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

High-temperature structural intermetallics

Increasing the temperature of jet engines requires materials that are stable against degradation. Towards this goal, growth of TiAl alloys with high strength and ductility, as well as superior creep resistance, is reported at high temperatures.

Polysynthetic twinned TiAl single crystals for high-temperature applications

Advances in gamma titanium aluminides and their manufacturing techniques

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.

Recent Progress in the Development of Gamma Titanium Aluminide Alloys

The compression behaviour of niobium alloyed γ-titanium alumindies

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

Phase decomposition and ordering reactions in β/B2-phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of laminate microstructure. The characteristic constituent of this microstructure are laths with a nanometer-scale substructure that are comprised of several stable and metastable phases. Microstructural control can be achieved by conventional thermomechanical processing and leads to a structurally and chemically very homogeneous material with excellent mechanical properties. The physical metallurgy of this novel type of alloy has been assessed by transmission electron microscope investigations and mechanical testing.

Jonathan Paul

Papers

Gamma titanium aluminide alloys : science and technology

Recent Progress in the Development of Gamma Titanium Aluminide Alloys

The compression behaviour of niobium alloyed γ-titanium alumindies

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

Nano‐Scale Design of TiAl Alloys Based on β‐Phase Decomposition