Theoretical and experimental study of the phase formation for Ti2YAl and Ti2Y′Ga (Y = Co,Fe; Y′ = Cr,Fe)

The site preference and phase formation of Ti₂YAl and Ti₂Y′Ga (Y = Co, Fe; Y′ = Cr, Fe) have been investigated by theoretical calculation and experimental study. It has been found that Ti₂CoAl and Ti₂FeAl are stable in Hg₂CuTi-type structure, while Ti₂CrGa and Ti₂FeGa compounds are most likely to form Cu₂MnAl-type structure. These results indicate that not all full-Heusler alloys follow the site preference rule, especially when the transition metals are low-valent metals. The differences of band structures and magnetic properties between Hg₂CuTi-type and Cu₂MnAl-type structures for the above compounds are also studied by first-principles calculation. It has been predicted that Ti₂FeAl, Ti₂FeGa and Ti₂CoAl are half-metallic compounds in Hg₂CuTi-type structure, while their half-metallic properties are completely destroyed as they formed Cu₂MnAl-type structure.     

On the solubility of chromium in the cubic carbide and M7C3 carbide in the WC–Co–Cr–Me systems (Me = Ti,Nb, Ta and Zr)

The solubility of Cr in cubic carbides in the systems WC–Co–TiC and WC–Co–NbC has been determined using equilibrium samples. Thermodynamic calculations were used to design the alloys through extrapolations of Gibbs energy expressions. The alloys were designed to have a microstructure containing the following phases: WC, liquid, M₇C₃, graphite and cubic carbide. The alloys were investigated using scanning electron microscopy and analyzed using energy-dispersive X-ray spectrometry.The present work shows how Cr solubility depends on which cubic carbide former is present.     

Effects of Ti,Zr, and Hf on the phase stability of Mo_ss + Mo3Si + Mo5SiB2 alloys at 1600 °C

Understanding the stability of the three-phase Mo_ss + Mo₃Si + Mo₅SiB₂ region is important for alloy design of Mo–Si–B-based refractory metal intermetallic composites. In this work, thermodynamic modeling is coupled with guided experiments to study phase stability in this three-phase region of the Mo–Si–B–X (X = Ti, Zr, Hf) system. Both the calculated and experimental results show that additions of Zr and Hf limit significantly the stability of the three-phase region because of the formation of the ternary phases MoSiZr and MoSiHf, while Ti addition leads to a much larger region of stability for the three-phase equilibrium.     

The structural,elastic, electronic and thermodynamic properties of hexagonal η-Cu6−xNixSn5 (x = 0, 0.5, 1, 1.5 and 2) intermetallic compounds

Based on first-principles calculations, the effects of various Ni concentrations on the structural, elastic, electronic and thermodynamic properties of hexagonal η-Cu₆Sn₅ compound have been systematically investigated. The results demonstrate that higher Ni concentration in the η-Cu_6−xNi_xSn₅ (x = 0, 0.5, 1, 1.5 and 2) leads to thermodynamically stable compounds, and Ni atoms preferentially occupy Cu₂ + Cu_1c sites forming the η-Cu₄Ni₂Sn₅ compound. It is also found that the unit cell volume and lattice parameter of the ‘a’ axis decrease with increasing Ni concentration, which are consistent with the other experimental results. Furthermore, the polycrystalline elastic properties are obtained from single-crystal elastic constants. Our results indicate that the addition of Ni enhances the mechanical stability, brittleness, modulus and Debye temperatures of η-Cu₆Sn₅ compound. Analyzing the electronic structure and charge density distribution provides the explanation that Ni develops distinct bonding energy to Cu and Sn in the structure.     

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.     

Role of crystalline precipitates on the mechanical properties of (Cu0.50Zr0.50)100−xAlx (x = 4, 5, 7) bulk metallic glasses

The mechanical behaviour upon compression of (Cu_0.50Zr_0.50)_100?xAl_x (x = 4, 5, 7) rods with 2 and 3 mm diameter was systematically studied and compared with the literature data. Fully amorphous bulk metallic glasses (x = 5, 7) show little permanent deformation and reproducible yield stress values. The remarkable fracture strain, observed for some apparently X-ray diffraction amorphous samples (x = 4), was found to be due to significant amounts (at least 20%) of the B2-CuZr crystalline phase. The effect of possible flaws on the external surface of the rods was evaluated by mechanical testing of either as cast and machined samples.     

Combined ab initio and experimental study of A2 + L21 coherent equilibria in the Fe–Al–X (X = Ti,Nb, V) systems

A combined theoretical and experimental study of ordering and phase separation in α-Fe alloys in the Fe–Al–X (X = Ti, Nb, V) systems is presented. The theoretical part is dedicated to the assessment of the BCC-based phase equilibrium diagram in the iron-rich zone of the ternary systems via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. The stability and the solid solubility of Al in the Fe₂X Laves phases are also included in the discussion of the ternary BCC ground state diagrams. The approach was employed in order to explore particular vertical sections of the ternary systems where a coherent two-phase microstructure can be generated with an optimal combination of volume fraction of Fe₂AlX (L2₁) Heusler type precipitates and Al content in the α-Fe (A2) matrix. The results indicate that in the Fe–Al–Ti and Fe–Al–V systems there are two kinds of phase separations of the BCC phase, (A2+ L2₁) and (B2 (FeAl structure) + L2₁). A tie-line separates both two-phase fields that shrinks and moves toward the Fe-X binary system while its direction remains almost parallel as the temperature increases. Selected experiments were performed on three alloys of the Fe–Al–Ti system belonging to vertical sections that contain this tie-line. The microstructure and composition of the matrix and precipitate phases were characterized by transmission electron microscopy (TEM) and Energy Dispersive Spectroscopy (EDS), the theoretical predictions were borne out.     

High-Temperature Deformation of Alloys of the Multicomponent Nb – Si – X (X = Mo,Ti, Hf,Zr, Cr,Al) System After Hip and After Induction Melting in the Mode of Directed Crystallization

Metal Science and Heat Treatment - The results of creep tests in the bending mode at 1150 – 1200°C for the Nb – Si – X alloy (X = Mo, Ti, Hf, Zr, Cr, Al) after hot isostatic...     

Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ₅–TiNi_2?xAl₅, x = 0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a = 0.3984(2) nm, c = 1.4073(3) nm, R_F2 = 0.0133). Titanium atoms were unambiguously located from neutron powder data. τ₅ is isotypic with the crystal structure of ZrNi₂Al₅. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ. = 0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi_2?xAl_5?y, x = 0.4, y = 0.4 and HfNi_2?xAl_5?y, x = 0.5, y = 0.2. The crystallographic relations among the structure types of Cu, TiAl₃, ZrNi₂Al₅ and Zr(Ni,Ga)₇ have been defined in terms of a Bärnighausen scheme.     

A comparison of the oxidation behaviours of Al2O3 formers and Cr2O3 formers at 700 °C – Oxide solid solutions acting as a template for nucleation

The oxidation behaviour of a number of ferritic iron based commercial steels and model alloys containing 6 and 9 wt% Cr and 0–2.5 wt% Al have been studied at 700 °C. The oxidation time ranged from 5 min to 500 h and the atmosphere consisted of flowing dry synthetic air. The oxide layers formed were analysed by SEM, GI-XRD and ToF-SIMS. The material without Al formed a (Cr,Fe)₂O₃ film with an Fe enrichment in the outer part of the layer. The Al containing alloys showed more complex oxidation behaviour. The oxidation started initially by formation of (Cr,Fe)₂O₃ with an Cr enriched inner part. With time Al was oxidized and dissolved in the inner Cr rich part of the oxide. This process continued until it eventually was transformed into α-Al₂O₃ with minute amount of Fe in the outer and Cr in the inner part of the oxide. The thickness of all oxide films ranged from 20 to 400 nm apart from the material that contained 9% Cr and no Al, which experienced breakaway oxidation after 500 h at 700 °C. This means that materials alloyed with small amounts of Al must also be considered to be protective at 700 °C, as the thicknesses of the Al₂O₃ oxides was comparable with the ones not containing Al, and as they do not experience breakaway corrosion.     

Manufacturing of the bulk amorphous Fe61Co10Zr2+xHf3−xW2Y2B20 alloys (where x = 1, 2, 3) their microstructure,magnetic and mechanical properties

In this paper the microstructure, magnetic properties and mechanical studies results for Fe₆₁Co₁₀Zr_2+xHf_3?xW₂Y₂B₂₀ (for x = 1, 2 or 3) alloys are presented. The samples used in the investigations were obtained by a suction-casting method. The samples were produced in the forms of rods with diameter of 1 mm and length of about 20 mm and plates with thickness of 0.5 mm and surface area of about 100 mm². The results show that the best soft magnetic properties were achieved by Fe₆₁Co₁₀Zr₃Hf₂W₂Y₂B₂₀ amorphous alloy in the form of a plate. This sample has the highest value of saturation magnetization (1.09 T) and the smallest values of coercivity (H_C = 1.5 A/m) and core losses. All investigated samples of amorphous alloys were characterized by substantially greater values of microhardness and, unfortunately, slightly lower values of wear resistance compared with their crystalline counterparts.     

Preparation and characterization of sol–gel derived (1 0 0)-textured Pb(Zr,Ti)O3 thin films: PbO seeding role in the formation of preferential orientation

Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary region were deposited onto Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates using a sol–gel method. A seeding layer was introduced between the most underlying surface of the PZT film and the platinum electrode surface to control the texture of the PZT thin film. The lead oxide seeding layer resulted in the formation of a single-phase perovskite and absolutely (1 0 0)-textured PZT film. SEM, XRD, XPS, and AES were used to characterize the evolution of the lead oxide layer and the PZT thin films. The growth kinetic mechanism of the (1 0 0)-textured PZT thin films was proposed phenomenologically. The ferroelectric and piezoelectric properties of the PZT films were also evaluated and discussed in association with different preferential orientations.     

Influence of the Exchange-Correlation Functional on the Energy of Formation and Magnetic Behavior of Binary D0<Subscript>3</Subscript> Intermetallic Compounds FeM<Subscript>3</Subscript> (M = Ti,Zr, Hf)

In recent years, ab-initio calculations based on the density functional theory became a commonly used tool in supporting, improving or even refuting experimental results in different research fields. In this work we discuss some accuracy aspects inherent to ab-initio electronic structure calculations regarding the understanding of different structural, electronic and magnetic physical properties. In particular, we discuss the dependence of the magnetic ground-state and the formation energy with the exchange-correlation functional for the binary intermetallic compounds FeTi₃, FeZr₃ and FeHf₃ with D0₃ crystal structure. All exchange-correlation schemes used were based on the generalized gradient approximation. It is the aim of the present paper to call the attention of the community to some fundamental aspects of the calculations that can influence the final results and the conclusions derives from it.