Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

High entropy alloys: A focused review of mechanical properties and deformation mechanisms

Processing of advanced materials using high-energy mechanical milling

During the past decade considerable research efforts have been directed towards achieving balanced engineering properties of two-phase γ-titanium aluminide alloys for future applications as structural materials. For optimization of mechanical properties such as yield and creep strengths, tensile ductility and fracture resistance, a basic understanding of the temperature dependent micromechanisms of plasticity and fracture, and their interplay with various microstructural constituents is required. In this review article, the current knowledge on dislocation types and slip systems, the development of deformation substructures, factors controlling the mobility and multiplication of dislocations, interface related plasticity, solid solution and precipitate strengthening mechanisms as well as microscopic aspects of creep and fracture will be addressed. These topics will be related to specific microstructures and associated engineering properties.

Microstructure and deformation of two-phase γ-titanium aluminides

Preface INTRODUCTION CONSTITUTION The Binary Ti-Al Phase Diagram Ternary and Multicomponent Alloy Systems THERMOPHYSICAL CONSTANTS Elastic and Thermal Properties Point Defects Diffusion PHASE TRANSFORMATIONS AND MICROSTRUCTURES Microstructure Formation on Solidification Solid State Transformations DEFORMATION BEHAVIOR OF SINGLE-PHASE ALLOYS Single-Phase Gamma(TiAl) Alloys Deformation Behavior of Single-Phase Alpha2(Ti3Al) Alloys Beta/B2 Phase Alloys DEFORMATION BEHAVIOR OF TWO-PHASE ALPHA(Ti3Al) + GAMMA(TiAl) ALLOYS Lamellar Microstructures Deformation Mechanisms, Contrasting Single-Phase and Two-Phase Alloys Generation of Dislocations and Mechanical Twins Glide Resistance and Dislocation Mobility Thermal and Athermal Stresses STRENGTHENING MECHANISMS Grain Refinement Work Hardening Solution Hardening Precipitation Hardening Optimized Nb-Bearing Alloys DEFORMATION BEHAVIOR OF ALLOYS WITH A MODULATED MICROSTRUCTURE Modulated Microstructures Misfitting Interfaces Mechanical Properties CREEP Design Margins and Failure Mechanisms General Creep Behavior The Steady-State or Minimum Creep Rate Effect of Microstructure Primary Creep Creep-Induced Degradation of Lamellar Structures Precipitation Effects Associated with the Alpha2 -> Gamma Phase Transformations Tertiary Creep Optimized Alloys, Effect of Alloy Composition and Processing Creep Properties of Alloys with a Modulated Microstructure FRACTURE BEHAVIOR Length Scales in the Fracture of TiAl Alloys Cleavage Fracture Crack-Tip Plasticity Fracture Toughness, Strength, and Ductility Fracture Behavior of Modulated Alloys Requirements for Ductility and Toughness Assessment of Property Variability FATIGUE Definitions The Stress-Life (S-N) Behavior HCF Effects of Temperature and Environment on the Cyclic Crack-Growth Resistance LCF Thermomechanical Fatigue and Creep Relaxation OXIDATION BEHAVIOR AND RELATED ISSUES Kinetics and Thermodynamics General Aspects Concerning Oxidation Summary ALLOY DESIGN Effect of Aluminum Content Important Alloying Elements - General Remarks Specific Alloy Systems Summary INGOT PRODUCTION AND COMPONENT CASTING Ingot Production Casting Summary POWDER METALLURGY Prealloyed Powder Technology Elemental-Powder Technology Mechanical Alloying WROUGHT PROCESSING Flow Behavior under Hot-Working Conditions Conversions of Microstructure Workabiliy and Primary Processing Texture Evolution Secondary Processing JOINING Diffusion Bonding Brazing and Other Joining Technologies SURFACE HARDENING Shot Peening and Roller Burnishing Residual Stresses, Microhardness, and Surface Roughness Surface Deformation Due to Shot Peening Phase Transformation, Recrystallization, and Amorphization Effect of Shot Peening on Fatigue Strength Thermal Stability of the Surface Hardening APPLICATIONS, COMPONENT ASSESSMENT, AND OUTLOOK Aerospace Automotive Outlook

Gamma titanium aluminide alloys : science and technology

Intermetallic titanium aluminides offer an attractive combination of low density and good oxidation and ignition resistance with unique mechanical properties. These involve high strength and elastic stiffness with excellent high temperature retention. Thus, they are one of the few classes of emerging materials that have the potential to be used in demanding high-temperature structural applications whenever specific strength and stiffness are of major concern. However, in order to effectively replace the heavier nickel-base superalloys currently in use, titanium aluminides must combine a wide range of mechanical property capabilities. Advanced alloy designs are tailored for strength, toughness, creep resistance, and environmental stability. These concerns are addressed in the present paper through global commentary on the physical metallurgy and associated processing technologies of γ-TiAl-base alloys. Particular emphasis is paid on recent developments of TiAl alloys with enhanced high-temperature capability.

Michael Oehring

Papers

Gamma titanium aluminide alloys : science and technology

Recent Progress in the Development of Gamma Titanium Aluminide Alloys

Novel design concepts for gamma-base titanium aluminide alloys

Alloy design concepts for refined gamma titanium aluminide based alloys

Microstructure and mechanical properties of a forged β-solidifying γ TiAl alloy in different heat treatment conditions