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 Pb−S (Lead-Sulfur) system

Thermodynamics of the substantially ordered Cu3Au-type alloys

The diffusion path and reaction kinetics of Pt/GaAs diffusion couples were investigated in the temperature range 400 to 1000° C using optical metallography, electron probe microanalysis (EPMA), and scanning electron microscopy (SEM). In all cases the formation of two phases, Pt3Ga and PtAs2, was observed. A layered structure Pt/Pt3Ga/ PtAs2/GaAs was observed in the temperature range 400 to 450° C and again between 700 and 800° C. However, in the intermediate temperature range from 450 to 650° C a two-phase mixture was observed with the sequence Pt/Pt3Ga/(Pt3Ga, PtAs2)/GaAs. This two-phase microstructure resulted in faster reaction rates than would have been predicted from the layered kinetics. The reversion from the agglomerate two-phase to a layered microstructure between 650 and 675° C may be explained by a rapid rate of grain coarsening in the two-phase microstructure at high temperature. This return to a planar morphology resulted in a substantial decrease in reaction rate with increasing temperature.

Morphological development during platinum/gallium arsenide interfacial reactions

The Cd-Se (cadmium-selenium) system

The solid and liquid phase equilibrium of Al-rich Al–Cu and Al–Cu–Mg alloys was investigated in order to examine the effect of small uncertainties in the solidus on model-calculated microsegregation during solidification. The phase boundaries were determined using EPMA on quenched two-phase alloys. The liquidus was obtained using a raster beam scan technique. The phase boundaries obtained for Al–Cu are in accord with experimental data available in the literature. The measured (Al) solidus in terms of Cu is found to be ∼0.5 at% higher than model-calculated values from a thermodynamic description reported in the literature. On the other hand, the measured (Al) liquidus in terms of Cu are in good agreement with model-calculations. Moreover, the difference between the measured and calculated solidus in ternary Al–Cu–Mg was found to be the same as that in binary Al–Cu. This difference is believed to be responsible for the deviations in the calculated microsegregation from experimental data for Al-rich Al–Cu–Mg alloys. The (Al ) phase in both the Al–Cu and Al–Cu–Mg systems was thus remodeled so that the calculated solidus using the new model description is in better agreement with experimental data. The microsegregation model-calculations show that even small differences in the phase boundaries of 0.5 at.% or less have a pronounced effect on the calculated concentration profiles of Cu in the dendrites. We thus conclude that in order to test the validity of a microsegregation model, accurate phase boundary data are needed!

Y. A. Chang

Papers

The Pb−S (Lead-Sulfur) system

Thermodynamics of the substantially ordered Cu3Au-type alloys

Morphological development during platinum/gallium arsenide interfacial reactions

The Cd-Se (cadmium-selenium) system

Importance of reliable phase equilibria in studying microsegregation in alloys: Al–Cu–Mg