EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

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1 Materials Science Forum Vol. 583 (2008) pp online at (2008) Trans Tech Publications, Switzerland 3D Reconstruction of Ni 4 Ti 3 Precipitates in a Ni 51 Ti 49 Alloy in a FIB/SEM Dual-Beam System S. Cao a, W. Tirry b, W. Van den Broek c and D. Schryvers d EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium a shanshan.cao@ua.ac.be, b wim.tirry@ua.ac.be, c wouter.vandenbroek@ua.ac.be, d nick.schryvers@ua.ac.be Keywords: Ni 4 Ti 3 precipitates, FIB, slice & view, 3D reconstruction Abstract. Ni 4 Ti 3 precipitates play an important role in the shape memory and superelastic behaviour of thermo-mechanically treated Ni-Ti material. The 3D morphology and distribution of such precipitates with lenticular shape and rhombohedral atomic structure in the austenitic B2 matrix of a binary Ni-rich Ni-Ti alloy has been elucidated via a slice & view procedure in a Dual-Beam FIB/SEM system. With the sequence of cross-section SE images obtained from the SEM, a 3D reconstruction has been achieved after proper alignment and image processing, from which both qualitative and quantitative analysis can be performed. Careful imaging is needed to ensure that all variants of the precipitates are observed with equal probability, regardless sample orientation. Moreover, due to the weak contrast of the precipitates, proper imaging conditions need to be selected to allow for semi-automated image treatment. Finally, a volume ratio of 10.2% for the Ni 4 Ti 3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni Ti for the matrix, leading to an increase of 113 degrees for the martensitic start temperature Ms. Also, the expected relative orientation of the different variants of the precipitates could be confirmed. In the near future, other quantitative measures on the distribution of the precipitates can be expected. Introduction Binary Ni-rich Ni-Ti alloys have been widely studied and applied for their unique properties in shape memory and superelasticity [1,2]. The presence of Ni 4 Ti 3 precipitates in the austenitic B2 matrix of these alloys affects these features due to their particular influence on the related B2 to B19 martensitic and R-phase transformations. Indeed, the replacement of Ti by Ni in the precipitates implies a Ni-depleted area up to more than 100 nm surrounding the precipitate [3], while the lattice mismatch between the precipitate and the matrix causes a strain gradient in the matrix (since this is softer than the precipitate [4]) which can again extend up to more than 100 nm away from the precipitate [5] as long as the precipitates don t grow too large. The Ni depletion can raise Ms while the lattice strain fits the strain for the ensuing R-phase deformation favoring the transformation of B2 - R due to weaker resistance to smaller lattice deformation [1,6]. Larger precipitates finally yield interface dislocations to relieve the increased stresses [7,8]. The precipitates themselves do not transform during the R-phase or martensitic transformation. Moreover, the actual distribution of the precipitates depends strongly on the thermo-mechanical history of the material. Short and intermediate term stress free aging can lead to heterogeneous microstructures with precipitates concentrating around grain boundaries, splitting the transformation into several steps upon cooling [9,10,11] while compressing or elongating the sample during heating can introduce preferential growth of a single variant [12,13,14]. Also, the volume fraction of precipitates will influence the overall composition of the matrix, having a strong effect on the martensite transformation temperatures [15]. In other words, the study of the three-dimensional distribution of these precipitates is of great importance for any application of Ni-Ti SMA. The Ni 4 Ti 3 precipitates normally have a lenticular morphology (micro-scale in length, nano-scale in width) with a central plane (habit plane) which is parallel to the {111} B2 planes of the matrix, as seen in the schematic of Fig.1a. There are 8 variants of Ni 4 Ti 3 precipitates in total, but since two All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /07/08,14:50:28)

2 278 Advances in Shape Memory Materials variants share one habit plane, only four different orientations will be observed, as seen in Fig. 1c. It has been known for a while that the Ni 4 Ti 3 precipitates have a rhombohedral atomic structure with a=0.670nm, α=113.9 [16] (or a=b=1.124nm, c=0.5077nm in the hexagonal description [5]), with the lattice parameter of the B2 structure of the matrix being equal to 0.301nm [5]. Recently, the internal atomic structure was refined using electron diffraction intensities in the MSLS approach better explaining the shrinking direction [17]. Fig. 1. Schematic drawing of the lens-shaped Ni 4 Ti 3 in the two zones: (a) the [1 0-1] B2 ; (b) the [1 1 1] B2 ; (c) Typical TEM BF image showing four variants of Ni 4 Ti 3 precipitates Experimental procedures A Ni 51 Ti 49 disc of 3 mm diameter and 150 micron thick is annealed at 500 C for 4 hours to introduce Ni 4 Ti 3 precipitates of approximate 5µm [1] (actually the discs are 300µm thick when they are aged at 500 C and afterwards they are ground to 15µm thickness, since even in 'vacuum' some oxidation of the outer region takes place). The sample is then electropolished with a solution of 93% acetic acid and 7% perchloric acid at 21V and 10 C till perforation to yield a sample suitable for reference transmission electron microscopy (TEM) work on the thin edges of the central hole. Still, more than sufficient bulk material is left for manipulation in a site-specific Dual-Beam Focused Ion Beam/Scanning Electron Microscope (DB FIB/SEM) system. In the present work a DB FIB/SEM xt Nova Nanolab 200 (FEI) is employed in order to conduct a serial sectioning procedure. It is a combination of a precise FIB operating with a Ga + ion beam and an ultra-high resolution field emission SEM (sub 2nm resolution), a combination by which one can carry out nanoscale fabrication [18], machining [19], 2D/3D-characterization and analysis [20,21], as well as TEM sample preparation [22,23,24]. The construction of a DB system is described in Fig.2.a. The sample is placed on the eucentric point of the stage where the ion column and the electron column converge at the angle of 52. This DB geometry is designed to provide optimal ion milling and electron and imaging resolution at the beams eucentric point.

3 Materials Science Forum Vol During the slice and view processing, the stage is tilted 52 to be normal with the ion beam for the in-plane erosion. As a result, the electron beam forms an angle of 38 with the stage (52 with the cross-section) for the imaging. Fig.2. Schematic (a) plus front view (b) of the U-cutting and the central bulk of Ni-Ti material that will be sliced away to form the 3D image volume. The orientations of the slicing Ga + beam and imaging electron beam under an angle of 52 are indicated. The main steps for obtaining the 2D cross-section images of the Ni-Ti alloy sample on the DB FIB/SEM system can be listed as: (a) deposition of a Pt protective layer; (b) creating a U-shape cutting for the slicing and imaging; (c) sequence slicing and imaging. In the first step, a Pt layer with a region of 10µm 20µm 0.4µm (width depth height) is deposited on top of the material with the ion beam for the purpose of protection of the surface of the material. After this, a U-shape cutting (shown in Fig.2.b) is created surrounding the Pt layer to yield a beam-like bulk piece for the subsequent slicing and imaging by the ion milling, with 10µm to the right and left side, 20µm on the front side and 7µm in depth. This U-shape can effectively decrease the damage brought by the sputtered ions to the cross-section as well as the re-deposition due to the confinement of the trench during the ion milling. The shadow on the cross-section during SE imaging and induced by the walls will also be reduced by this U-shape. In this manner, the contrast of the cross-section images can be improved significantly. After this rough U-cutting, a part of the Ni 4 Ti 3 precipitates as well as some TiC impurities are already visible on the front side of the cube (see Fig.2.b). The electropolishing has also induced some surface roughness yielding the precipitates visible, as seen at the bottom part of the SEM image in Fig 2.b. On top of the prepared cube, two small parallel trenches are milled to be used as a mark for the alignment in the subsequent off-line image processing. After several millings of 100nm each for cross-section cleaning, the slicing-and-imaging procedure is performed with the help of the Auto slice & view program to ensure the precision of slicing. The ion beam with an accelerating voltage of 30kV and a current of 300pA is applied for fine slicing, while the e-beam with an accelerating voltage of 5kV and a current of 1.6nA is applied for cross-section imaging in the immersion mode with a Through-the-lens (TLD) secondary electron detector for high resolution to give the best image quality. Using this procedure, a sequence of 55 2D images as shown in Fig.3 and with intervals of 100nm is obtained, finally yielding the 3D data after image processing. In the present work, image processing is performed using ImageJ [25] and Amira [26] programs. The principle of 3D reconstruction from 2D cross-section data can be described as shown in Fig. 4. The main procedure is as follows: (a) cross-section image alignment and resizing; (b) background elimination and contrast enhancement; (c) image segmentation and 3D reconstruction; (d) quantitative data extraction.

4 280 Advances in Shape Memory Materials Fig. 3. One of the sequence cross-section TLD SE images including reference points for alignment The cross-section images are aligned in Amira with the help of the reference points shown by arrowheads in Fig.3. Afterwards, a region of interest with the size of 4.65µm 4.29µm is selected from the aligned sequence. A resizing procedure for elongating the z direction by dividing by sin(52 ) is applied on this selected region to compensate for the angle between the e-beam and the cross-section. As a result, the dimension of the selected region is finally determined as 4.65µm 5.44µm. Fig. 4. Sketch of the 3D reconstruction procedure To ensure the precision of the 3D reconstruction, especially for the further quantitative analysis, some image processing is necessary to improve the image quality. In ImageJ, the function of Background subtraction is applied on the sequence of images for the purpose to eliminate the gradient of illumination within each image as well as between different slices. The contrast of images after processing is also improved, but the existing noise is still disturbing (see Fig. 5.b). The optimized image sequence is segmented in Amira with the label field function to differentiate different variants of Ni 4 Ti 3 precipitates and TiC impurities from the matrix. Afterwards

5 Materials Science Forum Vol the 3D reconstruction of the Ni-Ti alloy can be achieved through the Iso-surface function, which is shown in Fig. 6. Fig. 5. One of the resized 2D cross-section images before (a) and after (b) background subtraction Results and discussion Based on this 3D reconstruction, the volume ratio of different variants of precipitates and impurities can be calculated with the Tissue statistic function in Amira. It shows that all the precipitates take a volume ratio of 10.2% in total (Variant 1: 5.4%, Variant 2: 1.7%, Variant 3: 2.5%, Variant 4: 0.6%), while the TiC has a volume ratio of 0.2%. Considering the conservation of element and volume during the ageing procedure, with the alloy composition of Ni 51 Ti 49 before ageing, the overall composition of the matrix in the 3D volume of the present study after the precipitation reaction can be calculated as Ni Ti In the 3D reconstruction of the Ni-Ti alloy, the micron size lenticular disc shape of large Ni 4 Ti 3 precipitates is clearly observed. The discontinuous shape of the reconstructed precipitates is due to the relatively large step size of the ion slicing procedure. As indicated in the introduction, such large precipitates are incoherent and can induce interfacial dislocation to relax the strong stress fields, which can finally lead to less contribution of the R phase transformation. All four variants of the precipitates can be observed in the 3D volume. However, the respective volume ratios show large differences between different variants. This could possibly be due to the small size of the reconstructed region or due to a local inhomogeneity resulting from the presence of a small stress field in the vicinity of the TiC particles which could favour certain variants. The latter effect can indeed also be observed in two-dimensional TEM images, as seen in Fig. 7. Based on the observed volume fractions of Ni 4 Ti 3 precipitates and TiC impurities it is calculated that the remaining Ni fraction of the matrix is changed to 50.36% after the present aging. According to Fig. 5 of ref. [15], it can be estimated that this composition change finally rises Ms from 150K to 263K. By determining the normal to the central plane of the precipitates, also the angle between representative example precipitates of all 4 variants can be calculated. On average, an angle of ± 1.1 is obtained, which corresponds to the theoretical value of between {111} B2 family planes. The present 3D reconstruction of the Ni-Ti alloy in the FIB/SEM Dual-Beam system has already given some qualitative and quantitative information on the effect of micron scale Ni 4 Ti 3 precipitates. However, the resolution and the precision of the 3D reconstruction are still strongly limited by the image quality and the milling step. At present efforts are being concentrated to further improve the original image quality by optimizing the beam and detector conditions while also the slicing

6 282 Advances in Shape Memory Materials procedure needs to be improved in order to decrease the slicing step to obtain smooth reconstructed shapes. Fig. 6. 3D reconstruction for all the Ni 4 Ti 3 precipitates (a), 4 kinds of variants of precipitates (b-e), and TiC impurity (f) Fig. 7. BF TEM image showing Ni 4 Ti 3 precipitates preferentially oriented in the close vicinity of a larger carbide/oxide particle.

7 Materials Science Forum Vol Conclusions The 3D distribution and morphology of Ni 4 Ti 3 precipitates and TiC impurities in a Ni 51 Ti 49 alloy is investigated by slice & view in a FIB/SEM Dual-Beam instrument. From the volume fraction of the precipitates a change in matrix composition to Ni Ti is calculated and a Ms increasing of 113K is determined. The average angle between different families of precipitates is confirmed to be ± 1.1. In order to improve the quality and resolution of the 3D reconstruction, a sequence of cross-section images with higher quality and suitable ratio between magnification and milling step (decreasing both the magnification and milling step) is called for. Acknowledgments The present work was performed in the framework of MULTIMAT Multi-scale modeling and characterization for phase transformations in advanced materials, a Marie Curie Research Training Network (MRTN-CT ) of the EU. Support was also provided by the FWO project G The functional properties of shape memory alloys: a fundamental approach. S. Cao also likes to thank E. Montoya for support with the manipulation of the FIB dual beam system. References [1] K. Otsuka and X. Ren: Progress in Materials Science Vol. 50 (2005) p. 511 [2] K. Otsuka and C. M. Wayman: Shape memory materials (Cambridge University Press, 1998) [3] Z. Yang, W. Tirry and D. Schryvers: Scripta Materialia Vol. 52 (2005) p [4] Z. Yang, W. Tirry, D. Lamoen, S. Kulkova and D. Schryvers: Acta Materialia (2007), doi: /j.actamat (accepted for publication) [5] W. Tirry and D. Schryvers: Acta Materialia Vol. 53 (2005) p [6] W. Tirry: Strains on the nano and microscale in NiTi: an advanced TEM study, Ph.D.. thesis, Universiteit Antwerpen (2007) [7] W. H. Zou, X. D. Han, R. Wang, Z. Zhang, W. Z. Zhang and J. K. L. Lai: Materials Science and Engineering A Vol. 219 (1996) p. 142 [8] K. Gall, H. Sehitoglu, Y. I. Chumlyakov, I. Kireeva, and H. J. Maier: Journal of Engineering Materials and Technology Vol. 121 No. 1 (1999) p [9] J. Khalil-Allafi, A. Dlouhy, G. Eggeler: Acta Materialia, Vol. 50 (2002) p [10] G. Fan, Y. Zhoua, W. Chen, S. Yang, X. Ren, K. Otsuka: Materials Science and Engineering A Vol (2006) p. 622 [11] M. Nishida, T. Hara, T. Ohba, K. Yamaguchi, K. Tanaka and K. Yamauchi: Materials Transactions Vol. 44 No.12 (2003) p [12] J. Michutta, Ch. Somsen, A. Yawny, A. Dlouhy and G. Eggeler: Acta Materialia Vol. 54 (2006) p [13] J. Michutta, M. C. Carroll, A. Yawny, Ch. Somsen, K. Neuking and G. Eggeler: Materials Science and Engineering A Vol 378 (2004) p.152 [14] D. Y. LI, L. Q. Chen: Acta Materialia Vol. 45 (1997) p. 471 [15] W. Tang: Metallurgical and Materials Transactions A Vol. 28 No.3 (1997) p. 537

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