Effects of high magnetic field on modification of Al-Si alloy

1INTRODUCTIONAmong Al-Si alloys,the Al-Si eutectic alloyhas the best foundry capability.The mechanicalproperties of Al-Si eutectic and hypoeutectic alloyhighly relates to the shape,size and distribution ofSi phase in eutectic structure.Coarse acicular-likeeutectic Si dissevers Al matrix badly to inducestress concentration and debase the mechanicalproperties,especially the tenacity.Modification isthe process to change the shape and size of eutecticSi,namely to change the shape of eutec…     

Alignment behavior of MnBi phase in Bi-Mn alloy solidified in static magnetic field

1　INTRODUCTIONRegularstructuresinmaterialsarealwayspre paredtoimprovetheirproperties .Itiswellknownthatpowderedferromagneticmaterialscanbeorientedinastaticmagneticfield .Inrecentyearshighmag neticfieldwasappliedtoinducealignmentofparticlesinsomenonferromagneticmaterialswithanisotropicmagneticsusceptibilityinroomtemperature ,suchasparamagneticYBa2 Cu3O7ceramic[1] anddiamagneticgraphite[2 ] .Ifthematerialshavearesidualanisotropyintheirmagneticsusceptibilityatahightemperature ,theycanbetex…     

Corrosion and corrosive-wear behaviors of a high strength and toughness Cu–15Ni–8Sn alloy in seawater

The Cu–15Ni–8Sn alloy with high strength and toughness is a common bearing alloy, while it is often subjected to premature failure in service due to corrosion and wear. Therefore, the corrosion and wear behaviors of an ultra-high-strength and toughness Cu–15Ni–8Sn (wt%) alloy fabricated by hot isostatic pressing were investigated in this study. The results indicated that intergranular corrosion and pits were observed when the studied alloy was immersed into seawater for 30 days. When the Cu–15Ni–8Sn alloy slid in seawater, there was a synergistic effect between corrosion and wear. With the increase of normal load, the synergistic effect weakened initially and then enhanced the interaction between corrosion and wear changed from negative to positive, and the main wear mechanisms transferred from abrasive wear into delamination.     

Microstructural analysis of modification and grain refinement in a hypoeutectic Al–Si alloy

Abstract

It is well known that in hypoeutectic Al–Si alloys addition of titanium boride refines primary aluminium and addition of strontium modifies eutectic silicon. In this work their individual and simultaneous effect on both features has been investigated quantitatively. On the basis of experimental observations and quantitative analysis, it was concluded that an addition of 0·002 wt-% titanium almost halves grain size of primary aluminium in an ingot of A356, cast in a bar-shaped copper mould. Combining 0·002 wt-% titanium (as Al–3Ti–B) with 0·02 wt-% strontium resulted in a decrease in this grain refinement. However, an addition of 0·02 wt-% strontium has a slight refinement effect on primary aluminium phase. In order to quantify the effect of addition on eutectic silicon readily, as-cast specimens were heat-treated. It was observed that the number of eutectic silicon particles in Sr-modified specimens increased in a higher level compared to a Sr + Ti treated specimen. It was also found that Ti slightly influences the size of eutectic silicon particles.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Effect of Mg composition on sintering behaviors and mechanical properties of Al–Cu–Mg alloy

Al–3Cu–Mg alloy was fabricated by the powder metallurgy (P/M) processes. Air-atomized powders of each alloying element were blended with various Mg contents (0.5%, 1.5%, and 2.5%, mass fraction). The compaction pressure was selected to achieve the elastic deformation, local plastic deformation, and plastic deformation of powders, respectively, and the sintering temperatures for each composition were determined, where the liquid phase sintering of Cu is dominant. The microstructural analysis of sintered materials was performed using optical microscope (OM) and scanning electron microscope (SEM) to investigate the sintering behaviors and fracture characteristics. The transverse rupture strength (TRS) of sintered materials decreased with greater Mg content (Al–3Cu–2.5Mg). However, Al–3Cu–0.5Mg alloy exhibited moderate TRS but higher specific strength than Al–3Cu without Mg addition.     

Structure and texture in Ni–30% Co alloy ribbons subjected to annealing in high magnetic field

The structure and texture in the Ni–30% Co alloy subjected to cold rolling and annealings in strong dc magnetic field at the temperatures of above and below the Curie point are studied. It has been shown that, at all annealing temperatures, the average grain size after magnetic annealing is smaller than after annealing without filed. After the magnetic annealing of the alloy in the ferromagnetic state, the volume fraction of grains with cube orientation decreases and the volume fraction of the components of deformationinduced texture increases.     

Molecular dynamics simulation of Al–Co–Cr–Cu–Fe–Ni high entropy alloy thin film growth

Molecular dynamics (MD) simulations are used to study AlCoCrCuFeNi high entropy alloy (HEA) thin film growth on a silicon (100) substrate. Effect of the atomic composition is studied on morphology and atomic scale structure. Input data are chosen to fit with experimental operating conditions of magnetron sputtering deposition process. It is observed that the different structures are determined by the chemical composition and atomic size mismatch. The simulated results are in good agreement with the solid-solution formation rules proposed by Zhang et al. [1] for multi-principal component HEAs which based on the two parameters δ and Ω, respectively describing describe the comprehensive effect of the atomic-size difference in the n-element alloy and the effects of enthalpy and entropy of mixing on formation of multi-component solid-solutions. When Ω ≥ 1.1 and δ ≤ 6.6%, the multi-component solid solution phase could form. In contrast, the multi-component alloys forming intermetallic compounds and bulk metallic glasses (BMG) have larger value of δ and smaller value of Ω. The value of Ω for BMG is smaller than that of intermetallic compounds.     

Analysis of phase in Cu–15%Cr–0.24%Zr alloy

The vacuum medium-frequency induction melting technology was employed to prepare the Cu–15%Cr–0.24%Zr alloy. The scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to analyze the phase composition, morphology and structure of the alloy. The results reveal that the as-cast structure of the alloy consists of Cu matrix, Cr dendrite, eutectic Cr and Zr-rich phase. A large number of Cr-precipitated phases occur in the Cu matrix, and Cu₅Zr particles can be found in the grain boundary of Cu matrix. The HRTEM images prove that there is a semi-coherent relationship between Cu₅Zr and Cu matrix.     

Nucleation and growth mechanism of α-lamellae of Ti alloy TA15 cooling from an α + β phase field

The mechanism of nucleation and growth of α-lamellae when a TA15 Ti alloy with an equiaxed structure is cooled from an α + β phase field were studied by end quenching experiments and thermal simulation tests using a Gleeble-3500. The results showed that for the Ti alloy TA15 the nucleation and growth of α-lamellae involved four steps, including nucleation of α_GB, growth of α_GB, nucleation of α_WGB, and growth of α_WGB. Widmanstätten α, α_WGB, grew faster than grain boundary α, α_GB, and equiaxed α in common cases. It was found for the first time that the mode of nucleation of α_WGB for alloy TA15 was interface instability, i.e. α_WGB nucleated through surface instability and the protuberance of α_GB and equiaxed α, and the α_WGB nucleus did not have an independent and complete surface. A new model of the nucleation of α_WGB and phase transformation is proposed. The growth of α_WGB in the TA15 alloy started from a small protuberance and spread into a β grain with a sectorial morphology, to become lamellar instead of spiculate or oblate cuboid in shape. The nucleation rate of α_WGB determined the thickness of α_WG, with, to some extent, an inverse relation between the nucleation rate and thickness of α_WGB.     

Preparation and magnetic properties of amorphous Co–Cu–B alloy nano-powders

Magnetic Co–Cu–B powders were successfully prepared by the reduction of copper chloride and cobalt chloride in an aqueous solution of sodium borohydride. Using X-ray diffraction (XRD), selective area electronic diffraction (SAED), differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) it was shown that, when Co²⁺/Cu²⁺ ratio in the original mixed solution was greater, the resultant powders were amorphous alloy powders, having a nearly spherical morphology with diameters <50 nm. DSC trace of the as-prepared Co–Cu–B ultrafine amorphous alloy powders showed only one exothermic peak, and no sharp glass transition temperature could be identified. From the room-temperature magnetic hysteresis loops measured by vibrating sample magnetometer, it was found that the Co–Cu–B amorphous alloy powders exhibited hard magnetic behavior, and the saturation magnetization could be further improved by crystallization.     

Effect of barium on the refinement of primary aluminum and eutectics in a hypoeutectic Al-Si alloy

The effect of barium on the refinement of primary aluminum and on the modification of eutectics in a hypoeutec-tic aluminum-silicon alloy was investigated. The results indicate that barium not only modifies the eutectic silicon but also refines the primary aluminum and there is a relationship between the retained barium and the second spacing of primary aluminum. Experiments of barium-treated commercial Al-Si hypoeutectic alloy show that barium is a better modifier than sodium when there is a longer holding time.     

In situ observation of Cu segregation and phase nucleation at a solid–liquid interface in an Al alloy

This paper presents a systematic study comparing experimental in situ transmission electron microscopy observation of microstructural and compositional evolution with complementary thermodynamic calculations, to better understand the redistribution of solute elements and the nucleation behavior of different phases in a commercial Al-alloy powder (AA390). The results show that Cu segregation to the solid Si–liquid Al interface, as well as the significant undercooling achieved in the liquid under non-equilibrium conditions because the Al phase cannot nucleate homogeneously, play a important roles in nucleating Al₂Cu at the interface prior to the Mg₂Si phase in the alloy. Although Cu segregation can occur at various locations along the interface, the Al₂Cu phase appears to preferentially nucleate at a high-index Si–liquid interface as opposed to a low-index one. The Cu concentration during segregation remains essentially constant with time, indicating that the observed segregation behavior is a thermodynamic and not a kinetic phenomenon. These in situ observations and complementary thermodynamic calculations substantially enhance our understanding of potential crystal nucleation and growth processes.     

Refinement of primary Si in Cu–50Si alloys with novel Al–Zr–P master alloy

In this article, a novel Al-6Zr-2P master alloy with ZrP particles was successfully synthesized, and the refining performance of this novel master alloy for the primary Si in Cu-50Si alloys was also investigated. By means of the fracture plane observation, it is found that the ZrP phase would precipitate first in the solidification process, and then, the ZrAl 3 phase grows around them. Furthermore, it is observed that the refining effect can be remarkably improved by changing the addition sequence of the raw materials. After the melting of commercial Cu, the 2.0 wt% Al-6Zr-2P master alloy and crystalline Si were added in sequence, and the mean size of the primary Si in Cu-50Si alloy can be significantly refined from 255.7 to 75.3 lm. Meanwhile, the refining mechanism was discussed in detail.     

Nucleation and thickening of shear bands in nano-scale twin/matrix lamellae of a Cu–Al alloy processed by dynamic plastic deformation

Microstructural evolution associated with the shear banding in nano-scale twin/matrix (T/M) lamellae of a Cu–Al alloy processed by means of dynamic plastic deformation was investigated using transmission electron microscopy (TEM) and high-resolution TEM. The development of a shear band was found to be a two-stage process, namely a nucleation stage resulting in a narrow band composed of nano-sized (sub)grains intersecting the T/M lamellae, followed by a thickening stage of the narrow band into adjacent T/M lamellae regions. The nucleation stage occurred within a narrow region of an almost constant thickness (100–200 nm thick, referred to as “core” region) and consisted of three steps: (1) initiation of localized deformation (bending, necking, and detwinning) against the T/M lamellae, (2) evolution of a dislocation structure within the detwinned band, and (3) transformation of the detwinned dislocation structure (DDS) into a nano-sized (sub)grain structure (NGS). On the two sides of a core region, two transition layers (TRLs) exist where the T/M lamellae experienced much less shear strain. The interface boundaries separating the core region and the TRLs are characterized by very large shear strain gradients accommodated by high density of dislocations. Increasing shear strains leads to thickening of shear bands at the expense of the adjoining T/M lamellae, which is composed of thickening of the core region by transforming the TRLs into the core region with DDS and NGS, analogous to steps (2) and (3) of the nucleation process, and outward movement of the TRLs by deforming the adjoining T/M lamellae. Grain sizes in the well-developed shear bands are obviously larger than the lamellar thickness of original T/M lamellae.     

Cavity formation and early growth in a superplastic Al–Mg alloy

Knowledge of the exact physical mechanism of cavity formation and early growth is important for the prediction of the extent of internal damage following superplastic deformation. To this end, the early stages of cavitation in a superplastic Al–Mg–Mn–Cu alloy have been experimentally studied and reported here. Small cavities (<0.5 μm) were detected by scanning electron microscopy and the number of cavities per unit volume was monitored by image analysis through optical microscopy. Before deformation, some cavities were seen at the particle–matrix interfaces. However, during tensile deformation in the temperature range of 450–550°C (and strain rates ∼10⁻⁴ to 10⁻² s⁻¹), additional cavities emerge and grow. Most cavities are observed at the interface between particles and the matrix from submicrometer size range, and grow initially along the interface. This suggests that early cavity growth is by matrix/particle decohesion, possibly starting from interfacial defects, and this growth has rapid kinetics. The density of observable cavities increases with strain, i.e. “nucleation” is continuous. The number of cavities increases at higher strain rates and at lower test temperatures. This is due to the higher flow stresses, reduced strain-rate sensitivity and poorer diffusional accommodation process, which assist in the initial growth of the submicrometer and nanoscale interface defects. But the evidence for diffusional cavity growth in the initial stages was not found.     

Nucleation/growth mechanism of electrocrystallization for As–Sb alloy in hydrochloric acid system

The initial electrocrystallization of As–Sb alloy on glass carbon (GC) electrode in hydrochloric acid system was studied via cyclic voltammetry (CV) and chronoamperometry measurements. Current transients were presented in dimensionless formation, which showed that the initial nucleation/growth process of As–Sb alloy followed three-dimensional nucleation model with diffusion-controlled growth. Relevant nucleation parameters were calculated by analyzing related current transients. Particular attention was paid to the effect of Sb(III) concentration on the nucleation process during the co-electrodeposition. The quantitative results showed that Sb(III) played a positive effect on enhancing the nucleation rate of As–Sb alloy, leading to the evolution of alloy surface morphology from grain structure to compact layer structure.     

Microstructures and corrosion behaviors of Al–6.5Si–0.45Mg–xSc casting alloy

The microstructures and corrosion behaviors of the Al–6.5Si–0.45Mg casting alloys with the addition of Sc were investigated by using scanning electron microscopy, X-ray diffraction, electrochemical measurement techniques and immersion corrosion tests and compared with those of Sr-modified alloy. The results show that Sc has evident refining and modifying effects on the primary α(Al) and the eutectic Si phase of the alloy, and the effects can be enhanced with the increase of Sc content. When the Sc content is increased to 0.58 wt.%, its modifying effect on the eutectic Si is almost same as that of Sr. Sc can improve the corrosion resistance of the test alloy in NaCl solution when compared with Sr, but the excessively high Sc content cannot further increase the corrosion resistance of the alloy. The corrosion of the alloys mainly occurs in the eutectic region of the alloy, and mostly the eutectic α(Al) is dissolved. This confirms that Si phase is more noble than α(Al) phase, and the galvanic couplings can be formed between the eutectic Si and α(Al) phases.     

Dynamic grain growth and particle coarsening in Al–3.5Cu

Grain growth and particle coarsening in Al–3.5Cu at a temperature of 450 °C has been studied. Plastic deformation of this Zener-pinned system at strain rates of 10^?3 and 10^?4 s^?1 led to an increase in both the grain growth and particle coarsening rates. The results of mechanical tests and metallography, including in situ studies, showed that the material was deforming primarily by intragranular slip. The dynamic grain growth was ascribed to the geometric effect of deformation on the Zener pinning, and the rate sensitivity of the growth to the dynamic particle coarsening. The principal effect of deformation on particle coarsening was concluded to be increased diffusion due to the dislocation content.