Ni3Al-based Alloy Research Articles

Unidirectional Crystal Orientation of Dual-Phase Ni3Al-Based Alloy via Laser Irradiation

Dual-phase Ni3Al-based alloys feature extensive applicability even under high-temperature conditions. We selectively modified the microstructure of a representative dual-phase Ni3Al-based alloy from equi-axed grains to unidirectional grains, using a kW-class high-power laser irradiation technique. On employing the laser probe to linearly scan the Ni-9 at.% Al-16 at.% V alloy specimen, the laser-irradiated region was partially molten and then immediately solidified from the two edges of the molten pool toward the center. Laser irradiation under low-speed scanning increased the molten pool width. The grains in the laser-irradiated region extended preferentially from the two edges toward the center; their crystal orientation was similar to adjacent substrate grains, suggesting epitaxial growth. Therefore, the unidirectional orientation of grains could be extended via wide-range scanning using laser irradiation. This suggests that the microstructure of the alloy can be selectively modified to unidirectional orientated grains by optimizing laser irradiation conditions, such as the power density, scanning speed, and scanning paths. The hardness of the laser-irradiated region decreased due to the phase transformation from the ordered Ni3Al and Ni3V phases to the disordered fcc phase. However, the hardness improved to a value comparable to that of the alloy subjected to heat treatment at 980 °C for 1 h.

Interfacial microstructure evolution and mechanical properties of Ni3Al-based alloy TLP joints with BNi-2 interlayer

Ni3Al-based alloys were successfully bonded via transient liquid phase (TLP) bonding technique with a BNi-2 interlayer. The typical joint consisted of three characteristic zones including: athermal solidification zone (ASZ) composed of Ni3B phase, isothermal solidification zone (ISZ) with dual-phase γ+γ′ structure and diffusion affected zone (DAZ) containing borides precipitation. With the increase of bonding temperature or holding time, the ASZ was gradually replaced by the ISZ, and Ni3B with high hardness disappeared. The shear strength of the joint improved from 391 MPa at 1050 °C for 30 min to 712 MPa at 1100 °C for 120 min, and the fracture occurred in the ISZ. To eliminate the borides precipitation in the DAZ of the as-bonded joints, a post-bond heat treatment (PBHT) process was implemented. The microstructure of the ISZ was homogenized and the hardness distribution across the whole joint was uniform. Therefore, the shear strength of the joint bonded at 1100 °C for 120 min increased to 838 MPa after PBHT at 1150 °C for 540 min.

Strain-modulated Ni3Al alloy promotes oxygen evolution reaction

Oxygen evolution reaction (OER) is crucial for electrochemical water splitting. However, corrosive issues in the alkaline environment impede its applications. Here, we explore the corrosion-resistant multiphase Ni3Al-based intermetallic alloy (Ni3Al alloy) as self-supported OER catalysts for the first time. A two-step treatment, annealing at high-temperature followed by acid oxidation is performed to improve the catalytic activity. Further investigations demonstrate that the annealing process can tune the size of γ′ precipitates, which will modify the strain effects caused by γ/γ′ lattice mismatch to boost the OER activity. The oxidation of Ni during the acid pretreatment can facilitate the formation of active high-valent Ni sites during the OER process, and further enhance the OER performance. The optimized electrode exhibits a low overpotential of 280 mV at 10 mA cm−2, a small Tafel slope of 73.5 mV dec−1 and long-term stability of 180 h in 1 M KOH solution. The findings in this paper will inspire the exploration of high stability OER catalysts and the functional application of Ni3Al alloy.

Creep behaviors of multiphase Ni3Al-based intermetallic alloy after 1000 °C-1000 h long-term aging at intermediate temperatures

The microstructure evolutions during 1000 °C-1000 h long-term aging and relevant 800 ~ 840 °C/220 ~ 300 MPa creep behaviors of a novelly designed multiphase Ni3Al-based intermetallic alloy were investigated. Results showed this alloy presented excellent intermediate temperature creep properties even after 1000 °C-1000 h long-term aging. A kind of bimodal R-type γ′ rafting structures were observed in the 800 °C/220 MPa creep process of the long-term aged alloy, and the creep resistance levels of three kinds of γ′ rafting structures were evaluated to be in the order of bimodal R-type γ′ rafting structures > R-type γ′ rafting structure > N-type γ′ rafting structure. The 1000 °C-1000 h long-term aging can promote the inhomogeneous precipitation of Cr4.6MoNi2.1 phase at the edge γ′+γ dendrite area, which can play the role of pinning dislocations and inhibiting the further rafting and coarsening of the R-type γ′ rafts during creep. Differed from Ni-based alloys and other Ni3Al-based alloys, this long-term aged alloy exhibited abnormal four-stage creep characteristics, which consisted of atypical short steady-state creep stage with step fluctuant feature and atypical long accelerated creep stage with creep rate increasing at a constant rate.

A promising anode candidate for rechargeable nickel metal hydride power battery: An A5B19-type La–Sm–Nd–Mg–Ni–Al-based hydrogen storage alloy

Nickel metal hydride rechargeable batteries hold a prominent position in battery-powered electric vehicles market, owing to the noticeable advantages of high-power capability. To promote the utilization of the nickel metal hydride batteries, the anode materials – hydrogen storage alloys, are in the spotlight. Here, we report an A5B19-type La–Sm–Nd–Mg–Ni–Al-based alloy with superior rate performance. The alloy is specifically composed of Pr5Co19-and Ce5Co19-type structures after annealing during which the Ce5Co19-type structure transforms to Pr5Co19-type structure within 900–995 °C. We find that the alloy exhibits desirable rate performance as possessing more Ce5Co19-type structure that the Ce5Co19-type alloy (900 °C) delivers a high discharge capacity of 277.4 mAh g−1 even at 1500 mA g−1 which is 72.4 mAh g−1 higher than the Pr5Co19-type alloy (995 °C). Further exploration reveals the fascinating high rate dischargeability benefitting from the kinetics including the fast charge transfer and speedy hydrogen diffusion. Differing from the pattern of rate performance, the cycling stability of the alloy is enhanced by the growing Pr5Co19-type structure whose capacity retention rate is 89.4% at the 100th cycle. The work will encourage the progress of the nickel metal hydride batteries with superior rate performance and long cycling life.

Solidification and the Structure-Phase State of Ni3Al–Ni–NiAl Alloys with Chromium, Molybdenum, Tungsten, Rhenium, and Cobalt

The ternary Ni–Al–AE phase diagrams, where AEs are the main alloying elements of high-temperature cast (γ' + γ) γ'-Ni3Al-based alloys, are analyzed. Principal differences between the solidification schemes in the vicinity of the γ'-Ni3Al and β-NiAl phases and the differences in the characters of the influence of the main AEs at their contents to 10 at % on the phase composition and the structure of multicomponent γ'-Ni3Al-based VKNA-1V, VKNA-25, VKNA-4U, and IM730 alloys are detcted. The structural base of all the alloys are (γ' + γ) dendrites. In the first-type cobalt-free VKNA1V alloys, as well as in IC221M, IC218, IC438, and IC6SX alloys, coarse single-phase γ'-Ni3Al precipitates, which are the degenerate eutectic L$$ \rightleftarrows $$ (γ' + γ) + γ', form in the interdendritic space. In the second-type cobalt-containing alloys at the same (~17 at %) aluminum content (VKNA-4U, VKNA-25) or in the chromium-containing cobalt-free alloys with an increased (~21 at %) aluminum content (IM730), the following two-phase precipitates form: the nonequilibrium β-NiAl phase, which represents the degenerate L$$ \rightleftarrows $$ (γ' + β) + β eutectic, forms inside γ'-Ni3Al. The following solid-phase transformations are found to develop intensely in the second-type alloys on heating in the temperature range 1200–1300°C: the dissolution of the β-phase precipitates in the primary γ'-Ni3Al precipitates, the dissolution or a weakening of the interdendritic γ'-phase precipitates, and a change in the morphology and orientations of the γ-phase precipitates in dendrites and the interdendritic space.

The influence of temperature on the mechanical properties of polycrystalline and single-crystalline Ni3Al

Because of the excellent mechanical property at high temperature, single-crystalline Ni3Al-based alloys, which belong to Ni-based superalloys, are widely adopted in the fields of aerospace, aviation and military. Usually, single-crystalline Ni-based alloys are bonded or welded together by polycrystalline Ni-based alloys. In this study, the Young’s modulus, shear modulus, bulk modulus, and Poisson ratio of polycrystalline Ni3Al at the temperature ranging from 10 °C to 850 °C were calculated according to Voigt–Reuss approximation, and the relationships between the mechanical properties and the temperatures were clarified. Moreover, the influence of temperature on the universal anisotropy index and mechanical properties of single-crystalline Ni3Al was evaluated. Results show that, except for Poisson ratio, the increase of temperature linearly decreases the mechanical properties of polycrystalline Ni3Al, while it increases the universal anisotropy index of the single-crystalline Ni3Al. Moreover, different with the bulk modulus, the Young’s modulus and shear modulus of single-crystalline Ni3Al are obvious anisotropic, and their anisotropic characteristics are quite opposite. The increasing temperature decreases both maximum and minimum values of Young’s modulus, shear modulus, and bulk modulus of single-crystalline Ni3Al.

Influence of diffusion processes on structure of brazed joints and brazing filler metal after spreading over Ni3Al-based alloy

This paper presents the results of investigations on wetting with Pd-Ni-Cr(Me)-Ge system brazing filler metal of pure nickel, high-temperature alloy based on nickel aluminide (Ni3Al). Micro-X-ray spectrum examinations showed that spread of Pd-Ni-Cr(Ме)-Ge system brazing filler metal over pure nickel substrate, which has no aluminum, promoted crystallization of brazing filler metal in the form of a solid solution of palladium-nickel and single disperse inclusions of (PdNiCr)-14,13Ge phase. This inclusion precipitates along the boundaries of solid solution grains and can be the reason for strengthening phase. In the spread of this alloy over Ni3Al (Ni-Al-Mo-В)-based high-temperature alloy, containing around 10 wt.% of aluminum, there is another composition and morphological structure of brazing filler metal after crystallization. Thus, the solid solution crystallizes on the base metal substrate. Phase precipitations based on palladium (60.59%) saturated with aluminum (12.3%) is formed in it. In brazing of Ni3Al (Ni-10Al-14Mo-В)-based alloy using this brazing filler metal, the high concentration of aluminum in base metal and the presence of concentration gradient at interface result in active diffusion of the latter from base metal in the brazed seam. Diffusion processes lead to the substitution of germanium on aluminum in (PdNiCr)xGey phase. It results in the formation of (PdNiCr)xAly intermetallic phase, the concentration of aluminum which corresponds to the concentration of aluminum in base metal and it precipitates in a central zone of the brazed seam and leads to brittle fracture of the brazed joints. The use of the brazing filler metal on the Ni-Cr-Al-Me for brazing of nickel aluminide leads to the formation of a structure without intermetallic phases and eutectics in the central zone of the brazed seam.

Structure and Electrochemical Hydrogen Storage Properties of as-Milled Mg–Ce–Ni–Al-Based Alloys

At room temperature, crystalline Mg-based alloys, including Mg2Ni, MgNi, REMg12 and La2Mg17, have been proved with weak electrochemical hydrogen storage performances. For improving their electrochemical property, the Mg is partially substituted by Ce in Mg–Ni-based alloys and the surface modification treatment is performed by mechanical coating Ni. Mechanical milling is utilized to synthesize the amorphous and nanocrystalline Mg1−xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50 wt%Ni hydrogen storage alloys. The effects made by Ce substitution and mechanical milling on the electrochemical hydrogen storage property and structure have been analyzed. It shows that the as-milled alloys electrochemically absorb and desorb hydrogen well at room temperature. The as-milled alloys, without any activation, can reach their maximal discharge capacities during first cycling. The maximal value of the 30-h-milled alloy depending on Ce content is 578.4 mAh/g, while that of the x = 0.08 alloy always grows when prolonging milling duration. The maximal discharge capacity augments from 337.4 to 521.2 mAh/g when milling duration grows from 5 to 30 h. The cycle stability grows with increasing Ce content and milling duration. Concretely, the S100 value augments from 55 to 82% for the alloy milled for 30 h with Ce content rising from 0 to 0.08 and from 66 to 82% when milling the x = 0.08 alloy mechanically from 5 to 30 h. The alloys’ electrochemical dynamics parameters were measured as well which have maximum values depending on Ce content and keep growing up with milling duration extending.

Diffusion bonding of Ni3Al-based alloy using a Ni interlayer

Ni3Al-based alloys were direct diffusion bonded to themselves at 950–1100 °C for 10–60min under a pressure of 20 MPa. The effects of the joining parameters on shear strength and interface bonding ratio of the direct diffusion bonded joints were studied in detail. The maximum joint strength achieved was 689 MPa, when the joint was bonded at 1100 °C for 60min under 20 MPa, corresponding to an interface bonding ratio of 95%. However, the fracture surface of the bonded joint was characterized mainly by cleavage fracture, indicating that the fracture mode was brittle rupture. The formation mechanism of the direct diffusion bonded joint and the coarsening phenomenon of γ’ precipitates in the Ni3Al-based alloy were revealed. To inhibit the coarsening of γ’ caused by high bonding temperature, a Ni interlayer was introduced. When bonded at 1050 °C with a 30 μm thick Ni foil, the pure Ni interlayer completely vanished because of interdiffusion with the Ni3Al-based alloy; however, the performance of the joint was still inferior to the base metal because of the heterogeneity in the microstructure of the diffusion zone. Therefore, the fracture occurred mainly at the diffusion zone. When bonded at 1050 °C with a 3 μm thick electroplated Ni coating, the microstructure of the diffusion zone was consistent with that of the base metal because of sufficient interdiffusion. The shear strength basically reached the level of the direct diffusion bonded joint at 1100 °C. The fracture morphology changed from cleavage fracture to alternating dimples and facets after introducing the electroplated Ni coating.

Effect of the Deformation during Pressure Welding of a Wrought EP975 Nickel Alloy and a Single-Crystal Intermetallic VKNA-25 Alloy on the Structure and Properties of the Welded Joints

This work is a continuation of the studies of the possibility of formation of a one-piece blisk unit for gas turbine engines by solid-phase pressure welding (PW) of a single-crystal [001] Ni3Al-based VKNA-type blade alloy with a wrought EP975 disk nickel superalloy. The influence of the PW temperature at a strain of 10 and 24% on the structure and mechanical properties of welded specimens at room temperature is investigated. An increase in the process temperature by 50°C to 1175°C is shown not to change the structure of the VKNA-25 alloy, and the γ-phase grain sizes in the EP975 alloy increase from 7–8 to 15–18 μm. During tensile tests, the welded specimens fail through the VKNA-25 alloy, and the strength of the welded specimens fabricated at 1175°C is almost twice as high as that of the specimens fabricated by PW at 1125°C at the same strain of the EP975 alloy. Electron backscatter diffraction (EBSD) analysis of the solid-phase bonding zone demonstrates that the VKNA-25 alloy retains its single-crystal structure.

Precipitation of intersected plate-like γ′ phase in β and its effect on creep behavior of multiphase Ni3Al-based intermetallic alloy

The precipitation and evolution of intersected plate-like γ′ (γ′P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni3Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′P precipitates have the Bain orientation relationship (OR) of (100)γ′P//(100)β and [0-11]γ′P//[001]β with the interdendritic β-matrix. The precipitation of intersected γ′P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure.

Stress Concentration Induced by the Crystal Orientation in the Transient-Liquid-Phase Bonded Joint of Single-Crystalline Ni3Al

Single-crystalline Ni3Al-based superalloys have been widely used in aviation, aerospace, and military fields because of their excellent mechanical properties, especially at extremely high temperatures. Usually, single-crystalline Ni3Al-based superalloys are welded together by a Ni3Al-based polycrystalline alloy via transient liquid phase (TLP) bonding. In this study, the elastic constants of single-crystalline Ni3Al were calculated via density functional theory (DFT) and the elastic modulus, shear modulus, and Poisson’s ratio of the polycrystalline Ni3Al were evaluated by the Voigt–Reuss approximation method. The results are in good agreement with previously reported experimental values. Based on the calculated mechanical properties of single-crystalline and polycrystalline Ni3Al, three-dimensional finite element analysis (FEA) was used to characterize the mechanical behavior of the TLP bonded joint of single-crystalline Ni3Al. The simulation results reveal obvious stress concentration in the joint because of the different states of crystal orientation between single crystals and polycrystals, which may induce failure in the polycrystalline Ni3Al and weaken the mechanical strength of the TLP bonded joint. Furthermore, results also show that the decrease in the elastic modulus of the intermediate layer (i.e., polycrystalline Ni3Al) can relieve the stress concentration and improve the mechanical strength in the TLP bonded joint.

Structure and electrochemical hydrogen storage behaviors of Mg–Ce–Ni–Al-based alloys prepared by mechanical milling

The influences of milling time and Ce content on the electrochemical property and microstructure of as-milled Mg1−xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50 wt%Ni alloys were investigated systematically. The as-milled alloys have an outstanding activation property. The cycle stability conspicuously grows up with milling time and Ce proportion increasing. The capacity retention rate at 100th cycle of x = 0.02 alloy augments from 47% to 63% when prolonging milling time from 5 to 30 h and it grows from 55% to 82% for the 30 h milled alloy with Ce content growing from 0 to 0.08. The discharge capacity of x = 0.02 alloy grows up invariably with milling time prolonging, while that of the 30 h milled alloys has the maximal value of 578.4 mAh/g with Ce content increasing. Moreover, the electrochemical kinetic properties of alloys significantly improve with milling duration extending, while they have the maximal values with Ce proportion varying.

Effect of initial microstructure on the hot deformation behavior of a Ni3Al-based alloy

To investigate the effect of initial microstructure on the hot deformation behavior of a Ni3Al-based alloy, the isothermal compression tests were conducted on the studied alloy with two different microstructures at 1100 °C and 1200 °C with strain rate range of 0.01–1 s−1. The results show that the hot deformation behavior of the Ni3Al-based alloy is sensitive to the initial microstructure, particularly under low deformation temperature and high strain rate. The flow curves signified that the peak stress and the magnitude of the stress drop for the alloy with coarse γ′ precipitates are higher than those for the alloy with fine γ′ precipitates at a given temperature and strain rate. Besides, it is found that the critical conditions are affected by deformation temperature, strain rate and initial microstructure. Both the critical stress and strain of dynamic recrystallization significantly decreases with the increase of strain rate and decrease of deformation temperature. For different initial microstructures, the critical stress and strain for the alloy with coarse γ′ precipitates are higher than that for the alloy with fine γ′ precipitates. The critical ratios of the studied alloy were also determined. The dissolution of coarse γ′ precipitates easily occurs and the dissolution rate can be accelerated at the sufficiently high temperature, which leads to that the effect of initial microstructure on hot deformation behavior of the Ni3Al-based alloy is weakened.

Formation and widening mechanisms of envelope structure and its effect on creep behavior of a multiphase Ni3Al-based intermetallic alloy

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni3Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni3Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni3Al phase with ordered face-centered L12 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101]γ'-envelope//[01‾1]dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L12 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni3Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr23C6 carbides surrounded by the widened γ′-envelope.

Effect of Temperature on the Low-Cycle Fatigue Characteristics of Single Crystals Made of an Ni3Al-Based Rhenium-Containing Intermetallic Alloy

Abstract—The low-cycle fatigue (LCF) of the single crystals made of a γ'-Ni3Al-based VKNA-25 γ' + γ nickel superalloy is studied at 104 cycles and a temperature of 20, 750, and 900°C. A temperature anomaly in the LCF fatigue limit of the VKNA alloy, namely, its increase with temperature in the range from 20 to 750°C, is detected. This anomaly is related to a change from octahedral to cubic slip at the temperature of the peak in L12-ordered alloys and to a change in the properties of the γ' and γ phases. At normal temperature, the static tensile strength and the LCF characteristics of the [111] single crystals are higher than those of the single crystals with other crystallographic orientations (CGOs). The static tensile strengths of the [001] and [111] single crystals at 900°C and the strengths of the single crystals with all CGOs at 1000°C are almost the same, and the LCF fatigue limit of the [001] single crystals at 900°C is slightly higher than that of the [111] single crystals.

HIGH TEMPERATURE STRENGTH OF COMPOSITE MATERIAL WITH CELL STRUCTURE ON THE BASIS OF Ni<sub>3</sub>Al INTERMETALLIDIDE

The powder metallurgy method was used to obtain materials in the form of a single-phase alloy based on Ni3Al and in the form of composite material (Ni3Al + W) with cell structure based on it. The structural unit of the composite material was a round granule (grain) with average size of 25 μm from nickel alloy, on which the continuous tungsten coating with thickness of ~0.4 μm was deposited by chemical vapor deposition. Compression tests at room temperature have shown that the yield stress of composite material (Ni3Al + W) with cell structure at temperatures of 20 – 1000 °C is higher than of single-phase Ni3Al-based alloy (up to 1.7 times), but at higher test temperature the yield strength of the composite is compared with the yield strength of the nickel alloy. The specific yield strength (that is, normalized for the density of 7.8 g/cm3 for the alloy and of 9.5 g/cm3 for the compo site) behaves similarly. At the temperature of 1300 °C, single-phase Ni3Al-based alloy exhibits solid-liquid behavior under compression. Creep tests were carried out for compression under vacuum at temperatures of 1000 – 1200 °C. Using the pair and parametric methods of mathematical analysis of creep processes according to Hollomon, regression equations of creep rate, stress and temperature of the test were obtained. The ultimate strength of creep for the given tolerances for steady-state creep rate and inverse values were calculated. It is shown that at all test temperatures the composite material has lower creep rate (up to 7 times) and higher ultimate strength of creep (up to 2.5 times) than the nickel alloy on which it is based. Creep activation energies of the materials studied are determined using the exponential law of coupling of experimental values. The creep activation energy for the nickel alloy found is close to the activation energy of nickel self-diffusion in Ni3Al and materials based on it (230 ÷ 310 kJ/mol), and for the composite – to self-diffusion activation energy of tungsten (503 kJ/mol).

Effects of milling duration on electrochemical hydrogen storage behavior of as-milled Mg–Ce–Ni–Al-based alloys for use in Ni-metal hydride batteries

Mg-based hydrogen storage alloys are regarded as ideal materials for use as the negative electrode in nickel-metal hydride batteries. However, their poor electrochemical de-/hydriding performance at room temperature is a problem. Methods comprising the partial substitution with Ce for Mg and surface modification by mechanical coating with Ni have been applied to address the problem. In this study, nanocrystalline and amorphous Mg1-xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50 wt%Ni (named Mg1-xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50Ni) alloys were synthesized by mechanical milling. Electrochemical testing showed that the samples exhibited outstanding electrochemical de-/hydriding performance at room temperature. The maximum discharge capacities were obtained after only one de-/hydriding cycle and without any need for activation. The discharge capacity and cycle stability increased with the milling duration. In particular, for the alloy where x = 0.04, the discharge capacity increased from 378.8 to 578.4 mAh/g, and the capacity retention rate after the 100th cycle increased from 51% to 69% as the milling duration increased from 5 to 30 h. Furthermore, measurements based on the high rate discharge ability, electrochemical impedance spectra, potentiodynamic polarization curves, and potential-step measurements demonstrated that the electrochemical kinetic properties of the alloys could be improved by extending the duration of milling.

Microstructure and mechanical properties of Ni3Al-based alloy joint transient liquid phase bonded using Ni/Ti interlayer

Ni3Al-based alloy joints were fabricated by transient liquid phase (TLP) bonding with Ni/Ti composite interlayer. The typical interfacial microstructure of the bonded joint was primarily composed of three zones: isothermal solidification zone, athermal solidification zone and base metal. Various reaction products were observed in the bonded joint, including γ′-Ni3(Al, Ti), TiNi3, Cr-Mo-Fe rich precipitates and β-Ti. The TiNi3 phase in the athermal solidification zone was gradually reduced and eventually vanished by increasing the bonding parameters. The maximum room temperature shear strength of 828 MPa was obtained when the joint was bonded at 1220 °C for 60 min, while it was 625 MPa and 533 MPa at 650 °C and 800 °C, respectively. Cracks propagated primarily along the Ni3Al base metal and partially at the bonding interface when the joint was bonded at 1220 °C for 60 min, indicating the shear strength of the joint was almost equal to that of the base metal.