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 Se−Sn (Selenium-Tin) system

The S−Sn (Sulfur-Tin) system

Although not as soundly based, the cluster-site approximation (CSA) has the considerable advantage of computational simplicity over the cluster variation method (CVM) for the evaluation of mixing entropies in multicomponent solid solutions. The CSA is thus particularly suitable for the calculation of technologically important phase diagrams of multicomponent alloys, a task which necessarily depends on the optimization of the models' energy parameters with the available experimental data for the constituent binary and ternary systems. Comparison of results obtained by Monte Carlo (MC) simulations for the same energy parameters with those obtained by the CVM and CSA for the tetrahedron approximation for fcc alloy shows them to be of comparable accuracy---the CVM finds the solution to be more random whereas the original CSA (due to Yang and Li) finds the solution to be less random when compared with the MC results. It is shown how the CSA can be improved by the introduction of a single correction parameter. Some CSA-calculated prototype binary phase diagrams which agree very well with those obtained from MC calculations are presented together with two phase diagrams for the Cu-Au system which can be compared with similar diagrams previously calculated using the CVM. It is not suggested that the modified CSA should replace the use of large clusters in the CVM for ``first-principles'' phase diagram calculations for binary systems.

Improved cluster-site approximation for the entropy of mixing in multicomponent solid solutions

Portions of the Cu-In phase diagram were redetermined experimentally, specifically in the compositional region from 32.0 to 100 at % In. The experimental techniques used were differential scanning calorimetry (DSC), differential thermal analysis (DTA), powder X-ray diffraction (XRD), and electron probe microanalysis (EPMA).

Y. A. Chang

Papers

The Se−Sn (Selenium-Tin) system

The S−Sn (Sulfur-Tin) system

Improved cluster-site approximation for the entropy of mixing in multicomponent solid solutions

Phase equilibria of the cu-in system I: Experimental investigation

Computational and experimental investigation of microsegregation in an Al-rich Al–Cu–Mg–Si quaternary alloy