Ti2Ni-type Phase Research Articles

Evolution of microstructure and mechanical properties of a Ti80(CoFeNi)20 ultrafine eutectic composite during thermal processing

In this study, the influence of thermal processing on phase transformation and microstructural evolution of Ti80(CoFeNi)20 ultrafine eutectic composites was investigated. The as-cast alloy exhibited the typical hypoeutectic microstructure consisting of a number of β-Ti dendrites and the β-Ti + Ti2Ni-type ultrafine eutectic matrix. In comparison with the as-cast alloy, the alloy annealed at 1173 K for 1 h exhibited significant microstructural change from a hypoeutectic structure to a microscale dual-phase structure comprising the dispersed Ti2Ni-type phase and β-Ti matrix domain. The alloy aged at 873 K for 1 h exhibited a significant phase transformation from the β-Ti matrix to α-Ti and Ti2Ni-type phases by the eutectoid decomposition. These microstructural evolutions induced by heat treatment significantly influenced to the mechanical properties. Optimized mechanical properties (yield strength of 1530 MPa and plasticity of 5.7%) were achieved for the as-annealed alloy.

Martensitic phase transformation and shape memory properties of the as-cast NiCuTiHf and NiCuTiHfZr alloys

This study investigates the structure, martensitic phase transformation behavior, and shape memory properties of the quaternary Ni44.8Cu5Ti45.2Hf5 and quinary Ni44.8Cu5Ti40.2Hf5Zr5 and Ni45.5Cu5Ti39.5Hf5Zr5 shape memory alloys (SMAs) in the as-cast state. The alloys exhibited a dual-phase microstructure composed of matrix phase (austenite or martensite depending on the chemical composition) and Ti2Ni-type phase. The experimental results revealed that the alloys underwent a one-step B2↔B19′ transformation. The Ni44.8Cu5Ti45.2Hf5 alloy showed a maximum shape memory strain of 6.2% with a recovery ratio of 81% upon thermal cycling under a tensile stress of 300 MPa, while in the Ni44.8Cu5Ti40.2Hf5Zr5 alloy, a maximum shape memory strain of about 5% was obtained under 500 MPa with a recovery ratio of 86%. The as-cast Ni44.8Cu5Ti45.2Hf5 alloy showed superelastic response with 0.3% maximum transformation strain at 125℃. On the other hand, the Ni44.8Cu5Ti45.2Hf5 alloy showed the best superelastic response at 65℃ with a recoverable strain of about 1.9%. Two-way shape memory strains of 0.8% and 3.3% were obtained in the Ni44.8Cu5Ti45.2Hf5 and Ni44.8Cu5Ti40.2Hf5Zr5 alloys, respectively. The Ni45.5Cu5Ti39.5Hf5Zr5 alloy, however, did not reveal the shape memory effect. Results obtained from low-temperature X-ray diffraction (XRD) analysis indicated that the B2→B19′ transformation was considerably suppressed in this alloy and the volume fraction of B19′martensite did not exceed 8% even at an ultra-low temperature of − 150 °C.

Effect of Nitrogen Addition on TiNi Shape Memory Alloys

In present paper we will show how nitrogen effects microstructures, transformation temperatures, and mechanical properties of equiatomic Ti50–Ni50 and Ti-rich Ti51–Ni49 binary shape memory alloys. 0.5 at.% of nitrogen was added to prepare Ti50–Ni49.5–N0.5, and Ti51–Ni48.5–N0.5 (at.%) alloys by arc-melting. Microstructures were investigated by scanning electron microscope (SEM), phase constitutions were investigated by X-ray diffraction (XRD), transformation temperatures were investigated by differential scanning calorimeter (DSC) and mechanical properties were tested by tensile tests. Solutions treated Ti–Ni–N shape memory alloys contain TiNi matrix without nitrogen, Ti2Ni type phase containing a small amount of nitrogen and a new Ti2N type phase containing a small amount of nickel. Compared with Ti50–Ni50 and Ti51–Ni49 binary alloys, the martensitic transformation starts temperatures (Ms) of Ti50–Ni49.5–N0.5 and Ti51–Ni48.5–N0.5 ternary alloys decreased from 63.4 °C to 41.6 °C and from 85.3 °C to 79.4 °C, respectively. By adding N, fracture strain decreased and incomplete superelasticity was observed.

Microstructural characteristics and hardness of CoNiTi medium-entropy alloy coating on pure Ti substrate prepared by pulsed laser cladding

A ternary CoNiTi medium-entropy (MEA) alloy coating was successfully fabricated on pure Ti substrate by utilizing pulsed laser cladding. Phase constitutions and microstructural characteristics of the coating were analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, secondary electron imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that three zones with distinct microstructures are formed after the pulsed laser cladding treatment: cladding zone (CZ), bonding zone (BZ) and heat-affected zone (HAZ). Dendritic (BCC solid-solution phase)-interdendritic (Ti2Ni type phase) structures are formed in the CZ with many nanoparticles (Ti2Co type phase) dispersed inside the interdendritic structures. The BZ consists of fine acicular grains while the HAZ below the BZ is composed of irregular-shaped bulk grains. Hardness measurements reveal that hardness of the MEA coating reaches 571 ± 46 H V, which is ∼5 times that of the substrate (114 ± 4 H V). Comprehensive analyses show that such high hardness can be attributed to the combined contributions from solid-solution hardening of the BCC phase and second-phase hardening from the Ti2Ni and Ti2Co type intermetallic compounds.

Preparation and electrochemical hydrogen storage properties of MWCNTs-doped Ti49Zr26Ni25 alloy containing quasicrystal phase

Mechanical alloying technique and subsequent annealing were used to prepare the Ti49Zr26Ni25 quasicrystal. Composite materials of multi-walled carbon nanotubes (MWCNTs) doped Ti49Zr26Ni25 alloys were obtained via ball-milling to improve the hydrogen storage performance of Ti49Zr26Ni25. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to analyze the structural properties of the samples. The phase structures of the composite alloys were composed of icosahedral quasicrystal phase and Ti2Ni-type phase. Smaller alloy particles were obtained after doping MWCNTs. Moreover, a three-electrode battery system was carried out to test the electrochemical properties. The composites showed higher discharge capacity, stronger HRD, better cyclic stability and lower charge-transfer resistance than the original Ti49Zr26Ni25 alloy. In the composite electrodes, the optimal discharge capacity of 254.2 mAh/g and the highest capacity retention of 63.1% were achieved for 5 wt.% additive content of MWCNTs. The electro-catalytic function of MWCNTs, the reduction of particle size and raise of specific surface area for Ti49Zr26Ni25 alloy may provide larger electrochemically accessible area and rapid channel for hydrogen transportation, which are crucial for improving the electrochemical performance of the alloy.

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti1.4V0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti1.4V0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti1.4V0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti2Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti1.4V0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g−1 as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g−1 compared with that of 30 mA g−1, and the cycling life achieved above 200 mAh g−1 after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti1.4V0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti1.4V0.6Ni. Ti1.4V0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.

(Fe, Cr)6Nb6Ox phase of the filled Ti2Ni type with × ± 0.75 in the quaternary Cr–Fe–Nb–O system

Abstract The effect of O on the phase equilibria in Nb–Fe–Cr alloys, more specifically on the formation of a Ti2Ni type phase, has been studied. Nb–xFe–yCr alloys with × = 37.4 – 41.4 at.%, y = 7.6 – 8.6 at.% were subjected to long term annealing experiments at 950 8C after oxygenation. The phases were identified and characterized using scanning electron microscopy, X-ray diffraction and wavelength dispersive spectroscopy. In the pure Nb-37Fe-8Cr and Nb-41Fe-9Cr alloys respectively a two phase equilibrium between (Nb) and l FeNb and a three phase equilibrium between (Nb), l FeNb and Fe2Nb are established. In the oxygenated alloys a Ti2Ni type phase with composition (Fe, Cr)6Nb6Ox appears mainly at the expense of the l FeNb phase. With respect to composition and crystallographic parameters the (Fe, Cr)6Nb6Ox phase has large similarities with the Fe2.4Nb3.6O and Fe6Nb6O phase. From wavelength dispersive spectroscopy measurements, it is concluded that × equals 0.75 when (Fe, Cr)6Nb6Ox is in equilibrium with (Nb), l FeNb and Fe2Nb. × is higher when this phase is in equilibrium with NbO. The structure of (Fe, Cr)6Nb6Ox is also analyzed with Rietveld refinement. This analysis reveals that Cr atoms partially replace Fe atoms only on the 16d site and not on the 32e site.

Synthesis of Ti-Zr-Ni amorphous and quasicrystal powders by mechanical alloying, and their electrochemical properties

Mechanical alloying of Ti45Zr38–xNi17+x and Ti45–xZr38Ni17+x (0 ≤ x ≤ 8) elemental powders produced an amorphous phase, but subsequent annealing converted the amorphous phase into an icosahedral quasicrystal phase, along with a Ti2Ni-type phase. The discharge capacities, measured in a three-electrode cell at room temperature for both the amorphous and quasicrystal electrodes, increased with increasing Ni substitution for Zr or Ti. The highest discharge capacities, which were about 60 mAh/g for the amorphous electrode and 100 mAh/g for the quasicrystal electrode, were obtained from (Ti45Zr30Ni25) after substitution of Ni for Zr. For the Ti45Zr30Ni25 composition, the discharge performance of the quasicrystal electrode was stable over charge/discharge cycling, but that of the amorphous electrode gradually decreased with cycling. The structure of the quasicrystal phase in the electrodes was stable, even after 15 charge/discharge cycles, but the amorphous phase converted to a (Ti, Zr)H2 f.c.c. hydride.

Structural and compositional investigations of Zr 4Pt 2O: A filled-cubic Ti 2Ni-type phase

Syntheses, structural and compositional analyses of the filled cubic Ti 2Ni-type phase in Zr–Pt–O system have been studied, largely for the platinum-richer compositions. Diffraction quality crystals were grown by annealing an arc-melted composition Zr 4Pt 2O 0.66 at 1600 °C under Ar. The refined composition Zr 4.0Pt 1.95(1)O 0.93(6) ( a=12.5063(6) Å, Fd 3 ¯ m , Z=16) is close to the idealized composition Zr 4Pt 2O known in several other Zr–T–O systems ( T=late 4 d or 5 d transition element). (This composition has been erroneously reported by ICDD for years as Zr 6Pt 3O (No. 00-017-0557) and referred to as ε-Zr 6Pt 3O.) The product is only marginally poor in platinum and oxygen, in contrast to the 1960 reports of metallographic studies (∼Zr 4Pt 1.62O 0.44). Under arc-melting conditions, high yields of the cubic phase are obtained from samples with lower platinum concentrations (Zr 4Pt 1.74O 1.04), whereas samples near the refined cubic composition contain hexagonal Zr 5Pt 3O x as well (Mn 5Si 3-type). The hexagonal structure of binary Zr 5Pt 3 was also refined (Mn 5Si 3 type, P6 3/ mcm, a=8.210(1) Å, c=5.385(2) Å) and shown to be non-stoichiometric to at least Zr 5Pt 2.5.

Electrochemical hydrogen storage in (Ti 1− xV x) 2Ni ( x = 0.05–0.3) alloys comprising icosahedral quasicrystalline phase

For (Ti 1− x V x ) 2Ni ( x = 0.05, 0.1, 0.15, 0.2 and 0.3) ribbons, synthesized by arc-melting and subsequent melt-spinning techniques, an icosahedral quasicrystalline phase was present, either in the amorphous matrix or together with the stable Ti 2Ni-type phase. With increasing x values, the maximum discharge capacity of the alloy electrodes increased until reached 271.3 mAh/g when x = 0.3. The cycling capacity retention rates for these electrodes were approximately 80% after a preliminary test of 30 consecutive cycles of charging and discharging. Ti 1.7V 0.3Ni alloy electrode displayed the best high-rate discharge ability of 82.7% at the discharge current density of 240 mA/g.

Mg 8Rh 4B — A new type of boron stabilized Ti 2Ni structure

The new magnesium rhodium boron compound Mg 8Rh 4B has been synthesized by reaction of the metal powders with crystalline or amorphous boron or the RhB precursor. The crystal structure of Mg 8Rh 4B was solved using single-crystal X-ray diffraction data (space group Fd 3 ¯ m , a = 12.1711 ( 4 ) Å , Z = 8 , 174 reflections, R F = 0.016 ). The crystal structure can be described as a filled Ti 2Ni type where the interstitial sites 8 b ( 1 2 , 1 2 , 1 2 ), located at the center of two nested Mg 4Rh 4 tetrahedra, are occupied by boron atoms. Taking into account the absence of the Ti 2Ni-type phase in the binary Mg–Rh system, the boron atoms can be considered as stabilizing this structural motif. From the bonding analysis with the electron localization function the crystal structure is described as covalently bonded [Rh 4B] 3− anions, embedded in a cationic magnesium matrix.

Formation of 2–5 nm size pre-precipitates of cF96 phase in a Hf–Co–Al glassy alloy

The devitrification behaviour of Hf 55Co 25Al 20 glassy alloy was studied by X-ray diffractometry, transmission electron microscopy, differential scanning and isothermal calorimetry. Formation of a nanoscale structure with a pre-precipitate of the cF96 Ti 2Ni-type phase embedded in the residual glassy matrix was observed upon annealing. We obtained an Avrami exponent value of 2.6, indicating that the primary pre-precipitates are formed by steady state nucleation and diffusion-controlled growth of nuclei.

Nanoscale cF96 cubic versus icosahedral phase in devitrified Hf-based metallic glasses

The devitrification behaviour of the rapidly solidified quaternary hafnium-based alloys of Hf 65NM 17.5TM 10Al 7.5 composition (NM- noble metal, TM- 3d transition metal) was studied using differential scanning calorimetry. The purpose of the study was to evaluate the influence of composition on the devitrification products at the primary stage. The structure was analysed by using X-ray diffraction and transmission electron microscopy. The formation of the nanoparticles of the Hf-based quasicrystalline icosahedral or cubic cF96 Fd 3 −m Ti 2Ni-type phase about 10 nm in size was observed at the primary devitrification stage in different metallic glasses with close compositions. The icosahedral phase consists of 137-atom Bergmann rhombic triacontahedra. The alloys in systems in which the stable Hf-based cF96 phase exists do not show the formation of the icosahedral phase from the amorphous matrix. The cF96 phase and the icosahedral phase formed by primary devitrification from the amorphous phase may inherit the local order of the icosahedral clusters in the amorphous matrix.

Effect of preparation conditions on the short-range order in Zr-based bulk glass-forming alloys

The short-range order in as-prepared and annealed Zr–Cu–Al–Ni glassy alloys, obtained by quenching from the melt at different cooling rates as well as by solid state reaction via mechanical alloying, was investigated by perturbed angular correlations technique (PAC). All the samples were also characterised by X-ray diffraction and differential scanning calorimetry. For all the as-prepared amorphous samples, the PAC spectra are well described by a Czjzek distribution revealing that the short-range order corresponds to that of a dense random packing of ions. The observed electric field gradient seems to be independent of the preparation method and, when compared with previous measurements, of the Zr content. These trends are discussed in comparison with the similar behaviour observed in binary amorphous alloys. Samples annealed above the glass transition temperature but below the crystallisation step showed two broad distributions of frequencies, which were assigned, to a metastable Ti 2Ni-type phase.

High-pressure hydrogen loading in Ti 45Zr 38Ni 17 amorphous and quasicrystal powders synthesized by mechanical alloying

Amorphous and icosahedral phase (i-phase) powders, synthesized directly by mechanical alloying (MA) and after subsequent annealing, respectively, are hydrogenated at a temperature of 573 K and an initial pressure of 3.8 MPa. The i-phase powder contains a Ti 2Ni-type phase (fcc structure, lattice parameter, a=1.23 nm) as a minor phase. Hydrogen cycling for the i-phase powder decreases the coherence length and enhances the formation of an fcc hydride phase, namely (Ti, Zr)H 2. The amorphous powder, which transforms to the fcc hydride after hydrogenation, is transformed primarily into a Ti 2Ni-type crystal phase and a small amount of the i-phase after hydrogen desorption. Hydrogen cycling and mechanical alloying in a hydrogen gas atmosphere dramatically reduces the loading time of hydrogen for both the i-phase and the amorphous powders.

Hydrogen Pressure-Composition Isotherms for Ti<SUB>45</SUB>Zr<SUB>38</SUB>Ni<SUB>17</SUB> Amorphous and Quasicrystal Powders Produced by Mechanical Alloying

Pressure-composition isotherms (PCTs) for amorphous and icosahedral (i) quasicrystal powders produced by mechanical alloying of Ti45Zr38Ni17 powder mixtures were measured at temperatures of 473 K and 523 K at low-hydrogen pressures, lower than 0.1 MPa. Sloping plateau-like features on PCTs were observed at equilibrium hydrogen pressures lower than 1 kPa, below an H/M (hydrogen to metal atom ratio)≈1.2 and ≈1 for the amorphous and i-phase powders respectively. The plateau-like region for the i-phase powder was steeper and narrower than that for the amorphous powder, implying some small differences between the local structures of the i-phase and the amorphous phase. After the PCT measurements, an increase in the nearest-neighbor atom spacing and an expansion of the quasilattice were observed for the amorphous and i-phase powders respectively. Impurities from some unsynthesized elemental material and a Ti2Ni type phase were also present. These also absorbed hydrogen, shown by an expansion of their crystal lattices. However, no crystal hydride formation was observed in any of the powders.

Effect of Si and P on the Formation and Crystallization of Ti25Hf50Ni25 Metallic Glasses

ABSTRACTTi255Hf50Ni25 metallic glasses, prepared by rapid quenching, are strongly metastable with a 65°C separation between the glass transition temperature, Tg = 335°C, and the onset temperature for primary crystallization. The deep metastability of the glass is likely due to sluggish nucleation and growth kinetics, limited by the Hf diffusion. The glass crystallizes to a nano-scale microstructure consisting of an icosahedrally symmetric ordered phase. This phase is metastable and transforms to a stable Ti2Ni-type phase with annealing at higher temperatures. The primary crystallization to a metastable icosahedrally-ordered phase suggests that the local structure of the glass contains a high degree of icosahedral short-range order. Here, glass formation and crystallization were studied in alloys containing 2 at.% of Si and P.

Mössbauer spectroscopy of the Zr-rich region in Zr–Nb–Fe alloys with low Nb content

Intermetallic phases and solid solutions in the Zr-rich region of the Zr–Nb–Fe system with low Nb content are studied by Mossbauer spectroscopy complemented with X-ray diffraction, optical and scanning electron microscopy and electron microprobe analysis. The phases found in each sample were those expected from the corresponding binary Zr–Fe system. Furthermore, one of the samples showed a ternary cubic Ti2Ni type phase with a similar stoichiometry to the tetragonal Zr2Fe compound. Mossbauer parameters were suggested to this phase (IS: - 0.12 mm/s, QS: 0.30 mm/s), to the bcc Zr(β) phase (IS: (-0.11 α 0.01) mm/s, QS: (0.23 α 0.02) mm/s), and to the hcp Zr(βT) phase (IS: (-0.24 α 0.02) mm/s, QS: (0.45 α 0.02) mm/s).

Reinvestigation of the oxygen stabilized Ti 2Ni-type phase Ti 4Fe 2O 0.4 by neutron diffraction

The structure of the oxygen-poor cubic η-phase Ti 4Fe 2O 0.4 (filled Ti 2Ni-type, a 0 = 11.3326(5) A ̊ ) was reinvestigated by means of neutron powder diffraction profile refinement ( R ( nuc) = 2.5%). The spacegroup is Fd3m (number 227, O h 7, z = 16, origin at centre ( 3 ̄ m )) with titanium in 48f ( x, 1 8 , 1 8 ), x = 0.936 and 16d ( 1 2 , 1 2 , 1 2 ), iron in 32e ( x, x, x), x = 0.289 and oxygen in 16c (0, 0, 0), occupancy n = 0.407. No indication for a Ti Fe statistical occupation was found nor are the titanium (48f) octahedral voids (8a sites) occupied by oxygen.

On the change in physical properties of Ti4−xFe2 + xOy during hydrogenation I: Activation behaviour of ternary oxides Ti4−xFe2 + xOy and β-Ti

Abstract The data available in the literature on the composition and hydrogenation behaviour of the oxygen-stabilized Ti2Ni-type phase Ti4−xFe2+xOy and on the possible role of this alloy in the activation process of TiFe are contradictory. These data are critically discussed on the basis of a reinvestigation of the metal-rich part of the ternary system Ti-Fe-O and are compared with data obtained for crystalline and powdered material. Bulk specimens of Ti2FeO0.2 with fresh surfaces reacted easily without activation treatment at 373 K and a low hydrogen pressure (8 × 104 Pa) or at 298 K under hydrogen at 4 MPa. The hydrides obtained were kinetically hampered and are only partially reversible at technical operating conditions owing to very low absorption pressure plateaux. An increased oxygen content in addition to the substitution of titanium by iron led to a decrease in the hydrogen uptake. Crystalline β-Ti (20 at.% Fe) required severe activation treatment (773 K and 4 MPa) and decomposed into TiFe and TiH2−x under these conditions. It was observed that in bulk multiphase alloys the activation of Ti2FeO0.2 was still easier than that of β-Ti. However, when powdered material was used, β-Ti formed an f.c.c. hydride “Ti4FeH8.5” at 573 K and 4 MPa. The easy activation of β-Ti powder in contrast with that of the bulk material can be explained by a segregation model of magnetic particles. However, this argument is unsuccessful for Ti2FeO0.2 alloys which became deactivated after crushing.