- Email: [email protected]
Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition
Accepted Manuscript Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition Bingwen Lu, Xiufang Cui, Xiangru Feng, Meiling Dong, Yang Li, Zhaobing Cai, Haidou Wang, Guo Jin PII:
S0042-207X(18)30885-6
DOI:
10.1016/j.vacuum.2018.08.028
Reference:
VAC 8175
To appear in:
Vacuum
Received Date: 25 May 2018 Revised Date:
13 June 2018
Accepted Date: 16 August 2018
Please cite this article as: Lu B, Cui X, Feng X, Dong M, Li Y, Cai Z, Wang H, Jin G, Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition, Vacuum (2018), doi: 10.1016/j.vacuum.2018.08.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition Bingwen Lua,b, Xiufang Cuia∗, Xiangru Fenga, Meiling Donga, Yang Lia, Zhaobing
a
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Caia, Haidou Wangb,Guo Jina∗ Institute of Surface/Interface Science and Technology, Key Laboratory of Superlight
Material and Surface Technology of Ministry of Education, College of Material
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Science and Chemical Engineering, Harbin Engineering University, Harbin 150001,
b
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China.
National Key Laboratory for Remanufacturing, Armored Forces Engineering
Institute, Beijing, 100072, China Abstract
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Recently, additive manufacturing (AM) technology has drawn significant attention to fabricate shape memory alloys. In this study, plasma arc deposition (PAD), an AM technology, has been successfully applied to prepare the Ti51Ni49 alloy directly
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and rapidly at the first time. The synthesized PAD Ti51Ni49 alloy consisting of TiNi
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(B2), TiNi (B19’) and Ti2Ni phase, exhibited primary TiNi columnar dendrites structure with a certain number of Ti2Ni phases distributed in the interdendritic regions. The as-deposited Ti51Ni49 alloy displayed a two-step phase transformation evidently (B2→ R → B19’) during cooling process, and special linear superelasticity (up to 4.5%) with narrow hysteresis due to Ti2Ni phase embedded in TiNi matrix phase. This work suggests that the PAD is an effective technology to directly and ∗
Corresponding author Email: [email protected] (X. F. Cui). Email: [email protected] (G. Jin).
1
ACCEPTED MANUSCRIPT rapidly synthesize shape memory alloy. Key words: plasma arc deposition, shape memory alloy, phase transformation, superelasticity
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TiNi alloy as a functional material has been widely applied in the aerospace, biomedical and telecommunication sectors due to its unique shape memory effect and super-elasticity effect [1]. In recent years, additive manufacturing (AM) techniques
SC
have attracted significant attention on the direct formation of complex TiNi parts by
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means of its immanent advantages [2], like, selective laser melting, laser engineered net shaping and directed energy deposition technologies [3-7], however, there are no research reports being focused on the manufacture of the TiNi alloys by plasma arc deposition (PAD). Due to the potential in efficiency, convenience and cost-savings,
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PAD is a significant AM technology for rapid prototyping [8, 9]. Therefore, PAD has been gaining a great attention on synthesizing several materials, such as stainless steel, titanium alloys and aluminum alloys [10]. Naturally, PAD technology can also be
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used to product large-scale TiNi alloy components, and to broaden the application of
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TiNi alloys greatly.
Thus, the aim of the present work is to synthetize TiNi alloy via PAD with the
prealloyed TiNi powder. The microstructure, phase transformation behavior and superelasticity behavior of samples were investigated in detail. The Ti51Ni49 (at. %) prealloyed spherical powder with an average size of 75um was used as raw material. Ti51Ni49 alloy samples were fabricated on TC4 substrates using a PAD system in continuous mode (containing plasma arc welding source,
2
ACCEPTED MANUSCRIPT protection device, welding torch, powder feeding device, welding robot and a computer control unit) with an oxygen-content < 80ppm and high-purity Ar as protective atmosphere. The schematic of additive manufacturing path and physical
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map of deposited Ti51Ni49 alloy were shown in Fig. 1a and Fig. 1b, respectively. The main fabricating parameters of PAD were as follows: plasma gas flow of 0.3L/min, shield gas flow of 15 L/min, current of 150A, scanning speed of 3mm/s， powder
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feeding rate of 3.5 g/min and each deposited height of 2.5mm.
Fig. 1. (a) Schematic of deposition path and (b) PAD deposited sample of TiNi alloy.
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After as-deposited samples were mechanically polished and etched using Kroll’s reagent, the microstructures were investigated by a BX-51 optical microscope (OM).
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The morphologies and the corresponding chemical compositions were analyzed using a scanning electron microscope (SEM, HITACHI-S4800) equipped with an energy dispersion spectroscope (EDS). The phase structure was observed by an X-ray diffraction (XRD, BRUKER-D8) with Cu target Kα radiation at different temperature. The transformation temperatures (TTs) was detected by a differential scanning calorimetry (DSC, DSC204F1) with a heating and cooling rate of 10 oC/min from -80 C to 100 oC. Compression tests were conducted on samples of 4mm diameter and
o
3
ACCEPTED MANUSCRIPT 10mm length using a mechanical tester (MTS-800) with loading strain rate of 0.001 s-1 and unloaded rate of 100 N/s at Af +15 , which equipped with an annular furnace and an extensometer at temperature.
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Fig. 2a displays the XRD patterns of the as-deposited sample at room temperature. Phase analysis reveals the existence of TiNi (B2), TiNi (B19’) and Ti2Ni phases. Fig. 2b depicts the microstructures of across-section (Y-Z plane) of the
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as-deposited Ti51Ni49 alloy, which mainly consists of columnar dendritic structure,
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growing epitaxially along the deposition direction (Z direction). However, the microstructure of overlapping interface exhibits a finer cellular dendrite structure due to the dynamic heat transfer of the moving heat source and deposition characteristics of multi-layer material [11, 12]. When a layer is deposited, a thin top region of
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previous deposited layer is remelted. A thin nucleation zone at the interface between the layers forms when molten pool begins to solidify. The interface region has faster solidification velocity and greater temperature gradient than the inner region of each
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layer due to the thermolysis of previous layer. In turn, the heat is transferred to the
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previous layer when the new layer is deposited, thus leading to grain growth of previous layer. Therefore, a finer dendrite structure forms at the overlapping region. Fig. 2c shows that two different phases, light grey phase and dark grey phase, exist in samples. Numerous light grey phases are the major component of dendrites, and a certain number of dark grey phases distribute in the interdendritic regions. The corresponding element distribution maps in Fig. 2c is shown in Fig. 2d，which indicates that these two phases contain Ni and Ti elements, but light grey phase is
4
ACCEPTED MANUSCRIPT enriched in Ni element, whiledark grey phase regions is enriched in Ti element. Quantitative point analysis results show that the compositions (at. %) of Ti and Ni in the light grey phase are almost equivalent, so this phase can be confirmed to as TiNi
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phase. The Ti/Ni atomic ratio of the dark grey phase is 2:1, thereby, it is identified as Ti2Ni phase. Thus, one conclusion can be drew out that the microstructure of the as-deposited sample is mainly composed of TiNi columnar dendritic structure with
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some irregular Ti2Ni phase in the inter-dendritic regions.
Fig. 2. Microstructure of the as-deposited Ti51Ni49 alloy: (a) XRD spectra, (b) optical micrographs(Y-Z plane), (c) SEM image and (d) element distribution maps. Fig. 3a shows the TEM image of a typical microstructural features, indicating some relatively large precipitates distribute in matrix phase. The selected-area electron diffraction (SAED) patterns of A and B (marked in Fig. 3a) display that A
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along [111] zone axis is identified as a FCC structure, and B along [001] zone axis is confirmed as a BCC structure. Combining the analysis results of XRD and EDS, A is the Ti2Ni phase and B is the TiNi (B2) matrix phase. The magnified HRTEM image
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demonstrates that Ti2Ni particles embed in TiNi (B2) matrix phase and have a specific
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orientation relationship with matrix phase.
Fig. 3. (a) TEM micrographs with corresponding SAED patterns and (b) HRTEM
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micrographs of the as-deposited Ti51Ni49 alloy. A model on the formation of microstructure is proposed based on the above
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microstructural observations and binary equilibrium Ti-Ni phase diagram [13]. During the PAD process, when the temperature of the molten pool decline below 1310
,
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solidification in TiNi alloy starts with the reaction of primary liquid (L) → TiNi+ L (Ti) reaction [14]. The TiNi phase grows rapidly in a columnar crystal or dendrite mode without secondary dendrite arms, but some Ti-rich liquid phase is retain around the TiNi dendrite because the molten pool has higher content of Ti with relatively low Ni. Then TiNi dendrites growth with some secondary dendrite arms, and the Ti-rich liquid phase undergo an L (Ti) + TiNi →Ti2Ni peritectic reaction to form Ti2Ni around TiNi dendrite [15]. Finally the microstructure of primary TiNi columnar 6
ACCEPTED MANUSCRIPT dendritic structure with some irregular Ti2Ni phase in the inter-dendritic regions (Fig. 2b) forms. DSC measurement (Fig. 4a) demonstrates that two exothermic peaks and one
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endothermic peak distinctly appear on the cooling curve and heating curve, respectively. In order to explain the DSC peaks and phase transformations, XRD experiments were performed at the different temperature according to the DSC curve.
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The XRD patterns (Fig. 4a) displays that diffraction peaks corresponding to B2 and
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Ti2Ni are found in the patterns at 50 . Compared to the XRD pattern of the as-fabricated sample, the diffraction peak of B19’ phase disappears and the intensity of diffraction peak corresponding to B2 phase significantly increases. This means that B19’ → B2 transformation may occurs during heating process and the endothermic to 66
at DSC heating curve is ascribed to the phase transformation
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peak from 22
of B19’ → B2. In addition, the diffraction peaks corresponding to the R phase is found at 33 , which means that B2 → R transformation occurs during cooling process and
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the exothermic peak from 38
to 26
at DSC cooling curve is attributed to phase
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transformation of B2→ R [16]. When the temperature decline down to 8 ℃, the diffraction peak of R phase disappears and the intensity of diffraction peak corresponding to B19’ phase increases greatly, illustrating that R → B19’ transformation occurs at 8℃ and the exothermic peak from 17℃ to -13℃ at DSC cooling curve is due to the phase transformation of R → B19’ [17]. The two-step transformation (B2→ R → B19’) can be attributed to the existence of Ti2Ni phase in TiNi phase. Ti2Ni phases may form in Ti-rich TiNi alloys, embedding in TiNi phase
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ACCEPTED MANUSCRIPT and creating local stress fields to accelerate B2↔R transformation and suppress the
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B2↔ B19’ transformation [18, 19].
Fig. 4. (a) DSC curves and XRD patterns obtained at various temperatures (b) compressive stress-strain curves of PAD NiTi alloy.
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ACCEPTED MANUSCRIPT Figs. 4b displays the stress-strain curves of the as-deposited sample, and showing that the fully reversible behavior persists up to 4.5% strain. Particularly, Fig. 4b reveals that a feature of linear superelasticity with narrow stress hysteresis of the the results of
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as-deposited Ti51Ni49 alloy sample, which is similar to
LENS-fabricated TiNi alloys with Ti2Ni phase and other previous literature [20, 21]. The study concluded that the quasi-linear superelasticity is mainly attributed to
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continuous phase transformation and propagation of martensite, which is induced by
residual stress in precipitates [22].
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interaction between the nucleation of martensite in the TiNi matrix phase and the
In summary, the TiNi alloy is successfully fabricated via plasma arc deposition. The as-deposited Ti51Ni49 alloy consisted of TiNi (B2), TiNi (B19’) and Ti2Ni phase.
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The microstructure exhibited primary TiNi columnar dendrites structure with a certain number of Ti2Ni phases distribute in the interdendritic regions. The as-deposited Ti51Ni49 alloy displayed two-step transformation (B2→R→B19’) at cooling process
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and special linear superelasticity with narrow hysteresis due to Ti2Ni phase intimated
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contact with the TiNi matrix phase. Plasma arc deposition technology will be a highly potential technique for fabricating TiNi alloy component. Acknowledgements
This work was financially supported by National Natural Science Foundation of
China (No. 51775127). References [1] B.A. Bimber, R.F. Hamilton, J. Keist, T.A. Palmer, Anisotropic microstructure and
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ACCEPTED MANUSCRIPT superelasticity of additive manufactured NiTi alloy bulk builds using laser directed energy deposition, Mater. Sci. Eng. A 674 (2016) 125-134. [2] M. Elahinia, N. Shayesteh Moghaddam, M. Taheri Andani, A. Amerinatanzi, B.A.
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Bimber, R.F. Hamilton, Fabrication of NiTi through additive manufacturing: A review, Prog. Mater. Sci. 83 (2016) 630-663.
[3] R.F. Hamilton, B.A. Bimber, M. Taheri Andani, M. Elahinia, Multi-scale shape
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memory effect recovery in NiTi alloys additive manufactured by selective laser
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melting and laser directed energy deposition, J. Mater. Process. Tech. 250 (2017) 55-64.
[4] S. Dadbakhsh, B. Vrancken, J.P. Kruth, J. Luyten, J. Van Humbeeck, Texture and anisotropy in selective laser melting of NiTi alloy, Mater. Sci. Eng. A 650 (2016)
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225-232.
[5] A. Bagheri, M.J. Mahtabi, N. Shamsaei, Fatigue behavior and cyclic deformation of additive manufactured NiTi, J. Mater. Process. Tech. 252 (2018) 440-453.
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[6] S. Saedi, N. Shayesteh Moghaddam, A. Amerinatanzi, M. Elahinia, H.E. Karaca,
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On the effects of selective laser melting process parameters on microstructure and thermomechanical response of Ni-rich NiTi, Acta Mater. 144 (2018) 552-560. [7] M. Taheri Andani, S. Saedi, A.S. Turabi, M.R. Karamooz, C. Haberland, H.E. Karaca, M. Elahinia, Mechanical and shape memory properties of porous Ni50.1Ti49.9 alloys manufactured by selective laser melting, J. Mech. Behav. Biomed. Mater. 68 (2017) 224-231. [8] J.J. Lin, Y.H. Lv, Y.X. Liu, et al, Microstructural evolution and mechanical
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ACCEPTED MANUSCRIPT properties of Ti-6Al-4V wall deposited by pulsed plasma arc additive manufacturing, Mater. Des. 102 (2016) 30-40. [9] J. Lin, Y. Lv, Y. Liu, Z. Sun, K. Wang, Z. Li, Y. Wu, B. Xu, Microstructural
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evolution and mechanical property of Ti-6Al-4V wall deposited by continuous plasma arc additive manufacturing without post heat treatment, J. Mech. Behav. Biomed. Mater. 69 (2017) 19-29.
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[10] J. Guo, Y. Zhou, C. Liu, Q. Wu, X. Chen, J. Lu, Wire Arc Additive
Frequency, Materials 9 (2016).
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Manufacturing of AZ31 Magnesium Alloy: Grain Refinement by Adjusting Pulse
[11] G. Jin, Y. Li, H. Cui, X. Cui, Z. Cai, Microstructure and Tribological Properties of In Situ Synthesized TiN Reinforced Ni/Ti Alloy Clad Layer Prepared by Plasma
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Cladding Technique, J. Mater. Eng. Perform. 25 (2016) 2412-2419. [12] J.F. Wang, Q.J. Sun, H. Wang, J.P. Liu, J.C. Feng, Effect of location on microstructure and mechanical properties of additive layer manufactured Inconel 625
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using gas tungsten arc welding, Mater. Sci. Eng. A 676 (2016) 395-405.
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[13] P.R. Halani, Y.C. Shin, In Situ Synthesis and Characterization of Shape Memory Alloy Nitinol by Laser Direct Deposition, Metall. Mater. Trans. A, 43 (2011) 650-657. [14] B. Zhang, J. Chen, C. Coddet, Microstructure and Transformation Behavior of in-situ Shape Memory Alloys by Selective Laser Melting Ti–Ni Mixed Powder, J. Mater. Sci. Tech. 29 (2013) 863-867. [15] X. Xu, X. Lin, M. Yang, J. Chen, W. Huang, Microstructure evolution in laser solid forming of Ti-50wt% Ni alloy, J. Alloy. Compd. 480 (2009) 782-787.
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ACCEPTED MANUSCRIPT [16] J.I. Kim, Y. Liu, S. Miyazaki, Ageing-induced two-stage R-phase transformation in Ti–50.9at.%Ni, Acta Mater. 52 (2004) 487-499. [17] Z.H. Wu, D. Vokoun, et al, A Two-Way Shape Memory Study on Ni-Rich NiTi
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Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phase Transformation, J. Mater. Eng. Perform. 26 (2017) 5801-5810.
[18] H.-j. Moon, S.-j. Chun, et al, Effect of alloy composition on the B2-R
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transformation in rapidly solidified Ti-Ni alloys, J. Alloy. Compd. 577 (2013)
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S259-S264.
[19] Y. Liu, M. Blanc, G. Tan, J.I. Kim, S. Miyazaki, Effect of ageing on the transformation behaviour of Ti–49.5at.% Ni, Mater. Sci. Eng. A 438-440 (2006) 617-621.
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[20] P.R. Halani, I. Kaya, Y.C. Shin, H.E. Karaca, Phase transformation characteristics and mechanical characterization of nitinol synthesized by laser direct deposition, Mater. Sci. Eng. A 559 (2013) 836-843.
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[21] X. Zhang, H. Zong, L. Cui, X. Fan, X. Ding, J. Sun, Origin of high strength, low
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modulus superelasticity in nanowire-shape memory alloy composites, Sci. Rep. 7 (2017) 46360.
[22] Shijie Hao, et al, A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength, Science 339 (2013) 1191-1194.
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Highlights： ： 1. Plasma arc deposition has been successfully applied to fabricate the shape memory alloy for the first time.
4.5%) with narrow hysteresis.
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2. The as-fabricated sample displays linear super-elasticity (up to
3. Ti2Ni phase intimated contact with the TiNi (B2) matrix phase is
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the origin of the unusual linear superelasticity.
S0042-207X(18)30885-6
DOI:
10.1016/j.vacuum.2018.08.028
Reference:
VAC 8175
To appear in:
Vacuum
Received Date: 25 May 2018 Revised Date:
13 June 2018
Accepted Date: 16 August 2018
Please cite this article as: Lu B, Cui X, Feng X, Dong M, Li Y, Cai Z, Wang H, Jin G, Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition, Vacuum (2018), doi: 10.1016/j.vacuum.2018.08.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Direct rapid prototyping of shape memory alloy with linear superelasticity via plasma arc deposition Bingwen Lua,b, Xiufang Cuia∗, Xiangru Fenga, Meiling Donga, Yang Lia, Zhaobing
a
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Caia, Haidou Wangb,Guo Jina∗ Institute of Surface/Interface Science and Technology, Key Laboratory of Superlight
Material and Surface Technology of Ministry of Education, College of Material
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Science and Chemical Engineering, Harbin Engineering University, Harbin 150001,
b
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China.
National Key Laboratory for Remanufacturing, Armored Forces Engineering
Institute, Beijing, 100072, China Abstract
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Recently, additive manufacturing (AM) technology has drawn significant attention to fabricate shape memory alloys. In this study, plasma arc deposition (PAD), an AM technology, has been successfully applied to prepare the Ti51Ni49 alloy directly
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and rapidly at the first time. The synthesized PAD Ti51Ni49 alloy consisting of TiNi
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(B2), TiNi (B19’) and Ti2Ni phase, exhibited primary TiNi columnar dendrites structure with a certain number of Ti2Ni phases distributed in the interdendritic regions. The as-deposited Ti51Ni49 alloy displayed a two-step phase transformation evidently (B2→ R → B19’) during cooling process, and special linear superelasticity (up to 4.5%) with narrow hysteresis due to Ti2Ni phase embedded in TiNi matrix phase. This work suggests that the PAD is an effective technology to directly and ∗
Corresponding author Email: [email protected] (X. F. Cui). Email: [email protected] (G. Jin).
1
ACCEPTED MANUSCRIPT rapidly synthesize shape memory alloy. Key words: plasma arc deposition, shape memory alloy, phase transformation, superelasticity
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TiNi alloy as a functional material has been widely applied in the aerospace, biomedical and telecommunication sectors due to its unique shape memory effect and super-elasticity effect [1]. In recent years, additive manufacturing (AM) techniques
SC
have attracted significant attention on the direct formation of complex TiNi parts by
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means of its immanent advantages [2], like, selective laser melting, laser engineered net shaping and directed energy deposition technologies [3-7], however, there are no research reports being focused on the manufacture of the TiNi alloys by plasma arc deposition (PAD). Due to the potential in efficiency, convenience and cost-savings,
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PAD is a significant AM technology for rapid prototyping [8, 9]. Therefore, PAD has been gaining a great attention on synthesizing several materials, such as stainless steel, titanium alloys and aluminum alloys [10]. Naturally, PAD technology can also be
EP
used to product large-scale TiNi alloy components, and to broaden the application of
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TiNi alloys greatly.
Thus, the aim of the present work is to synthetize TiNi alloy via PAD with the
prealloyed TiNi powder. The microstructure, phase transformation behavior and superelasticity behavior of samples were investigated in detail. The Ti51Ni49 (at. %) prealloyed spherical powder with an average size of 75um was used as raw material. Ti51Ni49 alloy samples were fabricated on TC4 substrates using a PAD system in continuous mode (containing plasma arc welding source,
2
ACCEPTED MANUSCRIPT protection device, welding torch, powder feeding device, welding robot and a computer control unit) with an oxygen-content < 80ppm and high-purity Ar as protective atmosphere. The schematic of additive manufacturing path and physical
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map of deposited Ti51Ni49 alloy were shown in Fig. 1a and Fig. 1b, respectively. The main fabricating parameters of PAD were as follows: plasma gas flow of 0.3L/min, shield gas flow of 15 L/min, current of 150A, scanning speed of 3mm/s， powder
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feeding rate of 3.5 g/min and each deposited height of 2.5mm.
Fig. 1. (a) Schematic of deposition path and (b) PAD deposited sample of TiNi alloy.
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After as-deposited samples were mechanically polished and etched using Kroll’s reagent, the microstructures were investigated by a BX-51 optical microscope (OM).
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The morphologies and the corresponding chemical compositions were analyzed using a scanning electron microscope (SEM, HITACHI-S4800) equipped with an energy dispersion spectroscope (EDS). The phase structure was observed by an X-ray diffraction (XRD, BRUKER-D8) with Cu target Kα radiation at different temperature. The transformation temperatures (TTs) was detected by a differential scanning calorimetry (DSC, DSC204F1) with a heating and cooling rate of 10 oC/min from -80 C to 100 oC. Compression tests were conducted on samples of 4mm diameter and
o
3
ACCEPTED MANUSCRIPT 10mm length using a mechanical tester (MTS-800) with loading strain rate of 0.001 s-1 and unloaded rate of 100 N/s at Af +15 , which equipped with an annular furnace and an extensometer at temperature.
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Fig. 2a displays the XRD patterns of the as-deposited sample at room temperature. Phase analysis reveals the existence of TiNi (B2), TiNi (B19’) and Ti2Ni phases. Fig. 2b depicts the microstructures of across-section (Y-Z plane) of the
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as-deposited Ti51Ni49 alloy, which mainly consists of columnar dendritic structure,
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growing epitaxially along the deposition direction (Z direction). However, the microstructure of overlapping interface exhibits a finer cellular dendrite structure due to the dynamic heat transfer of the moving heat source and deposition characteristics of multi-layer material [11, 12]. When a layer is deposited, a thin top region of
TE D
previous deposited layer is remelted. A thin nucleation zone at the interface between the layers forms when molten pool begins to solidify. The interface region has faster solidification velocity and greater temperature gradient than the inner region of each
EP
layer due to the thermolysis of previous layer. In turn, the heat is transferred to the
AC C
previous layer when the new layer is deposited, thus leading to grain growth of previous layer. Therefore, a finer dendrite structure forms at the overlapping region. Fig. 2c shows that two different phases, light grey phase and dark grey phase, exist in samples. Numerous light grey phases are the major component of dendrites, and a certain number of dark grey phases distribute in the interdendritic regions. The corresponding element distribution maps in Fig. 2c is shown in Fig. 2d，which indicates that these two phases contain Ni and Ti elements, but light grey phase is
4
ACCEPTED MANUSCRIPT enriched in Ni element, whiledark grey phase regions is enriched in Ti element. Quantitative point analysis results show that the compositions (at. %) of Ti and Ni in the light grey phase are almost equivalent, so this phase can be confirmed to as TiNi
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phase. The Ti/Ni atomic ratio of the dark grey phase is 2:1, thereby, it is identified as Ti2Ni phase. Thus, one conclusion can be drew out that the microstructure of the as-deposited sample is mainly composed of TiNi columnar dendritic structure with
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some irregular Ti2Ni phase in the inter-dendritic regions.
Fig. 2. Microstructure of the as-deposited Ti51Ni49 alloy: (a) XRD spectra, (b) optical micrographs(Y-Z plane), (c) SEM image and (d) element distribution maps. Fig. 3a shows the TEM image of a typical microstructural features, indicating some relatively large precipitates distribute in matrix phase. The selected-area electron diffraction (SAED) patterns of A and B (marked in Fig. 3a) display that A
5
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along [111] zone axis is identified as a FCC structure, and B along [001] zone axis is confirmed as a BCC structure. Combining the analysis results of XRD and EDS, A is the Ti2Ni phase and B is the TiNi (B2) matrix phase. The magnified HRTEM image
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demonstrates that Ti2Ni particles embed in TiNi (B2) matrix phase and have a specific
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orientation relationship with matrix phase.
Fig. 3. (a) TEM micrographs with corresponding SAED patterns and (b) HRTEM
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micrographs of the as-deposited Ti51Ni49 alloy. A model on the formation of microstructure is proposed based on the above
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microstructural observations and binary equilibrium Ti-Ni phase diagram [13]. During the PAD process, when the temperature of the molten pool decline below 1310
,
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solidification in TiNi alloy starts with the reaction of primary liquid (L) → TiNi+ L (Ti) reaction [14]. The TiNi phase grows rapidly in a columnar crystal or dendrite mode without secondary dendrite arms, but some Ti-rich liquid phase is retain around the TiNi dendrite because the molten pool has higher content of Ti with relatively low Ni. Then TiNi dendrites growth with some secondary dendrite arms, and the Ti-rich liquid phase undergo an L (Ti) + TiNi →Ti2Ni peritectic reaction to form Ti2Ni around TiNi dendrite [15]. Finally the microstructure of primary TiNi columnar 6
ACCEPTED MANUSCRIPT dendritic structure with some irregular Ti2Ni phase in the inter-dendritic regions (Fig. 2b) forms. DSC measurement (Fig. 4a) demonstrates that two exothermic peaks and one
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endothermic peak distinctly appear on the cooling curve and heating curve, respectively. In order to explain the DSC peaks and phase transformations, XRD experiments were performed at the different temperature according to the DSC curve.
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The XRD patterns (Fig. 4a) displays that diffraction peaks corresponding to B2 and
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Ti2Ni are found in the patterns at 50 . Compared to the XRD pattern of the as-fabricated sample, the diffraction peak of B19’ phase disappears and the intensity of diffraction peak corresponding to B2 phase significantly increases. This means that B19’ → B2 transformation may occurs during heating process and the endothermic to 66
at DSC heating curve is ascribed to the phase transformation
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peak from 22
of B19’ → B2. In addition, the diffraction peaks corresponding to the R phase is found at 33 , which means that B2 → R transformation occurs during cooling process and
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the exothermic peak from 38
to 26
at DSC cooling curve is attributed to phase
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transformation of B2→ R [16]. When the temperature decline down to 8 ℃, the diffraction peak of R phase disappears and the intensity of diffraction peak corresponding to B19’ phase increases greatly, illustrating that R → B19’ transformation occurs at 8℃ and the exothermic peak from 17℃ to -13℃ at DSC cooling curve is due to the phase transformation of R → B19’ [17]. The two-step transformation (B2→ R → B19’) can be attributed to the existence of Ti2Ni phase in TiNi phase. Ti2Ni phases may form in Ti-rich TiNi alloys, embedding in TiNi phase
7
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B2↔ B19’ transformation [18, 19].
Fig. 4. (a) DSC curves and XRD patterns obtained at various temperatures (b) compressive stress-strain curves of PAD NiTi alloy.
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as-deposited Ti51Ni49 alloy sample, which is similar to
LENS-fabricated TiNi alloys with Ti2Ni phase and other previous literature [20, 21]. The study concluded that the quasi-linear superelasticity is mainly attributed to
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continuous phase transformation and propagation of martensite, which is induced by
residual stress in precipitates [22].
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interaction between the nucleation of martensite in the TiNi matrix phase and the
In summary, the TiNi alloy is successfully fabricated via plasma arc deposition. The as-deposited Ti51Ni49 alloy consisted of TiNi (B2), TiNi (B19’) and Ti2Ni phase.
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The microstructure exhibited primary TiNi columnar dendrites structure with a certain number of Ti2Ni phases distribute in the interdendritic regions. The as-deposited Ti51Ni49 alloy displayed two-step transformation (B2→R→B19’) at cooling process
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and special linear superelasticity with narrow hysteresis due to Ti2Ni phase intimated
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contact with the TiNi matrix phase. Plasma arc deposition technology will be a highly potential technique for fabricating TiNi alloy component. Acknowledgements
This work was financially supported by National Natural Science Foundation of
China (No. 51775127). References [1] B.A. Bimber, R.F. Hamilton, J. Keist, T.A. Palmer, Anisotropic microstructure and
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ACCEPTED MANUSCRIPT superelasticity of additive manufactured NiTi alloy bulk builds using laser directed energy deposition, Mater. Sci. Eng. A 674 (2016) 125-134. [2] M. Elahinia, N. Shayesteh Moghaddam, M. Taheri Andani, A. Amerinatanzi, B.A.
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ACCEPTED MANUSCRIPT properties of Ti-6Al-4V wall deposited by pulsed plasma arc additive manufacturing, Mater. Des. 102 (2016) 30-40. [9] J. Lin, Y. Lv, Y. Liu, Z. Sun, K. Wang, Z. Li, Y. Wu, B. Xu, Microstructural
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ACCEPTED MANUSCRIPT [16] J.I. Kim, Y. Liu, S. Miyazaki, Ageing-induced two-stage R-phase transformation in Ti–50.9at.%Ni, Acta Mater. 52 (2004) 487-499. [17] Z.H. Wu, D. Vokoun, et al, A Two-Way Shape Memory Study on Ni-Rich NiTi
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S259-S264.
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Highlights： ： 1. Plasma arc deposition has been successfully applied to fabricate the shape memory alloy for the first time.
4.5%) with narrow hysteresis.
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2. The as-fabricated sample displays linear super-elasticity (up to
3. Ti2Ni phase intimated contact with the TiNi (B2) matrix phase is
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the origin of the unusual linear superelasticity.