Intermetallic Phase Particles Research Articles

Microstructure Modification of Powder Compact of Al–Zn–Mg Nanostructured Alloy by a Semisolid Thermomechanical Processing

A thermomechanical process (TMP) consisting of three cycles of cold pressing at 154 MPa and liquid-phase sintering at 600 °C for 30 min in each cycle was applied to modify the microstructure of nanostructured Al–Zn–Mg alloy. The alloy powders were produced by mechanical alloying. Also, solid-state sintering at 550 °C for 90 min was done to compare the results with those obtained from the TMP. The powders and the thermomechanically (TM) processed samples were analyzed by XRD to reveal the present phases in addition to calculating the crystallite size changes by the Williamson–Hall method. Moreover, scanning electron microscope was employed to observe the morphology of the powder and the microstructures of the sintered and the TM processed samples. The results revealed that the TMP affected the microstructure noticeably as well as the microhardness by removing the continuous grain boundary porosities and uniform distribution of the intermetallic phase particles as well as obtaining a near globular microstructure after the second cycle. Also, the average grain sizes in the first and the second cycles of the TMP were lower than those of the sintered sample. Furthermore, nanocrystalline grains were stable up to the second cycle of the TMP.

Accelerated growth of oxide film on aluminium alloys under steam: Part I: Effects of alloy chemistry and steam vapour pressure on microstructure

Aluminium alloys were treated with steam of varying vapour pressures which resulted in the growth of aluminium oxyhydroxide layers of an average thickness of ~450–825nm. The microstructure and composition of the generated layers were characterised by GD-OES, FEG-SEM, GI-XRD and TEM. The thickness of the oxide layer as well as the compactness increased with steam vapour pressure. The increase in vapour pressure also resulted in a better coverage over the intermetallic particles. Oxide layer showed a layered structure with more compact layer at the Al interface and a nano-scale needle like structure at the top. The kinetics of formation of film under steam was rapid; approx. 350nm thick layers were generated within 5s of steam treatment, however increase in thickness of the oxide retarded further growth. The enrichment or depletion of different alloying elements at the surface of aluminium as a result of alkaline etching pre-treatment influenced the thickness and growth of the oxide. Moreover the steam treatment resulted in the partial oxidation of second phase intermetallic particles present in the aluminium alloy microstructure.

Influence of calcium addition and stirring on the cellular structure and foaming behavior of molten zinc

In this paper, the influence of calcium addition and melt stirring on the structure and foaming behavior of molten zinc was investigated. In this regard, zinc foam was produced by Alporas method (in which foam alloy melts and titanium hydride is used as a blowing agent). Optical microscopy and scanning electron microscopy were used to investigate the phase distribution and structure in the foams. Results showed that addition of calcium increased foamability and foam efficiency of the molten zinc. In contrast, stirring had no significant effect on the foaming behavior of the melt. Microstructural examinations indicated that improving the foaming behavior of molten zinc was attributed to the formation of CaZn13 intermetallic phase and ZnO particles in the foam structure, which increased viscosity and reduced drainage rate.

Observations of the mechanical response and evolution of damage of AA 6061-T6 under different strain rates and temperatures

Investigated in this paper is the mechanical and microstructural response of specimens cut from rolled AA 6061-T6 alloy subjected to uniaxial loading at different temperatures (25, 100, 200, and 300 °C) and strain rates (10–4, 10−3, 10−2, 10−1 s−1). It was found that the material exhibits a strain rate and temperature dependent behavior. Microstructure observations reveal grains elongated in the loading direction with the average grain size being independent of loading conditions. However, it was observed that loading conditions influence the distribution and shape of the intermetallic phase particles. Micro cracks are observed to occur and are linked to observed damage and, consequently, decrease of ductility. Investigating the root cause of this damage mechanism reveals that particles of intermetallic phase composition are the likely sites of crack initiation. The crack-initiation mechanism at temperatures less than 200 °C is identified as crack initiation emanating from these particles while at temperatures of about 300 °C the mechanism is identified as interfacial decohesion.

Comparative study of protection of AA 2024-T3 exposed to rare earth salts solutions

The inhibitive effects of the dibutyl phosphate anion in cerium dibutylphosphate [Ce(dbp)3] on AA 2024-T351 immersed in 0·05M NaCl were evaluated by comparison with immersion in cerium chloride (CeCl3) in 0·05M NaCl as well as 0·05M NaCl alone. Scanning electron microscopy and energy dispersive X-ray hyperspectral mapping were used to examine the surfaces after exposure to the different inhibitor solutions. It was found that the dibutylphosphate anion produced additional inhibition for the matrix and the S phase intermetallic particles. The most significant mode of corrosion in the absence of inhibitors was in the form of rings of corrosion product. CeCl3 largely suppressed the formation of the rings of corrosion product, and the Ce(dbp)3 entirely eliminated them (for the time scales investigated here), indicating the additional inhibitive action of the dibutylphosphate anion.

The ambient and high temperature deformation behavior of Al–Si–Cu–Mg alloy with minor Ti, Zr, Ni additions

The principal aim of the present work was to investigate the effects of minor additions of nickel and zirconium on the strength of cast aluminum alloy 354 at ambient and high temperatures. Tensile properties of the as-cast and heat-treated alloys were determined at room temperature and at high temperatures (190°C, 250°C, 350°C). The results show that Zr reacts only with Ti, Si and Al. From the quality index charts constructed for these alloys, the quality index attains minimum and maximum values of 259MPa and 459MPa, in the as-cast and solution-treated conditions; also, maximum and minimum values of yield strength are observed at 345MPa and 80MPa, respectively, within the series of aging treatments applied. A decrease in tensile properties of ∼10% with the addition of 0.4wt.% nickel is attributed to a nickel–copper reaction. The reduction in mechanical properties due to addition of different elements is attributed principally to the increase in the percentage of intermetallic phase particles formed during solidification; such particles act as stress concentrators, decreasing the alloy ductility. Tensile test results at ambient temperatures show a slight increase (∼10%) in alloys with Zr and Zr/Ni additions, particularly at aging temperatures above 240°C. Additions of Zr and Zr+Ni increase the high temperature tensile properties, in particular for the alloy containing 0.2wt.% Zr+0.2wt.% Ni, which exhibits an increase of more than 30% in the tensile properties at 300°C compared with the base 354 alloy.

The Role of Second Phase Intermetallic Precipitates in Fatigue Fracture Mechanism for Aluminum Alloy AW 7075

This paper presents the results of durability tests of AW7075 aluminium alloy, which is widely used in producing high-durable and particularly reliable constructions in aeronautical and automotive industries. The plain-strain fracture toughness, depending on the orientation of fracture direction with respect to the rolling direction of the material, and the fatigue crack growth rate were determined. Based on scanning electron microscope (SEM) studies the two stages of fatigue fracture are shown: the first is plastic striations and the second is phenomenon of connecting micropores that were formed around intermetallic second phase particles. As a results of research a model of fatigue crack for the second stage is presented.

IMPROVED CORROSION RESISTANCE OF FRICTION STIR ZONE OF RARE EARTH MAGNESIUM ALLOY AE42

Corrosion studies on friction stir welded AE42 magnesium alloy were carried out in 0.1 M ammonium carbonate solution. For better understanding, comparative analysis of corrosion rates was carried out before and after welding. Friction stir weld zone was more corrosion resistant than the parent metal under similar experimental conditions. Grain size was refined at weld zone. Second phase intermetallic particles were refined into tiny pieces of 1-2 micron size and evenly distributed throughout the matrix. Local electrochemical imbalance was eliminated due to the grain refinement and elimination of continuous grain boundary. These phenomena improved the corrosion resistance of weld zone. Scanning electron microscopic images confirmed mild and uniform corrosion of weld zone and rigorous inhomogeneous corrosion of parent metal.

THE IMPACT TOUGHNESS OF HYPOEUTECTIC AlSi7Mg0.3 ALLOY PROCESSED BY ECAP

The effect of the pre-ECAP heat treatment and ECAP (Equal Channel Angular Pressing) processing on the impact toughness and fracture morphology of the AlSi7Mg0.3 alloy was investigated. The pre-ECAP heat treatment (annealing at 550°C followed by quenching and ageing at 250°C) of as -cast alloy improved its ductility, which was necessary for its severe plastic deformation. The pre-ECAP heat treatment followed by ECAP processing (4 passes at room temperature using A-route) of as-cast alloy homogenized the distribution of silicon and intermetallic phase particles in the solid solution matrix and improved its impact toughness. The as-cast alloy sample was fractured mostly in brittle manner. The fracture cracks initiated and propagated mainly along eutectic particles networks. T he fracture surface of ECAPed sample showed a typical ductile characteristic. The fracture propagated along and/or across banded distribution of eutectic particles and across bands of solid solution matrix.

Friction Stir Processing to Increase the Application Temperature of Rare Earth Magnesium Alloy AE42

AE 42 is rare earth containing magnesium alloy which was developed for creep resistance and automobile applications. This alloy was subjected to friction stir processing and the effect of processing parameters on the properties was studied in detail. Mechanical and metallurgical properties of stir zone were evaluated and compared with parent metal. Thermal cycling of stir zone was carried out to study the stability of it at high temperature. Due to processing, stir zone microstructure was refined to 5 micron. The second phase inter-metallic particles Mg17Al12, Al11Ce3 and Al2Ce were refined to tiny pieces of 1-2 micron and evenly distributed in the matrix. The stir zone was stable up to 250°C during thermal cycling. Mechanical properties of friction stir zone were superior to parent metal.

Selective aluminum dissolution as a means to observe the microstructure of nanocrystalline intermetallic phases from Al–Fe–Cr–Ti–Ce rapidly solidified alloy

Rapidly solidified aluminum alloys are promising materials with very fine microstructure. The microscopy observation of these materials is complicated due to overlay of fcc-Al matrix and different intermetallic phases. A possible way to solve this problem is to dissolve the Al matrix. By this process powder formed by single intermetallic phase particles is obtained. In this paper a new aqueous based dissolving agent for Al-based alloy is presented. The influence of oxidation agent (FeCl3) concentration on quality of extraction process was studied.

R-HPDC of Magnesium Alloys

Flexibility of the Council for Scientific and Industrial Research’s Rheocasting System (CSIR-RCS) and its rheo-high pressure die casting (R-HPDC) technology is again demonstrated, as with aluminium alloys, by processing and shape casting of three different magnesium alloys (AM50A, AM60B, AZ91D) in a first attempt. All as-cast microstructures are characterised more by rosette shaped globules of the primary-(Mg) phase together with Mg17Al12 as evidence of non-equilibrium cooling rates. Surface liquid segregation is observed in the as-cast microstructure for all three alloys. Minor alloy additions of Mn, in composition specifications, results in the formation of Al8Mn5 intermetallic phase particles dispersed throughout the microstructure. All alloys were homogenised at 415 °C for 16 hours for the T4 condition. The Mg17Al12 phase dissolves with homogenisation while the Al8Mn5 intermetallic phase does not dissolve. The resulting tensile properties of all three alloys in the as-cast and T4 conditions are reported.

Effect of alloying elements on the formation of rolling texture in Mg-Nd-Zr and Mg-Li alloys

The effect of alloying on the texture of Mg-Nd-Zr and Mg-Li alloy sheets is studied using pole and inverse pole figures. The basal texture intensity in rolling of Mg-Nd-Zr alloys is shown to be substantially decreased due to the precipitation of dispersed intermetallic Mg12Nd phase particles. As a result, the workability characteristics during deep drawing can be increased. Lithium alloying causes the formation of a prismatic rolling texture, which is unusual for magnesium alloys, as a result of the phase transformation of the lithium-based bcc phase into the magnesium-based hexagonal close-packed phase that obeys the Burgers orientation relationships.

Influence of the structural-phase state on the strength and plasticity of the pressure-synthesized intermetallic compound Ni3Al

878 The strength of hightemperature nickel alloys con� taining up to 70% disperse γ'�phase particles based on the intermetallic compound Ni3Al is largely determined by the strength and plasticity of the intermetallicphase par� ticles, which, in turn, depend on their structural-phase state. If we understand the influence of the structural- phase state on the plasticity and strength of the interme� tallic compound Ni3Al, we may purposefully modify the strength of the intermetallic component and the high� temperature nickel alloy as a whole. The compound Ni3Al is an alloy with L12 super� structure, characterized by high ordering energy and an anomalous (positive) temperature dependence of the yield point: at both low and elevated temperatures, the polycrystalline compound Ni3 Al undergoes brittle fracture along the grain boundaries. Therefore, it is impossible to improve the plasticity and strength of this compound by using familiar methods of intense plastic deformation to reduce its grain size (1). In this context, a valuable tool is selfpropagating hightemperature synthesis of intermetallic compounds in powder mixtures of the pure elements. If we synchro� nize the hightemperature synthesis of intermetallic compounds (by thermal explosion of the initial powder mixture) with compaction and plastic deformation of the intermetallic product, within a single technological cycle, we may create structural-phase states of different scale within the intermetallic compound (2, 3). In the present work, we investigate the structural-phase state of the intermetallic compound Ni 3 Al synthesized under pressure, with plastic deformation of the product, and also study the influence of this state on the plasticity and strength of this compound.

Effect of Alloy Purity on Fracture Behavior of Overaged 7000 Alloy Plates

The article deals with the relations between microstructural parameters, microscopic fracture mechanisms, and resultant plane-strain fracture toughness, KIC, of hot-forged 7000 alloy plates in overaged condition. Three industrially produced alloys with the (Zn + Mg + Cu) content of ∼11.5 mass% and different contents of (Fe + Si) impurities (from 0.23 to 0.37 mass%) were used. The variation in alloy purity generated differences in the geometric characteristics of coarse intermetallic phase particles, exerting significant effect on fracture behavior of these alloys. To evaluate the fracture mechanism, characterization of the broken KIC test specimen surfaces was performed using scanning electron microscopy. Three micromechanisms governing fracture process were identified. The extent of primary voiding at coarse intermetallic phase particles increases systematically with the increase of (Fe + Si) level, whereas the extent of intergranular/transgranular fracture decreases. This observation was confirmed by a quantitative description of fractures by means of a profilometric method. It was found that morphological parameters of the fracture profiles clearly differentiate between different fracture modes and offer conclusive proof that coarse voiding at fractured intermetallic phase particles dominates fracture at low alloy purity. A higher fraction of coarse voiding is shown to result in an overall reduction of fracture toughness.

Failure analysis of pin prick defects in galvannealed sheet – A case study

A peculiar type of pin prick spots/dents on the surface of some galvannealed sheets was observed, causing significant appearance problems in the finished product. In the present study, the surface defect was characterized by visual inspection, optical microscopy, microhardness, scanning electron microscopy and EDX to understand the source and mechanism of the defects. In the visual inspection, these peculiar defects were found to align in one direction. The SEM examination exhibited three distinct regions of the surface defect apart from the matrix: (1) particle, (2) a dense layer surrounding the particle, and (3) a crater region surrounding the particle. The EDX analysis of these regions indicated enrichment of Al and Fe and depletion of Zn in the particle. The microhardness measurements of the dense layer and the matrix indicated an increase in hardness in the dense layer compared to the matrix. Based on the preliminary results, the following hypothesis was made: Dross/zinc dust falling from snout wall or intermetallic phase or reaction product particles embed in the craters on the strip surface; during subsequent skin-pass rolling these particles peel out the zinc coating in the craters, thus densifying and flattening the region surrounding the particle. The paper presents the results of the investigation.

Wet‐chemical synthesis and characterization of NiBi nano‐ and micro‐particles

AbstractPreparation and characterization of NiBi nano‐ and micro‐particles are presented. Firstly, predetermined compositions were obtained by simultaneous precipitation from solutions of Bi(NO3)3 and Ni(NO3)2. The precipitates were heated under oxygen flow and in air, and thereafter reduced to metals under hydrogen flow at 673 K. Correlation between the predetermined and the actual compositions of the products was found. Characterization of the precipitates was carried out employing X‐ray diffraction, scanning and transmission electron microscopy. Nickel‐bismuth intermetallic phase (NiBi) particles with nano‐ and micro‐dimensions were observed in the samples after reduction. The other possible Ni–Bi intermediate phase (NiBi3) did not form at these conditions although its presence was expected according to the phase diagram. This finding might be useful for the implementation of Bi‐based solders where the growth of the compound NiBi3 in the solder joints must be prevented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructural Characterization of 5083 Al Alloy Joints Friction Stir Welded

The Friction Stir Welding (FSW) process is a quite recent joining method whose particular characteristics yield to materials modifications not yet fully understood. This paper aims to present the research results of a study focused on the modifications induced in an aluminium alloy AA5083 – H111, when processed by FSW to build components for structural applications. The welded samples were firstly analysed by optical microscopy in order to define the different joined zones and to identify defects. Further tests included the measurement of the Vickers microhardness, grain and particle distribution and chemical analysis of the constituents, particularly of the intermetallic second phase particles, by Auger spectroscopy. Special attention has been given to the characterisation of several microstructural “non-homogeneities” like a laminated structure called onion ring and an identified anomaly (in the form of a line), whose presence was evaluated in order to determine its effects on the final joint properties. The results have shown important new details about the relationship between the microstructure and the final joint structural properties, contributing to improve the knowledge about the materials behaviour increasing the potential of utilization of the FSW process.

ＡＺ９１マグネシウム合金における腐食挙動とミクロ組織の関係

Magnesium alloys are remarkably corroded in chloride environments. The corrosion behavior of the AZ91 alloy is mainly dependent on Fe/Mn ratio and size and distribution of the intermetallic Al-Mn-Fe phase and Mg17Al12 (β-phases) particles. Grain size and heat-treatment condition also influence both the microstructure and corrosion behavior. The AZ91E was melted and casted into various permanent molds in order to obtained several cast ingots with different grain sizes (110 to 250 μm). Some ingots were heat-treated to control the distribution of β-phase particles. Corrosion characteristics were evaluated by a salt-water immersion testing and the open circuit corrosion potential measurement. Metallographic characteristics were evaluated by the average distance between intermetallic phase particles. The corrosion rate was increased with increasing the grain size in the as-cast (F) specimens. But it was independent of Fe/Mn ratio. Both of grain size and Fe/Mn ratio affected the corrosion behavior in the homogenized (T4) specimens. Independent of grain size and Fe/Mn ratio. Artificially aged (T6) specimens showed a good corrosion resistance. It was found that the β-phase act as a good barrier against salt-water corrosion. Corrosion potential of the Al-Mn-Fe phases depending on the composition.

Formation of eutectic intermetallic rosettes by entrapment of liquid droplets during cellular columnar growth

We study the formation of α-Al-(Fe,Mn)-Si phase intermetallic particles in a directionally-solidified model Al–0.5Fe–1.0Mn–0.2Si (wt%) alloy. In this the aluminium matrix has a cellular-columnar grain structure. Secondary phases are expected to form from solute-partitioned liquid at the cell boundaries. However, spherical rosettes of eutectic α-phase can be found within the cells. Evidence is presented that these are formed from liquid droplets trapped when a cell splits and then coalesces. The intercellular regions of the as-cast alloy are primarily occupied by metastable Al 6(Fe,Mn) eutectic particles. However, these may transform to the stable α-phase, either by a ternary peritectic transformation during solidification, or by the eutectoid 6-to-α transformation during the period in which the ingot cools to room temperature.