Elsevier

Acta Materialia

Volume 53, Issue 18, October 2005, Pages 4955-4961
Acta Materialia

Laser annealing of amorphous NiTi shape memory alloy thin films to locally induce shape memory properties

https://doi.org/10.1016/j.actamat.2005.07.022Get rights and content

Abstract

We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation.

Introduction

Shape memory alloys (SMAs) are active materials that derive their unique properties from a thermoelastic martensitic transformation. They have been studied extensively over the last 50 years with most attention focused on bulk materials. Recently, the shape memory effect has been demonstrated in thin films of these alloys [1], [2], [3], [4], [5], [6], [7], making them attractive candidates for use as actuators in microelectromechanical systems (MEMS). It is, however, difficult to induce an intrinsic two-way shape memory effect in thin films and a biasing spring is generally needed to restore the initial state after actuation. As a result, use of SMA actuators in MEMS has been limited to bimorph-like mechanisms. The technique of laser annealing of shape memory alloys (LASMA) recently emerged as a promising approach for the fabrication of planar mechanisms [8]. Using this technique, shape memory properties can be spatially distributed across a material: crystallized material has shape memory properties and can be used as an actuator, untransformed material is passive and provides the restoring force. Various aspects of the LASMA process, especially for thin films, have yet to be explored. In this paper, we present the results of a crystallization study in which a laser was used to crystallize selected areas of amorphous NiTi films.

Section snippets

Experimental

NiTi thin films with a thickness of approximately 1.5 μm were deposited on 1 mm thick fused quartz substrates by direct current magnetron sputtering. The background pressure of the sputter chamber was less than 5 × 10−8 Torr; the pressure of the Ar working gas was 1.5 mTorr. All depositions were performed at room temperature. The films were grown by co-sputtering an equiatomic NiTi alloy and an elemental Ti target. The composition of the films was controlled by varying the power to individual guns

Results and discussion

Fig. 1 compiles the optical and TEM observations of the structure of the annealed NiTi films as a function of laser power density and scan speed. At a given scan speed, the film transitions from amorphous to partially crystalline and eventually fully crystalline with increasing laser power. The TEM micrographs in Fig. 1 show the microstructure close to the center of the laser trace where the annealing temperature is the highest. At low laser power, only a few isolated grains are formed in an

Conclusion

In conclusion, we have investigated a laser annealing technique that allows us to selectively crystallize an amorphous Ti–Ni film in specific areas where shape memory properties are desired. The film undergoes homogenous nucleation and has a random crystallographic texture after crystallization. The crystalline films have a uniform microstructure across the annealed area for the range of laser annealing parameters used in this study. The material in the crystalline regions transforms to

Acknowledgment

The authors acknowledge financial support from the National Science Foundation (Grant DMR-0133559).

References (22)

  • J.A. Walker et al.

    Sensor Actuat A

    (1990)
  • A. Ishida et al.

    Thin Solid Films

    (1993)
  • S. Miyazaki et al.

    Mater Sci Eng A

    (1999)
  • Y. Bellouard et al.

    Mater Sci Eng A

    (1999)
  • K.M. Knowles et al.

    Acta Metall

    (1981)
  • J.X. Zhang et al.

    Acta Mater

    (2001)
  • J.Z. Chen et al.

    J Non-Cryst Solids

    (2001)
  • Y.C. Shu et al.

    Acta Mater

    (1998)
  • H. Inoue et al.

    Acta Mater

    (1996)
  • S. Miyazaki et al.

    Int J Plast

    (2000)
  • Y.Q. Fu et al.

    Surf Coat Technol

    (2003)
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    Present address: Micro- and Nano-Scale Engineering, Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

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