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 (old)|
|HGF - Program:||Aeronautics|
|HGF - Program Themes:||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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