0
Research Papers

Characterization and Prediction of Texture in Laser Annealed NiTi Shape Memory Thin Films

[+] Author and Article Information
Gen Satoh

Mechanical Engineering Department,  Columbia University, New York, NY 10027

Xu Huang, Ainissa G. Ramirez

 Mechanical Engineering Department, Yale University, New Haven, CT 06511

Y. Lawrence Yao

 Mechanical Engineering Department, Columbia University, New York, NY 10027

J. Manuf. Sci. Eng 134(5), 051006 (Sep 10, 2012) (11 pages) doi:10.1115/1.4007459 History: Received March 23, 2010; Revised July 26, 2012; Published September 10, 2012; Online September 10, 2012

Thin film shape memory alloys are a promising material for use in microscale devices for actuation and sensing due to their strong actuating force, substantial displacements, and large surface to volume ratios. NiTi, in particular, has been of great interest due to its biocompatibility and corrosion resistance. Effort has been directed toward adjusting the microstructure of as-deposited films in order to modify their shape memory properties for specific applications. The anisotropy of the shape memory and superelastic effects suggests that inducing preferred orientations could allow for optimization of shape memory properties. Limited work, however, has been performed on adjusting the crystallographic texture of these films. In this study, thin film NiTi samples are processed using excimer laser crystallization and the effect on the overall preferred orientation is analyzed through the use of electron backscatter diffraction and X-ray diffraction. A three-dimensional Monte Carlo grain growth model is developed to characterize textures formed though surface energy induced abnormal grain growth during solidification. Furthermore, a scaling factor between Monte Carlo steps and real time is determined to aid in the prediction of texture changes during laser crystallization in the partial melting regime.

Copyright © 2012 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 15

XRD spectra derived from Monte Carlo model volume fraction data for each MCS. Good agreement with experimentally determined normalized spectra (Fig. 1). Note increase in intensity of (110) peak at 42.8 deg, decrease in intensity of (200) peak at 61.9 deg, and near-constant intensity for (211) peak at 78.1 deg 2θ with number of MCS.

Grahic Jump Location
Figure 16

Intensity fraction versus MCS and energy density for MC and experimental measurements respectively. Good agreement between model and experimental values for all but highest energy density sample. Highest energy density sample is considered to be in the near-complete melting regime. Error bars denote standard deviation from four separate measurements.

Grahic Jump Location
Figure 17

Sample cross section from Monte Carlo model for nine MC steps. Each color represents a different surface normal orientation while each cell represents a 1.33 μm wide element from the MC mesh. Continuous areas with the same color represent grains. Note v-shaped columnar grain structure indicative of abnormal grain growth. Not show to scale.

Grahic Jump Location
Figure 14

Volume fraction of grains with selected orientations from Monte Carlo simulation as a function of MCS. Note increase in volume fraction of low surface energy orientation (110) and decrease in volume fraction for high surface energy orientation (200).

Grahic Jump Location
Figure 18

Percentage change in average surface grain size as a function of MCS and energy density for numerical and experimental results, respectively, showing increasing trend due to growth of preferentially oriented grains and occlusion of others. Grain size for highest energy density (1333 mJ/cm2 ) not observed due to significant amorphization of film.

Grahic Jump Location
Figure 19

(a) Optical micrograph of boundary between processed and unprocessed regions. Note significant roughening of processed regions due to step grain boundaries. (b) AFM cross section shows significant change in surface roughness in processed region with adjacent grains having ∼10 nm height difference.

Grahic Jump Location
Figure 13

Normalized X-ray diffraction spectra for samples processed at various energy densities. Spectra shifted up in order of increasing energy density. Note shift in peaks toward higher 2θ angles with increasing energy density as well as the peak broadening. Some amorphization of the material is considered to be occurring for highest energy density sample.

Grahic Jump Location
Figure 12

(110) and (200) oriented grains at (a) first MCS and (b) final MCS of the anisotropic Monte Carlo model. Growth of grains is observed for grains with low surface energy orientation (110). Grains with higher surface energy (200) decrease in size or disappear as number of MCS increases due to occlusion.

Grahic Jump Location
Figure 11

Surface grain map derived from anisotropic Monte Carlo simulation after nine Monte Carlo steps. Colors denote different surface normal orientations. Note grain shape and size distribution comparable to those observed experimentally through EBSD (Figs.  68).

Grahic Jump Location
Figure 10

ODF for processed film showing peaks at Φ = 45 deg for ϕ2  = 0, 90 deg and Φ = 90 deg for ϕ2 near 45 deg suggesting (110) preferred orientation. Note lack of dependence on ϕ1 indicating random in-plane orientation. ϕ1 , Φ, and ϕ2 are Euler angles defined in the Bunge convention.

Grahic Jump Location
Figure 9

EBSD maps of sample processed at 909 mJ/cm2 with different surface normal grain orientations highlighted. (a) (100) and (b) (110). Note overall larger grain size and greater density of (110) oriented grains. Multiplicity factor of (110) is twice that of (100) for BCC cubic materials.

Grahic Jump Location
Figure 8

(a) EBSD map for normal direction of film processed at 909 mJ/cm2 with Euler angle coloring. Grain boundaries (solid lines) are shown where adjacent measurements have a misorientation angle of 10 deg or more. Inverse pole figure (b) showing slight (110) texture normal to the film surface.

Grahic Jump Location
Figure 7

XRD spectra for (a) as-annealed and (b) as-deposited film obtained at room temperature. Note amorphous structure for as-deposited film and austenitic peaks for annealed sample.

Grahic Jump Location
Figure 6

EBSD grain map of furnace annealed sample. Grain boundaries (solid lines) are shown where adjacent measurements have a misorientation angle of 10 deg or more. Grain shapes and sizes comparable to those observed through optical microscopy.

Grahic Jump Location
Figure 5

DIC Optical Image of furnace-annealed sample etched using Kroll’s reagent. Note maximum grain size on the order of 10 μm. Clear delineation between adjacent grains. Distribution of grain sizes is also observed.

Grahic Jump Location
Figure 4

Flow chart for MC Model. Both isotropic and anisotropic models depicted. Isotropic model run first to set up initial grains for anisotropic model. Isotropic model neglects surface energy. Anisotropic model sets surface energy to be a function of orientation as shown in Fig. 2.

Grahic Jump Location
Figure 3

Transient reflectance measurement during laser processing of a single 320 μm square area near the complete melt threshold. Reflected signal from 632 nm HeNe laser source. Note melt duration of about 250 ns.

Grahic Jump Location
Figure 2

Surface energies for various orientations for typical base-centered cubic metal. Note significant anisotropy and minimum surface energy for (110) normal orientation.

Grahic Jump Location
Figure 1

Schematic representation of grains within two-dimensional Monte Carlo model. Orientation of each node is represented by an integer number between 1 and 24. Each node represents a 1.33 μm square area. Connected nodes with identical orientations represent grains. Grain boundaries (solid lines) separate nodes with different orientations and are added for clarity.

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In