We investigated the microstructural features in solution-treated and aged Ti35.4Ni49.4Zr15.2 alloys by conventional transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations. The effect of short-range order (SRO) and lattice modulation on the martensitic transformation behavior was discussed. The solution-treated specimen consisted of the B2 structure with mottled contrasts at room temperature. In the selected-area diffraction patterns, two wave-like diffuse scattering contrasts and extra spots were observed along the g* = 001B2 vector of the B2 structure. HAADF-STEM observations indicated an SRO phase with nanometer-scale order and lattice modulation with fine atomic displacements on the (001)B2 and (10)B2 planes of the B2 matrix. Scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis also indicated Zr clustering on the (001)B2 plane of the B2 matrix. The martensitic transformation temperatures increased as a result of aging at 873 K. The fine H-phase particles precipitated along the {001}B2 plane of the B2 matrix, followed by ordering of the (202)H plane of the H-phase. The SRO phase and lattice modulation originated from the attractive interaction induced by mixing enthalpies between each element as a result of the Zr clustering, and they are considered the precursor phenomena of the H-phase. The atomic displacements and strain arising from the SRO phase and lattice modulation suppress the martensitic transformation.

Vanadium oxides have attracted a great deal of attention because of their good electrical and magnetic properties, in addition to their excellent optical properties. VO2 undergoes a phase transformation from a tetragonal to a monoclinic structure at 341 K, leading to changes in its electrical and optical properties. We successfully produced oxygen-defective VO2_x fi lms with thicknesses of the order of several hundreds of nanometers on the surface of V metal foils heat-treated at 773 K for 4 to 20 h under an oxygen partial pressure of 10.13 Pa. The thickness and the grain size of the oxides increased with increasing holding time. High-angle annular dark-field scanning transmission electron microscopy observations indicated that the transformation from a tetragonal to a monoclinic structure was accompanied by an atomic displacement along the c- and a-axes of the tetragonal VO2 structure and that the monoclinic VO2 structure was distributed in nanodomains within the tetragonal VO2 grain. The existence of a stable tetragonal structure at room temperature probably originates from suppression of the atomic displacements accompanying the transformation through the introduction of oxygen deficiencies. The diffuse reflectance of oxygen-defective VO2_x fi lms was less than 30 % , indicating the absorption of visible and near-infrared lights.

Introducing oxygen deficiencies into metallic oxides dramatically improves their functional properties. We produced periodic oxygen-defective ZrO2−x films via a stepwise oxidation process involving annealing Zr metal foil in air and under a controlled oxygen partial pressure. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM revealed that the c-axis of the unit cell of monoclinic ZrO2 structure contracted upon introduction of oxygen vacancies. The β angle of the monoclinic structure also changed to 90.5°. The ordering of oxygen deficiencies related to the < 111 > fluorite direction on the ( 1 01 ) _P2_1 /c plane led to a periodic structure with a spacing of some nanometers. This work is the first report visualizing the ordering of oxygen vacancies in the monoclinic ZrO2−x structure. Our process can be used to control the content and ordering of oxygen deficiencies through the selection of appropriate oxidation conditions, thereby improving the functional properties of ZrO2−x films. (a) Bright-field conventional transmission electron microscopy image from the surface to depth of a few micrometers in the cross-section of sample A. (b) Higher-magnification image near the surface in (a). (c) Selected-area electron diffraction pattern for grain C in (b). High-angle annular dark-field scanning transmission electron microscopy image taken along (d) the [101] _P2_1 /c and (e) the [010] _P2_1 /c zone axis of the monoclinic ZrO2 structure, respectively. Atomic arrangements and lattice spacing of the [010] _P2_1 /c direction of the monoclinic ZrO2 structure with β angles; (f) β = 99.1°and (g) β = 90.5°, respectively.

We successfully produced a dopant-free oxygen-defective HfO2-x film that exhibit room-temperature ferromagnetism via a heat treatment of metallic Hf foil at 1673 K for 10 h under an oxygen partial pressure of 1.0 x 10(-4) atm. The sample, which exhibits similar to 50% transmission of visible light, demonstrates room-temperature ferromagnetism, with saturation and residual magnetizations of similar to 0.05 emu/g and similar to 0.005 emu/g, respectively. The HfO2-x film comprises columnar grains several hundred nanometers wide and several micrometers long. High-angle annular dark-field scanning electron microscopy observations indicated that the tetragonal HfO(2 )structure was distributed in nanodomains within the monoclinic HfO(2 )grains; that is, the films exhibited a hybridized microstructure. The tetragonal HfO(2 )structure would be mainly due to the introduction of oxygen deficiencies, which led to the room-temperature ferromagnetism.

In a thermoelastic martensitic transformation, there is a "self-accommodation" in which the microstructure itself relieves strain induced by the transformation. In this study, the crystal structure and self-accommodation microstructure of martensite in Ti30Ni50Zr20 alloy were investigated using X-ray diffraction, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy. In addition, the deformation microstructure was investigated by observing the samples after tensile tests. The crystal structure of the martensite in the Ti30Ni50Zr20 alloy was determined to be the B19′ monoclinic structure. In this alloy, plate- and polygonal-like variants with a width of a few microns were observed, and pairs of habit-plane variants (HPVs) forming {011}B19′ twins were observed at the interface. The self-accommodation has a mosaic-like morphology because of the combination of these pairs of HPVs. The (001)B19′ compound twins were formed as internal defects. These twins are considered to be lattice-invariant shear (LIS) of martensite in this alloy. No plateau region was observed in the tensile test. In the deformed specimen, the self-accommodation was collapsed by the movement of the HPV interface and the introduction of (100)B19′ compound twins was observed as an internal defect. This defect is considered to be not LIS but a deformation twin. The lack of a plateau region in the stress–displacement curve was attributed to these deformed microstructures.

In a thermoelastic martensitic transformation, there is a "self-accommodation" in which the microstructure itself relieves strain induced by the transformation. In this study, the crystal structure and self-accommodation microstructure of martensite in Ti30Ni50Zr20 alloy were investigated using X-ray diffraction, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy. In addition, the deformation microstructure was investigated by observing the samples after tensile tests. The crystal structure of the martensite in the Ti30Ni50Zr20 alloy was determined to be the B19' monoclinic structure. In this alloy, plate- and polygonal-like variants with a width of a few microns were observed, and pairs of habit-plane variants (HPVs) forming {011}B19' twins were observed at the interface. The self-accommodation has a mosaic-like morphology because of the combination of these pairs of HPVs. The (001)B19' compound twins were formed as internal defects. These twins are considered to be lattice-invariant shear (LIS) of martensite in this alloy. No plateau region was observed in the tensile test. In the deformed specimen, the self-accommodation was collapsed by the movement of the HPV interface and the introduction of (100)B19' compound twins was observed as an internal defect. This defect is considered to be not LIS but a deformation twin. The lack of a plateau region in the stress-displacement curve was attributed to these deformed microstructures.

Ferromagnetism can be induced by introducing oxygen vacancies into metallic oxides. The oxygen deficiencies introduced into ZrO2 samples thus far have been present in unquantifiable concentrations and randomly scat-tered near the surface. We successfully produced room-temperature-ferromagnetic 9 mu m-thick ZrO2-x films via a stepwise oxidation process consisting of annealing metallic Zr foil in air and under a controlled oxygen partial pressure. The ZrO2-x films are composed of a periodic structure with a spacing of 3-4 nm; this structure orig-inated from the presence and absence of oxygen deficiencies. The unique structure in the ZrO2-x phase consisting of the periodic arrangements of oxygen deficiencies can be produced by the stepwise oxidation using Zr metal. Our proposed process should open the door for new applications of defect-driven ferromagnetic films of undoped magnetic oxides with a periodic oxygen-defective structure.

It has been reported that functional and mechanical properties of Ti-Ni shape memory alloys could be degraded by hydrogen absorption. Many studies have indicated the formation of a hardened layer on the surface in hydrogen-absorbed Ti-Ni alloys. However, the underlying microstructural features and their impact on the functional and mechanical properties remain unclear. In the present study, ex situ and in situ electron microscopy characterizations of the high-hardness layer in a hydrogen-charged Ti-Ni alloy were performed for the first time. Electron diffraction and atomic resolution imaging revealed that the hardened layers have an orthorhombic structure with a four-layer stacking sequence and a martensitic-like crystallographic nature such as lattice correspondence including 12 variants of the orthorhombic phase against each orientation of the parent cubic (B2) phase. In situ and ex situ observations in an electron microscope confirmed that the orthorhombic phase undergoes athermal transformation upon cooling and stress- and/or strain-induced transformation upon deformation, which further characterizes the martensitic-like nature of this phase. The effect of hydrogen on the original phase transformation from the cubic (B2) to the monoclinic (B19 & PRIME;) structure is also discussed.

We produced Magnéli phase vanadium oxides with a thickness of approximately 1.0 μm on the surface of vanadium metal foil via a stepwise oxidation process consisting of annealing at 723 K in air and a subsequent heat treatment at 773 K under a low oxygen partial pressure of PO2 = 1.0 × 10−10 atm. The diffuse reflectance of samples with a Magnéli phase vanadium oxide was less than 20%, indicating the absorption of visible and near-infrared lights. Magnéli phase vanadium oxides with the desired vanadium-to-oxygen atomic ratio could be produced by controlling the heat-treatment conditions in our oxidation process.

A strong-interference film that absorbs visible light by using oxygen-deficient ZrO2-x was produced by heat treatment of Zr metal foil under a low partial pressure of oxygen. The oxygen-defective tetragonal ZrO2-x film contained nanocrystallites several nanometers in size and its thickness (t) increased from 30 to 140 nm on increasing the holding time during heat treatment. The color of the sample changed depending on the thickness of the oxygen-deficient ZrO2-x film from gold (t = 30 nm) through blue (t = 75 nm) to black (t = 140 nm). Thick ZrO2-x films (t > 140 nm) absorbed all visible light, leading to the observed black color, whereas in thinner films (t < 140 nm), strong interference resulted in changes in the sample color, for example, from gold to blue. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A black-TiZrO4 oxide film with light absorption in the visible and near-infrared regions was successfully formed on a surface using the atmospheric oxidation of a Ti–Zr alloy. The absorption of light in a wide range of wavelengths in the black-TiZrO4 oxide film was caused by a decrease in the energy gap owing to the oxygen deficiency, as indicated from the X-ray photoelectron spectroscopy analysis, and from a decrease in the refined lattice parameters, based on X-ray diffraction results. Moreover, no cracks or voids were observed within the TiZrO4 oxide film or at the interface between the oxide film and metallic Ti–Zr matrix, resulting in an ideal film. The rapid atmospheric oxidation in Ti–Zr alloy is attributed to both the promotion of the inward diffusion of oxygen ions by the substitution of Zr for Ti, and the faster diffusion of oxygen ions in a Ti–Zr-based oxide film.

A 3-μm-thick film of Magnéli Ti4O7 was successfully fabricated on the surface of Ti metal foil by a stepwise oxidation process involving annealing at 973 K in air followed by heating to 1173 K under a low oxygen partial pressure. Transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy showed that the Magnéli Ti4O7 phase consisted of a period of four layers of equiaxial grains several hundreds of nanometers in size. The black Magnéli Ti4O7 thin film formed by our process absorbed light in the visible and near-infrared regions.

Black-monoclinic oxygen defective HfO(2-x)film with about 5 mu m in a thickness was successfully formed on the surface of a metal Hf plate by controlled heat treatment in air. The black-monoclinic oxygen defective HfO2-x film was transformed into white body by further oxidation at higher temperature. Tauc plots using diffuse reflection data showed that impurity levels were formed within the energy gap for the black-monoclinic HfO2-x thin films. Based on the XPS analyses, the oxygen vacancy brought about the impurity levels. No cracks and voids were observed for the black-monoclinic HfO2-x films consisting of the fine columnar grains with the dimension of diameters of about 50 nm and lengths ranging from a few hundred nm to 1 mu m.

Dopant-free, transparent ZrO2 with a monoclinic structure was successfully developed through a two-step oxidation process by combining heat treatment in air and under a low oxygen partial pressure, using a Zr metal foil. The transmission of the sample was similar to 50% at 600 nm (in the region of visible light) and over 60% beyond the near-infrared region. This transparent ZrO2 with a monoclinic structure comprised equiaxed grains with dimensions of tens of nanometers and columnar grains with lengths of several micrometers. Moreover, it formed a dense film without cracks or voids, having fewer oxygen deficiencies. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

1Division of Materials Science and Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan 2Department of Materials Science and Engineering, Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan 3Department of Advanced Materials Science and Engineering, Faculty of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan

We investigated the texture and the shape change in the equiatomic TiPd alloy, and discussed the relationship between the shape change and the atomic movements associated with martensitic transformation. Thermomechanical analyzer tests indicate that the direction of the shape change was different between the 0° and 90° samples, cutting out parallel and perpendicular to the hearth side of button ingot, respectively. In the 0° sample, shrinking and expansion were observed during the reverse and forward martensitic transformations, respectively, whereas the opposite tendency was confirmed in the 90° sample compared to the 0° sample. During the isobaric test, the martensitic variants were oriented to a (010) plane with compressive loading, and the B2 parent phase crystals also became coarse. There is a close relationship between the shape change due to the crystal orientation by the isobaric test and the shear-shuffling direction due to martensitic transformation.

HAADF STEM (high angle annular dark field scanning transmission electron microscopy), LPSO (long period stacking ordered struc- ture), Zr Co based alloy TEM specimen preparation: Electropolishing in an electrolyte solution consisting of 20 H 2 SO 4 and 80 methanol by volume.