Analysis of Multi-Step Transformations in Single Crystal NiTi
Wagoner Johnson, A. and Hamilton, R. and Sehitoglu, H. and Biallas, G. and Maier, H.J. and Chumlyakov, Y.I. and Woo, H.S. (2005) Analysis of Multi-Step Transformations in Single Crystal NiTi. Metallurgical and Materials Transactions A, 36A (4), pp. 919-928.
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The effects of composition and heat treatment on the thermally induced phase-transformation behavior of single-crystal NiTi with compositions of 50.1, 50.4, 50.8, and 51.5 at. pct Ni are presented in this article. Differential scanning calorimetry (DSC) experiments reveal that a heat-treated 50.1 at. pct Ni alloy exhibits an unprecedented multiple-step transformation (MST) on both heating and cooling, with up to four peaks. This behavior is absent in the higher-Ni-content alloys. In polycrystalline NiTi alloys, MSTs have been attributed to microstructure heterogeneities such as grain boundaries and dislocations, which influence precipitation. In-situ scanning electron microscopy (SEM) results show that the MST in the 50.1 at. pct Ni alloy is associated with single-crystal defects such as dendrites and low-angle boundaries. A heterogeneous precipitate distribution is observed in transmission electron microscopy (TEM) images of the same low-Ni alloy, also associated with the defects, creating conditions that have been shown in other studies to promote the MST in polycrystals. These MSTs are not observed for high-Ni single-crystal alloys containing the same defects. In this article, we describe the origin of the extraordinary forward and reverse MSTs in the low-Ni alloy and the absence of the MST in high-Ni alloys. Transformation sequences are proposed based on the contrasting precipitate microstructures.
|Title:||Analysis of Multi-Step Transformations in Single Crystal NiTi|
|Journal or Publication Title:||Metallurgical and Materials Transactions A|
|In ISI Web of Science:||Yes|
|Page Range:||pp. 919-928|
|Keywords:||shape memory alloys, phase transformation, NiTi, scanning electron microscopy, transmission electron microscopy, differential scanning calorimetry, microstructure|
|HGF - Research field:||Aeronautics, Space and Transport|
|HGF - Program:||Aeronautics|
|HGF - Program Themes:||L AR - Aircraft Research|
|DLR - Research area:||Aeronautics|
|DLR - Program:||L AR - Aircraft Research|
|DLR - Research theme (Project):||L - Structures & Materials|
|Institutes and Institutions:||Institute of Materials Research > Mechanical Testing of Materials|
|Deposited By:||Dr.-Ing. Gerhard Biallas|
|Deposited On:||27 Jan 2006|
|Last Modified:||27 Apr 2009 04:55|
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