Microstructures and properties of high-entropy alloys

The chemical, physical, and mechanical characteristics of nickel-based superalloys are reviewed with emphasis on the use of this class of materials within turbine engines. The role of major and minor alloying additions in multicomponent commercial cast and wrought superalloys is discussed. Microstructural stability and phases observed during processing and in subsequent elevated-temperature service are summarized. Processing paths and recent advances in processing are addressed. Mechanical properties and deformation mechanisms are reviewed, including tensile properties, creep, fatigue, and cyclic crack growth. I. Introduction N ICKEL-BASED superalloys are an unusual class of metallic materials with an exceptional combination of hightemperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. These materials are widely used in aircraft and power-generation turbines, rocket engines, and other challenging environments, including nuclear power and chemical processing plants. Intensive alloy and process development activities during the past few decades have resulted in alloys that can tolerate average temperatures of 1050 ◦ C with occasional excursions (or local hot spots near airfoil tips) to temperatures as high as 1200 ◦ C, 1 which is approximately 90% of the melting point of the material. The underlying aspects of microstructure and composition that result in these exceptional properties are briefly reviewed here. Major classes of superalloys that are utilized in gas-turbine engines and the corresponding processes for their production are outlined along with characteristic mechanical and physical properties.

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Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties

Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

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Precipitation and Hardening in Magnesium Alloys

The objective of this review is to study interfacial reactions between pure Sn or Sn-rich solders, and common base metals used in Pb-free electronics production. In particular, the reasons leading to the observed interfacial reactions products and their metallurgical evolution have been analyzed. Results presented in the literature have been critically evaluated with the help of combined thermodynamic–kinetic approach based on the concept of local equilibrium and microstructural knowledge. The following conclusions have been reached: Firstly, the formations of intermetallic compounds in solid/liquid reaction couples are primarily controlled by the dissolution processes of base metals. Other factors that need be considered are the thermodynamic driving force for the formation of intermetallic compounds, their structures and concentration profiles in liquid. Secondly, annealing of solder interconnections in solid state can drastically change the microstructures formed in the solid/liquid reactions, especially if only one of the components in the solder takes part in the interfacial reactions. Thirdly, additional elements can have three major effects on the binary reactions between a base metal and Sn: (i) they can increase or decrease the reaction/growth rates, (ii) the additives can change the physical properties of the phases formed, and (iii) they can form additional reaction products or displace the binary equilibrium phases by forming new reaction products. Finally, if the local stable or metastable equilibrium is established at the interface, stability information together with kinetic considerations can provide a feasible approach to analyze interfacial reactions, which can have significant impact on the reliability of soldered electronics assemblies.

Interfacial reactions between lead-free solders and common base materials

The phase-field method is reviewed against its historical and theoretical background. Starting from Van der Waals considerations on the structure of interfaces in materials the concept of the phase-field method is developed along historical lines. Basic relations are summarized in a comprehensive way. Special emphasis is given to the multi-phase-field method with extension to elastic interactions and fluid flow which allows one to treat multi-grain multi-phase structures in multicomponent materials. Examples are collected demonstrating the applicability of the different variants of the phase-field method in different fields of materials science.

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Phase-field models in materials science

The Rh- Ga- As solid- state equilibrium phase diagram was determined at 800 °C with the use of powder X- ray diffraction (XRD), electron probe microanalysis (EPMA), and scanning electron microscopy (SEM). No ternary Rh- Ga- As phases were found, and very limited solid solubility was measured in GaAs and the constituent binary Rh- Ga and Rh- As compounds. GaAs, RhGa, and RhAs2 form a three- phase region that dominates the GaAs side of the phase diagram. The existence of this three- phase region confirms previous observations that RhGa and RhAs2 are the stable phases when Rh thin films are completely reacted on GaAs.

A Phase Investigation of the Rh-Ga-As System

An isothermal section of the InPPd phase diagram was determined at 160δC. Four ternary phases were identified. The phase Pd 5 In 2 P 2 identified in this study was compared with the phase Pd 2 InP reported in the literature to form during the reaction of palladium films with InP. Four binary phases were confirmed to be in equilibrium with InP: PdP 2 , Pd 3 In 7 , Pd 2 In 3 , and PdIn at the stoichiometric composition or when the phase is indium-rich.

Solid phase equilibria in the InPPd system

Phases of the nominal composition Ni{sub 3}MSb (where M'' represents aluminum, gallium, or indium) were found to occur in their respective ternary systems. They were analyzed by x-ray diffraction and were found to exhibit the B8{sub 1.5'} crystal structure, similar to that of Ni{sub 3}GaAs. These phases were shown by differential thermal analysis (DTA) to be stable between room temperature and their melting points, which were determined to be {gt}1150 {degree}C, 1066 {degree}C, and 1091 {degree}C for Ni{sub 3}AlSb, Ni{sub 3}GaSb, and Ni{sub 3}InSb, respectively. An analysis of their respective constituent binary systems suggested that these phases were not true ternary compounds, but rather represented specific compositions of extensive solid solutions of constituent binary phases. Speculations were made as to the solution behavior of these phases. The Ni{sub 3}MSb phases were predicted to form initially in nickel/MSb diffusion couples, in an analogy to what is observed in nickel/GaAs diffusion couples.

The existence of Ni3MSb phases in ternary nickel-M-antimony systems (where “M” represents aluminum, gallium, or indium)

Thermally stable, low-resistance ohmic contacts to GaAs (Si-doped, n=1.6–1.8×1018 cm−3) were formed using PdIn metallization sputter-deposited from an alloy target. Average specific contact resistances (ρc) in the 10−6 Ω cm2 range were reached upon annealing at 600 °C or higher. Contacts annealed under the optimum condition of 850 °C for 15 s exhibited an average ρc of 2.5×10−6 Ω cm2. The 100 h of thermal aging at 400 or 500 °C increased their average ρc to 3.0×10−6 and 1.0×10−5 Ω cm2, respectively. The ohmic behavior of the annealed contacts was ascribed to the exchange of In and Ga atoms between the metallization and the semiconductor and the concomitant formation of InxGa1−xAs, whose presence at the contact interface was confirmed using cross-sectional transmission electron microscopy.

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Thermally stable pdin ohmic contacts to n-gaas via exchange mechanism

Interfacial reactions between Co and GaAs have been studied using bulk diffusion couples of Co (∼0.5 mm thick)/GaAs and thin-film Co(∼-40 nm) on (001)GaAs as well as the epitaxial formation of CoAs and CoGa on (001)GaAs. In addition, phase diagram of Co-Ga-As at 600°C was determined. The thin-film results are rationalized in terms of the phase diagram and bulk diffusion data.

Y. A. Chang

Papers

A Phase Investigation of the Rh-Ga-As System

Solid phase equilibria in the InPPd system

The existence of Ni3MSb phases in ternary nickel-M-antimony systems (where “M” represents aluminum, gallium, or indium)

Thermally stable pdin ohmic contacts to n-gaas via exchange mechanism

Interfacial Reactions Between Co and GaAs