Structural,electronic and magnetic properties of the Mn54−xAl46Tix (x = 2; 4) alloys

The structural, electronic and magnetic behavior of the as-cast and annealed Mn₅₂Al₄₆Ti₂ and Mn₅₀Al₄₆Ti₄ alloys have been studied through electronic band structure calculations, X-ray diffraction and magnetic measurements in the temperature range 4–850 K and magnetic field up to 7 T. Band structure calculations show a preference for Ti atoms to occupy the Mn sites in the plane of Al atoms with their magnetic moments (−0.68 μ_B/Ti) coupled antiparallel relative to the Mn magnetic moments in the plane of Mn atoms (2.33 μ_B/Mn). The as-cast and annealed samples were phase mixtures with different values of the hard ferromagnetic τ phase content. Except the as-cast and annealed at 1050 °C Mn₅₂Al₄₆Ti₂ alloys, all the analyzed samples include, along with the τ and γ₂ phases, a soft κ phase (CsCl - structure type) with T_C around 530 K. The best magnetic characteristics were obtained for Mn₅₂Al₄₆Ti₂ alloy annealed at 470 °C for 6 h: M_S = 116 A m²/Kg at 4 K and T_C = 668 K, in good agreement with the values reported in the literature for the τ phase of the MnAl system. The effects of the composition and of the preparation route on the electronic and magnetic properties are discussed in comparison with the properties of Mn₅₄Al₄₆ parent alloy.     

Study of mechanical property and crystallization of a ZrCoAl bulk metallic glass

The mechanical property of Zr₅₆Co₂₈Al₁₆ bulk metallic glasses (BMGs) under compression test at room temperature was investigated. The alloy exhibited high fracture strength of approximately 2136 MPa and a pronounced plastic strain of 10.2%. No strain-hardening behavior was observed. The evolution of the morphology of the shear bands on the lateral surface of the as-cast samples was studied using scanning electron microscopy (SEM). The plasticity can be attributed to the formation and interaction of multiple shear bands during deformation. The crystallization behavior was studied by differential scanning calorimetry (DSC) at different heating rates. The activation energies of the glass transition (E_g), the onset of the crystallization (E_x) and the two stages of the crystallization (E_p1 and E_p2) were calculated to be E_g = 303.2 ± 13.5, E_x = 316.4 ± 37.9, E_p1 = 336.2 ± 36.2 and E_p2 = 362.0 ± 29.5 kJ/mol, respectively. The crystallization behavior research of this alloy indicates that the precipitation of the B2-ZrCo phase may be further utilized to promote the ductility of the ZrCoAl BMG composites.     

Glass forming ability and microstructure of hard magnetic Nd60Al20Fe20 glass forming alloy

Glass forming ability (GFA), magnetic properties and microstructure of Nd₆₀Al₂₀Fe₂₀ as-cast rod were investigated and further compared with Nd₆₀Al₁₀Fe₃₀ glass forming alloy. The rod prepared by suction casting with a diameter of 3 mm exhibits the typical amorphous nature in XRD pattern, distinct glass transition in DSC traces and hard magnetic properties. It is found that the diameter of cast Nd₆₀Al₂₀Fe₂₀ glassy rod is much larger than the critical section thickness (Z_c) of bulk metallic glass (BMG) predicted from DSC measurements. A few nano-crystalline particles with the structure and composition similar to A_x phase in Nd–Fe alloys were found embedded randomly in amorphous matrix and could be the origin of hard magnetic properties of the as-cast rods. The GFA of the alloy appears to be enhanced by the precipitation of metastable nano-particles with small positive forming enthalpy and the real Z_c of the alloy could be less than 1 mm predicted by parameter γ.     

Phase equilibria and thermodynamic calculation of the Co–Ta binary system

Experimental investigation and thermodynamic evaluation of the Co–Ta binary phase diagram was carried out. Equilibrium compositions obtained in two-phase alloys and diffusion couples were measured by electron probe microanalyzer (EPMA). A very narrow λ₃(C36) + λ₂(C15) two-phase region is confirmed to be present around 26.5 at.% Ta at temperatures between 950 °C and 1448 °C. Equilibrium relationships above 1500 °C among the liquid, Laves (λ₁(C14), λ₂ and λ₃, whose stoichiometry is described by Co₂Ta), μ(D8_b) and CoTa₂(C16) phases were investigated by microstructural examination in as-cast Co-(24–60 at.%)Ta alloys. The solvus temperature of the γ′ Co₃Ta (L1₂) phase precipitated in the 5.8 at.%Ta γ(Co) and the peritectoid temperature of the Co₇Ta₂ phase in an 8.5 at.%Ta alloy were determined to be 1013 °C and 1033 °C, respectively, by differential scanning calorimeter (DSC). Fine precipitates of the γ′ phase precipitated in the γ (A1) matrix were observed by transmission electron microscope (TEM). Analyzing the present experimental results synthetically, the γ′ Co₃Ta phase was identified to be a metastable phase, of which the γ/γ′ transition temperature of the stoichiometric Co₃Ta alloy was estimated to be 2000 °C. Thermodynamic assessment of the Co–Ta binary system was carried out based on the present results as well as on experimental data in the literature. Calculated results of not only stable but also metastable equilibria were found to be in good agreement with the revised phase diagram. The evaluated stability of the metastable γ′ Co₃Ta coincides with the enthalpy of formation (ΔH(γ'Co₃Ta) = −23.44 kJ/mol) calculated by the ab initio method.     

Effect of Fe additions on microstructure and mechanical properties of a multi-component Nb–16Si–22Ti–2Hf–2Al–2Cr alloy at room and high temperatures

The phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe alloys (where x = 1, 2, 4, 6 at.%, hereafter referred to as 1Fe, 2Fe, 4Fe and 6Fe alloys, respectively) prepared by arc-melting were investigated. It was observed that the nominal Fe content affected the solidification path of the multi-component alloy. The as-cast 1Fe alloy primarily consisted of a dendritic-like Nb_SS phase and (α+γ)-Nb₅Si₃ silicide, and the as-cast 2Fe and 4Fe alloys primarily consisted of an Nb_SS phase, (α+γ)-Nb₅Si₃ silicide and (Fe + Ti)-rich region. In addition to the Nb_SS phase, a multi-component Nb₄FeSi silicide was present in the as-cast 6Fe alloy. When heat-treated at 1350 °C for 100 h, the 1Fe and 6Fe alloys almost exhibited the same microstructures as the corresponding as-cast samples; for the 2Fe and 4Fe alloys, the (Fe + Ti)-rich region decomposed, and Nb₄FeSi silicide formed. The fracture toughness of the as-cast and heat-treated Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe samples monolithically decreased with the nominal Fe contents. It is interesting that at room temperature, the strength of the heat-treated samples was improved by the Fe additions, whereas at 1250 °C and above, the strength decreased, suggesting the weakening role of the Nb₄FeSi silicide on the high-temperature strength. As the nominal Fe content increased from 1 at.% to 6 at.%, for example, the 0.2% yield strength increased from 1675 MPa to 1820 MPa at room temperature; also, the strength decreased from 183 MPa to 78 MPa at 1350 °C.     

Microstructures and mechanical properties of conventionally solidified Al63Cu25Fe12 alloy

In this study, the microstructures and mechanical properties of conventionally solidified Al₆₃Cu₂₅Fe₁₂ alloy after different heat-treatments were investigated. The microstructures of the as-cast and subsequently heat-treated samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). The XRD results showed the presence of quasicrystalline icosahedral phase (i-phase) together with crystalline phases corresponding to β-AlFe(Cu) solid solution phase (β-phase) and τ-AlCu(Fe) solid solution phase (τ-phase). The SEM investigations clearly showed the formation of i-phase with pentagonal dodecahedra structure. However, the i-phase together with β-phase was also observed in the heat-treated samples and the peak intensity of the β-phase decreased with increasing heat-treatment temperature. From the DTA curves, the melting point of i-phase was determined as 890 °C for this alloy composition. Mechanical properties of the as-cast and subsequently heat-treated samples were measured by a Vickers indenter. Results showed that the microhardness (HV) and the elastic modulus (E) of the as-cast sample were around 598 kg fmm^?2 (5.86 GPa) and 104 GPa, respectively. In addition, the characteristic of material plasticity (δ_H) value was calculated to be 0.54.     

Crystal structure and thermoelectric properties of β-MoSi2

A powder sample of a metastable phase β-MoSi₂ having the C40-type crystal structure was prepared by heating Mo sheets with a Na–Si melt at 858 K for 12 h to determine details of the crystal structure. The lattice constants (a = 4.6016(3) Å and c = 6.5700(3) Å) and Si atom coordinate (y = 0.1658(2)) of β-MoSi₂ were determined by Rietveld analysis of the X-ray powder diffraction. The thermoelectric properties were refined for a bulk sample prepared by sintering the β-MoSi₂ powder at 773 K and 600 MPa. The electrical resistivity of the sintered β-MoSi₂ sample with a relative density of 65% of the theoretical one was 2.5 mΩ cm at 300 K, and slightly increased with increasing temperature from 300 to 725 K. The Seebeck coefficients changed from +60 to +89 μV/K in the temperature range from 330 to 725 K. The maximum thermoelectric power factor was 2.2 × 10^?6 W cm^?1 K^?2 at 725 K.     

Probing structure and microstructure of epitaxial Ni–Mn–Ga films by reciprocal space mapping and pole figure measurements

The crystal structure and complex twinning microstructure of epitaxial Ni–Mn–Ga films on (1 0 0) MgO substrates was studied by X-ray diffraction using 2θ scans, pole figure measurements and reciprocal space mapping (RSM). The orientation distribution of all variants is visualized by RSM, which forms the basis for a better understanding of the crystallographic relation between variants and substrate. Above the martensitic transformation temperature the film consists of single austenite phase with lattice constant a = 5.81 Å at 419 K. At room temperature some epitaxially grown residual austenite with a = 5.79 Å remains at the interface with the substrate, followed by an intermediate layer exhibiting orthorhombic distortion, a_trans = 6.05 Å, b_trans = 5.87 Å, c_trans = 5.73 Å and a major fraction of 14M (7M) martensite, a = 6.16 Å b = 5.79 Å c = 5.48 Å. The seven-layered modulation of this metastable martensite structure is directly observed by RSM. The intermediate phase observed close to interface indicates the existence of an instable, pre-adaptive martensite phase with a short stacking period.     

Microstructure and Mechanical Properties of AZ91 Alloys by Addition of Yttrium

Effects of yttrium (Y) on the microstructure and properties of as-cast Mg-Al-Zn (AZ91) alloys were studied. Y additions not only change the microstructure but also influence the mechanical properties of AZ91 alloy. AZ91 unmodified alloys under as-cast state indicate that eutectic phase Mg₁₇Al₁₂ is continuous and reticulated. Yttrium addition to AZ91 casting alloys has an important influence on the primary-phase and precipitation. When the Y content is 0.3 wt.%, no Y-containing compound was observed. When the Y content is 0.6 and 0.9 wt.%, Al₂Y phase formed in the alloy and the growth morphology of eutectic Mg₁₇Al₁₂ phase is modified. When the Y content is further increased to 1.2 wt.%, the Al₂Y phase becomes coarser and Mg₁₇Al₁₂ transforms into a cotton-shape structure. The results showed that Y can improve significantly as-cast microstructure of AZ91 alloys, refining Mg₁₇Al₁₂ phase and increasing in hardness and strength and decreasing in impact toughness and elongation.     

Deformation-induced structural transformation in boron-doped Ni76Al24

The paper describes the effect of cold deformation on the stability of L1₂ structured hypostoichiometric Ni₇₆Al₂₄(B) alloy. During cold rolling, the order parameter decreases with the degree of deformation. After 85% cold rolling, the material transforms to DO₂₂ structure from its initial L1₂ structure. The lattice parameters are measured as a = 3.567 Å and c = 7.188 Å having a c/a ratio of ∼2.015. After annealing, the metastable structure transforms back to its original L1₂ structure. The DO₂₂ structure can be derived from the L1₂ structure by local rearrangement of the Ni atoms as antisites. The activation energy of the reverse transformation is similar to that for the diffusion of Ni atoms in Ni₃Al.     

Evolution of microstructure,hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al_0.5CoCrFeNi alloy as well as Al_0.5CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al_0.5CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al_0.5CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl^? ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.     

Nd2Ni2MgH8 hydride: Synthesis,structure and magnetic properties

Hydrogen interaction with intermetallic compound Nd₂Ni₂Mg crystallizing in the tetragonal Mo₂FeB₂ type of structure leads to the formation of hydride/deuteride containing 7.4–8 H(D)/f.u. Hydrogen absorption is accompanied by a monoclinic deformation of the unit cell with the crystal structure data refined from Synchrotron XRD (Sp. gr. P2₁/c; a = 11.60733(5) Å; b = 7.66057(3) Å; c = 11.78743(5) Å, β = 92.4126(4)°, V = 1047.194(8) ų) and high resolution powder XRD data. The formed interstitial hydride disproportionates during heating forming NdH₂. Nd₂Ni₂Mg is an antiferromagnet with the Néel temperature T_N ≈ 19 K. Presence of another magnetic phase transition at 14 K together with two metamagnetic transitions indicates a complicated magnetic phase diagram. The hydrogenation reduces T_N to 1.0 K.     

Evolution of phases in Al55Ni30Pd15 alloy at temperatures up to 600 °C

The originally as-cast Al₅₅Ni₃₀Pd₁₅ alloy was investigated during continuous heating from room temperature to 600 °C and after annealing at 600 °C for respective 455, 2650, and 4050 h. In the investigation the synchrotron X-ray diffraction and the high-resolution scanning electron microscopy inclusive of the energy-dispersive X-ray spectroscopy were used. In all the investigated conditions β-(Ni,Pd)Al (Pm3m) and Al₃(NiPd)₂ (P3m1) phases were identified. After 2650 h of annealing the former phase was found to be separated into two isostructural modifications differing from one another in Pd- and Ni-contents. The annealing for 455 h contributed to the decrease of lattice parameters in the Al₃NiPd phase compared to the original as-cast condition.     

Determination of the crystallographic structure of the new U6Fe5Al8Si9 compound

The crystal structure of a new quaternary U₆Fe₅Al₈Si₉ orthorhombic compound described by Immm space group was solved and refined on the basis of X-ray powder diffraction data. The unit cell parameters (a = 12.2407(2) Å, b = 18.3614(3) Å, c = 4.0662(1) Å) and the atomic coordinates were optimized through Rietveld refinement procedure with the reliability factors R_p = 8.1%, R_b = 6.2% and R_wp = 11.4%. Density Functional Theory calculations were used to distinguish between positions of Si and Al atoms in the final model of the structure. The positions of heavy atoms observed in high-resolution TEM-images fit their positions on the respective projections of the proposed structural model.     

Identification of the structure of a new Al–U–Fe phase by electron microdiffraction technique

The particles of an unknown intermetallic phase with the approximate composition Al₁₀Fe₂U were observed in a ternary Al–Fe–U alloy. The structure of this phase was investigated in a transmission electron microscope using a microdiffraction technique based on analysis of the symmetry and relative positions of reflections in the zero-order and high-order Laue zones. The phase has an orthorhombic C-centered unit cell with lattice parameters a=8.900, b=10.190 and c=8.993 Å; its crystal symmetry can be described by the Cmcm space group.     

Interdiffusion and reaction between Zr and Al alloys from 425° to 625 °C

Zirconium has recently garnered attention for use as a diffusion barrier between U–Mo nuclear fuels and Al cladding alloys. Interdiffusion and reactions between Zr and Al, Al-2 wt.% Si, Al-5 wt.% Si or AA6061 were investigated using solid-to-solid diffusion couples annealed in the temperature range of 425° to 625 °C. In the binary Al and Zr system, the Al₃Zr and Al₂Zr phases were identified, and the activation energy for the growth of the Al₃Zr phase was determined to be 347 kJ/mol. Negligible diffusional interactions were observed for diffusion couples between Zr vs. Al-2 wt.% Si, Al-5 wt.% Si and AA6061 annealed at or below 475 °C. In diffusion couples with the binary Al–Si alloys at 560 °C, a significant variation in the development of the phase constituents was observed including the thick τ₁ (Al₅SiZr₂) with Si content up to 12 at.%, and thin layers of (Si,Al)₂Zr, (Al,Si)₃Zr, Al₃SiZr₂ and Al₂Zr phases. The use of AA6061 as a terminal alloy resulted in the development of both τ₁ (Al₅SiZr₂) and (Al,Si)₃Zr phases with a very thin layer of (Al,Si)₂Zr. At 560 °C, with increasing Si content in the Al–Si alloy, an increase in the overall rate of diffusional interaction was observed; however, the diffusional interaction of Zr in contact with multicomponent AA6061 with 0.4–0.8 wt.% Si was most rapid.     

Laves phases in the ternary systems Ti–{Pd,Pt}–Al

Formation and crystal structure of Laves phases in the systems Ti–{Pd,Pt}–Al were investigated employing XPD (X-ray powder diffraction), XSCD (X-ray single crystal diffraction) and EPMA (electron probe microanalysis) techniques. Laves phases with MgZn₂ type (space group: P6₃/mmc) and its variant with the Nb(Ir,Al)₂-type (a√3 × a√3 × c supercell of MgZn₂-type, space group: P6₃/mcm) were found in both systems. Formation of a particular structure type is dependent on temperature and composition. Laves phases with the Nb(Ir,Al)₂-type form around 25 at.% of Pd,Pt at 950 °C. The MgZn₂-type Laves phase Ti(Pt,Al)₂ was not observed at 950 °C, but it forms in as-cast alloys at a slightly lower Pt content, Ti_37.8Pt_19.0Al_43.2. In the Ti–Pd–Al system at 950 °C the MgZn₂-type phase exists at the Pd-poor side of the homogeneity region whilst the Nb(Ir,Al)₂-type phase is slightly richer in Pd. Phase relations associated with the Ti–Pt–Al Laves phase were established at 950 °C and reveal a new compound TiPtAl that derives from hexagonal ZrBeSi-type (ordered Ni₂In-type, a = 0.43925(4) nm, c = 0.54844(5); space group P6₃/mmc; R_F2 = 0.015 from single crystal data). Atom distribution in the compound shows a slight deviation from full atom order Ti(Pt_0.97Al_0.03)(Al_0.98Pt_0.02).     

Microstructural evolution and mechanical properties of an Nb–16Si in-situ composite with Fe additions prepared by arc-melting

This paper deals with phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–xFe in-situ composites (where x = 2, 4, 6 at.%, referred as to 2Fe, 4Fe and 6Fe alloys, hereafter) prepared by arc-melting. It is found that with additions of Fe, Nb₄FeSi silicide arises and microstructures of as-cast samples are consisted of dendritic-like Nb_SS phase, Nb₃Si block, and Nb₄FeSi matrix in the 2Fe and 4Fe alloys, and of the dendritic-like Nb_SS phase and Nb₄FeSi matrix in the 6Fe alloy. When heat-treated at 1350 °C for 100 h, part of the Nb₃Si phase decomposes in the 2Fe and 4Fe alloys, and the 6Fe alloy shows no change in microstructure as compared with the as-cast one. The Nb₄FeSi silicide is found to be brittle, its fracture toughness and elastic modulus are first obtained, having values about 1.22 MPa m^1/2, and 310 GPa, respectively. The fracture toughness of the bulk as-cast and heat-treated Nb–16Si–xFe samples are changed slightly by the Fe additions, which is in a range of 9.03–10.19 MPa m^1/2. It is interesting that at room temperature, strength is improved by the Fe additions, whereas at 1250 °C and 1350 °C the strength decreases. As the Fe content increased from 2 at.% to 6 at.%, for example, the 0.2% yield strength increases from 1410 MPa to 1580 MPa at room temperature, decreases from 479 MPa to 385 MPa at 1250 °C.     

Crystal structure and properties of U2Co3Al9

A new ternary compound with stoichiometry U₂Co₃Al₉ has been synthesized. It adopts the orthorhombic Y₂Co₃Ga₉-type structure (space group Cmcm, Z = 4, a = 12.824(2) Å, b = 7.515(1) Å, c = 9.249(2) Å). Measurements of dc- and ac-magnetic susceptibility, electrical resistivity, and magnetoresistivity on polycrystalline samples have been performed. The Curie–Weiss law is strictly followed, with θ_CW = −48 K and μ_eff = 3.2 μ_B. A small kink observed in the temperature dependence of the resistivity is attributed to a phase transition at T_t = 8 K. The magnetoresistivity was found to be negative at all temperatures examined below 45 K, with a sharp minimum at T_t = 8 K.     

Phase equilibria in the system Au–Cu–Si and structural characterization of the new compound Au5±xCu2±xSi

The ternary Au–Cu–Si system was investigated by means of powder X-ray diffraction (XRD) for phase identification, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) for microstructures and chemical compositions, light optical microscopy (LOM), and differential thermal analysis (DTA) for the determination of thermal effects. Three isothermal sections were constructed at 250 °C, 400 °C and 650 °C. A new ternary compound τ, Au_5±xCu_2±xSi, was identified and its crystal structure was determined by means of single crystal X-ray diffraction. It adopts a new crystal structure type in space group Pnma, Pearson symbol oP32 and shows a composition range between Au_5.6Cu_1.4Si and Au_4.4Cu_2.6Si at 250 °C. Lattice parameters were found to vary between a = 9.64–9.50 Å, b = 7.61–7.64 Å and c = 6.90–6.89 Å from the Au-rich to the Au-poor composition limit. Three vertical sections, at 10 and 30 at.% Si and at 10 at.% Cu, were constructed based on DTA data and four invariant ternary phase reactions were identified. A partial ternary reaction scheme (Scheil diagram) and a partial liquidus projection are given.