A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

Crystallization processes and phase evolution in amorphous Fe–Pt–Nb–B alloys

Fe–Pt system is nowadays widely studied due to its potential applications as magnetic recording media. The hard magnetic FePt L1₀ phase has extremely promising potential as permanent magnet with high magnetocrystalline anisotropy. Of recent interest is also the developing of the hard magnetic phase from an amorphous precursor by appropriate crystallization processes. The melt-spun amorphous Fe₆₈Pt₁₃Nb₂B₁₇ alloy has been submitted to dynamical annealing and its phase transformation during the process has been monitored by differential scanning calorimetry and in situ energy-dispersive X-ray diffraction of the synchrotron radiation. In the first stage of crystallization, -Fe and cubic FePt phases are formed from the amorphous precursor. At around 600 °C superlattice Bragg reflections corresponding to tetragonal FePt are indexed in the XRD spectra and -Fe phase diminishes drastically. Finally, between 900 °C and 975 °C the tetragonal superlattice peaks disappear and cubic FePt phase is formed again. This reversible order–disorder transformation is accompanied by a strong uniaxial lattice expansion of the cubic FePt unit cell. The system show promising features for the co-existence of hard and soft exchange coupled magnetic phases crystallized from FePt-based amorphous precursors.     

Rapid solidification and mechanical alloying of Al–Co–Ag ternary alloys for skeletal silver cobalt synthesis

Various precursors of Al₇₅Co_xCu_20−x (x=0–7.5) ternary alloys for formation of the skeletal and metastable materials with large specific surface area were prepared by slow, rapid solidification or mechanical alloying. Their structures were examined by SEM observation, EPMA, X-ray diffraction and DSC measurements. The as-prepared structures of these alloys strongly depended on the applied processes. Three equilibrium phases of AlAg₂, Al₉Co₂ and Al were formed by slow solidification. The formation of the AlAg₂ and Al₉Co₂ phases was suppressed and the super-saturated Al monophase was obtained by rapid solidification in a wide composition range. In the case of mechanical alloying, cobalt atoms were fully dissolved in the Al phase but the formation of the AlAg₂ phase was not suppressed. It was interpreted that the precipitation of the Ag₂Al phase from the super-saturated Al phase was enhanced by plastic strain introduced during mechanical alloying. In mechanical alloying of the Al based alloys including silver, it was difficult to dissolve silver atoms into the Al phase beyond the equilibrium solubility. Rapid solidification proved to be an excellent process to prepare the precursor for chemical leaching.     

The zirconium–platinum phase diagram

Phase relationships were studied in Pt-rich, near-equiatomic Zr–Pt alloys. The composition range of the previously unreported rhombohedral compound Zr₃Pt₄, isomorphous with Zr₃Pd₄ above room temperature, extends to the Zr-rich composition Zr₉Pt₁₁ by the formation of lattice vacancies on certain Pt sites. A metastable tetragonal Zr₉Pt₁₁ compound is formed, however, when vacancy formation is inhibited. The rhombohedral structure undergoes a displacement transformation on cooling between 90 and 140 °C to a low-temperature structure that is presumably triclinic. The orthorhombic compound ZrPt is stable from room temperature to 1590 °C where it transforms to a cubic B2-type structure. Structural data are given for the compounds ZrPt, Zr₉Pt₁₁, Zr₃Pt₄ and Zr₇Pt₁₀, and a complete Zr–Pt phase diagram is presented.     

Boron content dependence of crystallization, glass forming ability and magnetic properties in amorphous Fe-Zr-B-Nb alloys

The glass forming ability (GFA) was investigated in Fe_91−xZr₅B_xNb₄ alloys with B contents of 0–36 at.%. The GFA changes with B content, and fully amorphous alloys were prepared by melt spinning for B contents between 5 and 30 at.%. The amorphous alloys crystallize with a primary crystallization mode in the low B content range of 5≤x≤20 at.%, but in the eutectic mode in the high B content range of 20<x<30 at.%. A single new metastable Fe-Zr-B-Nb cubic phase with a lattice constant of 1.0704 nm, a saturation magnetization of 137 emu/g and a coercivity of 7.3 Oe at room temperature is formed when crystallizing in a polymorphous mode at x=30 at.%. The glass transition temperature (T_g), crystallization temperature (T_x), Curie temperature (T_c) and saturation magnetizations (M_s) of the amorphous alloys increase with increasing B content, but the coercivity (H_c) decreases. As the B content exceeds 20 at.%, not only increase the T_g, T_x and GFA sharply, due to the change of crystallization mode, but also the concentration dependence of the T_c and M_s changes. It is concluded that the amorphous alloys have better GFA, thermal stability and soft magnetic properties for the high B contents of 25–30 at.% than for the low B contents of 5–20 at.%.     

Effects of replacing Ni by Co on the crystallization behaviors of Al–Ni–La amorphous alloys

Effects of replacing Ni by Co on the crystallization behaviors of three Al–Ni–La amorphous alloys, i.e. Al₈₅Ni₉La₆, Al₈₆Ni₉La₅ and Al₈₇Ni₈La₅ were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the glass-forming ability decreases when Ni is replaced by excessive Co. Meanwhile replacing Ni by Co improves the thermal stability, enlarges the supercooled liquid region ΔT_x and promotes the precipitation of the metastable phase(s) as the primary phase. The apparent activation energy E_a1 of the first reaction changes complicatedly during the replacement and is strongly dependent on the type of the primary phase, i.e. diffusion of atoms.     

Structures and superparamagnetic properties of overaged Fe–Al–Mn–C alloys

Microstructures and superparamagnetic properties in aged-hardened Fe–9%Al–30%Mn– (x)C,Si alloys, resulting from overaging at a temperature of 823 K for 48 h to 313 days, have been investigated by transmission electron microscopy (TEM), X-ray diffraction patterns, and vibrating sample magnetometry (VSM). The results reveal that the precipitate κ-phase [(Fe,Mn)₃AlC] decomposition in this alloy, overaged at 823 K for one week, resulted from two separate mechanisms: (1) wetting of the antiphase boundary segment (APBs) of D0₃ [(Fe/Mn)₃Al] domains by the B2 [(Fe/Mn)Al] phase; and (2) precipitation of the B2 [(Fe/Mn)Al] phase within the domain. A superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for ≈120–313 days. The super-soft magnetic property was mainly attributable to the ferromagnetic spinel-ordered (B2 [(Fe/Mn)Al]+D0₃ [(Fe/Mn)₃Al]) phases and ordered B2 with monoclinic ′Mn structures.     

Effect of AI content on the phase structure and the hydrogenation characteristic of La（Mg1-xAlx） alloys

La（Mg1-xAlx） （x=0.2, 0.4, 0.6, 0.8） alloys have been prepared using induction melting followed by annealing. It is found that partial substitution of Mg by Al does not lead to a change in crystal structure, and the alloys have a single LaMg phase when x 〈 0.4. The lattice parameter of the LaMg phase decreases obviously after the partial substitution of Mg by Al. However, further substitution of Mg by Al leads to the coexistence of multiple phases when x ≥ 0.6. The alloys consist of the LaMg, LaAl, LaAl2, and La5Al4 phases. The LaMg phase decreases, whereas the La5Al4 phase increases with the increase in x. The Al-substituted La（Mgo.6Al0.4） alloy can be hydrogenated into the tetragonal LaH3, cubic LaH3, MgH2, and LaPd under 5 MPa at 473 K for 5 d.     

Devitrification of Al-Ce Amorphous Ribbon Investigated Using In situ High Energy X-ray Diffraction

Metastable phases formed during vitrification or devitrification open an avenue to study the intrinsic structural hierarchy in amorphous materials. The phase transformation sequence of Al89Ce11 amorphous ribbon was investigated using differential scanning calorimetry, in situ high energy X-ray diffraction (HEXRD) and ex situ high-resolution scanning transmission electron microscope. The results reveal the devitrification pathway following the reaction: amorphous → ε-Al₆₀Ce₁₁ +  fcc-Al → η-Al₄₁Ce₅ +  fcc-Al → Al₁₁Ce₃ +  fcc-Al. It has been found that both ε-Al₆₀Ce₁₁ and η-Al₄₁Ce₅ metastable phases have same Ce-centered 1-6-6-6-1 motif, suggesting that the structural motif can be inherited. Formation of metastable phase with large unit cell is related to the short-range orders developed during solidification. Structural heredity provides a new method to tailor the microstructure and properties of Al-based alloys based on genetic mechanism.     

Phase evolution of Mg–Al–Zr nanophase alloys prepared by mechanical alloying

Al–Mg and Al–Mg–Zr alloys were processed by mechanical alloying. The phase constitution of the powders was strongly dependent on the composition of the starting mixture. In as-milled powders, an Al(Mg) solid solution was formed with up to 40 at% Al, which after annealing transformed to the equilibrium β-Al₃Mg₂ phase. For high Mg concentrations (60–90 at%) the dominant phase was γ-Al₁₂Mg₁₇ in accordance with the equilibrium phase diagram. The addition of Zr led to the appearance of Zr–Al intermetallics causing Mg to precipitate out of the Al(Mg) solution. The effect of zirconium was also to refine the structure and to retard grain growth.     

Phase formation during mechanical alloying and subsequent low-temperature heat treatment of Al–27.4at%Fe–28.7at%C powders

Phase formation during high energy ball milling of a ternary elemental powder mixture with a composition of Al–27.4at%Fe–28.7at%C and during low temperature heat treatment of the milled powder was studied. It was found that an amorphous phase formed during prolonged milling. During heating the shorter time milled powder, Al and Fe reacted first, forming the AlFe phase and then at a higher temperature, AlFe reacts with Fe and C, forming the AlFe₃C_0.5 phase. During heating the longer time milled powder which contains a substantial amount of amorphous phase, the amorphous phase partially crystallizes first, forming the AlFe and AlFe₃C_0.5 phases, and then AlFe reacts with the remaining amorphous phase, forming the AlFe₃C_0.5 phase. Overall, mechanical alloying of Al, Fe and C elemental phases enables formation of an amorphous phase, while low temperature heat treatment of mechanically milled powder facilitates formation of AlFe and AlFe₃C_0.5 phases.     

Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Electrical, magnetic, thermal and thermoelectric properties of the “Bergman phase” Mg32(Al,Zn)49 complex metallic alloy

The Mg–Al–Zn system of intermetallics contains an exceptional crystalline phase Mg₃₂(Al,Zn)₄₉, named the Bergman phase, whose crystal structure is based on a periodic arrangement of icosahedral Bergman clusters within the giant-unit-cell, so that periodic and quasiperiodic atomic orders compete in determining the physical properties of the material. We have investigated electrical, magnetic, thermal and thermoelectric properties of a monocrystalline Bergman phase sample of composition Mg_29.4(Al,Zn)_51.6, grown by the Bridgman technique. Electrical resistivity is in the range ρ ≈ 40 μΩ cm and exhibits positive-temperature-coefficient with T² dependence at low temperatures and T at higher temperatures, resembling non-magnetic amorphous alloys. Magnetic susceptibility χ measurements revealed that the sample is a Pauli paramagnet with a significant Landau diamagnetic orbital contribution. The susceptibility exhibits a weak increase towards higher temperature. Combined analysis of the ρ(T) and χ(T), together with the independent determination of the Pauli susceptibility via the NMR Knight shift suggests that the observed temperature dependence originates from the mean-free-path effect on the orbital susceptibility. The electronic density of states (DOS) at the Fermi energy E_F was estimated by NMR and was found to amount 72% of the DOS of the fcc Al metal, with no evidence on the existence of a pseudogap. Thermal conductivity contains electronic, Debye and hopping of localized vibrations terms, whereas thermopower is small and negative. High structural complexity of the Bergman phase does not result in high complexity of its electronic structure.     

The Er–Ni–B system

The phase diagram of the ternary Er–Ni–B system at 1070 K has been built using the results of X-ray diffraction. As a result of our research the existence of twelve ternary borides has been confirmed, the composition of the boride ErNi₇B₃ (space group I4₁/amd, own structure type) has been refined and three new compounds have been found, namely: ErNi_6.5B₃ (cubic structure), Er_0.917Ni_4.09B (space group P6/mmm, own structure type), ErNi₈B₂ (unknown structure). The two latter borides exist only in as-cast samples.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Structure of the ternary phase Co3Al8Ga and formation of the gallium-containing decagonal phase d-Co(Al, Ga)3(m)

The structure of the ternary phase Co₃Al₈Ga, Pearson symbol oI96, space group Immm, a=12.0081(7) Å, b=7.5701(6) Å, c=15.394(1) Å is isotypic with Co₂NiAl₉. Powder diffraction data are reported for this ternary intermetallic compound. Using liquid quenching, the metastable pseudoternary decagonal phase d-Co(Al, Ga)₃(m) was obtained in the aluminium-rich portion of the ternary system Co–Al–Ga. Gallium substitutes for aluminium atoms in the d-Co(Al, Ga)₃(m) phase up to a mol fraction x_Ga=0.10. In the phase of the binary system Co–Al richest in aluminium, Co₂Al₉, the aluminium atomic positions can be occupied by gallium up to a gallium content of x_Ga=0.05.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Solidification of levitated Nd–Fe–B alloy droplets at significant bulk undercoolings

Nd–Fe–B alloys with composition of Nd₁₆Fe₇₆B₈, Nd₁₈Fe₇₃B₉ and Nd₂₂Fe₆₇B₁₁ were melted and solidified using an electromagnetic levitation technique. Two types of solidification behavior were observed depending on the bulk undercooling achieved prior to solidification. The samples with small undercoolings were solidified by the predominant primary formation of the Nd₂Fe₁₄B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd₂Fe₁₇B_x compound (x1) plus the subsequent formation of Nd₂Fe₁₄B. The critical undercooling for the primary Nd₂Fe₁₇B_x formation was determined to be 40, 70 and 130 K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd₂Fe₁₇B_x compound was estimated to be 1423, 1393 and 1283 K, respectively. The Nd₂Fe₁₇B_x compound was found to decompose into a mixture of Fe plus Nd₂Fe₁₄B due to the slow cooling rates of the samples. It was suggested that the metastable Nd₂Fe₁₇B_x compound may have a lower interfacial energy than that of the stable Nd₂Fe₁₄B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.