Influence of similar atom substitution on glass formation in (La–Ce)–Al–Co bulk metallic glasses

The glass-formation range of bulk metallic glasses (BMGs) based on lanthanum and cerium was pinpointed in the La–Al–Co, Ce–Al–Co and pseudo-ternary (La–Ce)–Al–Co systems, respectively, by copper mold casting. Through the stepwise substitution of La for solvent Ce in (La_xCe_1−x)₆₅Al₁₀Co₂₅ alloys (0 < x < 1), the fully glassy rods of the (La_0.7Ce_0.3)₆₅Al₁₀Co₂₅ alloy can be successfully produced up to 25 mm in diameter by tilt-pour casting. Compared with the glass-forming ability (GFA) of single-lanthanide-based alloys, La₆₅Al₁₀Co₂₅ and Ce₆₅Al₁₀Co₂₅, the coexistence of La and Ce with similar atomic size and various valence electronic structure obviously improves the GFA of (La_xCe_1−x)₆₅Al₁₀Co₂₅ BMGs, and this cannot be explained by the conventional GFA criteria for BMGs, e.g. atomic size mismatch and negative heats of mixing. A thermodynamic model was proposed to evaluate this substitution effect, which gives a reasonable explanation for the obvious improvement of GFA induced by the coexistence of similar atoms.     

Deformation behavior of isothermally forged Ti–5Al–2Sn–2Zr–4Mo–4Cr powder compact

The isothermal deformation behavior of hot isostatic pressed (HIPed) Ti–5Al–2Sn–2Zr–4Mo–4Cr(Ti-17) powder compact was investigated by compression testing in the temperature range of 810–920 °C and constant strain rate range of 0.001–1 s^?1. The true stress–true strain curves of the powder compact exhibit flow oscillation and flow softening phenomenon in both beta field and beta + alpha field. The flow softening behavior is related to the globularization of the primary acicular microstructure and deformation heating. The apparent activation energy for deformation in beta field is estimated to be 149 kJ mol^?1, indicating that the deformation is controlled by diffusion. The high apparent activation energy of 537 kJ mol^?1 for deformation in beta + alpha field may be related to the dynamic recrystallization of the primary acicular microstructure. Constitutive equations with the form of Arrhenius-type hyperbolic-sine relationship are proposed to delineate the peak flow stress as a function of the strain rate and the temperature for isothermal forging HIPed Ti-17 powder compact.     

Formation of bulk metallic glasses in Cu45Zr48−xAl7REx (RE = La,Ce, Nd,Gd and 0 ≤ x ≤ 5 at.%)

We report a series of bulk metallic glass-forming alloys of compositions (Cu₄₅Zr_48−xAl₇RE_x, RE = La, Ce, Nd, Gd and 0 ≤ x ≤ 5 at.%). By using a conventional copper mold sucking method, alloys with diameters ranging from 5 to 10 mm can be readily solidified into an amorphous structure without detectable crystallites. The best glass-forming ability is obtained for the alloys Cu₄₅Zr₄₆Al₇RE₂. Possible effects of RE addition on the glass-forming ability are discussed. In addition, the compositional effect on mechanical properties of Zr–Cu–Al–Gd alloys is presented.     

Fe-based bulk metallic glass composites without any metalloid elements

Glass-formation of Fe-based bulk metallic glasses (BMGs) and their composites is strongly dependent on their metalloid content. A good Fe-based glass former usually needs a metalloid content of ～20 at.% or above; however, the high content of the metalloids usually has side-effects on the performance of the alloys. In this paper, we developed a series of metalloid-free Fe-based BMG matrix composites with a diameter of more than 10 mm in the Fe–Co–La–Ce–Al–Cu system. During cooling, phase separation took place, i.e. Fe-rich and Fe-depleted liquids formed. Upon subsequent cooling, the body-centered cubic (bcc)-Fe(Co, Al) solid solution and Ce(Fe, Co)₂ intermetallic phase precipitated out of the Fe-rich liquid and the remaining Co–La–Ce–Al–Cu liquid was eventually vitrified to form the amorphous matrix. Due to elemental partitioning, it was found that, for a given Fe content, there exists an optimum compositional range for Co and Ce so that a large fraction of the glassy matrix can form. In addition, the final microstructure of the current composites was strongly dependent on the cooling rates applied: excessively fast cooling rates restricted the diffusion and precipitation of the Fe atoms, which reduced formation of the glassy matrix.     

Solidification paths of multicomponent monotectic aluminum alloys

Solidification paths of three ternary monotectic alloy systems, Al–Bi–Zn, Al–Sn–Cu and Al–Bi–Cu, are studied using thermodynamic calculations, both for the pertinent phase diagrams and also for specific details concerning the solidification of selected alloy compositions. The coupled composition variation in two different liquids is quantitatively given. Various ternary monotectic four-phase reactions are encountered during solidification, as opposed to the simple binary monotectic, L′ → L′′ + solid. These intricacies are reflected in the solidification microstructures, as demonstrated for these three aluminum alloy systems, selected in view of their distinctive features. This examination of solidification paths and microstructure formation may be relevant for advanced solidification processing of multicomponent monotectic alloys.     

La-based bulk metallic glasses with critical diameter up to 30 mm

We report composition optimization, thermal and physical properties of new La-based bulk metallic glasses with high glass forming ability (GFA) based on a ternary La₆₂Al₁₄Cu₂₄ alloy. By refining the (Cu, Ag)/(Ni, Co) and La/(Cu, Ag) ratios in the La–Al–(Cu,Ag)–(Ni, Co) pseudo-quaternary alloy, the formation of 30 mm diameter of La₆₅Al₁₄(Cu_5/6Ag_1/6)₁₁(Ni_1/2Co_1/2)₁₀ bulk metallic glass (BMG) alloy is achieved using water quenching. The origin of the high GFA was investigated from the kinetic, structural and thermodynamic points of view, and was found to be due to the smaller difference in Gibbs free-energy between the amorphous and crystalline phases in the pseudo-quaternary alloy. These alloys exhibit low glass transition temperatures, below 430 K, and relatively wide supercooled liquid regions of 40–60 K. Mechanical tests on these alloys show a fracture strength of 650 GPa, Vicker’s hardness 200 kg mm⁻², Young’s modulus 35 GPa, shear modulus 13 GPa and Poisson ratio 0.356. The La-based BMGs are useful for both scientific and engineering applications.     

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

Hot compression tests on samples of the TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloy have been done within the temperatures of 750–950 °C and strain rate ranges of 0.1–10 s^?1 to 40–60% height reduction. The experimental results show that the flow stress behavior can be described by an exponential law for the deformation conditions. The hot deformation activation energy (Q) derived from the experimental data is 538 kJ mol^?1 with a strain rate sensitivity exponent (m) of 0.107. Optical microstructure evidence shows that dynamic recrystallization (DRX) takes place during the deformation process. Moreover, only α DRX grains are founded in the titanium alloys. The influences of hot working parameters on the flow stress behavior and microstructural features of TC11 alloy, especially on the type of phase present, the morphologies of the α phase, grain size and DRX are analyzed. The optimum parameters for hot working of TC11 alloy are developed.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

Formation and mechanical properties of Ni-free Zr-based bulk metallic glasses

Glass formation and mechanical properties of Zr–Al–Co–Cu–Ag bulk metallic glasses (BMGs) were investigated. The glass-forming ability (GFA) of Zr₅₅Al₂₀Co₂₀Cu₅ alloy is significantly improved with minor addition of Ag, indicating by the impressive increase of the critical diameter of glass formation from 5 mm for Zr₅₅Al₂₀Co₂₀Cu₅ to 16 mm for (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₇Ag₃ and (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₅Ag₅ alloys. The Zr–Al–Co–Cu–Ag BMGs exhibit high compressive strength of 2160–2280 MPa and distinct plasticity of 0.6–2.5%. The Zr-based BMGs with outstanding GFA and mechanical properties as well as low-level cytotoxicity elements are expectative for industrial and biological applications.     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Very rapid growth of aligned carbon nanotubes on metallic substrates

Aligned carbon nanotubes were grown on metallic substrates using a microwave plasma-enhanced chemical vapor deposition system. The substrates were Ni and Cu, and the catalyst was an Fe–Si alloy thin film. The effects of substrate and catalyst characteristics and growth temperature were studied. We show, via the use of a microwave shield, and with optimized catalyst thickness and growth temperature, that carbon nanotubes (CNTs) with a length of up to 2.15 mm and an unprecedently high growth rate of 177 μm min^?1 can obtained. Non-isothermal growth was performed to investigate the growth kinetics and therefore to obtain the activation energies of CNTs grown on Ni and Cu. Very similar activation energies for the growth of CNTs on Ni and Cu substrates were determined to be 101.5 (1.05 eV) kJ mol^?1 and 102.3 (1.06 eV) kJ mol^?1, respectively.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.     

Hydrogen diffusion and trapping in a precipitation-hardened nickel–copper–aluminum alloy Monel K-500 (UNS N05500)

Hydrogen uptake, diffusivity and trap binding energy were determined for the nickel–copper–aluminum alloy Monel K-500 (UNS N05500) in several conditions. The total atomic hydrogen (H) concentration increased from 0 to 132 wppm as the hydrogen overpotential decreased to ?0.5 V in alkaline 3.5% NaCl electrolyte at 23 °C. The room-temperature H diffusion coefficient ranged from 0.9 to 3.9 × 10^?14 m² s^?1 for single-phase solid solution, aged, and cold worked then aged microstructures. Diffusivity was independent of lattice H concentration but depended weakly on metallurgical condition, with slower H diffusion after aging. The apparent activation energy for H diffusion was in the range of 29–41 ± 1.5 kJ mol^?1 at the 95% confidence level. The lower value approached nearly perfect lattice transport, while the high value was strongly influenced by traps of low-to-intermediate strength. Atomic hydrogen trapping at metallurgical sites, strongly suggested to be spherical-coherent γ′ (Ni₃Al) precipitates, was evident in the aged compared to the solution heat treated + water-quenched condition. Both thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy interpreted as Ni₃Al (10.2 ± 4.6 kJ mol^?1) interfaces was significantly less than the activation energy for perfect lattice diffusion (25.6 ± 0.5 kJ mol^?1) in this nickel-based alloy system.     

Kinetics and microstructure of laser chemical vapor deposition of titanium nitride

Titanium nitride (TiN) films were deposited onto Ti–6Al–4V substrates by laser chemical vapor deposition using a cw CO₂ laser and TiCl₄, N₂ and H₂ reactant gases. Laser-induced fluorescence (LIF) and pyrometry determined relative titanium gas phase atomic number density and deposition temperature, respectively. Auger electron spectroscopy found substoichiometric films, caused by diffusion of nitrogen through TiN grain boundaries to the titanium alloy substrate. The morphology is a polyhedral structure with crystallite sizes ranging from 10 to 1000 nm. The activation energy was calculated to be 122 ± 9 kJ mol⁻¹ using growth rates measured by film height and 117 ± 23 kJ mol⁻¹ using growth rates measured by LIF signals. Above N₂ and H₂ levels of 1.25% and below TiCl₄ input of 4.5%, the growth rate has a half-order dependence on nitrogen and a linear dependence on hydrogen. The rate-determining steps of TiN growth are discussed.     

Coercivity enhancement in boron-enriched stoichiometric REFeB melt-spun alloys

Considerable enhancement of magnetic properties was attained in initially stoichiometric nanophase RE₁₂Fe₈₂B₆ melt-spun alloys (RE = Nd, Nd + Pr) by means of an excess B content (10 at%) and additions of Zr and Co (2% and 7%, respectively). The intrinsic coercivity exhibited a marked improvement (with respect to the stoichiometric 6 at% B alloy), within the range 50–65%, with a maximum of 1161 ± 14 kA m⁻¹ for the B-rich and Zr-containing alloy, together with an excellent combination of remanence and energy density values of 0.90 ± 0.01 T and 137 ± 4 kJ m⁻³, respectively. Further Co addition led to a Curie temperature increase, while preserving high coercivity (1176 ± 31 kA m⁻¹) and useful energy densities (119 ± 4 kJ m⁻³). Results were interpreted on the basis of alloy microstructural features and on variations of the intrinsic magnetic properties, supported by micromagnetic calculations.     

Phase separation kinetics in a Fe–Cr–Al alloy

The α–α′ phase separation kinetics in a commercial Fe–20 wt.% Cr–6 wt.% Al oxide dispersion-strengthened PM 2000? steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748 K for times up to 3600 h. A progressive decrease in the Al content of the Cr-rich α′ phase was observed at 708 and 748 K with increasing time, but no partitioning was observed at 673 K. The variation in the volume fraction of the α′ phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264 kJ mol^?1, obtained after fitting the experimental results to an Austin–Rickett type equation, indicates that phase separation in PM 2000? is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.     

Phase separation and microstructure evolution of rapidly quenched Gd–Hf–Co–Al alloys

Phase separated metallic glasses were prepared in the Gd–Hf–Co–Al system by rapid quenching of the melt. Due to the strong positive enthalpy of mixing between the main constituent elements Gd and Hf (ΔH_Gd–Hf = +11 kJ/mol) a heterogeneous microstructure is formed consisting of two amorphous phases, which are Gd-enriched and Hf-enriched, respectively. The size of the phase separated regions varies from 0.1 μm to 5 μm, depending on alloy composition and cooling rate. Different types of microstructure, such as an interconnected structure or a droplet structure were obtained as a function of cooling rate. The microstructure of the phase separated metallic glasses is determined not only by their composition, the critical temperature, and the shape of the miscibility gap, but also by the viscosity and diffusivity of the melt.     

Oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si,Mn, Ni,Ge, Y,Zr, La,and Ta) alloys at 1173 K in air

The oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si, Mn, Ni, Ge, Y, Zr, La, and Ta) alloys was investigated at 1173 K in air for up to 630 ks under a heat-cycle condition between 1173 K and room temperature. During the sulfidation processing the TiAl–2 at.%Ta alloy formed Ta-aluminides on the TiAl₃ layer, while the alloys containing Mn, Ni, Y, and Zr formed a TiAl₃ (TiAl₂ included) layer including a small amount of the third element, like the TiAl binary alloy. The cross-sectional microstructure of the TiAl–2 at.%Ta alloy shows the sequence: oxide scale/TiAlTa/TiAl₂/alloy substrate; and the cross sections of the alloys containing Mn, Ni, Y, and Zr are: oxide scale/Ti₃Al/alloy substrate. The TiAl–2 at.%Ta alloy showed some scale exfoliation at the initial stage of oxidation, but very little exfoliation after long oxidation times, whereas alloys containing other third elements such as Si and Ge showed little exfoliation at the first several cycles and then tended to exfoliate significantly, resulting in very rapid oxidation. The TiAlTa/TiAl₂ layers formed by the reaction between the Ta-aluminide and TiAl₃ improve the oxidation properties of the TiAl–2 at.%Ta alloy.     

Effect of C,Ti, Zr and B alloying on fracture mechanisms in hot-rolled Fe–40 (at.%)Al

This paper reports a study of fracture behavior of FeAl-based intermetallic alloys with the addition of carbon, titanium, zirconium and boron (Fe–40Al–1C, Fe–40Al–1Ti and Fe–40Al–Zr–B). The alloys were prepared by modified processing technology of vacuum induction melting and hot rolling in special stainless steel sheath. Tensile and fracture toughness tests were carried out at 20 °C, 400 °C, 600 °C, 700 °C and 800 °C. The alloy showed best fracture toughness and tensile properties with Zr and B addition. The fracture toughness at 600 °C was comparable with values in stainless steels and nickel-based superalloys. The fractographic analysis revealed the change of fracture micromechanisms with temperature. Moreover, under specific conditions, the fracture micromechanisms were different in tensile and fracture toughness specimens.     

Fatigue crack growth behavior of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass-forming alloy

Fatigue crack growth behavior was studied for a Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass in ambient air, demonstrating a fatigue threshold of ΔK_TH = 1.4 MPa√m and a Paris law exponent of 1.7. A nearly stress intensity-independent crack growth regime occurred at 2.5 × 10^?8 m cycle^–1, suggesting an environmental influence of ambient air on the fatigue crack growth, as has been observed for Zr–Ti–Ni–Cu–Be bulk metallic glasses. However, this environmental fatigue effect was shifted to 25× higher growth rates due to the different chemistry.