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Research Papers

Annealing Effect on the Shape Memory Properties of Amorphous NiTi Thin Films

[+] Author and Article Information
Gen Satoh

Department of Mechanical Engineering, Columbia University, New York, NY 10025gs2358@columbia.edu

Andrew Birnbaum

Department of Mechanical Engineering, Columbia University, New York, NY 10025ajb2118@columbia.edu

Y. Lawrence Yao

Department of Mechanical Engineering, Columbia University, New York, NY 10025yly1@columbia.edu

J. Manuf. Sci. Eng 132(5), 051004 (Sep 20, 2010) (9 pages) doi:10.1115/1.4002189 History: Received June 22, 2009; Revised July 01, 2010; Published September 20, 2010; Online September 20, 2010

Thin film shape memory alloys have recently become a promising material for the actuation of devices on the microscale such as micropumps and microvalves. Their utilization, however, has been limited due to the difficulty in tailoring their properties for different applications. Control over the transformation temperatures as well as mechanical and shape memory properties is required to enable their widespread use. This study examines the effects of heat treatment time and temperature on the properties of amorphous, Ti-rich NiTi thin films on silicon substrates. The effects on the transformation temperatures are investigated through the use of temperature dependent optical microscopy and temperature dependent X-ray diffraction. The indentation modulus and hardness, as well as dissipated energy and depth recovery, are obtained through nanoindentation and atomic force microscopy. The role of microstructure and composition in altering both the mechanical and shape memory properties of the films is discussed.

Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

NiTi phase diagram. Note near-constant solubility limit of Ti on Ti-rich side of NiTi (20).

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Figure 3

(a) Length of martensitic surface relief as a function of annealing parameters. Error bars denote standard error. Optical micrographs of samples annealed at (b) 460°C for 5 min and (c) 560°C for 60 min (surface relief length denoted by bracket).

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Figure 4

Representative DIC optical micrographs of a martensitic sample upon heating and cooling

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Figure 5

Transformation temperatures versus annealing temperature for films annealed for 5 min. Measured through in situ temperature dependent optical microscopy. Transformation temperatures could not be measured for samples annealed at 760°C.

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Figure 6

Transformation temperatures versus annealing time for films annealed at 560°C. Measured through in situ temperature dependent optical microscopy. Transformation temperatures could not be measured for samples annealed at 760°C.

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Figure 7

XRD spectra for films annealed for 5 min at various temperatures. Note growth of austenitic peaks (42.8°C and 61.8°C) and decay of martensitic peaks with increasing annealing temperature. Spectra shifted for clarity.

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Figure 8

XRD spectra for films annealed at 560°C for various times. Note growth of austenitic peaks (42.8 deg and 61.8 deg) and decay of martensitic peaks with increasing annealing time. Spectra shifted for clarity.

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Figure 10

Transformation temperatures of samples annealed for 5 min as a function of annealing temperature. Comparison between XRD and optical methods. Austenitic start (As) and martensitic start (Ms) omitted for clarity. Transformation temperatures could not be measured for samples annealed at 760°C.

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Figure 11

Optical micrograph of residual impressions from 1 μm and 100 nm (inset) deep indentations on annealed NiTi film before heating. Film annealed at 460°C for 5 min.

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Figure 12

Representative load-displacement curves for 100 nm indentation in annealed films. Note smaller residual indentation depth of austenitic film due to superelasticity.

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Figure 13

Modulus and hardness of films annealed at various temperatures and times

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Figure 14

Dissipated energy ratio versus annealing time at various temperatures. Note reduced dissipation for higher temperature heat treatments due to austenitic structure.

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Figure 15

Cross section of typical AFM scan of indentations in martensitic films before and after heating showing depth recovery. Film annealed at 460°C for 5 min.

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Figure 16

Recovery ratio as a function of annealing temperature and time. Error bars denote standard error.

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Figure 17

Percentage of deformation accommodated by different mechanisms for films annealed for 5 min

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Figure 18

Percentage of indentation deformation accommodated by different mechanisms as a function of annealing time at 460°C

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Figure 1

Schematic diagram of the stress-temperature phase diagram for NiTi

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Figure 9

Transformation temperatures of samples annealed at 560°C as a function of time. Comparison between XRD and optical methods. Austenitic start (As) and martensitic start (Ms) omitted for clarity.

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