La Alloy Research Articles

Oxidation behavior and electrical property of ferritic stainless steel interconnects with a Cr–La alloying layer by high-energy micro-arc alloying process

Chromium volatility, poisoning of the cathode material and rapidly decreasing electrical conductivity are the major problems associated with the application of ferritic stainless steel interconnects of solid oxide fuel cells operated at intermediate temperatures. Recently, a novel and simple high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO 3-based coatings for the type 430 stainless steel interconnects using a LaCrO 3–Ni rod as deposition electrode. In this work, a Cr–La alloying layer is firstly obtained on the alloy surface by HEMAA using Cr and La as deposition electrode, respectively, followed by oxidation treatment at 850 °C in air to form a thermally grown LaCrO 3 coating. With the formation of a protective scale composed of a thick LaCrO 3 outer layer incorporated with small amounts of Cr-rich oxides and a thin Cr 2O 3-rich sub-layer, the oxidation rate of the coated steel is reduced remarkably. A low and stable electrical contact resistance is achieved with the application of LaCrO 3-based coatings, with a value less than 40 mΩ cm 2 during exposure at 850 °C in air for up to 500 h.

The Skin Effect in an Mg-RE High Pressure Die Cast Alloy

Cross-sectional microhardness maps of cast-to-shape flat tensile specimens have been obtained for a binary Mg-3.44 mass% La alloy. Higher microhardness numbers were generally found near the casting surface, at the corners and along the segregation band. The higher hardness values were ascribed to the finer solidification microstructure near the surface and to localized positive macro segregation. The majority of lower hardness numbers was found at the core region. Lower hardness values were ascribed to the coarser grain size prevalent at the core and to dispersed microporosity. The non uniformity of the harder surface layer in both depth and hardness appeared related to local homogeneities in the grain size distribution caused by the scattered presence of large externally solidified grains.

Mechanical properties as a function of microstructure in the new Mg–Al–Ca–La alloy solidified under different conditions

This paper aimed the study of the ZAXLa05413 magnesium alloy (Mg–0.5wt%Zn–4wt%Al–1wt%Ca–3wt%La) by analyzing the influence of solidification parameters on mechanical properties. In this way the studied alloy was solidified under slow cooling condition in order to make a thermal analysis and to obtain a completely equiaxial macrostructure, in the other hand, under a unidirectional solidification condition to obtain columnar and equiaxial structures. The ingots were submitted to metallographic analysis (macro and microstructure), mechanical characterization by tensile testing and hardness measurements in specimens extracted at different positions along the height of the ingot. Expressions correlating the mechanical behavior with microstructure parameters were determined permitting the establishment of general correlations among grain size and secondary dendritic arm spacing with solidification processing variables. The results confirm a direct correlation between the structural refinement (macro and micro) and the increase in ultimate tensile strength (σu) and also the yield strength (σe) of the magnesium alloy ZAXLa05413.

Fabrication of advanced La-incorporated Hf-silicate gate dielectrics using physical-vapor-deposition-based in situ method and its effective work function modulation of metal/high-k stacks

Lanthanum (La) incorporation into Hf-silicate high-permittivity (high-k) gate dielectrics was conducted using a physical-vapor-deposition (PVD)-based in situ method. PVD-grown metal Hf, La, and Hf–La alloys on base SiO2 oxides received in situ annealing to form high-quality HfLaSiO dielectrics, and subsequent deposition of metal gate electrodes was carried out to fabricate advanced metal/high-k gate stacks without breaking vacuum. The in situ method was found to precisely control La content and its depth profile and to tune the effective work function of metal/high-k stacks. Remarkable leakage current reduction of almost seven orders of magnitude compared with conventional poly-Si/SiO2 stacks and excellent interface properties comparable to an ideal SiO2/Si interface were also achieved at an equivalent oxide thickness of around 1.0 nm. Our x-ray photoelectron spectroscopy analysis revealed that, as previously suggested, effective work function modulation due to La incorporation is attributed to the interface dipole (or localized sheet charge) at the bottom high-k/SiO2 interface, which is crucially dependent on the La content at the interface. Moreover, it was found that high-temperature annealing causing interface oxide growth leads to redistribution of La atoms and forms the uppermost La-silicate layer at the metal/high-k interface by releasing the dipole moment at the bottom high-k/SiO2 interface. Based on these physical and electrical characterizations, the advantages and process guidelines for La-incorporated dielectrics were discussed in detail.

CeRuGe and CeRuSi: Heavy fermion systems with some unusual features

We have investigated the physical properties of CeRuSi, its homologue CeRuGe and some doped La alloys. All of them present similar properties: large paramagnetic temperatures θp ≈ −100 K and the level off of χ(T) below ≈ 10 K in coincidence with the maximum decrease of p(T), which starts to drop below 25 K. The respective γ(CeRuSi) ≊ 0.18 and γ(CeRuGe) ≊ 0.15 J/molK2 coefficients reveal a heavy fermion character, with a Kondo temperature TK ≊ 50 K comparable to the crystal field splitting. In both compounds a broad maximum in CP(T)/T is observed around 5 K, whose origin is yet not clear. In contrast to the usual behavior observed in Kondo lattice systems, when Si is replaced by Ge TK ∝ θp and 1/γ practically does not decreases despite the unit cell volume of CeRuGe is about 3% larger than that of CeRuSi. We discuss possible origins for these unusual features.

Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints

The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce0.8Zn0.2)Sn3 and (La0.9Zn0.1)Sn3 intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber- and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface of oxidized (La0.9Zn0.1)Sn3 intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However, aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging processes, AuZn8 intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate from the solder matrix to the solder/pad interfaces of the aged specimens.

The effects of lanthanum on microstructure and electrochemical properties of Al–Zn–In based sacrificial anode alloys

In order to improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloys, Al–5Zn–0.03In–1Mg–0.05Ti– xLa ( x = 0.3, 0.5 and 0.7 wt.%) alloys were developed. Microstructures and electrochemical properties of the alloys were investigated. The results show that the optimal microstructures and electrochemical properties are obtained in Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy. The main precipitate phase is Al 2LaZn 2 particles. The excellent electrochemical properties of Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy is mainly attributed to fine grains and grain boundaries containing fine Al 2LaZn 2 precipitates. At the same time the fine grains can improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloy.

Compressive creep behavior of Mg–Sn–La alloys

The compressive creep behaviors of as-cast Mg–5Sn, Mg–5Sn–2La, Mg–6.5Sn–2La and Mg–8.5Sn–2La alloys are investigated in this paper. The compressive creep resistance of Mg– xSn–2La ( x = 5 wt.%; 6.5 and 8.5 wt.%) alloy is much better than that of as-cast Mg–5Sn alloy at the applied stresses from 25 to 35 MPa and the temperature of 473 K, which is mainly due to the feather-shaped, rod-like and massive rare earth phases formed in Mg–Sn–La alloys. The calculated stress exponent n is 4.2, 5.6, 6.2, and 5.7, respectively for the as-cast Mg–5Sn, Mg–5Sn–2La, Mg–6.5Sn–2La and Mg–8.5Sn–2La alloys, which indicate that the dislocation creep is the rate controlling mechanism of these alloys.

Electrochemical behavior and application of lead–lanthanum alloys for positive grids of lead-acid batteries

The effects of different lanthanum content (0, 0.00600, 0.0112, 0.0195 and 0.0540 wt.%) on the electrochemical behavior of lead–lanthanum alloy in sulfuric acid solutions were investigated by linear potential sweep (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The morphology of the corrosion layer and corrosion section of Pb and Pb–La alloys were analyzed by scanning electron microscope (SEM) after corrosion testing. It was found that the addition of La inhibits the oxygen evolution reaction on the surface of Pb alloy electrodes, and La amounts of 0.00600 and 0.0540 wt.% in Pb–La alloy electrodes can lead to a more effective inhibition. The results of the LSV, CV and EIS experiments show that the addition of La can inhibit the growth of the anodic Pb(II) oxides and PbO 2 film. The resistance of the anodic film on the Pb–La electrodes is much lower than that on the Pb electrode. SEM for the corrosion layer indicates that the corrosion product on pure Pb and Pb–0.0195% La alloy is uniform and compact. The corrosion products on the alloys with La contents of 0.00600, 0.0112 and 0.0540 are loose and porous that the active materials can easily sit in the apertures to contact the grid surface intimately with the effective. The results demonstrate that Pb–La alloys show the potential for application as the positive grid material in maintenance-free lead-acid batteries.

Microstructure, tensile properties and compressive creep resistance of Mg-(5–8.5)%Sn-2%La alloys

The microstructure, tensile properties and compressive creep resistance of permanent-mould cast Mg-(5–8.5)%Sn-2%La (mass fraction) alloys were investigated. The results show that Mg-(5–8.5)%Sn-2%La alloys are all composed of α-Mg phase, Mg 2Sn and Mg-La-Sn compounds. Compared with those of Mg-5%Sn binary alloy, the grain size and the content of Mg 2Sn compound in Mg-5%Sn-2%La alloy are decreased. With the increase of Sn content in Mg-(5–8.5)%Sn-2%La alloys, the content of Mg 2Sn compound increases, while that of Mg-Sn-La compound changes little. In addition, the investigation suggests that the thermally stable Mg-Sn-La and Mg 2Sn compounds can improve the tensile properties and compressive creep resistance of the alloys.

TEM studies of nanostructure in melt-spun Mg–Ni–La alloy manifesting enhanced hydrogen desorbing kinetics

The hydrogen storage properties of a magnesium-rich Mg–Ni–La alloy prepared by melt-spinning are significantly improved by nanostructure formation during crystallization and activation. It can absorb and desorb ∼5 wt% hydrogen at temperatures as low as 200 °C in moderate time periods. Transmission electron microscopic (TEM) studies on this alloy indicate that the nanostructure, consisting of LaH 3 and Mg 2NiH 4 nano-particles dispersed homogeneously in MgH 2 matrices after hydrogenation, is rather stable at temperatures below 300 °C but undergoes coarsening and segregation of these particles and matrices above ∼400 °C. These structural changes have been confirmed by electron energy-loss spectroscopic (EELS) imaging as well as high-resolution TEM techniques. A new EELS peak associated with a plasmon excitation in the MgH 2 phase (H-plasmon) is found for the first time in this study. By imaging the H-plasmon peak, the hydrogen distribution in the alloy has been clearly visualized. We have succeeded in observing the hydrogen desorption process at ∼400 °C in-situ in the microscope using this EELS imaging technique.

Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–La alloys

Ti-based icosahedral quasicrystalline phase (I-phase) exhibited excellent hydrogen storage property for special structure. Unfortunately, the application as the negative electrode material of the nickel-metal hydride batteries was limited due to the poor electrochemical kinetics. Meanwhile, rare-earth element was beneficial to the electrochemical properties of Ti, Zr-based alloy. In present work, Ti 45Zr 30Ni 25La x ( x = 3, 5 and 7) alloys were prepared by melt-spinning. The effect of adding La on the electrochemical performance of melt-spun alloys was investigated in detail. The phase structure of the melt-spun alloys was analyzed by XRD and DSC. The results showed that the amorphous forming ability increased with increasing x value. The maximum discharge capacity decreased from 109 mAh g −1 to 99 mAh g −1 with increasing x value. High-rate dischargeability and cycling stability were improved by adding La, which may be resulted from the increase of amorphous phase in the alloy electrodes.

Investigation of the Development Processes of Self-Organizing, Protective Oxide Scales and Wear Resistant Surface Layers on the Fe-44%Cr-1%Ni-4%Al-0.3%La Alloy

Using the methods of scanning electron microscopy (SEM), Auger electron spectrometry (AES), fast electron diffraction (FED) in the “on reflection” regime and wavelength dispersive spectrometry (WDS) a complex investigation of the hierarchical sequence of amorphous Beilby layer formation has been studied due to the self-organizing dissipative processes, associated with extensive cold work, on the surface of an Fe-Cr-Ni-Al-La alloy, with high (>40%) chromium content. It was established that, the surface layer (≤1μm thickness) of the mechanically polished specimen of Fe-44%Cr-1%Ni-4%Al-0.3%La alloy consists of the amorphous Beilby layer and that its adjacent matrix layer, crushed due to the plastic deformation, formed an entropy “excited” functional system, which at the temperature of 1200°C in laboratory atmosphere permits the formation of an oxide surface layer with a micro-wrinkles modulated structure of uniform thickness, in the form of mixture of nanocrystallites (100÷500nm) made of oxides of atoms constituting the basic metallic matrix. Beneath this layer a thin alumina scale is observed to form. Increasing the oxidation temperature causes the regrowth of nanocrystallites and also the recrystallization processes, accompanied by solid-phase reactions between oxide nano-particles. This leads to scale delamination at the superficial oxide thin uniform alumina layer interface. The Al2O3 layer is characterized by high adherence with metallic substrate and provides protective features against both high temperature (1200°C) oxidation of the matrix and resistance to abrasion. By the pretreatment at 1200°C of the investigated alloy’s surface modified specimens, there forms a low thickness (several microns) scale which has ultra fine graininess (~1μ) with no porosity and blocked grain boundaries short-circuit diffusion paths. This gives to the scale the ability to protect the metallic matrix against high temperature gas (and other aggressive environment) corrosion.

Oxidation of Ag–Sn–La Alloy Powders

The gas atomized Ag-9.26wt%Sn-0.44wt%La alloy powders were oxidized in air between 400 and 900 °C. The oxidation thermodynamics, kinetics and microstructure of the alloy were investigated. We suggested that the addition of La may accelerate oxidation of Sn and prevent the formation of the dense SnO2 film. The suitable oxidation temperature of the alloy powders is 800 °C in air. After internal oxidation, many cracks were observed on the surface of the alloy powders. In addition, the whole oxidation process of the alloy powders is controlled by the oxygen diffusion. The diffusion coefficient of oxygen in the alloy powders at 800 °C is about 1.5 times larger than that at 700 °C on the initial stage of internal oxidation, while that is 4.5 times on the subsequent stage.

Formation of Whiskers and Hillocks on the Surface of Sn-6.6RE Alloys

As-cast Sn-6.6Ce, Sn-6.6Lu, and Sn-6.6La alloys contain peritectic structures of near β-Sn matrix embedded with CeSn3, Lu4Sn5, and LaSn3 clusters, respectively, among which those of LaSn3 are the smallest in size. After air storage at room temperature, Sn-6.6Ce and Sn-6.6Lu reveal a much higher tendency than Sn-6.6La to form long threadlike whiskers on the surface of CeSn3 phase. Sn-6.6La shows the earlier appearance of coarse hillocks during storage at 150 °C in comparison to the Sn-6.6Ce and Sn-6.6Lu, but these hillocks cease to grow after a short duration. In contrast, the hillocks in Sn-6.6Ce and Sn-6.6Lu can grow to large sizes. This discrepancy correlates to the lower oxidation rate of LaSn3 intermetallic phase than those of CeSn3 and Lu4Sn5, which is attributed to the higher chemical activity of La as compared to Ce and Lu. In addition, the smaller size of the LaSn3 clusters also leads to the lower amount of tin atoms released and lower compressive stress accumulated after oxidation, which result in the slower whisker and hillock growth in Sn-6.6La alloy as compared to those in Sn-6.6Ce and Sn-6.6Lu.

Preparation of Mg–23.5Ni–10(Cu or La) hydrogen-storage alloys by melt spinning and crystallization heat treatment

Mg–23.5 wt%Ni–10 wt%Cu and Mg–23.5 wt%Ni–10 wt%La alloys were prepared by melt spinning and crystallization heat-treatment. The Mg–23.5Ni–10Cu alloy (5.18 wt%) after planetary ball milling has a higher hydrogen-storage capacity than the Mg–23.5Ni–10La alloy (4.98 wt%) at 573 K. The activated Mg–23.5Ni–10Cu alloy has a slightly lower hydriding rate in the beginning, but a higher hydriding rate after about 5 min than the activated Mg–23.5Ni–10La alloy at 573 K. The activated Mg–23.5Ni–10Cu alloy has a much higher dehydriding rate and a larger quantity of the decomposed hydrogen than the activated Mg–23.5Ni–10La alloy at 573 K.

HRTEM Study of Nanostructures in Hydrogen-Absorbing Mg-Ni and Mg-Ni-La Alloys Prepared by Melt-Spinning

The structure of melt-spun and crystallized Mg-10%Ni and Mg-10%Ni-5%La alloys is studied using HRTEM, coupled with ED and EELS techniques, for specimens subjected to hydrogenation and dehydrogenation. The presence of nano-sized (5-10nm) Mg2Ni grains dispersed in the matrix of Mg nano-grains is observed before hydrogenation. This structure is almost preserved after hydrogenation and dehydrogenation at 300°C. In the hydrogenated specimen, nanoboundaries lying between MgH2 and Mg2NiH4 nano-grains are observed. They appear to provide main routes for the hydrogen transport in these nanostructured materials.

Preparation by gravity casting and hydrogen-storage properties of Mg–23.5wt.%Ni–(5, 10 and 15wt.%)La

Abstract Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La alloys were prepared by gravity casting and their hydrogen-storage properties were examined after pulverizing. The gravity cast Mg–23.5Ni–(5, 10 and 15)La alloys consist of α-Mg, Mg2Ni and Mg17La2 phases. The activated Mg–23.5Ni–10La alloy has the highest hydrogen-storage capacity of 4.96 wt.%H (from P–C–T curve) and the highest initial hydriding rate (hydrogen content 3.83 wt.%H at 10 min) with an initial hydrogen pressure in the channel of 11 bar H2 at 573 K. This is attributed to its containing the largest amount of the Mg17La2 phase, which is easily dissociable during the hydriding reaction.

THERMODYNAMICS AND AMORPHIZATION OF THE NICKEL–LANTHANUM ALLOYS

The thermodynamic properties of liquid and crystalline Ni–La alloys were studied over the whole range of compositions and a wide temperature range, 685–1854 K, by Knudsen mass spectrometry and an integral variant of the effusion method realized under ultrahigh oilless vacuum. Complete and reliable thermodynamic description of all intermediate phases in the Ni–La system was obtained. The thermodynamic functions of the melts were described by the associated solution model. The existence of the complexes NiLa and Ni2La in the melts was found. The contributions of various intercomponent chemical interaction types (covalent and metallic) to thermodynamic functions were estimated. The present results were applied to computation of the phase diagram. Obtained thermodynamic description of the Ni–La alloys allowed the analysis of their transformation into amorphous state. It was determined that the magnitude of the associate formation entropy should be used for quantitative concept development of the propensity of metallic melts to amorphization.

First-principles study of structural stabilities and electronic characteristics of Mg–La intermetallic compounds

A first-principles pseudopotential plane-wave method based on density functional theory is used to investigate the crystal and electronic structure of Mg–La intermetallic compounds with different types of structure. The obtained lattice constants a 0 for all intermetallics considered are very close to the corresponding experimental values. The calculation of cohesive energies indicated that the structure stability of Mg–La alloy will become higher with increasing La element independent on crystal structure type. The calculations of formation heats showed that the alloying abilities of Mg 3La, Mg 2La and MgLa were much stronger than Mg 12La and Mg 17La 2, while the Mg 17La 2 alloying ability was slightly higher than Mg 12La alloy. The energy band and the densities of states (DOS) of these intermetallic compounds revealed that the bonding occurred mainly among the valence electrons of Mg 3s, 2p and La 5d, 4f orbits, and the discrepancy in the stability of Mg–La intermetallic compounds could be attributed to the variation of bonding electron numbers at low-energy region of Fermi level.