textural evolution evaluated by ebsd and xrd after thermal treatment in ni-ti shape memory alloy
TRANSCRIPT
Textural Evolution Evaluated by EBSD and XRD after Thermal
Treatment in Ni-Ti Shape Memory Alloy
S.B. Ribeiro1,a, T.G. Andrade1,b, A.S. Paula1,c, J.F.C. Lins1,d, K.K. Mahesh2,e
and F.M. Braz Fernandes2,f 1 EEIMVR/UFF, Av. dos Trabalhadores, 420, CEP 27.255-125, Volta Redonda, RJ, Brazil
2 CENIMAT – I3N, Campus da FCT/UNL, 2829-516 Monte de Caparica, Portugal
Keywords: Ni-Ti, shape memory alloy, EBSD, XRD, microstructure, texture.
Abstract. The Nickel-Titanium (Ni-Ti) alloys are the most attractive amongst shape memory alloys (SMA) due to their good functional properties coupled with high strength and good ductility. The transformation temperatures in Ni-Ti SMA can be altered by chemical composition and thermal and/or mechanical treatments adequate to obtain reversible martensitic transformation in one or more steps. The goal of the present work is to investigate the evolution of texture in Ni-Rich Ni-Ti (50.8at%Ni-Ti) SMA showing different phase transformation temperatures as a result of different thermal/mechanical history: straight-annealed (as-received condition) and subsequent thermal treatment at 500ºC for 30 minutes in air. The microstructural and textural results were obtained by Electron Backscattering Diffraction on Scanning Electronic Microscopy (ESBD/SEM) and by X-Ray Diffraction (XRD) at room temperature. Mechanical properties were measured by Vickers micro hardness tests at room temperature.
Introduction
The crystallographic texture study in near-equiatomic Ni-Ti is an important subject matter, due to their influence on the exhibited shape memory effect (SME) and superelasticity (SE) in these alloys. The main goal in these studies is to tailor the fabrication processes in order to produce a final texture that will promote higher shape recovery on martensitic and reverse phase transformation (B19’↔B2 or B19’↔R↔B2), where isotropic or anisotropic recovery may be desired depending on the material application.
Coherent/semicoherent nanometric precipitates due to aging can be present on Ni-rich Ni-Ti SMA that increase the internal stress on B2 matrix resulted of the thermal cycle at low temperature (up to 500ºC) or short dwell time. The internal stress inside B2 matrix difficult the one step (B19’↔B2) transformation and promote two (B19’↔R↔B2) or multiple (B19’↔R, B19’↔B2 and R↔B2) steps transformation by intermediate phase formation (R-phase) [1].
As Ni-Ti alloy depends on coordinated atomic movements for phase transformations, which defines its unique mechanical behavior, any significant alignment of the atomic planes from texture in the polycrystalline material can have a marked influence on the mechanical response by either limiting or promoting that phase transformation [2]. Beyond the thermal/mechanical treatments, the chemical composition also has an important influence on the crystallographic texture development by these alloys. NiTi SMA in austenitic field has a bcc type texture [3,4]: α-fiber I <110>||RD ({001}<112>-{112}<110>-{111}<110>), α-fiber II <110>||RD ({111}<110>-{110}<110>), γ-fiber <111>||ND ({111}<110>-{111}<112>), and η-fiber <100>||RD ({001}<100>-{011}<100>).
Neutron Diffraction, X-Ray Diffraction (XRD), Synchrotron Radiation, Transmission Electronic Microscopy (TEM) and Electron backscattering diffraction (EBSD) on Scanning Electronic Microscopic (SEM) are found to be useful techniques for studying the crystallographic structure and texture of polycrystalline NiTi alloys [2,5,6].
Materials Science Forum Vols. 702-703 (2012) pp 884-887Online available since 2011/Dec/06 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.702-703.884
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 TTP,www.ttp.net. (ID: 128.210.126.199, Purdue University Libraries, West Lafayette, USA-28/08/14,16:14:57)
To address some of these problems, in the present study, EBSD is used to determine mesotexture at room temperature (RT), which enables to correlate mesotexture and microstructure/macrotexture aspects in Ni-Rich Ni-Ti alloy. This will help to understand the martensitic transformation in two steps (B2→R→B19’) taking place in straight annealed and heat treated (at 500ºC) conditions [7].
Materials and Methods
The samples for the present study were extracted from straight annealed Ni-Rich NiTi alloy (50.8at%Ni-Ti, thickness 1.0 mm) plates supplied by Memory-Metalle GmbH, Germany. The as-received (AR) samples were subjected to a thermal treatment at 500ºC for 30 minutes in air followed by water quenching (HT500).
Crystallographic orientations were obtained from XRD and Kikuchi patterns caused by EBSD (Electron Backscatter Diffraction) in SEM (Scanning Electronic Microscopy) with commercial system – EDAX TSL OIM v5-2008.
The microstructural and EBSD analysis were performed on sample plate surface at RT using a SEM Carl Zeiss (model EVO MA 10 with LaB6 filament, installed in LMME/UFF, at 20 kV accelerated voltage) with Secondary Electron detector (650 spot size and 8 mm working distance) and EBSD detector (610 spot size, 21 mm working distance, 3000x magnification and step size 0.1 µm). The surface of the samples plates for SEM analysis were mechanically polished to remove the oxide layer and then electropolished in a solution of 20% H2SO4 and 80% methanol at RT at 30 V / 30s.
XRD analysis was performed using a Bruker diffractometer (rotating anode – XM18H, CuKα radiation, 30 kV/100 mA, D5000 goniometer) with θ/2θ scanning (2θ = 15 to 90º) and texture analysis at RT (B2 phase). The rolling direction (RD) was kept aligned in φ = 0º and the transversal direction (TD) in φ = 90º. The samples for XRD analysis were subjected to chemical etching (10% vol HF + 45% vol HNO3 + 45% vol H2O), in order to remove the oxide layer.
Vickers Microhardness tests were proceeding in the samples in study at RT using a microhardness tester Buhler LTD model Micromet 3, installed in EEIMVR/UFF. The test load and creep time were 200 gf and 20 s, respectively; for each sample, 10 measurements were performed.
In the present study, EBSD maps, XRD profile and microhardness results on B2 field at RT were correlated with previous publications by the authors [7] associated to texture analysis and phase transformation sequence and temperatures.
Results and Discussion
The Ni-rich NiTi alloy under investigation in AR and HT500 conditions are found at RT to possess a mixture of B2 & R phases. The phase transformation characteristics obtained by employing Differential Scanning Calorimetry (DSC) and Electrical Resistivity (ER) were presented in previous publication [7]. According to these results, in AR condition, one step, B19'→B2, while heating, and two step, B2→R→B19', while cooling, phase transformations were observed. In HT500 condition, an overlapping two step, B19'→R→B2, while heating, and B2→R→B19' phase transformations while cooling were observed. Further, for HT500, the temperature intervals in which the phase transformations take place are narrower than those for the AR. In the heat treated specimens, the observed sharper DSC peaks corresponding to the phase transformations are attributed to its lower density of structural defects following recrystallization [1,3]. This behavior is emphasized by Vickers microhardness results: (i) 320,5±6,2 HV for AR sample; and (ii) 278,0±14,7 HV for HT500 sample. XRD profile results are shown in Fig. 1(a&b) (θ/2θ scans) revealing the presence of the Ni4Ti3 precipitate and B2 phase on these samples at RT.
Materials Science Forum Vols. 702-703 885
Fig. 1 – (a & b) XRD profiles obtained from AR and HT500 samples; (c & d) Pole Figures
(110)B2 obtained from AR and HT500 samples, respectively, at RT ((c & d) adapted from [7]).
In Fig. 2(a&d), microstructural images obtained by secondary electrons from SEM at RT are presented. EBSD results are plotted as orientation image microscopy (OIM –Figs. 2(b&e)) and confidence index (CI – Figs. 2 (c&f)) maps, in order to observe the orientation of B2 grains and the precipitation aspects, respectively. Orientations of the grains in Fig. 2 (b&e) are represented in different colors, as illustrated in, Fig. 2(g), the inverse pole figure (IPF), and black regions are related with CI<0.1 in OIM maps. All the grains are indexed/identified as B2 phase according to the Kikuchi pattern for each scanned point. In Fig. 2 (c&f), white colour means 1 (perfect fit with B2 structure) and black colour means 0 (not fit with B2 structure). The low CI values can be correlated with crystallographic orientation disturbance by stress fields around the Ni4Ti3 precipitates inside the B2 matrix [1].
(a)
(b) (c)
(h)
(g)
(e)
(f) (d) Fig. 2 – (a & d) Microstructural images from SEM; (b & e) EBSD OIM maps and (c & f) CI maps, for AR (a-c) and HT500 (d-f) samples, respectively. Fig. 2 (g) – IPF associated with OIM maps and
(h) CI scaling. Fig. 1 c, shows the pole figure (110)B2 corresponding to the AR sample. The AR sample
exhibited the observed texture components close to χ = 18º associated with texture components {210}<110>B2 and other components close to χ = 30º associated with texture components {211}<110>B2, with RD close to <110>. Pole Figure (110)B2 corresponding to the HT500 sample is presented in Fig. 1 d. After annealing at 500ºC, the intensity of texture components close to χ = 30º decrease [7]. This behavior can be related with the Ni4Ti3 precipitation growing inside the B2 matrix, which relaxes the internal stresses in some regions on B2 grains [1], where {111} planes are
886 Textures of Materials - ICOTOM 16
nearly parallel to the plate surface, as shown in OIM maps and evidenced by zero values (black regions) in IC map from EBSD results (Fig. 2f). This is consistent with XRD data (Figs. 1a&b), where (131) Ni4Ti3 peak has been detected. The precipitation growing induced by heat treatment, promotes a B19’ start temperature transformations (Ms increase) and a narrower temperature range for the reverse transformation, that are in agreement with the literature [1,7].
Summary
For the Ni-rich Ni-Ti alloy under study, the heat treatment at 500ºC promoted recrystallization in the straight annealed AR conditions and Ni4Ti3 precipitation growing that contributes to stress relief inside the B2 grains reflecting on the decreased of the texture components {211}<110>B2, with RD along <110>, and increased of {210}<110>B2 inducing the B19’ transformation at slightly higher temperatures. EBSD results are in consistence with the above observations. However, the precise quantification phases in Ni-Rich Ni-Ti SMA (with coherent/semicoherent nanometric precipitates and/or high microstrain by plastic deformation) must to be improvement in EBSD/SEM associated with the LaB6 filament electron beam.
Acknowledgement
The authors S.B.R, T.G.A., A.S.P and J.F.C.L acknowledged Faperj and CAPES for the financial support (APQ-1 2009/02 E-26/110.414/2010) and fellowships. Pluriannual financial support from the FCT/MCTES to CENIMAT/I3N and through the research project Smart Composites (PTDC/CTM/66380/2006) is gratefully acknowledged by K.K.M. and F.M.B.F. K.K.M. gratefully acknowledges the fellowship under the scheme, ‘Ciência 2007’ with Ref. No. C2007-443-CENIMAT-6/Ciência2007. The authors would like to thank the LMME / UFF for the technical support during electron microscopy works.
References
[1] K. Otsuka, X. Ren, Physical Metallurgy of Ti-Ni-based Shape Memory Alloys, Progress in Materials Science 50 (2005) 511-678.
[2] S.W. Robertson, V. Imbeni, H.-R. Wenk, R.O. Ritchie, Crystallographic texture for tube and plate of the superelastic/shape-memory alloy Nitinol used for endovascular stents, Journal of Biomedical Materials Research Part A 72A (2005) 190-199.
[3] K. Kitamura, S. Miyazaki, H. Iwai, M. Kohl, Effect of Heat-treatment on the Texture in Rolled Ti-Ni Thin Plates, SMST-97: Proceedings of the Second International Conference on Shape Memory and Superelastic Technologies (1997) 47-52.
[4] S. Miyazaki, K. Otsuka, C.M. Wayman, The Shape Memory Mechanism Associated with the Martensitic Transformation in Ti-Ni Alloys – I. Self-Accommodation, Acta Metallurgica 37 (1989) 1873-1884.
[5] P. Šittner, Y. Liu, V. Novák, On the Origin of Lüders-like Deformation of NiTi Shape Memory Alloys, Journal of the Mechanics and Physics of Solids 53 (2005) 1719-1746.
[6] S.C. Mao, X.D. Han, J.F. Luo, Z. Zhang, Microstructure and texture evolution of ultra-thin TiNi hot-rolled sheets studied by automated EBSD , Material Letters 59 (2005) 3567–3571.
[7] A.S. Paula, C.M.L. Santos, J.H.P.G. Canejo, K.K. Mahesh, F.M. Braz Fernandes, C.S.C. Viana, Textural Evolution in Ti-rich and Ni-rich Ni-Ti Shape Memory Alloys Submitted to Thermomechanical Treatment with Marforming Steps, In: Proceedings of 62th ABM Annual Congress (2007) 3426-3434.
Materials Science Forum Vols. 702-703 887
Textures of Materials - ICOTOM 16 10.4028/www.scientific.net/MSF.702-703 Textural Evolution Evaluated by EBSD and XRD after Thermal Treatment in Ni-Ti Shape Memory
Alloy 10.4028/www.scientific.net/MSF.702-703.884
DOI References
[1] K. Otsuka, X. Ren, Physical Metallurgy of Ti-Ni-based Shape Memory Alloys, Progress in Materials
Science 50 (2005) 511-678.
http://dx.doi.org/10.1016/j.pmatsci.2004.10.001 [4] S. Miyazaki, K. Otsuka, C.M. Wayman, The Shape Memory Mechanism Associated with the Martensitic
Transformation in Ti-Ni Alloys – I. Self-Accommodation, Acta Metallurgica 37 (1989) 1873-1884.
http://dx.doi.org/10.1016/0001-6160(89)90072-2 [5] P. Šittner, Y. Liu, V. Novák, On the Origin of Lüders-like Deformation of NiTi Shape Memory Alloys,
Journal of the Mechanics and Physics of Solids 53 (2005) 1719-1746.
http://dx.doi.org/10.1016/j.jmps.2005.03.005 [6] S.C. Mao, X.D. Han, J.F. Luo, Z. Zhang, Microstructure and texture evolution of ultra-thin TiNi hot-
rolled sheets studied by automated EBSD , Material Letters 59 (2005) 3567–3571.
http://dx.doi.org/10.1016/j.matlet.2005.06.029