,17] In this study, we observe three-step/three-step and four-step/fourstep transformations in single-crystal NiTi. According to many studies on the MST in polycrystalline NiTi shape-memory alloys, the main considerations are microstructural heterogeneity and variations in local composition.[2,3,7,14–16] Microstructures containing precipitates are inherently heterogeneous, and some authors claim this as the source for MSTs.[15,17,18] In other cases, the MST is attributed to an inhomogeneous distribution of precipitates. Highangle grain boundaries[7,19] or dislocations[2,20] are cited as the most likely sources for inhomogeneous precipitation of Ti3Ni4 in polycrystals. In single crystals, these
Data Loading...
