Al-Au System Research Articles

Phase-field modeling of reactive wetting and growth of the intermetallic Al2Au phase in the Al-Au system

When an Al-droplet is in contact with an Au-solid substrate, the liquid phase reacts with the substrate and an intermetallic layer is formed at the solid-liquid interface due to diffusion and reaction. This phenomenon has been commonly observed in the soldering process and the wetting is termed as reactive wetting, in contrast to the inert wetting where the droplet does not react with the base materials and the substrate is flat. Young's law can be used to interpret the contact angle in the static state, but is not able to predict the dynamics in the process of reactive wetting. In this work, we present a multi-phase model including phase transition and fluid dynamics to investigate the effect of the formation of the intermetallic Al2Au phase and capillary flow on the reactive wetting in the Al-Au system. In order to capture phase boundaries of solid-, liquid- and intermetallic-vapor, phase-field simulations are performed based on a ternary (Al-Au-X) phase diagram concept and using experimental data. It has been found that the increase of both the liquid-intermetallic interfacial tension as well as the capillary flow lead to an inhibition effect for the growth of the intermetallic phase.

A phase-field study on the formation of the intermetallic Al2Au phase in the Al–Au system

The growth mechanism of the intermetallic Al2Au phase in the Al–Au system during the process of soldering is investigated by a multiphase phase-field model. With the aid of the assessed thermodynamic data (Li et al., 2004), the binary phase diagram of the Al–Au system is reconstructed by assuming a regular solution model for the intermetallic Al2Au phase. The simulation setup is composed by placing an Al-liquid droplet on top of an Au-solid substrate. To capture the phase boundary evolution of the droplet and substrate interface with respect to the surrounding vacuum phase, the binary phase diagram is extended to a ternary model system at one particular temperature between the melting points of Al and Au. Due to diffusion of Al and Au at the droplet/substrate interface, intermetallic phase is formed. In simulations, we investigate the lateral and vertical growth velocities of the intermetallic phase as a function of the surface energy of liquid–intermetallic. Further, the instability of the liquid(Al)–solid(Au) interface is analyzed by examining the shrinking speed of the liquid–solid interface affected by the diffusivity of Au.

ChemInform Abstract: Turning Gold into “Diamond”: A Family of Hexagonal Diamond‐Type Au‐Frameworks Interconnected by Triangular Clusters in the Sr—Al—Au System.

AbstractDifferent intermetallic compounds containing three‐dimensional tetrahedral frameworks of Au atoms similar to hexagonal diamond are obtained from melts of the Sr‐Au‐Al systems: (I) hexagonal SrAl3‐xAu4+x (0.06 ≤ x ≤ 0.46; space group P$\bar 6$2m, Z = 3); (II) orthorhombic SrAl2‐yAu5+y (y ≤ 0.05; space group Pnma, Z = 4); (III) Sr2Al2‐zAu7+z (z = 0.32; space group C2/c, Z = 4); and (IV) rhombohedral Sr2Al3‐wAu6+w (w ≤ 0.18; space group R$\bar 3$c, Z = 6).

Turning Gold into “Diamond”: A Family of Hexagonal Diamond-Type Au-Frameworks Interconnected by Triangular Clusters in the Sr–Al–Au System

A new homologous series of intermetallic compounds containing three-dimensional (3-d) tetrahedral frameworks of gold atoms, akin to hexagonal diamond, have been discovered in four related Sr-Au-Al systems: (I) hexagonal SrAl3-xAu4+x (0.06(1) ≤ x ≤ 0.46(1), P62m, Z = 3, a = 8.633(1)-8.664(1) Å, c = 7.083(2)-7.107(1) Å); (II) orthorhombic SrAl2-yAu5+y (y ≤ 0.05(1); Pnma, Z = 4, a = 8.942(1) Å, b = 7.2320(4) Å, c = 9.918(1) Å); (III) Sr2Al2-zAu7+z (z = 0.32(2); C2/c, Z = 4, a = 14.956(4) Å, b = 8.564(2) Å, c = 8.682(1) Å, β = 123.86(1)°); and (IV) rhombohedral Sr2Al3-wAu6+w (w ≈ 0.18(1); R3c, Z = 6, a = 8.448(1) Å, c = 21.735(4) Å). These remarkable compounds were obtained by fusion of the pure elements and were characterized by X-ray diffraction and electronic structure calculations. Phase I shows a narrow phase width and adopts the Ba3Ag14.6Al6.4-type structure; phase IV is isostructural with Ba2Au6Zn3, whereas phases II and III represent new structure types. This novel series can be formulated as Srx[M3]1-xAu2, in which [M3] (= [Al3] or [Al2Au]) triangles replace some Sr atoms in the hexagonal prismatic-like cavities of the Au network. The [M3] triangles are either isolated or interconnected into zigzag chains or nets. According to tight-binding electronic structure calculations, the greatest overlap populations belong to the Al-Au bonds, whereas Au-Au interactions have a substantial nonbonding region surrounding the calculated Fermi levels. QTAIM analysis of the electron density reveals charge transfer from Sr to the Al-Au framework in all four systems. A study of chemical bonding by means of the electron-localizability indicator indicates two- and three-center interactions within the anionic Al-Au framework.

Glass formability and the Al–Au system

The aluminum–gold system exhibits various features that suggest high glass formability, such as a deep eutectic, formation of icosahedral clusters in the intermetallic compound near the eutectic minimum and a strongly negative heat of mixing. However, it is very difficult to form a glass with this system. Various issues related to glass formability are discussed using the Al–Au system as a negative test-case. In particular, the atomic level pressure was calculated from first principles for the first time for Al2Au, AlAu2 and AlAu4 intermetallic compounds. The atomic level pressure is very high in these compounds, suggesting frustrated electronic states which destabilize both crystalline and glassy phases.

Al-Au-Cu (Aluminum-Gold-Copper)

The Al-Au system [2005Oka] depicts the following intermediate phases: AuAl2 (C1, CaF2-type cubic), AuAl (AuAl-type monclinic), Au2Al (a, b, and c modifications with MoSi2-type or related structures), Au8Al3 (rhombohedral, space group R 3c), Au4Al (cubic, space group P213), and b (80-81.2 at.% Au; bcc). The Al-Cu phase diagram [2004Ria] depicts a number of intermediate phases: CuAl2 (h, C16-type tetragonal), CuAl(HT) (g1, orthorhombic), CuAl(LT) (g2, monoclinic), Cu5Al4(HT) (f1, orthorhombic, space group Fmm2), Cu5Al4 (LT) (f2, orthorhombic, space group Imm2), e1(HT) (bcc), e2(LT) (B82, Ni2In-type hexagonal), Cu3Al2 (d, rhombohedral), Cu9Al4(HT) (c0, D82, Cu5Zn8-type cubic), Cu9Al4(LT) (c1, D83-type cubic), and Cu3Al (b, bcc). In the above, HT= high-temperature and LT= low-temperature. Au and Cu form a continuous face-centered cubic (fcc) solid solution at high temperatures. At lower temperatures, at least three ordered structures Au3Cu (L12, AuCu3-type cubic), AuCu-I (L10, AuCu-type tetragonal) and AuCu3-I (L12-type cubic) are known, with formation temperatures of 240, 385 and 390 C respectively [Massalski2].

Microstructure and mechanical properties of sputtered intermetallic Al–Au coatings

Intermetallic Al-based coatings such as Al–Ni, Al–Cr, and Al–Pt are widely used as bond coats and/or as oxidation protection of turbine blades. Investigations during earlier work indicated promising thermal stability and oxidation resistance of intermetallic Al2Au-based coatings. Hence, here we study in detail the microstructure and mechanical properties of unbalanced magnetron sputtered Al–Au films with Al∕Au at. % ratios of 1.85, 2.00, and 4.32. The film with an Al∕Au ratio of 2.00 has a single-phase Al2Au dense columnar structure in the as-deposited state and a hardness of ∼8GPa. With increasing Al and Au contents the morphology changes into a more equiaxed structure with additional Al, AlAu, and Au phases, respectively. During growth, excess Al of the film with an Al∕Au ratio of 4.32 forms lamellar nanosegregations (∼3nm thick) in the Al2Au domains. The simultaneous nucleation and growth of Al2Au, AlAu, and Au phases in the coatings with an Al∕Au ratio of 1.85, with their different growth mechanisms, leads to the formation of pores having an average size of ∼150nm. These Al- and Au-rich films, compared to Al2Au, have reduced hardnesses of ∼4 and ∼2GPa, respectively, due to the additional soft metallic phases and pores present. Our results show that within the Al–Au system the single-phase intermetallic Al2Au films exhibit the best structural and mechanical properties for protecting oxidation sensitive materials.

Thermodynamic assessment of the Al–Au system

The Al–Au binary system has been thermodynamically assessed by means of the computer program Thermo-Calc. The Redlich–Kister polynomial was used to describe the solution phases, liquid (L) and fcc. The sublattice-compound energy model was employed to describe the compounds with homogeneity ranges, (Al2Au), ϕ(AlAu2) (ϕ = α, β and γ) and β(AlAu4). The compounds, AlAu, Al3Au8 and AlAu4, were treated as stoichiometric phases. The parameters of the Gibbs energy expressions were optimized according to all the available experimental information of both the equilibrium data and the thermodynamic results. A set of self-consistent thermodynamic parameters of the Al–Au system has been obtained. The calculations agree well with the respective experimental data.

Sputter-deposited Al–Au coatings

Transition metal nitride-based, wear-resistant hard coatings on cutting tools and other substrates often lack self-lubricating properties at elevated temperatures and distinct colorations allowing product differentiation. In the present work, the possibility of achieving these objectives using coatings based on the purple-red Al 2Au phase within the Al–Au system was investigated. Coatings within this system have been deposited onto cemented carbide and Inconel substrates using unbalanced magnetron sputtering. The coatings were characterized with respect to their topography and morphology, chemical and phase composition, hardness, optical, oxidation and tribological properties. Al 2Au-containing coatings have been deposited with dense, fine-grained structures yielding a hardness of 4 GPa and pink coloration. Vacuum annealing at temperatures of 500 °C results in a chance to a pronounced purple coloration. The coatings are stable up to about 850 °C, where the onset of oxidation occurs. Low friction coefficients when testing against alumina counterparts were achieved in the temperature range between 500 and 700 °C. The concept of applying Al 2Au-containing coatings as a colored self-lubricating layer on top of a hard coated cemented carbide tool warrants further investigations.

Sputtered Coatings Based on the Al2Au Phase

ABSTRACTTransition metal nitride-based, wear-resistant hard coatings on cutting tools and other substrates often lack distinct colorations allowing product differentiation and self-lubricating properties. In the present work, the possibility of achieving these objectives for sputtered coatings based on the purple-red Al2Au phase within the Al-Au system was investigated. Coatings were characterized with respect to morphology, chemical and phase composition, hardness, optical, oxidation and tribological properties. Al2Au-containing coatings were deposited with dense, fine-grained structures yielding a hardness of 4 GPa and pink coloration. The coatings were stable up to about 850°C, where the onset of oxidation occurs. Low friction coefficients against alumina were achieved between 500 and 700°C. The concept of applying Al2Au-containing coatings as a colored self-lubricating layer on top of a hard coated cemented carbide tool warrants further investigations.

Lattice site occupation of compound forming elements implanted into aluminium

The lattice site occupation of Ag, Au and Pd implanted into Al at various temperatures has been studied using ion channeling spectroscopy. The system Al Ag, Al Au and Al Pd were chosen in the order of increasing values for the negative heat of solution, ΔH sol, and with small values for the size mismatch energy, ΔH size. High substitutional fractions were observed after implantation of Ag at 77 K (| s = 0.93), 293 K (| s = 0.88) and after implantation of Au (| s = 0.93) and Pd (| s = 0.87) at 5 K. Implantation of Au and Pd at 77 K as well as annealing from 5 K to 77 K revealed reduced | s values due to impurity-self-interstitial atom interactions. A further reduction of | s occurred during annealing from 77 K to 293 K in the Al Au system due to interactions of the impurities with mobile vacancies. It is concluded that supersaturated alloys can be formed by ion implantation at 5 K for systems with very large negative ΔH sol-values and small values for ΔH size.

Surface plasma oscillations on disordered binary alloy surfaces

The bulk plasma frequencies of two disordered binary alloy systems, Al-Ag and Al-Au, are studied using their surface-plasmon properties. It is found that the Al-Ag alloy system agrees well with the free electron gas model but the Al-Au system does not.