Microstructure and microhardness in horizontally solidified Al–7Si–0.15Fe–(3Cu; 0.3Mg) alloys

Transient horizontal directional solidification (THDS) experiments have been carried out with Al–7wt.%Si–0.15Fe, Al–7wt.%Si–3wt.%Cu–0.15wt.%Fe and Al–7wt.%Si–0.3wt.%Mg–0.15wt.%Fe alloys, to identify experimental relationships between growth rates (G_R), cooling rates (C_R), tertiary dendrite arm spacings (λ₃) and microhardness (HV). Optical microscopy and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) were used to perform a comprehensive microstructural characterisation of the β-Al₅FeSi, ω-Al₇Cu₂Fe, θ-Al₂Cu, π-Al₈Mg₃FeSi₆ and α-Mg₂Si intermetallic phases. The addition of Cu and Mg to the Al–7wt.%Si–0.15wt.%Fe alloy led to the precipitation of ω and π phases from the β phase. It has been found for all analysed alloys that power experimental functions given by λ₃?=?constant.(G_R)^-1.1 and λ₃?=?constant.(C_R)^-0.55 best describe the variation of λ₃ with corresponding thermal and microstructural parameters.     

Laser additive processing of functionally-graded Fe–Si–B–Cu–Nb soft magnetic materials

Laser additive manufacturing is a novel tool for processing compositionally-graded alloys that are challenging to process via a conventional route. This article discusses a novel combinatorial approach for assessing composition–microstructure–magnetic property relationships, using laser deposited compositionally-graded Fe–Si–B–Nb–Cu alloys (by changing the silicon to boron ratios). The microstructure of Fe–Si–B–Nb–Cu alloys with a lower Si to B ratio consists of dendritic α-Fe₃Si grains, with B and Nb partitioning to the inter-dendritic regions, resulting in the formation of Fe₃B grains. As the Si/B ratio increases, the (Fe, Nb) enriched eutectic phase was observed along with α-Fe₃Si grains; and no Fe₃B was observed. These microstructural changes with varying Si/B ratios significantly affect the magnetic properties of these laser-deposited soft magnetic alloys.     

Microstructure and precipitation in Al–Li–Cu–Mg–(Mn,Zr) alloys

Abstract

Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al₃Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.     

An investigation on microwave sintering of Fe, Fe–Cu and Fe–Cu–C alloys

The powder characteristics of metallic powders play a key role during sintering. Densification and mechanical properties were also influenced by it. The current study examines the effect of heating mode on densification, microstructure, phase compositions and properties of Fe, Fe–2Cu and Fe–2Cu–0·8C systems. The compacts were heated in 2·45 GHz microwave sintering furnaces under forming gas (95%N₂–5%H₂) at 1120 °C for 60 min. Results of densification, mechanical properties and microstructural development of the microwave-sintered samples were reported and critically analysed in terms of various powder processing steps.     

Extrusion of AI–Zn–Mg–Cu–Zr alloys

Abstract

Four aluminium alloys of different zinc/magnesium ratio have been studied under various extrusion conditions. The alloys were cast in steel book moulds and subjected to initial thermomechanical treatments. Studies were made of hot extrusions and cold hydrostatic extrusions and in each case the changes in the extrusion parameters were analysed. An attempt has been made to explain some of the extrusion defects which appeared in various extruded sections. The extrusion speed was found to be crucial, since sections developed surface cracks at higher speeds. The extrusion speed was also found to vary inversely with the extrusion ratio, with higher speeds at low ratios. A well defined solute–depleted weld zone was observed on each of the four faces of a square tube extruded using a porthole die. Thermal treatment was not found to improve this weak weld zone. Tubes extruded using a floating-mandrel die withstood pressure testing up to 550 MPa.

MST/43     

Precipitation-hardening in cast AL–Si–Cu–Mg alloys

Age-hardenable aluminum–silicon alloys have attracted increasing attention in recent years, particularly as a result of the demand for lighter vehicles as part of the overall goal to improve fuel efficiency and to reduce vehicle emissions. Among these aluminum cast alloys, the 319-type alloys have become the object of extensive investigation considering their practical importance to the transport industry. All the experimental variables, such as solidification condition, composition, and heat treatment, are known to have an influence on precipitation behavior; precipitation-hardening, however, is the most significant of these because of the presence of excess alloying elements from the supersaturated solid solution which form fine particles and consequently act as obstacles to dislocation movement. The precipitation-hardening behavior of a Sr-modified 319-type alloy containing 0.4% Mg was investigated for this study using transmission electron microscopy. Non-conventional aging cycles were applied so as to evaluate the degree of the improvement in strength potentially obtainable. The results show that the main strengthening phase is θ-Al₂Cu occurring in the form of plates; other phases were observed as minor constituents in this alloy, including the binary β-Mg₂Si, the ternary S-CuAlMg₂, and the quaternary Q-Al₅Cu₂Mg₇Si₇.     

Effect of Fe content on the fracture behaviour of Al–Si–Cu cast alloys

Castings were prepared from 319.2 alloy melts, containing Fe levels of 0.2–1.0 wt%. Sr-modified (∼200 ppm) melts were also prepared for each alloy Fe level. The end-chilled refractory mold used provided directional solidification and a range of cooling rates (or dendrite arm spacings, DAS) within the same casting. Impact test samples were machined from specimen blanks sectioned from the castings at various heights above the chill end provided DASs of 23–85μm. All samples were T6-heat-treated before testing keeping with Aluminum Association recommendations. The results show that at low Fe levels and high cooling rates (0.4% Fe, 23 μm DAS), crack initiation and propagation in unmodified 319 alloys occurs through the cleavage of β-Al₅FeSi platelets (rather than by their decohesion from the matrix). The morphology and the size of the platelets (individual or branched) are important in determining the direction of crack propagation. Increasing the DAS to 83μm leads to cleavage fracture. In this case, the fracture path follows a transgranular plane that is usually a well-defined crystallographic plane as judged by the relatively large smooth surfaces of the β-Al₅FeSi phase platelets. Cracks also propagate through the fracture of undissolved CuAl₂ or other Cu-intermetallics, as well as through fragmented Si particles. In Sr-modified 319 alloys, cracks are mostly initiated by the fragmentation or cleavage of perforated β-phase platelets, in addition to that of coarse Si particles and undissolved Cu-intermetallics.The amount of undissolved Cu- intermetallics is directly related to the applied cooling rate. Slow cooling rate (DAS ≈83µm) results in the precipitation of Cu- containing phases on the β-platelets, amplifying the likehood for crack propagation through these loacations.     

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

The influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi₂ monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi₂ phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.     

Glass forming ability of Fe–Co–Zr–Nd–B alloys and bulk permanent magnets derived from amorphous precursor

The glass forming ability (GFA) and magnetic properties for Fe_48−x Co₂₇Zr₃Nd _x B₂₂ (x = 0–6) alloys were investigated. It was found that the proper addition of Nd (4–5 at.%) was very effective in improving GFA. The as-cast Fe₄₄Co₂₇Zr₃Nd₄B₂₂ and Fe₄₃Co₂₇Zr₃Nd₅B₂₂ alloys exhibited good soft magnetic behavior, while showed hard magnetic property after annealing at 760 °C for 10 min. Bulk permanent magnets were obtained from crystallization of amorphous alloys, which could provide a promising way for the bulk magnet produced by the simple process of copper mold casting and subsequent heat treatment.     

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.     

The electrochemical properties of melt-spun Al–Si–Cu alloys

Melt spinning was used to prepare Al_75−XSi₂₅Cu_X (X = 1, 4, 7, 10 mol%) alloy anode materials for lithium-ion batteries. A metastable supersaturated solid solution of Si and Cu in fcc-Al, α-Si and Al₂Cu co-existed in the alloys. Nano-scaled α-Al grains, as the matrix, formed in the as-quenched ribbons. The Al₇₄Si₂₅Cu₁ and Al₇₁Si₂₅Cu₄ anodes exhibited initial discharge specific capacities of 1539 mAh g⁻¹, 1324 mAh g⁻¹ and reversible capacities above 472 mAh g⁻¹, 508 mAh g⁻¹ at the 20th cycle, respectively. The specific capacities reduced as the increase of the Cu content. AlLi intermetallic compound was detected in the lithiated alloys. It is concluded that the lithiation mechanism of the Al–Si-based alloys can be affected by the third component. The structural evolution and volume variation can be mitigated due to the formation of non-equilibrium state and the co-existence of nano-scaled α-Al, α-Si, and Al₂Cu for the present alloys.     

Calorimetric and magnetic study of solid Fe–Cr–Co alloys

Abstract

An adiabatic calorimeter was used to measure the molar heat capacities and heats of transformation of iron alloys containing up to 14 at.-% Cr and 15 at.-% Co over the temperature range 700–1500 K. A Sucksmith magnetic balance was used to measure the magnetic properties of these alloys and of Fe–Co and Fe–Cr alloys previously investigated calorimetrically. In all but one of the ternary alloys the α → γ transformation occurred at a temperature below the Curie temperature (T_c), but the heats of transformation, corrected to the transformation temperature of pure iron (1184 K), correlated well with the difference between the extrapolated Curie temperature and 1184 K, as had been observed previously for alloys with clearly defined values of T_c. The fact that all observations fell on the same curve supported the proposal that a significant amount of magnetic order remains after the α → γ transformation for most iron alloys.

MST/198     

Microstructure and solidification behavior of Cu–Ni–Si alloys

The microstructure and solidification behavior of Cu–Ni–Si alloys with four different Cu contents was studied systematically under near-equilibrium solidification conditions. The microstructures of these Cu–Ni–Si alloys were characterized by SEM and the phase composition was identified by XRD analysis. The phase transition during the solidification process was studied by DTA under an Ar atmosphere. The results show that the microstructure and solidification behavior is closely related to the composition of Cu–Ni–Si alloys. The microstructure of Cu–Ni–Si alloys with higher than 40% Cu content consists of primary phase α-Cu(Ni, Si) and eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si).When the Cu content is about 40%, only the eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si)) is present. DTA analysis shows there are three phase transitions during every cooling cycle of alloys with higher than 40% Cu content, but only one for 40% Cu content. Cu–Ni–Si alloy with 40% Cu solidifies by a eutectic reaction, but Cu–Ni–Si alloys with higher than 40% Cu content solidify as a hypoeutectic reaction.     

Oxidation and abrasive wear of Fe–Si and Fe–Al intermetallic alloys

In the present work, intermetallic alloys Fe–Si and Fe–Al (Fe₃Si–C–Cr and Fe₃Al-C), produced by induction melting, were evaluated regarding their oxidation and abrasive resistance. The tests performed were quasi-isothermal oxidation, cyclic oxidation, and dry sand/rubber wheel abrasion. As reference, the ASTM A297-HH grade stainless steel was tested in the same conditions. In the oxidation tests, the Fe–Al based alloy presented the lowest oxidation rate, and the Fe–Si based alloy achieved the best results in the abrasion test, showing better performance than the HH type stainless steel.     

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al₈₆Mm₄Ni_10–x Fe _x alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of -Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of -Al and intermetallic phase observed especially in Al₈₆Mm₄Ni₅Fe₅ and Al₈₆Mm₄Ni₂Fe₈ alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.     

Eutectic Al–Si–Cu–Fe–Mn alloys with enhanced mechanical properties at room and elevated temperature

In this paper, we report a novel kind of eutectic Al–Si–Cu–Fe–Mn alloy with ultimate tensile strength up to 336 MPa and 144.3 MPa at room temperature and 300 °C, respectively. This kind of alloy was prepared by metal mold casting followed by T6 treatment. The microstructure is composed of eutectic and primary Si, α-Fe, Al₂Cu and α-Al phases. Iron-rich phases, which were identified as BCC type of α-Fe (Al₁₅(Fe,Mn)₃Si₂), exist in blocky and dendrite forms. Tiny blocky Al₂Cu crystals disperse in α-Fe dendrites or at the grain boundaries of α-Al. During T6 treatment, Cu atoms aggregate from the super-saturation solid solution to form GP zones, θ″ or θ′. Further analysis found that the enhanced mechanical properties of the experimental alloy are mainly attributed to the formation of α-Fe and copper-rich phases.     

Crystallization of amorphous CoSiB alloys: Effect of Fe additions

The results of transmission electron microscopy and X-ray diffraction studies of the crystallization behavior of different CoSiB alloys with and without Fe additions are reported. It is shown that a metastable bcc phase is formed by crystallization in all of the alloys with Fe additions. The bcc phase is not observed in the alloys without Fe additions.     

Density and viscosity of ternary Al–Cu–Si liquid alloys

Densities and viscosities of ternary Al–Cu–Si liquid alloys have been investigated over a wide temperature and composition range. Density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. In this ternary system, binary interaction parameters as well as a third (ternary) interaction parameter need to be taken into account for the excess volume to describe the liquid densities. The temperature dependences of the viscosities are well described by the Arrhenius law. A maximum of the activation energy of viscous flow is found in that compositional range in which intermetallic phases exist in the solid state.     

Thermal scanning studies of percolated Fe–Cu granular alloys

Fe–Cu alloys produced by mechanical alloying were studied with Mössbauer thermal scanning spectroscopy (MTS). This technique consists in recording Mössbauer effect absorption at a fixed energy while the temperature of the sample is changed. Hyperfine magnetic field behavior can be closely followed. Four mechanically alloyed samples Fe_xCu_100−x with iron concentration x=30, 35, 40, and 45 at.% were studied. Absorption graphs (intensity vs. temperature) are similar for all samples: intensity sharply falls with the increase of the temperature, following the collapse of the sextet lines. This collapse occurs at a very precise temperature, which coincides with a magnetic–nonmagnetic transition line in a former magnetic phase diagram. Conventional Mössbauer spectroscopy confirms the appointed phase change, displaying a doublet at temperatures above the critical.     

Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.