Al-rich Intermetallic Phases Research Articles

TiAl-based semi-finished material produced by reaction annealing of Ti/Al layered composite sheets

In this study multi-layered Ti/Al sheets prepared by accumulative roll bonding (ARB) underwent a two-step heat treatment (HT) to form intermetallic compounds. The microstructure and crystal structure of the samples were examined by scanning and transmission electron microscopy as well as energy-dispersive X-ray spectroscopy and synchrotron diffraction. In the first solid-state reaction annealing step, Ti-rich ARB samples containing 60 at% Ti and 40 at% Al were held at 600 °C for 12 h under a uniaxial pressure between 0 MPa and 50 MPa applied along the normal direction of the sheets. At this stage, Al is completely consumed by forming mainly Al-rich intermetallic phases and to a lower extent other titanium aluminides such as Ti3Al and TiAl. In the second step, high-temperature annealing produces TiAl and Ti3Al as major phases during both pressureless annealing at 1100 °C, 1200 °C and 1300 °C and annealing under an uniaxial pressure of about 100 MPa at 1200 °C. Pore formation during the reaction annealing can be significantly reduced by the applied pressure. As a result, a TiAl-based semi-finished material was fabricated.

Structure characterization of novel alluminides in the Nd-Re-Al system by electron crystallography methods

In the Al-rich A-Re-Al alloys (where A = lanthanide or actinide), several unique structure types of ternary alluminides crystalize. In the course of the Nd-Re-Al phase diagram investigation, the new Nd2Re3Al15 phase was revealed. Despite the prolonged heat treatment, the studied alloy did not attain equilibrium state, and contained multiple intermetallic phases, resulting in severe peak overlapping in the powder X-ray diffractogram. Thus, structure analysis was performed using combination of several electron crystallography approaches. First part of this work includes an investigation of the major phase constituting the studied alloy. Using electron diffraction tomography (EDT), atomic model of a pseudo binary (Nd,Re)Al12 phase was determined and found to be in perfect agreement with the reported earlier binary structure, solved from neutron data. Following the successful structure solution of the pseudo-binary phase, the new Nd2Re3Al15 structure was analyzed using the same methodology. The unit cell geometry was determined as orthorhombic with a = 12.801 Å, b = 22.545 Å and c = 9.103 Å. The space group was evaluated as Cc2m and partial model was obtained. It was revealed that the new Nd2Re3Al15 structure is related to A7+xRe12Al61+y. Typical topological features, observed in many Al-rich intermetallic phases, were also revealed in the atomic model of the new phase. Moreover, these features were clearly observed on the high resolution transmission electron micrographs, providing indirect confirmation of the proposed partial model.

Influence of zinc on intermetallic compounds formed in friction stir welding of AA5754 aluminium alloy to galvanised ultra-high strength steel

ABSTRACTIn this study, lap joints between AA5754 and DP1000 ultra-high strength steels were produced by friction stir welding. In order to investigate the roles of zinc on intermetallic phase formation and joint properties, steel substrates were used, two being galvanised coated and one uncoated. Joint performance has been evaluated in term of maximum tensile shear loading. The effects of the process parameter, translational speed; chemical compositions; and intermetallic phase formation on the mechanical properties have been investigated. The results show that joints with a galvanised layer exhibit higher strength as compared to the non-coated steel. A thicker galvanised layer promotes the presence of zinc in the aluminium matrix, resulting in better joint properties. The level of zinc contents in the aluminium matrix depends on process temperature and material circulation characteristics. Two stable Al-rich intermetallic phases, Al5Fe2 and Al13Fe4, were detected at the interface regardless of the coating conditions.

Electron-counting in intermetallics made easy: the 18-n rule and isolobal bonds across the Os–Al system

Abstract Electron count is one of the key factors controlling the formation of complex intermetallic structures. The delocalized nature of bonding in metals, however, has made it difficult to connect these electron counts to the various structural features that make up complex intermetallics. In this article, we illustrate how structural progressions in transition metal-main group intermetallics can in fact be simply understood with the 18-n bonding scheme, using as an example series the four binary phases of the Os–Al system. Our analysis begins with the CsCl-type OsAl phase, whose 11 electrons/Os count is one electron short of that predicted by the 18-n rule. This electron deficiency provides a driving force for Al incorporation to make more Al-rich intermetallic phases. In the structures of Os2Al3 (own type) and OsAl2 (MoSi2 type), each additional Al atom contributes three electrons, two of which go towards cleaving Os–Os isolobal bonds, with the third alleviating the original electron deficiency of OsAl. Across the series, the framework of isolobal Os–Os bonds is reduced from a primitive cubic network (n=6, OsAl) to layers of cubes (n=5, Os2Al3) to individual square nets (n=4, OsAl2). Upon adding more Al to form Os4Al13, the Os–Os contacts are further reduced to dumbbells at the interfaces between fluorite-type columns. At this point, the added Al raises the electron count beyond that needed for filled octadecets on the Os atoms; the excess electrons are accommodated by Al–Al bonds. Throughout this work, we emphasize how the 18-n scheme can be applied from structural inspection alone, with theoretical calculations confirming or refining these conclusions.

Production of MA956 Alloy Reinforced Aluminum Matrix Composites by Mechanical Alloying

Aluminum matrix composites (AMC) are attractive structural materials for automotive and aerospace applications. Lightweight, environmental resistance, high specific strength and stiffness, and good wear resistance are promising characteristics that encourage research and development activities in AMC in order to extend their applications. Powder metallurgy techniques like mechanical alloying (MA) are an alternative way to design metal matrix composites, as they are able to achieve a homogeneous distribution of well dispersed particles inside the metal matrix. In this work, aluminum has been reinforced with particles of MA956, which is an oxide dispersion strengthened (ODS) iron base alloy (Fe-Cr-Al) of high Young’s modulus and that incorporates a small volume fraction of nanometric yttria particles introduced by mechanical alloying. The aim of this work is to investigate the use of MA to produce AMC reinforced with 5 and 10 vol.% of MA956 alloy particles. Homogeneous composite powders were obtained after 20 h of milling. The evolution of morphology and particle size of composite powders was the typical observed in MA. The composite powders produced with 10 vol.% MA956 presented a more accentuated decrease in particle size during the milling, reaching 37 μm after 50 h. The thermal stability of the composite and the existence of interface reactions were investigated aiming further high temperature consolidation processing. Heat treatment at 420 °C resulted in partial reaction between matrix and reinforcement particles, while at 570 °C the extension of reaction was complete, with formation in both cases of Al-rich intermetallic phases.

Effect of iodine on the corrosion of Au–Al wire bonds

Corrosion study was performed on Au–Al wire bonds, thin layers of sputter deposited Au and Al, and Au–Al intermetallic nuggets. The test environment was iodine-vapour in air (1mg/L) at 85°C with varying relative humidity, and 500mg/L of KI in water. GDOES, XRD, SEM EDS, wire bond shear, and electrochemical testing were used to characterize the samples. Failures of Au–Al wire bonds were found to be primarily attributed to the corrosion of Al via formation of Al iodides and consequent formation of Al oxides and/or hydroxides. Most susceptible to corrosion are Al metallization and Al rich intermetallic phases.

A facile route to produce Fe–Al intermetallic coatings by laser surface alloying

This work studies the feasibility of obtaining iron aluminide coatings on pure iron substrate by means of laser surface alloying (LSA). Pure Al was plasma spray coated on pure Fe substrate and subsequently laser alloyed by using multi mode continuous wave Nd-YAG laser at different output power and scanning rates. The aluminum content of the alloyed layer shows gradual change from surface to the inside of substrate. Detailed optical, SEM–EDX and XRD investigations were performed to characterize the phases formed during laser alloying. At 300 W laser output power, Al rich intermetallic phases mainly FeAl3, Fe2Al5 had formed with scanning rate in the range of 100–400 mm/min. However, at high power outputs (500 W) Fe–Al intermetallic phase had formed, which was found to have sound and crack free interface with the substrate.

Thermodynamic assessment of the Al–Mo system and of the Ti–Al–Mo System from 0 to 20 at.% Ti

A re-assessment of the thermodynamic description of the binary Al–Mo and of the ternary Ti–Al–Mo system from 0 to 20 at.%Ti was performed. The new Al–Mo description reproduces a congruent melting of the AlMo phase and the calculated phase diagram is in excellent agreement with the experimental data on the Al-rich region showing the formation of several Al-rich intermetallic phases. The new Al–Mo description was used as the constituent binary in the Ti–Al–Mo system and the thermodynamic parameters of the η, β, and δ phases were re-optimized. Calculations using the new description are in good agreement with experimental work performed in the region from 0 to 20 at.% Ti showing the extension of the β phase from the Ti–Mo binary to the Al–Mo binary, the β phase solidification of several alloys, and the transition reaction between the β, Al 8Mo 3, δ, and η phases at 1540 K.

Formation of Aluminide Coatings on Ferritic–Martensitic Steels by a Low-Temperature Pack Cementation Process

A pack cementation process was developed to coat commercial 9% Cr ferritic–martensitic steel T91 at temperatures below its tempering temperature (∼ 750 °C) to avoid any potential detrimental effect on the mechanical properties. Two Cr–Al binary masteralloys with different Al levels were used to explore possibilities of forming FeAl-based coatings without the Al-rich intermetallic phases, such as Fe 2Al 5. In contrast to the pure Al masteralloy, which led to the formation of Fe 2Al 5 coatings at 650 °C, a coating consisting of a thin Fe 2Al 5 outer layer and an FeAl inner layer was formed at 700 °C by utilizing a Cr–25Al masteralloy. When the Cr-15Al masteralloy was employed, thin FeAl coatings (∼ 12 μm) containing < 50 at.% Al were achieved at 700 °C. The effect of the amount of masteralloys on coating growth was investigated using a series of packs containing 2NH 4Cl– x(Cr–15Al)–(98– x)Al 2O 3, where x ranged from 10 to 70 wt.%. Both coating thickness and surface Al content increased, when the amount of the masteralloy was increased from 10 to 40 wt.%. However, no further increase was observed once the masteralloy amount was greater than 40 wt.%.