Experimental investigation on the phase equilibria of the Mn–Ni–Zn system at 400 °C

Partial isothermal section of the Mn–Ni–Zn system at 400 °C was experimentally established by means of XRD and SEM/EDS techniques. Three ternary compounds, i.e. T, τ₁ and τ₂, were found to exist at 400 °C for the first time. The compound T having an approximate formula of Mn₇Ni₇Zn₈₆ was indexed as fcc structure with a lattice parameter of a = 1.81476 (1) nm. τ₁, the structure of which is unknown, has an approximately stoichiometric composition of about 29 at.% Mn, 38 at.% Ni and balanced Zn. τ₂ has fcc structure and a composition range of about 46–40 at.% Mn and constant 30 at.% Zn. Extended single-phase regions of the phases Mn₅Zn₂₁, Ni₂Zn₁₁, NiZn₃, NiZn and β-Mn were observed. The maximum solubility of Ni in Mn₅Zn₂₁ and those of Mn in Ni₂Zn₁₁, NiZn₃ and NiZn were determined to be 10, 6, 26 and 25 at.% at 400 °C, respectively.     

The ductility and shape-memory properties of Ni–Mn–Co–Ga high-temperature shape-memory alloys

Ni–Mn–Co–Ga alloys with Ni/Mn or Ni and Mn substituted by Co were investigated as candidates for high-temperature shape-memory alloys. Ni_56?xCo_xMn₂₅Ga₁₉ alloys with x < 8 consist of single phase martensite, whereas Ni_56?xCo_xMn₂₅Ga₁₉ (x ? 8), Ni₅₆Mn_25?yCo_yGa₁₉ (y = 4, 8) and Ni_56?z/2Mn_25?z/2Co_zGa₁₉ (z = 4, 6) alloys consist of a two-phase mixture of martensite and γ phase. The mechanical and shape-memory properties of Ni₅₆Mn_25?yCo_yGa₁₉ and Ni_56?z/2Mn_25?z/2Co_zGa₁₉ alloys, which were hot-rolled into 0.5 mm thin plates by conventional hot rolling process, were investigated. The ductility and hot-workability of Ni–Mn–Co–Ga alloys were greatly improved by increasing the amount of ductile γ phase. Dynamic tensile tests and scanning electron microscopy observations of fracture surfaces confirm that the existence of γ phase plays a key role in improving the ductility of Ni–Mn–Co–Ga alloys.     

Effects of amorphous Co–C on the structural and electrochemical characteristics of La0.8Mg0.2Ni0.8Mn0.1Co0.5Al0.1 hydrogen storage alloy

Amorphous Co–C powder prepared by ball milling was introduced to improve the performance of La_0.8Mg_0.2Ni_0.8Mn_0.1Co_0.5Al_0.1 hydrogen storage alloy. The structural and electrochemical properties of the as-prepared La_0.8Mg_0.2Ni_0.8Mn_0.1Co_0.5Al_0.1–x wt.% Co–C composites were investigated systematically. Scanning electron microscopic images show La_0.8Mg_0.2Ni_0.8Mn_0.1Co_0.5Al_0.1 alloy was coated by Co–C particles. X-ray diffraction patterns suggest that the composite almost remained original phase structures of La_0.8Mg_0.2Ni_0.8Mn_0.1Co_0.5Al_0.1 and Co–C in both charge and discharge processes. The maximum discharge capacity of the composites reached 414 mAh g^?1 at a current density of 50 mA g^?1 at 298 K. The cyclic stability and the discharge capacities of the composite electrodes were noticeably improved in comparison with single La–Mg–Ni-based alloy due to increased corrosion resistance and the catalysis of the Co–C powder. Cyclic voltammogram and potentiodynamic polarization studies on the composite indicate that the electrochemical kinetics was improved and the corrosion resistance was increased. The cycling performance of the composite electrode at high current density is good as well.     

Mg–Ni–Gd–Ag bulk metallic glass with improved glass-forming ability and mechanical properties

The improvement of glass-forming ability (GFA) and mechanical properties by using Ag to substitute Mg in the Mg–Ni–Gd bulk metallic glass (BMG) were studied. The Mg₆₉Ni₁₅Gd₁₀Ag₆ bulk metallic glass (BMG) could be cast into glassy rod up to 7 mm. The activation energies of Mg₆₉Ni₁₅Gd₁₀Ag₆ metallic glass were calculated by the Kissinger’s method to be E_g = 2.24 eV, E_x = 1.65 eV, E_p1 = 1.36 eV, E_p2 = 1.59 eV, E_p3 = 1.26 eV and E_p4 = 1.99 eV. The compressive fracture strength and the plastic strain of Mg₆₉Ni₁₅Gd₁₀Ag₆ BMG reached 846 MPa and 0.37% respectively.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.     

Study on the binary intermetallic compounds in the Mg–Ce system

Electron-probe microanalysis (EPMA) and X-ray diffraction (XRD) studies conducted on Mg–38 wt%Ce and Mg–66 wt%Ce alloys demonstrated the existence of two distinct intermetallic phases, Mg_3.6Ce, Mg₃Ce, with the compositions Mg_3.6–3.7Ce, Mg_3.0–3.2Ce, respectively. XRD indicates that Mg_3.6Ce is likely a defect-vacancy structure of Mg₃Ce. The μ-Mg₃Ce phase, with the Mg_3.0–3.3Ce composition and a possible orthorhombic structure, has also been discovered, which is considered a metastable high-temperature form of the Mg₃Ce phase. Based on these results a version of the phase diagram is suggested for the Mg–Ce system in the composition range of 38–70 wt%Ce which correlates well with the solidification microstructures and phases of the two alloys.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.     

Structure and yield strength of directionally solidified Ni3Al intermetallic premelted with MoSi2 phase

Ni₃Al and MoSi₂ intermetallic phases were arc melted in Ni₃Al/MoSi₂ molar proportion of ten. Pure binary Ni₃Al and the quaternary alloy were subjected to directional solidification using the floating zone method at growth rates of 10–50 mm h⁻¹. The phase composition and structure of the crystals were analyzed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. The yield strength was studied in compression at 293–1293 K and in tension at room temperature. Compared to the binary Ni₃Al crystals, the quaternary Ni–Al–Mo–Si crystals revealed up to 5 times higher compressive yield strength at 400–800 K. Ni–Mo–Si C14 Laves phase precipitation in the L1₂ Ni₃Al+L1₀ NiAl matrix duplex phase was found in quaternary crystals. This precipitation is assumed to cause the observed mechanical behaviour.     

H2-induced environmental embrittlement in ordered and disordered Ni3Fe: An electronic structure approach

The different behaviors in H₂-induced environmental embrittlement in ordered and disordered Ni₃Fe are associated with differences in their electronic structures. The experimental study on electronic structures of ordered and disordered Ni₃Fe has been carried out by electron energy-loss spectroscopy (EELS). The onset energy of Ni L_2,3 edges from ordered phase is 0.3 eV lower than that from disordered phase, while the 3d occupancy of Ni atoms in ordered phase is 0.07 electrons/atom less than that in disordered phase. Severe H₂-induced environmental embrittlement in ordered phase is attributed to rather negative dissociative adsorption energy of hydrogen at surfaces, which arises from upward shifting of the valence band center of Ni.     

Martensite stabilization and thermal cycling stability of two-phase NiMnGa-based high-temperature shape memory alloys

The martensite stabilization and thermal cycling stability of four types of two-phase NiMnGa-based high-temperature shape memory alloy, including Ni_56+xMn₂₅Ga_19?x (x = 0, 1, 2, 3, 4), Ni₅₆Mn_25?yFe_yGa₁₉ (y = 4, 8, 9, 12, 16), Ni₅₆Mn_25?zCo_zGa₁₉ (z = 4, 6, 8) and Ni₅₆Mn_25?wCu_wGa₁₉ (w = 2, 4, 8) alloys, were investigated. It is found that the martensite stabilization is closely related to the strength of the alloy and the volume fraction of γ phase; and increases as the alloy strength decreases. It is also found that in Ni₅₆Mn_25?yFe_yGa₁₉ alloys, with increasing Fe content to 12 and 16 at.%, the volume fraction of γ phase increases and the martensite stabilization decreases. The thermal cycling stability differs among different alloy systems and is related to the microstructural changes during thermal cycling and to the strength of the γ phase. Poor thermal cycling stability is observed in Ni_56+xMn₂₅Ga_19?x (x > 0), Ni₅₆Mn_25?zCo_zGa₁₉ and Ni₅₆Mn_25?wCu_wGa₁₉ alloys due to the formation of the ordered γ′ phase and the high strength of the γ phase. Results further show that Fe addition to Ni₅₆Mn₂₅Ga₁₉ alloy can broaden the (bcc + γ) two-phase region and shift it to the Ni–Ga and Ni–Mn sides, hence stabilizing the two-phase region to lower temperatures. These effects can retard the formation of the ordered γ′ phase in the Ni₅₆Mn_25?yFe_yGa₁₉ system during thermal cycling, thus leading to good thermal cycling stability.     

Hydrogen diffusion in Mg–H and Mg–Ni–H alloys

Coefficients of hydrogen diffusion in Mg₂NiH₄ (D_I), MgH₂ (D_M), and in (Mg + Mg₂Ni)−H eutectic (D_E) were measured in the temperature interval 449–723 K. Experimental material was prepared by two techniques: by melting and casting and by ball-milling and compacting into pellets. Hydrogen charging was carried out from the hydrogen gas phase at high temperature and pressure. The Mg₂NiH₄ pellets were prepared in different regimes resulting either in a structure with a high fraction or with a low fraction of twinned low-temperature phase LT2. It was found that the LT2 slows down the hydrogen desorption rate considerably – values of D_I in low-temperature untwinned phase LT1 are by a multiplication factor of about 20 higher than those in the twinned phase LT2. Obtained values of D_M are significantly lower than the literature values reported for the pure Mg. Hydrogen diffusion coefficients in interphase boundary were estimated from D_E.     

Structure of a new ternary compound with high magnesium content,so-called Gd13Ni9Mg78

The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h^?1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h^?1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a¹ and b¹ axis with vectors of modulation q1 = 0.42a¹ and q2 = 0.42b¹ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.     

The crystal structure of the β′ phase in Al–Mg–Si alloys

The crystal structure of β′, one of the metastable phases formed during precipitation hardening of Al–Mg–Si (6xxx) alloys, has been determined using electron diffraction (ED). With the aid of high-resolution electron microscopyimages and the composition estimated from energy dispersive X-ray analysis, an initial model for the structure of the β′ phase was obtained. The data from digitally recorded ED patterns were then used for a least-squares refinement of the atomic parameters of the model with a software program package developed in Delft (MSLS), taking into account dynamic scattering. The β′ structure has a composition of Mg₉Si₅. The unit cell is hexagonal, space group P6₃/m, with unit cell parameters a = 0.715 nm and c = 1.215 nm. The composition and structure were confirmed by ab initio calculations.     

Phase transformations in mechanically milled and annealed single-phase β-Al3Mg2

Mechanical deformation drastically affects both microstructure and thermal stability of polycrystalline β-Al₃Mg₂. Upon milling, the β-Al₃Mg₂ phase transforms into a nanoscale supersaturated Al(Mg) solid solution with 40 at.% Mg. Upon heating, the milled powders display a complex thermal behavior characterized by four distinct exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperatures, the β′-phase, a hexagonal phase with composition Al₃Mg₂, is formed and no traces of the solid solution can be detected, indicating that the solid solution is metastable and transforms into more stable phase(s). Finally, the subsequent exothermic events reveal the formation and growth of the β-Al₃Mg₂ phase.     

Microstructure and remarkably improved hydrogen storage properties of Mg2Ni alloys doped with metal elements of Al,Mn and Ti

Mg₂Ni_0.7M_0.3 (M=Al, Mn and Ti) alloys were prepared by solid phase sintering process. The phases and microstructure of the alloys were systematically characterized by XRD, SEM and STEM. It was found that Mg₃MNi₂ intermetallic compounds formed in Mg₂Ni_0.7M_0.3 alloys and coexisted with Mg and Mg₂Ni, and that radius of M atoms closer to that of Mg atom was more beneficial to the formation of Mg₃MNi₂. The hydrogen storage properties and corrosion resistance of Mg₂Ni_0.7M_0.3 alloys were investigated through Sievert and Tafel methods. Mg₂Ni_0.7M_0.3 alloys exhibited remarkably improved hydrogen absorption and desorption properties. Significantly reduced apparent dehydriding activation energy values of ?46.12, ?59.16 and ?73.15 kJ/mol were achieved for Mg₂Ni_0.7Al_0.3, Mg₂Ni_0.7Mn_0.3 and Mg₂Ni_0.7Ti_0.3 alloys, respectively. The corrosion potential of Mg₂Ni_0.7M_0.3 alloys shifted to the positive position compared with Mg₂Ni alloy, e.g. there was a corrosion potential difference of 0.110 V between Mg₂Ni_0.7Al_0.3 alloy (?0.529 V) and Mg₂Ni (?0.639 V), showing improved anti-corrosion properties by the addition of Al, Mn and Ti.     

Bredigite-structure Ca14Mg2[SiO4]8:Eu2+,Mn2+: A tunable green–red-emitting phosphor with efficient energy transfer for solid-state lighting

A single-phase green–red-emitting phosphor, Ca_13.7Eu_0.3Mg_2?xMn_x[SiO₄]₈ (CMS:Eu²⁺,Mn²⁺), was prepared by a solid-state reaction, and its energy transfer from Eu²⁺ to Mn²⁺ was investigated as a function of Mn²⁺ concentration. To explore the substitution of an Mn²⁺ site for each Mg site, a determination of the number of Mg substitutional sites was carried out using the Rietveld refinement method and bond valence sums. The dipole–dipole interaction was a dominant energy transfer mechanism of the electric multipolar character of CMS:Eu²⁺,Mn²⁺. The critical distance was calculated as 7.5 Å when using critical concentrations of Eu²⁺ and Mn²⁺. When CMS:Eu²⁺,Mn²⁺ was incorporated with an encapsulant in ultraviolet (λ_max = 400 nm) light-emitting diodes (LEDs), white light with a color rendering index of 67 under a forward bias current of 20 mA was obtained. The results of this work indicate that CMS:Eu²⁺,Mn²⁺ could be applicable to a single-phase phosphor for white LEDs under a near-ultraviolet source.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Hydrogenography of MgyNi1−yHx gradient thin films: Interplay between the thermodynamics and kinetics of hydrogenation

We monitor the hydrogenation of PTFE/Pd-capped Mg_yNi_1?yH_x gradient thin films using the change in optical transmission as a function of time, temperature and hydrogen pressure, to study the relation between kinetics and thermodynamics of hydrogenation of this multiphase hydride system. The interplay between kinetics and thermodynamics is used to extrapolate the hydrogenation equilibrium pressures via the H-absorption rate. Pressure–optical-transmission–isotherms determined independently at different temperatures provide a cross-check for the equilibrium pressure and the enthalpy of H-absorption. We find that the hydrogenation reaction is destabilized with respect to the Mg₂Ni → Mg₂NiH₄ reaction for Mg fractions close to the Mg₂Ni–Mg eutectic point. From a comparison with calculated enthalpies obtained from density functional theory, we conclude that the experimentally observed destabilization originates from the well-mixed microstructure around the eutectic point.     

The Y–Cu–Mg system in the 0–66.7 at.% Cu concentration range: The isothermal section at 400 °C

Synthesis and characterization of about fifty alloys were performed in order to construct the isothermal section of the Y–Cu–Mg ternary system at 400 °C in the 0–66.7 at.% Cu concentration range. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction (XRPD) techniques were used to examine microstructures, identify phases and define their compositions and crystal structures. Phase equilibria in the investigated compositional region are characterized by the absence of extended ternary solid solutions and by the presence of at least ten ternary phases. Crystal structures of the previously reported Y₂Cu₂Mg, Y₅Cu₅Mg₈, Y₅Cu₅Mg₁₃, Y₅Cu₅Mg₁₆ and YCuMg₄ phases were confirmed. A ternary phase with homogeneity range around the YCu₄Mg stoichiometry was found, crystallizing in the cF24--MgCu₄Sn structure type; at 400 °C this phase coexists with a ternary solid solution based on the binary Laves phase Cu₂Mg, which dissolves about 5 at.% Y. The equiatomic YCuMg phase was also found to exist: from the analysis of X-ray powder patterns it is suggested to crystallize in the hP9--ZrNiAl structure type (a = 0.74449(4) nm, c = 0.39953(2) nm). Two other stoichiometric ternary phases were detected, of approximate compositions Y₂₅Cu₁₈Mg₅₇ and Y₁₃Cu₉Mg₇₈, whose crystal structures are still unknown. In the Mg-rich region, a ternary phase forms characterized by a large homogeneity region.