As-solidified Alloys Research Articles

Assessment of Directionally Solidified Eutectic Sm–Fe(Co)–Ti Alloys as Permanent Magnet Materials

Sm–Fe–Ti and Sm–Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> –Ti alloys were prepared via arc-melting and directionally solidified on a water-cooled copper hearth. The as-solidified alloys featured cells of the Sm(Fe,Co,Ti) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> –Ti(Fe,Co) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+δ</sub> –(α-Fe) lamellar eutectic. The lamellae of Sm(Fe,Co,Ti) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> phase with a crystal structure of the ThMn12 type were less than 0.2 μm thick, and had their [001] easy-magnetization directions oriented along the temperature gradient of the solidification. The eutectic microstructure led to an increased coercivity, especially in the Co-added alloys. Below 250 °C, this coercivity was found not to vary much with temperature with a temperature coefficient of -0.18 %/°C. However, the modest absolute values, reaching only 0.7 kOe, are insufficient for utilization of the directionally solidified alloys as anisotropic permanent magnets.

Oxidation of Al-Co Alloys at High Temperatures.

In this work, the high temperature oxidation behavior of Al71Co29 and Al76Co24 alloys (concentration in at.%) is presented. The alloys were prepared by controlled arc-melting of Co and Al granules in high purity argon. The as-solidified alloys were found to consist of several different phases, including structurally complex m-Al13Co4 and Z-Al3Co phases. The high temperature oxidation behavior of the alloys was studied by simultaneous thermal analysis in flowing synthetic air at 773–1173 K. A protective Al2O3 scale was formed on the sample surface. A parabolic rate law was observed. The rate constants of the alloys have been found between 1.63 × 10−14 and 8.83 × 10−12 g cm−4 s−1. The experimental activation energies of oxidation are 90 and 123 kJ mol−1 for the Al71Co29 and Al76Co24 alloys, respectively. The oxidation mechanism of the Al-Co alloys is discussed and implications towards practical applications of these alloys at high temperatures are provided.

Large elastocaloric effect in directionally solidified all-d-metal Heusler metamagnetic shape memory alloys

Solid-state cooling based on the caloric effect of phase transformation materials has attracted considerable interest with the increased demand for energy-efficient and environmentally friendly cooling technologies. Here, we have systematically studied the microstructural evolution, martensitic transformation (MT) behaviors, and elastocaloric effect (eCE) of directionally solidified Ni35.5Co14.5Mn35Ti15 all-d-metal Heusler metamagnetic shape memory alloys. Electron backscatter diffraction (EBSD) analysis revealed the coexistence of Heusler-type 〈001〉-oriented dendritic crystal and Ti-poor interdendritic phase for as-solidified alloys. Upon homogenization annealing, the sample presents a columnar-like morphology with 〈105〉 textured five-layer-modulated (5 M) martensite along the growth direction. Besides, the multi-modulated martensite with 7 M and 8 M structures was identified by transmission electron microscopy (TEM). Using a unique technique of in situ digital image correlation (DIC) with the combination of infrared thermography, MT and eCE behaviors were studied. Ni35.5Co14.5Mn35Ti15 alloy yielded a large adiabatic temperature change (ΔTad) of 11.5 K with a low critical stress (σcr) of 38 MPa and a moderate stress hysteresis (Δσhy) of 54 MPa when subjected to uniaxial stress, resulting in the largest value of |ΔTad/σcr| = 0.31 K MPa−1. This improved eCE is attributed to the enhanced compatibility in the oriented polycrystalline alloy and the high mobility of the low-energy twin boundary in the multi-modulated martensite.

Relationship between Phase Occurrence, Chemical Composition, and Corrosion Behavior of as-Solidified Al-Pd-Co Alloys.

The microstructure, phase constitution, and corrosion performance of as-solidified Al70Pd25Co5 and Al74Pd12Co14 alloys (element concentrations in at.%) have been investigated in the present work. The alloys were prepared by arc-melting of Al, Pd, and Co lumps in argon. The Al74Pd12Co14 alloy was composed of structurally complex εn phase, while the Al70Pd25Co5 alloy was composed of εn and δ phases. The corrosion performance was studied by open circuit potential measurements and potentiodynamic polarization in aqueous NaCl solution (3.5 wt.%). Marked open circuit potential oscillations of the Al70Pd25Co5 alloy have been observed, indicating individual breakdown and re-passivation events on the sample surface. A preferential corrosion attack of εn was found, while the binary δ phase (Al3Pd2) remained free of corrosion. A de-alloying of Al from εn and formation of intermittent interpenetrating channel networks occurred in both alloys. The corrosion behavior of εn is discussed in terms of its chemical composition and crystal structure. The corrosion activity of εn could be further exploited in preparation of porous Pd–Co networks with possible catalytic activity.

Microstructure, Phase Occurrence, and Corrosion Behavior of As-Solidified and As-Annealed Al-Pd Alloys

In the present work, we studied the microstructure, phase constitution, and corrosion performance of Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 alloys (metal concentrations are given in at.%). The alloys were prepared by repeated arc melting of Al and Pd granules in argon atmosphere. The as-solidified samples were further annealed at 700 °C for 500 h. The microstructure and phase constitution of the as-solidified and as-annealed alloys were studied by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction. The alloys were found to consist of (Al), e n (~ Al3Pd), and δ (Al3Pd2) in various fractions. The corrosion testing of the alloys was performed in aqueous NaCl (0.6 M) using a standard 3-electrode cell monitored by potentiostat. The corrosion current densities and corrosion potentials were determined by Tafel extrapolation. The corrosion potentials of the alloys were found between − 763 and − 841 mV versus Ag/AgCl. An active alloy dissolution has been observed, and it has been found that (Al) was excavated, whereas Al in e n was de-alloyed. The effects of bulk chemical composition, phase occurrence and microstructure on the corrosion behavior are evaluated. The local nobilities of e n and δ are discussed. Finally, the conclusions about the alloy’s corrosion resistance in saline solutions are provided.

Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part II. Intermetallic Coarsening Behavior of Rapidly Solidified Solders After Multiple Reflows

Controlling the size, dispersion, and stability of intermetallic compounds in lead-free solder alloys is vital to creating reliable solder joints regardless of how many times the solder joints are melted and resolidified (reflowed) during circuit board assembly. In this article, the coarsening behavior of Cu x Al y and Cu6Sn5 in two Sn-Cu-Al alloys, a Sn-2.59Cu-0.43Al at. pct alloy produced via drip atomization and a Sn-5.39Cu-1.69Al at. pct alloy produced via melt spinning at a 5-m/s wheel speed, was characterized after multiple (1-5) reflow cycles via differential scanning calorimetry between the temperatures of 293 K and 523 K (20 °C and 250 °C). Little-to-no coarsening of the Cu x Al y particles was observed for either composition; however, clustering of Cu x Al y particles was observed. For Cu6Sn5 particle growth, a bimodal size distribution was observed for the drip atomized alloy, with large, faceted growth of Cu6Sn5 observed, while in the melt spun alloy, Cu6Sn5 particles displayed no significant increase in the average particle size, with irregularly shaped, nonfaceted Cu6Sn5 particles observed after reflow, which is consistent with shapes observed in the as-solidified alloys. The link between original alloy composition, reflow undercooling, and subsequent intermetallic coarsening behavior was discussed by using calculated solidification paths. The reflowed microstructures suggested that the heteroepitaxial relationship previously observed between the Cu x Al y and the Cu6Sn5 was maintained for both alloys.

Preparation of a bulk Fe83B17 soft magnetic alloy by undercooling and copper-mold casting methods

Bulk Fe83B17 eutectic alloy rods with diameters up to 3mm were prepared by undercooling solidification combined with Cu-mold casting. The results showed that the rapid solidification led to an increase in the nucleation rate, an inhibition of the grain growth and a competition between a stable Fe2B phase and a metastable Fe3B phase. Then, pure nano-lamellar eutectic microstructures and the metastable Fe3B phase were successfully obtained in as-solidified alloys, which resulted in improved soft magnetic properties.

Icosahedral Quasicrystal Grains Growth and Evolution in Mg-Zn-Y

Unlike the growth of crystal, the growth of icosahedral quasicrystal seems to have its own characteristics. Petal-like icosahedral quasicrystal grains (i-phase grains) morphology usually has been observed in as-solidified quasicrystalline alloys. The distinguished morphology is belonged to a given i-phase grains growth evolution mechanism. We first introduced a novel model named “coherent” model to illustrate i-phase grains growth evolution.

A study on the order–disorder phase transformations of rapidly solidified Sm–Co-based permanent magnets

The structural transformation from the metastable, disordered TbCu 7-type SmCo 7 structure to the equilibrium, ordered Th 2Zn 17-type Sm 2Co 17 structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu 7-type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm 2Co 17 structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.

Magnetic behavior of rapidly solidified Pr–Co alloys with the TbCu7-type structure

Rapid solidification has been utilized to produce a series of Pr–Co alloys between the Pr2Co17 and PrCo5 stoichiometries. In this system, PrCo5 has easy axis magnetization while Pr2Co17 has easy-plane magnetization. Alloys of the form (PrxCo1−x)94Ti3C3 with x ranging from 0.105 corresponding to the Pr2Co17 compound to x=0.167 corresponding to the PrCo5 compound were produced by melt spinning at a tangential wheel speed of 40m∕s. The rapid solidification and alloying additions were found to suppress the formation of the Pr2Co17 ordered phase, leading to the formation of the disordered TbCu7-type structure over a range of Pr∕Co ratios. Hysteresis loops were characterized by a smooth demagnetization curve reflective of single-phase demagnetization. Heat treatment at 800 °C led to the formation of the Pr2Co17 and PrCo5 phases, and the presence of the soft magnetic Pr2Co17 phase drastically decreased the coercivity. The soft magnetic behavior was consistent with in-plane magnetization of the Pr2Co17 structure that formed during heat treatment. However, the relatively high coercivity observed in the as-solidified alloys with the disordered TbCu7-type structure suggests that dumbbell disorder may create easy axis magnetization, and changes in saturation magnetization also imply that dumbbell configuration is important to the magnetic properties.

Thermal stabilities and discharge capacities of melt-spun Mg–Ni-based amorphous alloys

Mg–Ni–M (M=Ca, La or Pd) ternary alloys were synthesized by the melt-spinning technique. All as-solidified alloys possessed an amorphous single phase by the additional effect of the third element, though it was difficult to obtain an amorphous Mg 67Ni 33 binary alloy by melt-spinning. We examined the thermal stability and electrochemical cyclic life property of the ternary amorphous alloys. The crystallization temperature of the amorphous alloys increases with increasing M content. All the alloys except Mg 67Ni 28Pd 5 examined in the present study maintain the amorphous structure even after hydrogen absorption at 373 K for Mg–Ni–Ca and Mg–Ni–Pd and at 423 K for Mg–Ni–La. The crystallization temperature increases by absorbing hydrogen, indicating that the alloys are thermally stabilized by hydrogen absorption. In the electrochemical cyclic life measurements up to five cycles, the Mg–Ni–Pd amorphous alloys exhibit high discharge capacities ranging from 100 to 400 mA h/g as well as small cyclic life degradation tendency, though the Mg–Ni–Ca and Mg–Ni–La amorphous alloys possess small discharge capacities of 10–100 mA h/g with significant cyclic life degradation. It is thus concluded that the good cyclic life property of the amorphous hydrogen storage alloys can be obtained by application of the melt-spinning technique to Mg-based alloys with appropriate alloy compositions.

Solidification behavior of Al-Si-Fe alloys and phase transformation of metastable intermetallic compound by heat treatment

Solidification behavior of Al-20 wt% Si-8 wt% Fe and Al-30 wt% Si-5 wt% Fe alloys during cooling with a cooling rate of 10 K/min has been studied using optical microscopy, X-ray diffractometry, differential thermal analysis, scanning electron microscopy and energy dispersive X-ray spectroscopy. In Al-20Si-8Fe alloy, metastable δ-Al4FeSi2 phase with tetragonal structure formed first from melt, followed by primary Si precipitation and then remaining liquid solidified finally into ternary eutectic of α-Al, Si and δ phases. However, in Al-30Si-5Fe alloy, primary Si formed first, followed by the δ phase precipitation and then eutectic solidification. During isothermal heat treatment of as-solidified alloys, phase transformation from the δ phase to equilibrium β phase began at the interface between δ phase and α-Al matrix and progressed toward the inside of δ phase with co-precipitation of Si particles due to the difference in composition between δ-Al4FeSi2 and β-Al5FeSi phases.

Microstructural Development of Mo(ss) + T2 Two-Phase Alloys

ABSTRACTThe microstructure evolution involving Mo-B-Si as-solidified alloys with compositions in the Mo solid-solution(ss) + T2 two-phase field has been examined following arc-melting and rapid solidification processing (RSP). Several solidification pathways in the arc-melted alloys have been identified. Compositional segregation during conventional solidification results in the formation of additional phases such as borides in the arc-melted alloys which require a prolonged solid-state annealing to obtain equilibrated two-phase microstructures. The RSP methods employed, splat-quenching (SQ) and powder drop tube processing (DTP), allow for significant microstructural modifications that facilitate the attainment of uniform dispersions of Mo(ss) phase in a T2 phase matrix.

Effect of magnesium content on the ageing behaviour of water-chilled Al-Si-Cu-Mg-Fe-Mn (380) alloy castings

A study of the effect of magnesium concentration on the ageing behaviour as measured by the hardness of 380 alloy was conducted for three levels of magnesium, namely 0.06 (base alloy), 0.33 and 0.5 wt%, for water-chilled castings (dendrite arm spacing ∼ 10–15 μm). Differential scanning calorimetry analysis of as-cast samples was carried out to determine the changes in the reactions of the phases obtained during alloy solidification, employing heating rates of 0.1 and 1.0°Cs−1, up to approximately 700°C. Two heat treatments were applied to the as-cast alloys: T5 comprising ageing at 25 (room temperature), 155, 180, 200 and 220°C, for times up to 200 h, and T6 comprising solution heat treatment at 480 °C or 515°C for 8 h, followed by quenching in warm water at 60°C, followed by immediate artificial ageing at 155 or 180°C for varying times up to 100h. The results show that the higher hardness values obtained with T6 treatment can be explained by the excess precipitation of magnesium-containing phases in the as-solidified alloys. This precipitation could be eliminated under the high cooling-rate conditions prevalent in die-casting operations so that T5 treatment may be used to replace T6 treatment to produce the same hardness values. In addition, solution heat treatment in the low-temperature range (480–515°C) is adequate to produce the required changes in silicon morphology and dissolution of magnesium in the matrix. No significant difference in hardness behaviour was observed when the magnesium content was increased beyond 0.3 wt%.

Pd添加ラネー銅触媒の組織におよぼす凝固条件とPd量の影響

Cu-70 at%Al alloys with various amount of Pd were solidified at different cooling rates and structures of the as-solidified alloys and catalysts made by leaching Al from them were examined by SEM, DSC and X-ray diffraction as a basic study for improvement of the catalytic activity of Raney copper catalysts for the hydration reaction of acrylonitrile which is important for production acrylamide. The specific surface area of the catalysts were measured by the BET Ar-absorption technique. The activity of the catalysts was evaluated by the conversion ratio from acrylonitrile to acrylamide by the hydration reaction. The results are as follows.(1)In the conventional slowly solidified alloys, most of the added Pd was distributed in primary ε phase and its solubility in the θ phase was quite few. On the other hand, in the rapidly solidified alloys, the formation of the primary ε phase was almost suppressed and Pd was supersaturated in the θ phase.(2)Raney copper with soluted Pd was newly obtained by leaching the θ phase with Pd. This suggests that the rapid solidification process is an effective process to get new catalysts.(3)Specific surface area of Raney copper produced by leaching rapidly solidified alloys increased with Pd content. It was about 3 times higher in the 1.5 at%Pd alloy than that in the Pd-free alloy.(4)The catalytic activity of 1.5 at%Pd catalysts showed a maximum, but its value per unit specific surface area decreased with Pd content.