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“Anomalous stress-strain behavior of NiTi shape memory alloy close to the border of superelastic window”. Wang X, Yao X, Schryvers D, Verlinden B, Wang G, Zhao G, Van Humbeeck J, Kustov S, Scripta Materialia 204, 114135 (2021). http://doi.org/10.1016/J.SCRIPTAMAT.2021.114135
Abstract: In this work, we report an anomalous phenomenon on superelastic cycling of NiTi shape memory alloys when deforming at the temperature close to the border of superelastic window. New unexpected effects are found-(i) critical stress for inducing martensite transformation during the second loading cycle is higher than that of the first cycle; ( ii ) the plateau stress of the second cycle decreases to the original level when the strain overcomes the limit of the first cycle; ( iii ) transition from good superelasticity in the first cycle to fully irreversible strain in the second. We propose that defects generated during the first superelastic cycle close to the border of superelastic window impede following stress-induced martensitic transformations, leading to the increase of critical stress beyond yield stress of the B2 matrix, and thus functional fatigue of NiTi alloys. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 3.747
DOI: 10.1016/J.SCRIPTAMAT.2021.114135
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“Identification of nano-width variants in a fully monoclinic martensitic Ni50Ti50 alloy by scanning electron microscope-based transmission Kikuchi diffraction and improved groupoid structure approach”. Zhao ZX, Ma X, Cao S, Li YY, Zeng CY, Wang DX, Yao X, Deng ZJ, Zhang XP, Materials Letters 281, 128624 (2020). http://doi.org/10.1016/J.MATLET.2020.128624
Abstract: Nano-width martensite plates in a fully martensitic Ni50Ti50 alloy are indexed successfully by using the off-axis transmission Kikuchi diffraction in scanning electron microscope (i.e., SEM-based TKD). The data obtained by SEM-TKD are effectively interpreted using an improved approach based on the framework of the theoretical groupoid structure method, where the equivalent variants transformed from the monoclinic variants are introduced to calculate all theoretical axis/angle pairs of rotation, and to formulate a complete list of source martensite to target martensite pairs. Consequently, B19' monoclinic martensite variants in NiTi alloys are identified unambiguously, by using numerical comparison between the experimental and theoretical rotation components, without the reference of retained parent phase. (C) 2020 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3
DOI: 10.1016/J.MATLET.2020.128624
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“Quantitative FIB/SEM three-dimensional characterization of a unique Ni₄Ti₃, network in a porous Ni50.8Ti49.2 alloy undergoing a two-step martensitic transformation”. Cao S, Zeng CY, Li YY, Yao X, Ma X, Samaee V, Schryvers D, Zhang XP, Materials Characterization 169, 110595 (2020). http://doi.org/10.1016/J.MATCHAR.2020.110595
Abstract: The three-dimensional (3D) nanostructure of Ni4Ti3 precipitates in a porous Ni50.8Ti49.2 alloy has been re-constructed by “Slice-and-View” in a Focused Ion Beam/Scanning Electron Microscope (FIB/SEM). The 3D configuration of these precipitates forming a network structure in the B2 austenite matrix has been characterized via 3D visualization and quantitative analysis including volume fraction, skeleton, degree of anisotropy and local thickness. It is found that dense Ni4Ti3 precipitates occupy 54% of the volume in the B2 austenite matrix. Parallel Ni4Ti3 precipitates grow alongside the surface of a micro-pore, yielding an asymmetric structure, while nano voids do not seem to affect the growth of Ni4Ti3 precipitates. The small average local thickness of the precipitates around 60 nm allows their coherency with the matrix, and further induces the R-phase transformation in the matrix. On the other hand, the B2 matrix exhibits a winding and narrow structure with a skeleton of 18.20 mm and a thickness similar to the precipitates. This discontinuous matrix segmented by the Ni4Ti3 network and pores is responsible for the gradual transformation by stalling the martensite propagation.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 4.7
DOI: 10.1016/J.MATCHAR.2020.110595
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“A hidden single-stage martensitic transformation from B2 parent phase to B19 ' martensite phase in an aged Ni51Ti49 alloy”. Zeng CY, Cao S, Li YY, Zhao ZX, Yao XY, Ma X, Zhang XP, Materials letters 253, 99 (2019). http://doi.org/10.1016/J.MATLET.2019.06.055
Abstract: The aged Ni-rich NiTi shape memory alloys (SMAs) exhibit the multi-stage martensitic transformation (MMT), which has important influences on functional properties and practical applications of the NiTi SMAs. A hidden single-stage martensitic transformation from B2 parent phase to B19' martensite phase is found in an aged Ni51Ti49 alloy, which happens concurrently with a commonly observed two-stage martensitic transformation B2-R-B19' (R: martensite phase) and actually composes one stage of a multi-stage martensitic transformation (MMT) together with the two-stage one. B2-B19' martensitic transformation occurs in the NiTi matrix containing Ni4Ti3 precipitates with relatively large inter-particle space, while B2-R-B19' transformation takes place in the NiTi matrix with Ni4Ti3 precipitates having relatively small inter-particle space. (C) 2019 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.572
DOI: 10.1016/J.MATLET.2019.06.055
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Yao X (2019) An advanced TEM study on quantification of Ni4Ti3 precipitates in low temperature aged Ni-Ti shape memory alloy. 149 p
Keywords: Doctoral thesis; Electron microscopy for materials research (EMAT)
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“Optimization of Automated Crystal Orientation Mapping in a TEM for Ni4Ti3 Precipitation in All-Round SMA”. Yao X, Amin-Ahmadi B, Li Y, Cao S, Ma X, Zhang X-P, Schryvers D, Shape memory and superelasticity 2, 286 (2016). http://doi.org/10.1007/s40830-016-0082-z
Abstract: Automated crystal orientation and phase mapping in TEM are applied to the quantification of Ni4Ti3 precipitates in Ni–Ti shape memory alloys which will be used for the implantation of artificial sphincters operating using the all-round shape memory effect. This paper focuses on the optimization process of the technique to obtain best values for all major parameters in the acquisition of electron diffraction patterns as well as template generation. With the obtained settings, vast statistical data on nano- and microstructures essential to the operation of these shape memory devices become available.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Times cited: 2
DOI: 10.1007/s40830-016-0082-z
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“Optimization of Automated Crystal Orientation and Phase Mapping in TEM Applied to Ni-Ti All Round Shape Memory Alloy”. Yao X, Li Y, Cao S, Ma X, Zhang X-ping, Schryvers D, MATEC web of conferences
T2 –, Proceedings of ESOMAT 2015 10th European Symposium on Martensitic Transformations, September 14-18, 2015, Antwerp, Belgium 33, 03022 (2015). http://doi.org/10.1051/matecconf/20153303022
Abstract: A new application which focuses on an artificial sphincter fabricated by Ni-Ti SMAs for human implantation is under investigation by applying the all-round shape memory effect with precise control of the phase transformation temperatures. In this study, a Ni51at.%-Ti alloy was fabricated by arc melting with fast solidification, followed by a proper strained aging which induces the two way shape memory effect needed for this particular application. Differential scanning calorimetry was used to investigate the thermal behavior and transmission electron microscopy was used for studying the microstructure of the alloys. With the latter the novel technique of automated crystal orientation microscopy is used and optimized to obtain phase and orientation mapping of the various structures.
Keywords: P1 Proceeding; Electron microscopy for materials research (EMAT)
Times cited: 1
DOI: 10.1051/matecconf/20153303022
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“Microstructural Characterization and Transformation Behavior of Porous Ni50.8Ti49.2”. Yao X, Cao S, Zhang XP, Schryvers D, Materials Today: Proceedings 2, S833 (2015). http://doi.org/10.1016/j.matpr.2015.07.411
Abstract: Porous Ni50.8Ti49.2 bulk material was prepared by powder metallurgy sintering. Solid solution and aging treatments were applied to improve the phase homogeneity and phase transformation behavior. Scanning and transmission electron microscopy, aided by energy dispersive X-ray analysis, were used to study the microstructure and chemical phase content of the alloys. In-situ cooling was carried out to observe the phase transformation behavior. As-received material contains dispersed Ni2Ti4O particles while Ni4Ti3 precipitates appear after aging. Close to pore edges, the latter have a preferential orientation due to the induced stress fields in the matrix.
Keywords: P1 Proceeding; Electron microscopy for materials research (EMAT)
DOI: 10.1016/j.matpr.2015.07.411
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