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.     

Nanocrystallization of Pd74Si18Au8 metallic glass

The crystallization process of Pd₇₄Si₁₈Au₈ amorphous alloy has been investigated by transmission electron microscopy, small angle X-ray scattering and three-dimensional atom probe techniques. Although literature suggests that the alloy decomposes into two glassy phases prior to the crystallization, we found that the crystallization occurs directly from a single amorphous phase by the primary crystallization of fcc Pd–Au solid solution, followed by the polymorphous crystallization of the remaining amorphous phase to a Pd₃Si phase.     

Phase transformations in the Zn–Cd–Sb system

Phase equilibria of the Cd–Sb–Zn system have been investigated by metallographic examinations, DSC, XRD and WDS measurements. At 250 °C, the ternary diagram shows two three-phase fields, (Zn)+(Cd)+Zn₄Sb₃ and (Cd)+ Zn₄Sb₃+(Zn,Cd)Sb. Continuous solid solution has been found between ZnSb and CdSb. Solubility of Cd in Sb₃Zn₄ was determined to be about 43 at.%. A variant of the reaction scheme is proposed for the Cd–Sb–Zn system to understand phase relations observed at 250 °C.     

Experimental determination of phase equilibria in the Fe–Nb–V ternary system

The phase equilibria in the Fe–Nb–V ternary system were investigated by means of optical microscopy, electron probe microanalysis and X-ray diffraction. Four isothermal sections in the Fe–Nb–V ternary system at 1000 °C, 1100 °C, 1200 °C and 1300 °C were firstly experimentally established. Present experimental results indicate that: (1) there is a large (Nb, V) continuous bcc solid solution; (2) there are the larger solubilities of V in the FeNb and Fe₂Nb phases. The newly determined phase equilibria in this system will provide important support for the development of hydrogen storage materials and microalloyed steels.     

Phase equilibria of the long-period stacking ordered phase in the Mg–Ni–Y system

Phase equilibria in the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The results show that a long-period stacking ordered (LPSO) phase with 14H structure is thermodynamically stable in the Mg–Ni–Y system in a wide temperature range, but it dissolves varying from 492 to 559 °C depending on the alloy composition. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg₉₁Ni₄Y₅. The isothermal sections of the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been finally established, and a eutectic reaction, Liquid ↔ α-Mg + 14H + Mg₂Ni, has been determined occurring at 492 °C with a liquid composition about Mg_84.8Ni_12.0Y_3.2.     

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′(L1₂)-particles (Ni₃Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.     

Magnetic ordering and magnetic properties of ErAuxNi1−xIn (0 ≤ x ≤ 1) solid solution

The ErAu_xNi_1−xIn (0 ≤ x ≤ 1) quasiternary compounds crystallize in the hexagonal layered crystal structure of ZrNiAl-type. ErAuIn was reported to be an antiferromagnet with T_N = 3 K and magnetic moments having triangular arrangement within the basal plane (the magnetic order is described by the propagation vector ). On the contrary ErNiIn is a ferromagnet with T_C = 9 K and magnetic moments pointing along the c-axis. The magnetic ordering in ErAu_xNi_1−xIn (0 < x < 1) solid solution, has been investigated by neutron diffractometry in the temperature range between 1.5 and 15 K. Moreover, bulk magnetic measurements have been carried out in the range 1.72–400 K. All alloys of intermediate composition were found to be antiferromagnets with T_N between 4.6 and 7 K. Below 2 K their magnetic order is described by the propagation vector and magnetic moments are aligned along the c-axis. However, for alloys with 0.2 ≤ x ≤ 0.7 the propagation vector was found to turn into with increasing temperature.     

Structural and Mössbauer studies on mechanical milled SmCo5/α-Fe nanocomposite magnetic powders

The SmCo₅/α-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo₅ and α-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and ⁵⁷Fe Mössbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo₅ and α-Fe phases, Mössbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of α-Fe(Co) phase as a function of milling time. It has been found that the fractional volume of α-Fe(Co) solid solution tends to increase at the expense of the initial α-Fe phase upon progressive milling.     

Microstructural aspects of the corrosion of Alloy 800

Transmission electron microscopy studies on solution-annealed Alloy 800 revealed small (100–200 nm), spherical-shaped titanium carbide (face centered cubic structure) and large (200 nm–5 μm), faceted titanium nitride (hexagonal structure) particles randomly distributed in the austenite matrix. The volume fraction of former particles was found to be greater than that of the latter. Corrosion studies of the alloy in acidic, chlorides and acidic chloride environments at room temperature indicated that the passivity of Alloy 800 was adversely affected by the addition Cl⁻ ions. X-ray photoelectron spectroscopy revealed that the surface film formed on the alloy at the onset of passivity consisted of Cr³⁺ (as Cr₂O₃), without any Fe³⁺/Fe²⁺ or Ni²⁺. Scanning electron microscopy studies indicated initiation of pitting at large, faceted particles, not at small, spherical-shaped ones.     

Thermodynamic investigations in the solid state of the lanthanum–magnesium–zinc system

The La–Mg–Zn phase diagram is experimentally investigated at 595 K, x_La > 4% and the corresponding isothermal section is partially determined. This section includes 5 substitutional solid solutions based on the binary compounds (LaMg, LaZn, LaMg₃, LaMg_10.3 and La₂Zn₁₇) and three ternary phase (La₈(Mg,Zn)₉₂, La₃(Mg,Zn)₁₁ and La_4.27Mg_2.89Zn₃₀). The enthalpies of mixing in the ternary solid solutions are calculated at 298 K on the basis of tin solution calorimetry experiments.     

Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

The isothermal section of the Gd–Ni–V ternary system at 773 K

The phase equilibria of the Gd–Ni–V system at 773 K were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The experimental results show no existence of ternary compounds at 773 K. The existence of 14 single-phase regions, 25 two-phase regions and 12 three-phase regions was determined. The maximum solubility of V in (Ni), Gd₂Ni₁₇, GdNi₅ and GdNi₂ was measured to be about 16 at.%, 2 at.%, 3 at.% and 2.5 at.%, respectively, while that of Gd in (Ni), Ni₃V, Ni₂V, Ni₂V₃, NiV₃ and (V) was less than 1 at.%. An isothermal section of the Gd–Ni–V system at 773 K has been presented according to the present work.     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.     

Thermodynamic properties of the ternary system InSb–NiSb–Sb in the temperature range 640–860 K

The ternary InSb–NiSb–Sb system has been studied by X-ray diffraction and by potentiometry. The electromotive forces (EMF) have been measured in the temperature range 640<T/K<860 by using the following galvanic cell:

with x (0.075<x<0.498) and y (0<y<0.359). The investigated samples are located on the following lines of the Gibbs triangle: InSb–Ni_0.33Sb_0.66, InSb–Ni_0.48Sb_0.52, InSb–NiSb, Sb–(InSb)_0.75(NiSb)_0.25, Sb–(InSb)_{0. 5}(NiSb)_0.5, Sb–(InSb)_0.25(NiSb)_0.75. From these measurements, the values of the partial molar thermodynamic functions (Δμ°_m,In, ΔH°_m,In, ΔS°_m,In) (data at reference pressure p⁰=10⁵ Pa), for the liquid InSb alloy, for the three solid heterogeneous regions InSb–NiSb₂–Sb, InSb–NiSb_1±δ?–NiSb₂, InSb–NiSb_1±δ, for six ternary liquid–solid alloys, have been calculated.     

Magnetic properties and microstructural characteristics of bulk Nd–Al–Fe–Co glassy alloys

Bulk Nd–Al–Fe–Co glassy alloys with diameter up to 5 mm were investigated by magnetic measurements, magnetic force microscopy (MFM) and high resolution electron microscopy (HREM) at room temperature. The results from the measurement of vibrating sample magnetometer show that these samples with compositions Nd₆₅Al₁₀Fe_25-xCo_x (x=0–10 at.%) and Nd₆₀Al₁₀Fe₂₀Co₁₀ display hard magnetic properties with H_C of 300 kAm⁻¹, M_S of 10 Am² kg⁻¹, and M_r of 7 Am² kg⁻¹. The MFM measurements of the Nd₆₀Al₁₀Fe₂₀Co₁₀ bulk metallic glass (BMG) reveal the existence of magnetic domains with a period of about 0.36 μm, and the ordered clusters with the averaged size of about 5 nm was observed by the HREM on the sample. The domain structure or cluster is believed to be associated with the appearance of hard-magnetic properties in this alloy system. The existence of the large-size domains demonstrates that magnetic moment of a great deal of ordered atomic clusters in the BMG has been aligned by exchange-coupling.     

Determination of phase equilibria in the Co-rich Co–Al–W ternary system with a diffusion-couple technique

Phase equilibria in the Co-rich Co–Al–W ternary system were determined with a unique diffusion-couple technique in which Co–27Al and Co–15W binary alloys (at. %) were first coupled for interdiffusion and then heat-treated for precipitation. After a diffusion process at 1300 °C for 20 h, concentration gradients of Al and W were formed in the γ-Co(A1) matrix in the vicinity of the coupled interface. After a heat treatment at 900 °C for 500 h the γ′-Co₃(Al,W)(L1₂) phase was formed with a coarsened shape in contact with the γ, CoAl(B2) and Co₃W(D0₁₉) phases. Additionally, it appeared with a submicron cuboidal shape within the γ matrix. After 2000 h, however, the coarsened γ′ phase became infrequent and the three phases of γ, CoAl and Co₃W came into frequent contact with each other. These results clearly demonstrate that the γ′ phase is metastable and the three phases of γ, CoAl and Co₃W are thermodynamically in equilibrium at 900 °C in the Co–Al–W ternary system.     

Complex -phases in the Al–Pd-transition–metal systems: Towards a combination of an electrical conductor with a thermal insulator

-Phases in the Al–Pd–(Mn,Fe,Co,Rh,…) alloy systems form in wide compositional ranges and belong to the interesting class of complex intermetallics that are characterized by giant unit cells with quasicrystals-like cluster substructure. In order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the physical properties of the material, we performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the -phases in the Al–Pd–Fe, Al–Pd–Co and Al–Pd–Rh systems. Magnetic measurements reveal that the materials are diamagnetic, containing tiny fractions of magnetic transition–metal atoms (of the order 10–100 ppm). Electrical resistivity is moderate, of the order 10² μΩ cm, and shows weak temperature dependence (in most cases of a few %) in the investigated temperature range 4–300 K. An interesting feature of the -phases is their low thermal conductivity, which is at room temperature comparable to that of thermal insulators amorphous SiO₂ and Zr/YO₂ ceramics. While SiO₂ and Zr/YO₂ are also electrical insulators, -phases exhibit electrical conductivity typical of metallic alloys, so that they offer an interesting combination of an electrical conductor with a thermal insulator. The reason for the weak thermal conductivity of the -phases appears to be structural: large and heavy atomic clusters of icosahedral symmetry in the giant unit cell prevent the propagation of extended phonons, so that the lattice can no more efficiently participate in the heat transport. The thermoelectric power of the investigated -phase families is small, so that these materials do not appear promising candidates for the thermoelectric application.     

Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti₄₉Ni₄₁Cu₁₀ shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)₂ precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2₁ ↔ B19₁ ↔ B19′₁ and B2₂ ↔ B19₂ ↔ B19′₂ transformations corresponding to the regions near and far from Ti(Ni,Cu)₂ precipitates, respectively. Experimental results show that the more the annealing time, the more the B2₁ ↔ B19₁ ↔ B19′₁ transformations and finally only B2₁ ↔ B19₁ ↔ B19′₁ transformations retain with the transformation temperatures close to those of Ti₅₀Ni₄₀Cu₁₀ SMA.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

A contribution to the rare earth intermetallic chemistry: Praseodymium-magnesium alloy system

Pr-Mg alloys were studied in the range 0–100 at. % Mg. By using X-ray powder diffraction, optical and scanning electron microscopy, electron probe micro-analysis and differential thermal analysis, the different intermediate phases were identified and their crystal structures confirmed or determined. The following phase equilibria have been also determined: PrMg (cubic, cP2, CsCl type, melting point 765°C), PrMg₂ (cubic, cF24, MgCu₂ type, peritectic formation 740°C), PrMg₃ (cubic, CF16-BiF₃ type, melting point 790°C), Pr₅Mg₄₁ (tetragonal, tI92 Ce₅Mg₄₁-type, peritectic formation 575°C) and PrMg₁₂ (tetragonal, tI26 ThMn₁₂-type, peritectic formation 565°C) PrMg₂ undergoes a eutectoidal decomposition at 670°C. Three eutectic reactions were observed to occur at 735°C and 40·0 at. % Mg, at 725°C and 59·5 at. % Mg and at 560°C and 95·0 at. % Mg, respectively. The (β-Pr) terminal solid solution was observed to decompose eutectoidally at 510°C and 19·5 at. % Mg. The data obtained in this study are compared with those relating to other previously studied R-Mg systems.The crystallochemical characteristics of the binary phases formed by Mg with the rare earths and with the alkaline earths are briefly discussed.