Magnesium-rare earth phase diagrams: Experimental investigation of the Ho-Mg system

The Ho-Mg phase diagram was determined in the range 0 to 100 at.% Mg by differential thermal analysis (DIA), X-ray powder diffraction, optical and scanning electron microscopy, and electron probe microanalysis (EPMA). Four intermediate phases were found to exist, and their crystal structures were confirmed or determined as the following: (1) β phase, Ho₂Mg, cubic,cI2, W type, peritectic formation 1170 °C; (2) HoMg, cubic,cP2, CsCl type, peritectic formation 845 °C; (3) HoMg2, hexagonal,hP12, MgZn₂ type, peritectic formation 695 °C; and (4) Ho₅Mg₂₄, cubic, cI58, α Mn-type, peritectic formation 600 °C. The β phase undergoes a eutectoidal decomposition at 685 °C and 29.0 at.% Mg. A eutectic reaction was observed to occur at 555 °C and 90.0 at.% Mg. The data obtained in this study are compared with those of other previously studied RE-Mg phases and briefly discussed. “Mischmetal” is the tradename of an inter-rare earth alloy ideally containing: 27 at.% La, 48 at.% Ce, 5 at.% Pr, 16 at.% Nd, and 4 at.% other rare earths. This composition is close to that of a typical rareearth ore.     

The phase diagram of the terbium–gold alloy system

The terbium-gold phase diagram has been investigated in the 0–100 at% Au field by differential thermal analysis (DTA), X-ray diffractometry (XRD), optical microscopy (LOM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Eight intermetallic phases were found, namely: Tb₂Au orthorhombic oP12–Co₂Si, peritectic decomposition at 1000°C, TbAu, L.T. form, orthorhombic oC8–CrB type and H.T. form, cubic cP2–CsCl type, congruent melting at 1590°C, Tb₃Au₄ trigonal hR42–Pu₃Pd₄ type, peritectic decomposition at 1340°C, Tb₇Au₁₀ tetragonal tI136–Gd₇Au₁₀ type, peritectic decomposition at 1210°C, TbAu₂ tetragonal tI6–MoSi₂ type, congruent melting at 1265°C, TbAu₃ orthorhombic oP8–TiCu₃ type, congruent melting at 1215°C, Tb₁₄Au₅₁ hexagonal hP65–Gd₁₄Ag₅₁ type, peritectic decomposition at 1175°C, and TbAu₆ tetragonal tP56–SmAu₆ type, peritectic decomposition at 855°C. Four eutectic reactions were found to occur at 880°C and 20.0 at% Au, at 1195°C and 62.5 at% Au, at 1160°C and 71.0 at% Au and finally at 805°C and 89.0 at% Au. A catatectic reaction occurs in the Tb-rich region. The experimental results are discussed and compared with the general behaviour of the other R–Au systems.     

The role of Mg2Si formation in the hydrogenation of Mg film and Mg nanoblade array on Si substrates

In this paper, both an Mg film and an Mg nanoblade array have been first fabricated directly on Si substrates and hydrogenated under 20 bar hydrogen pressure at temperatures ranging from 200 °C to 350 °C. It is found that Mg₂Si alloy starts to form at T = 200 °C in both the Mg samples, which produces a two-layered structure in the hydrogenated films with the bottom dense layer of Mg₂Si. To prevent Mg alloying with Si, a layer of 200 nm thick Ti film was deposited in between the Mg samples and Si substrates as a diffusion barrier, and their hydrogenation results show that Mg₂Si formation is suppressed greatly and even eliminated in nanoblades, though Mg₂Si hillock defects are observed in the hydrogenated films, which could be formed progressively through the pinholes in the Ti film. To improve the diffusion barrier, a unique structure, consisting of layers of Ti nanorod array and Ti film, has been designed for Mg-based nanostructure deposition. The hydrogen cycling study demonstrates that the structure of 450 nm Ti nanorods on 1 μm Ti film can endure enough number of cycles for the hydrogen storage kinetic and thermodynamic study of film-based Mg nanostructures with/without nanocatalyst, and thus one can gain a fundamental understanding of hydrogen interacting with Mg intrinsic nanostructures and nanocatalysts.     

Experimental investigation of the Tb-Mg phase diagram

The Tb-Mg alloys were studied in the range 0 to 100 at.% Mg, by using X-ray powder diffraction, optical and scanning electron microscopy, electron probe microanalysis (EPMA), and differential thermal analysis (DTA). The following intermediate phases were identified and their crystal structures confirmed: TbMg (cubic,cP2 CsCl type), TbMg₂ (hexagonal,hP12-MgZn₂ type), and TbMg₃ (cubic,cF16-BiF₃ type). Two phases, moreover, were confirmed or identified in a composition range very close to 83 at.% Mg: χ₁ phase (cubic,cF440-GdMg₅ type) and χ₂ phase (cubic,cI58-αMn type). All the phases show peritectic formation with the possible exception of the χ₂ phase. The following phase equilibria were also determined: a eutectic reaction at 530 °C and 90.5 at.% Mg and a eutectoidal decomposition of the (βTb) phase at 670 °C and ∼28 at.% Mg.     

Phase equilibria on the ternary Mg–Mn–Ce system at the Mg-rich corner

Three isopleths at the Mg-rich corner of Mg–Mn–Ce ternary system were investigated via thermal analysis, SEM/EPMA and XRD. A ternary eutectic reaction was observed at 1 wt.% Mn and 23 wt.% Ce and 592 °C. A solid-solution type ternary intermetallic compound, (Mg,Mn)₁₂Ce, was observed with 0.5 at% solid solubility of Mn in the tetragonal Mg₁₂Ce. With the aid of thermodynamic modeling and experiments, a revised phase diagram for the binary Mg–Ce system and the isopleths of 0.6, 1.8 and 2.5 wt.% Mn were proposed up to 25 wt.% Ce.     

Hydriding/dehydriding behavior of Mg2CoH5 produced by reactive mechanical milling

Complex Mg₂CoH₅ hydride was obtained by a combined procedure that included a milling stage of a 2Mg–Co mixture under argon followed by reactive mechanical alloying (RMA) under hydrogen, both at room temperature. During RMA, MgH₂ is produced at short milling times (10 h) and Mg₂CoH₅ (50 wt%) after 90 h. Improvement in the yield and the formation times could be associated with both refinement of microstructure and enhancement of intermixing of Mg–Co during pre-milling stage. DSC studies of Mg₂CoH₅ phase produced by RMA show that the starting decomposition temperature is about 205 °C.Absorption and desorption PCIs were determined under static (300 °C) and dynamic (230–330 °C) conditions. An important hysteresis and two plateaus were observed and correlated with formation/decomposition of Mg₂CoH₅ (high-pressure plateau) and Mg₆Co₂H₁₁ (low-pressure plateau) hydrides. For comparing hydrogen sorption kinetics, Mg₂CoH₅ (65 wt%) was also obtained by a sintering method at 410 °C and 6.0 MPa of hydrogen pressure. Absorption was very fast in the temperature range of 150–350 °C, independently of synthesis procedure. However, desorption curves showed a better behavior for RMA powders.MgCo was observed after decomposition of Mg₂CoH₅ under particular thermal treatments, while MgCo₂ phase was not detected. The results of this study reinforce the idea that kinetics factors related with atomic mobility play a key role in the formation of Mg–Co intermetallics.     

Solidification of Mg-28%Zn-2%Y alloy involving icosahedral quasicrystal phase

Applying XRD, DTA, SEM and TEM techniques, an investigation on the solidification microstructure and solidification sequence of Mg-rich Mg-28%Zn-2%Y （mole fraction） alloy was carried out. It is found that, a-Mg dendrites, Mg7Zn3 phase and icosahedral quasicrystal phase coexist in the as-solidified alloy, where the icosahedral quasicrystal, whose structure is indentified to be a face-centered type, originates from a peritectic reaction occurring at 416 ℃. The primary phase of this peritectic reaction has the composition of Mg20Zn66Y14, which is coincident with the H phase reported by TSAI as （Zn, Mg）5Y. Furthermore, the single I-phase grain morphology was observed and its growth evolution was also discussed.     

Hydrogenation of CaLi2−xMgx (0 ≤ x ≤ 2) with C14 Laves phase structure

CaLi_2−xMg_x (0 ≤ x ≤ 2) which has the C14-type Laves phase structure has been successfully synthesized and hydrogenated. The C14-type Laves phase structure was kept after hydrogenation of CaLi_2−xMg_x (x = 0.2, 0.5, 1). After hydrogenation of CaLi₂ and CaMg₂, the Laves phase disappeared. The CaH₂ and LiH phases were formed from CaLi₂ and the CaH₂ and Mg phases from CaMg₂, respectively. CaLi_2−xMg_x (0 < x < 2) ternary alloys formed stable hydride phases with the C14-type Laves phase structure in contrast to CaLi₂ and CaMg₂ binary alloys.     

Electrical resistivity, oxidation resistivity and hardness of single crystal compounds in the Er–Rh–B system

Single crystals of ternary borides ErRh₃B (cubic,Pm3m), ErRh₃B₂ (monoclinic, C2/m) and ErRh₄B₄ (tetragonal, P4₂/nmc) have been grown from copper solution by slow cooling method. The electrical resistivity, oxidation resistivity and Vickers microhardness were studied. The electrical resistivities at room temperature of the (100) face of ErRh₃B, (001) face of ErRh₃B₂ and (100) face of ErRh₄B₄ are 25.6 μΩ·cm, 50.0 μΩ·cm and 106.8 μΩ·cm, respectively. According to thermogravimetric and differential thermal analyses, the oxidation of ErRh₃B, ErRh₃B₂ and ErRh₄B₄ start at 1030°C, 373°C and 690°C, respectively. The weight gain of the same compounds after heating in air up to 1200°C is 0.7%, 15.44% and 5.4%, respectively. The values of the Vickers microhardness for the (100) face of ErRh₃B, the (100) face of ErRh₃B₂ and the (110) face of ErRh₄B₄ are 4.8–5.0 GPa, 10.4–10.9 GPa and 10.9–11.3 GPa, respectively. The effect of boron content and crystal structure of each compound on the electrical resistivity, oxidation resistivity and Vickers microhardness are discussed.     

Constitution of the high-Al region of Al–Cu–Rh

Phase equilibria between 540 and 1010 °C were studied in Al–Cu–Rh alloys containing more than 50 at.% Al. Congruent equiatomic AlRh dissolves more than 40 at.% Cu and extends up to 58 at.% Al at the high-Cu part of its compositional range. High-temperature cubic C-Al₅Rh₂ (C-phase) dissolves up to 13 at.% Cu, “Al₃Rh” (₆-phase) up to 15 at.% Cu and Al₉Rh₂ up to 1.5 at.% Cu. The solubility of the third element in other binary Al–Rh and Al–Cu phases is below 0.5 at.%. Close to the high-Cu limit of the C-phase region the fcc C₂-phase structurally related to the C-phase is formed. Stable decagonal phase (D₁-phase) is formed below 1005 °C in a compositional range extending from Al₆₅Cu₁₆Rh₁₉ to Al₆₂Cu₂₃Rh₁₅, which shifts to higher Cu concentrations with decreasing temperature. An additional ternary phase forming around the Al₇₀Cu₂₀Rh₁₀ composition below 660 °C was revealed. Partial 1010, 990, 900, 800, 700, 600 and 540 °C isothermal sections were determined.     

Microstructural evolution behavior of Mg–5Si–1Al alloy modified with Sr–Sb during isothermal heat treatment

The microstructural evolution behavior of Mg–5Si–1Al alloys modified with Sr–Sb during isothermal heat treatment was investigated in the present study. Although the morphology of eutectic Mg₂Si phase varied with isothermal holding temperature increasing from 620 to 670 °C, no spheridization occurred for primary Mg₂Si polyhedrons in Sr–Sb-modified alloy even when the temperature reaches 660 or 670 °C. Such an abnormal phenomenon of primary Mg₂Si, during partial remelting isothermal treatment, might be ascribed to both the spheridization restriction effect caused by incorporation of Sb in Mg₂Si and stability of octahedral primary Mg₂Si crystals faced by {1 1 1} planes.     

Hydriding and electrochemical characteristics of a homogeneous amorphous Mg2Ni-Ni composite

Hydriding and electrochemical characteristics of a homogeneous amorphous Mg₂Ni-Ni composite prepared by ball-milling of Mg₂Ni alloy with Ni (70 wt.% vs. Mg₂Ni) were investigated. It was found that the Mg₂Ni-Ni composite absorbed hydrogen to the amount of 4.0 wt.% vs. Mg₂Ni [2.4 wt.% vs. (Mg₂Ni+70 wt.% Ni)] at a high rate under a hydrogen pressure of 3 MPa at 30°C. Furthermore, this alloy exhibited an extremely high discharge capacity of 1082 mAh g(Mg₂Ni)⁻¹ [636 mAh g(Mg₂Ni+70 wt.% Ni)⁻¹] at 30°C, which exceeded the theoretically calculated value of 999 mAh g(Mg₂Ni)⁻¹ on the basis of Mg₂NiH₄ for crystalline Mg₂Ni.     

Effect of Zn concentration on the microstructure and phase formation of Mg–5Gd alloy

The effect of Zn additions (0, 1 and 2 wt%) on the microstructure and phase formation of Mg–5Gd alloy has been investigated using X-ray diffraction (XRD), differential thermal analyzer (DTA) and scanning electron microscope (SEM) techniques. Zinc free alloy shows the negligible amount of Mg₅Gd phase formation whereas 1 wt% Zn addition leads to formation of more volume of Mg₅Gd phase having dissolution temperature at 518 °C. Two weight percentage of Zn addition yielded more volume of MgZn₂ phase having the dissolution temperature at 333 °C but less volume of Mg₅Gd phases. Moreover, 1 wt% Zn addition is found to be more favorable to the age hardening behavior as compared to 2 wt% Zn added alloy.     

X-ray diffraction and dielectric studies across morphotropic phase boundary in (1 − x) [Pb(Mg0.5W0.5)O3]-xPbTiO3 ceramics

We present here the results of comprehensive X-ray diffraction and dielectric studies on several compositions of (1 − x)[Pb(Mg_0.5W_0.5)O₃]-xPbTiO₃ (PMW-xPT) solid solution across the morphotropic phase boundary. Rietveld analysis of the powder X-ray diffraction data reveals cubic (space group Fm3m) structure of PMW-xPT ceramics for the compositions with x ≤ 0.42, tetragonal (space group P4mm) structure for the compositions with x ≥ 0.72 and coexistence of the tetragonal and cubic phases for the intermediate compositions (0.46 ≤ x ≤ 0.68). Temperature dependence of the dielectric permittivity above room temperature exhibits diffuse nature of phase transitions for the compositions in the cubic and two phase region while the compositions with tetragonal structure at room temperature exhibit sharp ferroelectric to paraelectric phase transition. The PMW-xPT compositions with coexistence of tetragonal and cubic phases at room temperature exhibit two anomalies in the temperature dependence of the dielectric permittivity above room temperature. Using results of structural and dielectric studies a partial phase diagram of PMW-xPT ceramics is also presented.     

Thermodynamic and mechanical properties of lanthanum-magnesium phases from density functional theory

Thermodynamic and mechanical properties of the six known phases in the La-Mg phase diagram, viz. LaMg, LaMg₂, LaMg₃, La₅Mg₄₁, La₂Mg₁₇, and LaMg₁₂, and their elemental antecedents, Mg and La, are computed with density functional theory (DFT) using the PBE and PBEsol exchange-correlation functionals. Phase stability analyses show that both LaMg₂ and La₅Mg₄₁ are metastable at low temperatures which is consistent with experiments and vibrational spectra. We generalize an existing approach for computing the crystallographic dependence of Young's modulus and Poisson's ratio, which is presently limited to cubic systems, to address any space group symmetry using 0 K elasticity tensor components (C_ij) from DFT. Isothermal and isentropic C_ij(T) are computed with the quasiharmonic approximation (QHA) as are the linear thermal expansion of the cubic compounds, the average linear thermal expansion for the non-cubic compounds, the bulk modulus, and the constant pressure heat capacity. A critical comparison of theoretical results from the PBE and PBEsol functionals is made with available experimental data.     

The interpolated Tm---Mg phase diagram

The alloying behavior of the different rare earths, which varies according to systematic patterns, can be used to predict constitutional properties of rare earth alloys with the same element. The applicability to prediction of the Tm---Mg phase diagram is presented here. The predicted version is compared with that determined experimentally by analyzing a few key compositions using X-ray powder diffraction, optical and scanning electron microscopy, electron probe microanalysis (EPMA) and differential thermal analysis (DTA). The following phases were identified and their crystal structures confirmed or determined: β phase, ≈Tm₂Mg (cubic, cI2, W type), TmMg (cubic, cP2, CsCl type), TmMg₂ (hexagonal, hP12, MgZn₂ type) and ≈Tm₅Mg₂₄ (cubic, cI58, -Mn type). All the phases show peritectic formation. Two other equilibria were also observed: a eutectic reaction at 575 °C and 89 at.% Mg and the eutectoid decomposition of the β phase at 670 °C and approximately 30 at.% Mg.     

Analysis of hydrogen distribution on Mg–Ni alloy surface by scanning electron-stimulated desorption ion microscope (SESDIM)

Hydrogen distribution and behavior on a Mg–Ni alloy surface are studied by using a time-of-flight electron-stimulated desorption (TOF-ESD) microscopy and a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). The desorbed hydrogen ions are energy-discriminated and distinguished into two characters in the adsorbed states, which belong to Mg₂Ni grains and the other to oxygen-contaminated Mg phase at the grain boundaries. Adsorbed hydrogen is found to be stable up to 150 °C, but becomes thermally unstable around at 200 °C.     

The Al–R–Mg (R=Gd, Dy, Ho) systems. Part I: experimental investigation

Key experiments were carried out on the three Al–R–Mg (R=Gd,Dy,Ho) systems and the results obtained used for the thermodynamic optimisation reported in a separate paper in this issue [Caccasmani G, De Negri S, Saccone A, Ferro R. Intermetallics this issue.]. The samples were characterized by differential thermal analysis (DTA), X-ray powder diffraction (XRD), light optical microscopy (LOM), scanning electron microscopy (SEM) and quantitative electron probe microanalysis (EPMA). The isothermal sections at 400 °C are all characterized by extended homogeneity regions at a constant rare earth content. The extension of the (Mg,Al)R solid solution, cP2-CsCl type, varies with the R atomic number. Ternary compounds (τ) of Al₂(R,Mg) stoichiometry (hexagonal Laves phases with MgNi₂-type structure) have been found to exist at 400 °C in all the systems. Their temperatures of formation were detected by DTA measurements.     

The isothermal section at 450°C of the Yb–Pr–Mg system

In the framework of a systematic investigation of the ternary alloys formed by magnesium with two different metals of the rare earth family, data obtained in a study of the Yb–Pr–Mg system are presented. In the isothermal section at 450°C, phases have been identified which are based on the ternary extensions of the binary compounds PrMg, PrMg₃, Pr₅Mg₄₁ and YbMg₂. The PrMg₁₂ compound on the contrary shows very small ternary solid solubility. Two ternary compounds have also been identified: τ₁, (Yb_xPr_1-x)Mg₂ (0.15⩽x⩽0.48), cF24–MgCu₂ Laves type and τ₂,≈(Yb_xPr_1-x)Mg_4.5 (0.12⩽x⩽0.40), cF440–GdMg₅ type. The trends of the lattice parameters and the general features of the isothermal section are discussed and compared with those of other magnesium ternary alloys with rare earth elements. The behaviour of the different MeMg₂ Laves phases is particularly highlighted.     

Investigation of the microwave dielectric properties of Ca1−xMgxLa4Ti5O17 ceramics for application in coplanar patch antenna

In this paper, the dielectric properties of Ca_1−xMg_xLa₄Ti₅O₁₇ ceramics at microwave frequency have been studied. The diffraction peaks of Ca_(1−x)Mg_xLa₄Ti₅O₁₇ ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic were observed at different sintering temperatures. A maximum density of 5.3 g/cm³ can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. A maximum dielectric constant (_r) and quality factor (Q × f) of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h are 56.3 and 12,300 GHz (at 6.4 GHz), respectively. A near-zero temperature coefficient of resonant frequency (τ_f) of −9.6 ppm/°C can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. The measurement results for the aperture-coupled coplanar patch antenna at 2.5 GHz are presented. With this technique, a 3.33% bandwidth (return loss <−10 dB) with a center frequency at approximately 2.5 GHz has been successfully achieved.