Laser annealing of amorphous NiTi shape memory alloy thin films to locally induce shape memory properties
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)
- et al.
Sensor Actuat A
(1990) - et al.
Thin Solid Films
(1993) - et al.
Mater Sci Eng A
(1999) - et al.
Mater Sci Eng A
(1999) - et al.
Acta Metall
(1981) - et al.
Acta Mater
(2001) - et al.
J Non-Cryst Solids
(2001) - et al.
Acta Mater
(1998) - et al.
Acta Mater
(1996) - et al.
Int J Plast
(2000)
Surf Coat Technol
Cited by (82)
Surface amorphization of bulk NiTi induced by laser radiation
2023, Surfaces and InterfacesThe effect of high temperature annealing on surface wettability, mechanical and chemical properties of laser ablated nitinol surface
2023, Optics and Laser TechnologyToward tunable mechanical behavior and enhanced elastocaloric effect in NiTi alloy by gradient structure
2022, Acta MaterialiaCitation Excerpt :It is reasonable to anticipate that a trans-scale martensitic transformation occurs in the GS NiTi [49], since the grain size span is greater than an order of magnitude from the top surface to the interior (Figs. 3 and S3). The observed structural-gradient-dependent material properties in Fig. 5 are the collective behavior of the entire thickness of the integrated GS specimens, as a result of the synergy between fine and coarse grains [31,50]. During straining within superelasticity, the longitudinal deformation across the thickness is mutually accommodated by overall layers [50], and the transformation characteristics of fine and coarse grains will appear when their proportions are overwhelmingly dominant.
Bulk metallic glass composites containing B2 phase
2021, Progress in Materials ScienceFunctionally graded NiTi alloy with exceptional strain-hardening effect fabricated by SLM method
2020, Scripta Materialia
- 1
Present address: Micro- and Nano-Scale Engineering, Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.