Primary Intermetallic Particles Research Articles

Изучение влияния содержания гафния и эрбия на формирование микроструктуры при литье алюминиевого сплава 1590 в медный кокиль

Introduction. High-magnesium aluminum alloys are widely used in the automotive, building and aerospace industries due to its low specific gravity and high strength. The characteristics of such alloys can be improved by small additions of scandium and zirconium. However, scandium is very expensive, so in new generation alloys its amount is tended to be reduced. In the recently developed 1590 aluminum alloy, this was achieved by addition of erbium and hafnium. The objective of the paper is to study the effect of erbium and hafnium concentrations on the modification of the cast structure in 1590 aluminum alloy at high solidification rates. Research Methods. The paper investigates the microstructure, chemical composition and size of intermetallic compounds in specimens from ten alloy 1590 modifications with different hafnium and erbium contents cast into a copper chill mold with a solidification rate of 10 °C/sec. The grain structure was studied using an optical microscope. The chemical composition and size of the intermetallic phases were studied using a Tescan Vega 3 scanning electron microscope. Results and discussion. It is established that as the amount of hafnium and erbium increases, the cast structure is modified. In general, grain refinement with the addition of hafnium and erbium can be explained by a higher degree of supercooling between the solid and liquid phases. At a hafnium content of 0.16 %, the dendritic structure begins to transform into an equiaxed grain structure. This transformation can be explained by the appearance of primary intermetallic compounds of the Al3Sc type in the liquid phase. Such intermetallic compounds are identified at a concentration of erbium and hafnium equal to 0.16 %. Moreover, in all alloys eutectic intermetallic compounds are identified that contained manganese and iron and had no effect on the cast structure. Comparison with previously obtained results on the grain size of specimens cast into a steel mold shows that with higher solidification rate, the structure modification in 1590 alloy is getting less efficient. This is explained by an increase in the concentration of transition elements in the solid solution, primarily scandium, necessary for the formation of primary intermetallic particles.

Formation of primary intermetallic phases in the interaction of Fe-containing melts of recycled Al-Si alloys with Al2O3 and Al2O3-C filter materials: A µ-CT study

Fe is the most detrimental impurity element in recycled Al-Si alloys due to the formation of brittle, primary, Fe-containing, intermetallic particles during solidification. Their removal from the Al-Si melts e.g. by filtration can reduce the Fe content. New active filter materials can facilitate the formation of these particles for their removal and contribute to the production of high-quality, recycled Al-Si alloys. The interaction of the alloy with the filter material can lead to modification of the thermodynamics of the alloy or of the kinetics of the particle formation. Time-resolved, three-dimensional microstructural investigations have been carried out to study the formation of primary intermetallic particles in Al7.1Si1.5Fe and Al7.1Si0.75Fe0.75Mn alloy melts in contact with Al2O3 and Al2O3-C filter substrate material during a melt conditioning treatment at 620 °C. The microstructures, in particular the primary intermetallic particles αc and αh, have been characterized by computed tomography (CT) and supplementary scanning electron microscopy (SEM). As expected by thermodynamics, the total volume fraction of primary particles remains unchanged by the interaction with the substrate materials. However, kinetic advantages for Fe-removal efficiency can be achieved by an accelerated and preferred selective particle formation in contact with the Al2O3-C material. Furthermore, particle formation is discussed in view of its different stages: nucleation, growth, and ripening.

THERMAL STABILITY OF THE MICROSTRUCTURE OF AV ALUMINUM ALLOY LOCALLY REINFORCED WITH SiC PARTICLES BY FRICTION STIR PROCESSING

Friction stir processing (FSP) is one of the innovative methods for surface hardening commercial aluminum alloys. It modifies the surface of the alloy in the solid phase state by severe plastic deformation at elevated temperatures. However, most alloys have low thermal stability of the microstructure after FSP. In some cases, abnormal grain growth occurs after high-temperature heat treatment. This study demonstrates the potential to enhance thermal stability of AV (avial, Al-Mg-Si) aluminum alloy microstructure via local surface alloying with high-strength SiC particles by means of FSP and T6 heat treatment. Experimental trials determined the optimal FSP settings for commercial aluminum alloys (rotation speed 1000 rpm, feed rate 16 mm/min) and T6 heat treatment (consisting of solid solution treatment and artificial aging) on AK12D (Al-Si) aluminum alloy. Advanced techniques in optical metallography, scanning electron microscopy, and orientation microscopy (EBSD analysis) were utilized to examine microstructures. The study compared two structural states, AV and AV-SiC alloys, after FSP and T6 heat treatments. The analysis revealed that FSP resulted in the formation of a basin-shaped stir zone whose boundaries extended uniformly to the workpiece surface. FSP has been found to result in the fragmentation and uniform distribution of primary intermetallic phase particles. Additionally, FSP has been shown to contribute to the crushing of SiC particles and their non-uniform distribution in the stir zone. The introduction of highstrength particles into the AV alloy during FSP has also been found to increase its thermal stability. Following a hardening heat treatment, a fine-grained microstructure with an average grain size of about 1.60±0.03 microns is observed.

Rheological Behaviour of Secondary AlSi7Mg Alloy for Semi-Solid Processing

Semisolid castings are usually produced using primary Al alloys to ensure significant mechanical performances. However, the need to increase the use of secondary alloys is becoming more and more urgent to reduce the carbon footprint of the manufacturing process. In fact, it is well known that the production of primary alloys is far more demanding in terms of energy and emissions than the recycling route. To extend the use of secondary alloys to semisolid processing, it is necessary to thoroughly understand their properties and how they can influence a material with peculiar properties as the semisolid one. Besides microstructural and mechanical features, the rheological behaviour also plays a major role when dealing with processing metals in the semisolid state. Therefore, in the present study, a rheological characterization of secondary AlSi7Mg commercial alloy was carried out and compared to that of the conventional primary alloy. In details, a different content of Fe, Cu and Mn was considered, as these impurities easily form primary intermetallic particles, which can remain dispersed in the liquid matrix of the semisolid metal. The aim of this work is to understand if this can affect the rheological properties of the considered semisolid alloy. Flow curves and yield stresses were obtained from the experimental results to compare the behaviour of the different alloys.

Interactions of primary intermetallic compound particles and double oxide films in liquid aluminum alloys

One of the main challenges in processing aluminum alloys in the liquid state is the avoidance of the formation of double oxide films and their incorporation into the final parts. Bifilms in liquid alloys can act as heterogeneous nucleation sites for primary intermetallic compounds (IMCs), which phenomenon can be utilized for their aided sedimentation. In this study, TiAlSi IMCs were used for the sedimentation treatment of a liquid Al–Si–Mg–Cu alloy. The interactions between IMCs and bifilms were studied by computed tomography (CT) and scanning electron microscopy (SEM), while the changes in melt quality were assessed by the CT analysis of reduced pressure test (RPT) samples. Additionally, the grain refinement effect of Ti-addition was studied by grain size measurements. It was found that (Al, Si)3Ti particles became attached to bifilms, and their simultaneous sedimentation resulted in lowered porosity. However, the attachment of IMCs to the surface oxide layer caused Ti-macrosegregation in the top region of the melt. The attachment of (Al, Si)3Ti intermetallic particles is caused by the heterogeneous nucleation of IMCs on oxide films and the partial engulfment of oxide film segments during the growth of the (Al, Si)3Ti particles. Investigations of the pore surfaces support the theory of bifilm-initiated pore formation.

Microstructure and Properties after Friction Stir Processing of Twin-Roll Cast Al–Mn–Cu–Be Alloy

We studied the effect of friction stir processing (FSP) on the microstructure and properties of high-speed twin-roll cast strips made of an experimental Al–Mn–Cu–Be alloy. The samples were examined using light, scanning, and transmission electron microscopy, microchemical analysis, X-ray diffraction, and indentation testing. During FSP, the rotational speed varied, while other parameters remained constant. The uniformity of the microstructure increased with the growing rotational speed. In the stir zone, several processes took place, and the most important were: recrystallisation of the matrix grains, fragmentation of the primary intermetallic particles Al15Mn3Be2 and their more uniform distribution in the stir zone, fracture, and dispersion of the eutectic icosahedral quasicrystalline phase (IQC), transformation of tiny Al15Mn3Be2 and IQC particles into the τ1-Al26Mn6Cu4 phase and precipitation of Al–Mn–Cu precipitates. In the thermomechanically affected zone, new dislocations formed as well as dispersion of the IQC eutectic phase and recrystallisation of the matrix grains. In the heat-affected zone, dissolution of θ’-Al2Cu precipitates occurred. The hardness variation was not severe between the stir and heat-affected zones.

Evolution of pitting corrosion resistance and mechanical properties in ultrafine-grained commercially pure aluminium during annealing

The aim of the present study was to determine the evolution of resistance to pitting corrosion and changes in the mechanical properties of ultrafine-grained aluminium during annealing. In contrast to the numerous papers devoted to the topic of the corrosion resistance of severely deformed aluminium alloys, a unique approach has been taken in this study. The size and distribution of the primary intermetallic particles, which are crucial in terms of corrosion resistance, remain constant during annealing. Therefore, the influence of other microstructural features could be investigated and compare with the results of mechanical properties. It was shown that the ultrafine-grained structure of commercially pure aluminium was stable up to 200 °C. Higher annealing temperatures caused significant grain growth and a reduction in dislocation density, which resulted in a drop in mechanical strength. Also, the corrosion resistance slightly decreased, since with an increase in annealing temperature a decrease in the corrosion potential, pit size, and the area damaged by corrosion attack were noted. The best combination of mechanical strength and resistance to pitting corrosion has been achieved for as-deformed state.

Insight into the localized strain effect on micro-galvanic corrosion behavior in AA7075-T6 aluminum alloy

Micro-galvanic corrosion behaviors between two primary intermetallic particles (Mg2Si, α(Al-Fe-Cu)) and the Al matrix were investigated under elastic and plastic deformations in AA7075-T6. Strain localization was mainly concentrated within Mg2Si and at the interface of α(Al-Fe-Cu)/Al matrix under deformations. The experimental Volta potential differences between Mg2Si, α(Al-Fe-Cu) and Al decreased by 0–16 mV and 2–27 mV in the elastic stage but increased by 0–39 mV and 0–81 mV in the plastic stage, which were consistent with the calculation results. Accordingly, the pitting corrosion resistance was improved under elastic deformation but deteriorated under plastic deformation.

Mechanism of ultrasound-induced microstructure modification in Al–Zr alloys

This work probes the effect of ultrasonic melt treatment (UST) on primary intermetallic particles and α-Al using pure Al and Al–Zr alloys (Al–0.3Zr–0.1Ti, Al–0.5Zr, Al–0.5Zr–0.5Mg–0.9Si) solidified at various cooling rates of 0.2–70 K s−1. The application of UST is shown to decrease the size and increase the number density and volume fraction of D023-structured primary Al3Zr and Al3(Zr,Ti) particles with a high nucleation potency for α-Al formation. High-resolution transmission electron microscopy analysis reveals that the cavitation-induced wetting and dispersion of γ-Al2O3 inoculant particles contribute to the refinement of primary intermetallic particles. The grain size of Al–Zr alloys increases with increasing cooling rate because of the concomitantly reduced formation of primary intermetallic inoculant particles. The UST-induced refinement of primary Al3Zr particles and the increased number density lead to grain refinement in Ti-free Al–Zr alloys. For the Ti-containing Al–Zr alloy, UST-induced grain refinement is achieved only at a very low cooling rate of 0.2 K s−1, possibly because of the side effects of UST on the growth-restricting influence of Ti. The fast cooling–induced solidification enables significant age-hardening by L12-Al3Zr nanoprecipitation (~5 nm) and is not affected by UST. Thus, this study shows that UST can be used to reduce the degree of grain coarsening of age-hardenable Al–Zr alloys at high cooling rates.

Fatigue behavior of AW7075 aluminum alloy in ultra-high cycle fatigue region

Advanced electron microscopy methods were used with the aim to explain the differences in the response of strengthened AW 7075 – T6511 aluminum alloy to fatigue loading at 5 Hz and 20 kHz. The shift of the S–N curve to higher number of cycles to fracture for the specimens tested at high frequency was experimentally determined. This effect is not connected with a change of the fatigue crack initiation mechanism and site from the surface to material interior. The absence of slip markings and cracking of primary intermetallic particles on the surface of cycled specimens were characteristic features. A difference in the dislocation density and dislocation arrangement in the vicinity of the fatigue crack initiation sites was shown to be the only observable effect. However, consistent with the strengthened structure of the alloy, no specific dislocation structure due to the cyclic loading was observed, regardless of the loading frequency and stress amplitude.

Effect of structural-phase transformations on the microhardness of the Al-22%Si-3%Cu-1.7%Ni alloy after high-pressure torsion and annealing

The squeeze casting Al-22%Si-3%Cu-1.7%Ni alloy was subjected to high-pressure torsion (HPT) and subsequent annealing for investigation of their effect on the microstructure and microhardness. Specimens with a diameter of 8 mm and a thickness of 0.45 mm, cut from a squeeze cast ingot (region of ingot without primary silicon particles), were severely deformed by HPT via 5 revolutions under a pressure of 4 GPa at room temperature. The edge parts of the deformed samples were annealed at temperatures of 300, 400 and 500 °C for 5 minutes. HPT resulted in the refinement and partial dissolution of the eutectic silicon and primary intermetallic particles in an aluminum matrix with the formation of abnormal supersaturated solid solution and fragmentation of the structure that caused an increase in microhardness from 149±10 HV for the squeeze casting state to 285±14 HV for the deformed state. Subsequent annealing leads to the decomposition of the abnormal supersaturated solid solution, which occurred simultaneously with the recovery and recrystallization of the fragmented structure, which led to a decrease in the microhardness to 183±5, 108±5 and 132±6 HV at temperatures of 300, 400 and 500 °C, respectively.

The effect of precipitates on voiding, twinning, and fracture behaviors in Mg alloys

The effect of precipitates on fracture behaviors was comparatively investigated among three kinds of Mg alloys with different precipitates, i.e., Mg-Gd alloy with prismatic plate-shaped precipitates, Mg-Zn alloy with [0001]α rod-shaped precipitates, and Mg-Gd-Zn-Zr alloy with basal plate-shaped precipitates. By comparing the fracture behaviors of the alloys before and after aging treatment, it was evident that the presence of precipitates greatly promoted the formation of microvoids that were initiated at primary intermetallic particles. Voiding was the most accelerated in the Mg-Gd alloy, where the precipitate-dislocation interactions are the strongest. While in the Mg-Gd-Zn-Zr alloy with the weakest precipitate hardening, the precipitate-facilitated voiding was the least significant. The precipitate-dependent voiding suppressed the twinning behaviors, causing the volume fraction of deformation twins decreased in turn from Mg-Gd-Zn-Zr to Mg-Zn, and finally to Mg-Gd alloys. The tensile ductility of present Mg alloys approximately scaled with the volume fraction of deformation twins, and was highly sensitive to the precipitates. The fracture scenario of present Mg alloys was proposed that the voiding suppressed twinning and the competition between voiding and twinning can be mediated by precipitates. In the Mg-Gd-Zn-Zr alloy with limited voiding, the twinning dominated the deformation process and concomitantly resulted in a great ductility. The formation of microvoids or the fracture of primary intermetallic particles was quantitatively analyzed by applying a Weibull model, where both the alloy strength and the volume fraction of primary intermetallic particles were considered to rationalize the remarkable difference in fracture behaviors among the present three Mg alloys. Furthermore, the coupling contribution of precipitates and twins to the work hardening was modelled in the aged Mg-Gd-Zn-Zr alloy that displayed a large room temperature tensile ductility of ~ 13.5%.

Effect of hot extrusion and subsequent T6 treatment on the microstructure evolution and tensile properties of an Al-6Si-2Cu-0.5Mg alloy

In this work, an Al-6Si-2Cu-0.5Mg alloy was subjected to hot extrusion at various temperatures (400°C, 450°C and 500°C) followed by subsequent T6 heat treatment, and the effect of aforementioned thermal-mechanical treatment on the microstructure evolution and tensile properties was systematically investigated. Microstructure observed suggested that both eutectic Si particles and primary intermetallic particles (Q phase) were refined by hot extrusion. Further T6 treatment led to the dissolution of the Q phase and the formation of nano-precipitates. Tensile test revealed that the alloy extruded at 450°C (after T6 treatment) exhibited the best combination of strength and ductility. Increasing or decreasing the extrusion temperature resulted in the deterioration of ductility. The effect of the extrusion temperature and T6 treatment on the mechanical properties was discussed based on the microstructure evolution.

A synchrotron X-radiography study of the fragmentation and refinement of primary intermetallic particles in an Al-35 Cu alloy induced by ultrasonic melt processing

The authors acknowledge financial support from the UK Engineering and Physical Science Research Council (EPSRC) for the Ultra-Cast project (grants EP/L019884/1; EP/L019825/1; EP/L019965/1). Mi also would like to acknowledge the financial support by the Royal Society Industry Fellowship awarded in 2012-2016. The experiments were performed on the Psiche beamline at Synchrotron SOLEIL, France (Proposal No. 2016076). Beamline I12 at Diamond Light Source, UK, is also thanked for loan of the high speed camera.

Fatigue Behavior of Precipitation Hardening Alloys in the LCF and VHCF Regime

LCF/HCF strength of precipitation hardening alloys is primarily controlled by its heat treatment condition. However, for a nickel-based superalloy and a wrought aluminium alloy it will be shown, that the VHCF behavior cannot solely be explained by the precipitation morphology. Damage accumulation is dominated by microstructure related slip localization, grain morphology and microstructural flaws. In contrast to the LCF behavior, the prediction of cyclic strength in the VHCF regime requires a detailed analysis of the competing microstructural crack initiating characteristics. Hence, new fatigue life prediction models have to be developed, which consider a statistical analysis of the failure-relevant inhomogeneities. In the case of the two materials studied, VHCF behavior is dominated by isolated and inhomogeneously distributed irreversible slip accompanied by a low dislocation density. The formation of single slip bands in favorably oriented grains in Nimonic 80A results in a decrease of the VHCF strength for the peak-aged condition. The overaged condition shows better VHCF strength due to a more homogeneous distribution of slip bands, as dislocations pile up at the overaged precipitates due to the very low strain amplitudes, while in the LCF regime the Orowan mechanism results in a weaker cyclic strength compared to the peak-aged condition. Crack initiation of the aluminium alloy EN AW-6082 on the one hand depends on the size and distribution of primary intermetallic particles of the Al-Fe-type acting as local stress raiser embedded in a strong matrix in case of the peak-aged condition. In contrast, such stress peaks are less failure-relevant due to the softer solid solution depleted matrix. Hence, the heat treatment alone does not define VHCF behavior.

Microstructure investigations on two different aluminum wrought alloys after very high cycle fatigue

Fatigue studies were conducted under load control with a servo-hydraulic testing machine with smooth specimens of the aluminum wrought alloys EN AW-6056-T6 (Rm=399MPa) and EN AW-6082-T5 (Rm=356MPa) up to a maximum number of N=2×108 and 109cycles, respectively. The results show for both aluminum alloys that the fatigue strength decreases with increasing number of cycles after the kneepoint of the S–N-curve and that approximately at this point a transition of the crack initiation site from the surface to the subsurface occurs. All fractured specimens were investigated with a scanning electron microscope (SEM). Large defects like primary intermetallic particles could not be found at the crack initiation sites. The difference between the subsurface non-defect crack initiation sites of both alloys is to be found in the fractographic structure. The internal crack initiation site of EN AW-6056-T6 shows a multiplicity of cleavage-like planes contrary to the flat area found at the alloy EN AW-6082-T5. These results were already presented in [1]. The scope of this present paper is new microstructural investigations and the formulation of a failure model which respects the results of different scales of microscopy. Electron back scatter diffraction analysis (EBSD analysis) and transmission electron microscopy (TEM) were done for both alloys to characterize the microstructure and to compare unstressed and tested materials. These differences are discussed with regard to the differences in failure mechanisms.

Multiscale characterization of nanostructured Al-Si-Zr alloys obtained by rapid solidification method.

Properties of engineering metallic alloys (e.g., fracture toughness, corrosion resistance) are often limited by the presence of primary intermetallic particles which form during conventional solidification. Rapid solidification brings about much more homogenous amorphous and/or nanocrystalline structure with reduced density of primary particles. Rapidly solidified thin ribbons obtained by melt spinning are usually considered as intrinsically homogenous. However, due to different cooling conditions at the wheel surface and on the side exposed to the ambient environment, structure of such ribbons may vary significantly across its thickness. The materials studied in this study were 30–40 μm thickness ribbons of nanocrystalline hyper- and hypo-eutectic Al–Si–Zr alloys produced by melt-spinning method. Transmission electron microscopy and high resolution scanning transmission electron microscopy were used to characterize the structure homogeneity across the ribbons. Thin foils for transmission observations were prepared by focused ion beam system. Microstructural observations confirmed nanocrystalline character of Al–Si–Zr alloys. However, these observations revealed inhomogeneity of the structure across the ribbon width.

Effect of Sc and Zn Additions on Microstructure and Hot Formability of Al-Mg Sheet Alloys

Trace elements of Sc or Zn were added to Al-Mg sheet alloys. Microstructural examination showed that the Sc addition greatly refined the grain size, especially in the Zn-containing alloy where large amounts of subgrains and fine grains were formed. Meanwhile, a number of large primary intermetallic particles formed in the Sc-adding alloys. In order to evaluate the high-temperature formability, warm tensile tests were carried out at temperatures ranging from 275 °C to 350 °C and at strain rates of 0.015 to 1.5 s−1. The test results showed that, in the alloys with the single addition of Zn or Sc, Zn slightly increased the flow stress but decreased the ductility and Sc worsened both the flow stress and the ductility. However, in the alloy with the combined addition of Zn and Sc, the flow stress was significantly increased at almost all testing conditions and the ductility was also increased at a higher temperature of 350 °C and lower strain rate of 0.015 s−1. The results of superplastic tensile tests and biaxial stretch tests demonstrated that the alloy with both Zn and Sc additions exhibited good high-temperature formability. The effect of Zn and Sc additions on the microstructure and the hot formability is discussed.

Effect of primary intermetallic particles on surface microstructure and appearance of aluminium extrusions

Different amounts of coarse intermetallic particles were produced in an aluminium alloy by variations of the iron content. Microstructural examination of the extrusion surface shows that Fe-rich intermetallic particles reduce the grain size, randomize the texture, retard the recrystallization process and decrease the difference in the fraction of high-angle grain boundaries. After alkaline etching, the primary intermetallic particles decrease the surface gloss and its variation on the same extrusion surface, reducing the tendency for streaking defects.

Structure and thermal stability of primary intermetallic particles in an Mg–Zn casting alloy

The structure and thermal stability of primary intermetallic particles formed in an Mg–8 wt.%Zn casting alloy have been characterized using transmission electron microscopy. Most primary intermetallic particles in the as-cast microstructure are of the Mg7Zn3 phase, and they decompose into α-Mg and Mg4Zn7 phases during post-casting heat treatments. The resultant Mg4Zn7 phase is metastable and, during prolonged heat treatments, it is gradually replaced by the equilibrium phase MgZn that has a hitherto unreported monoclinic structure.