Self-propagating exothermic reactions: the synthesis of high-temperature materials by combustion

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†

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

The deformation behaviour of intermetallic superlattice compounds

The present article will describe aspects of the science and technology of titanium aluminide (TiAl) alloy system and summarise the low and high temperature mechanical and environmental properties exhibited by different alloy generations. In terms of processing developments, conventional gravity casting and near net shape casting would be discussed in detail. Also newer and non-conventional forging and additive manufacturing routes would be briefly highlighted. Extensive investigations of TiAl alloys have enabled their commercial implementation in aerospace and automotive industries. The GEnx™ engine is the first commercial aircraft engine that used TiAl (alloy 48–2–2) for their low pressure turbine blades. Among non GE engines, recently, new β-stabilised TiAl alloy (TNM) is being used to manufacture LPT blades for PW1100G™ engines. TiAl materials and design processes can reduce engine weight and improve engine performance.

TiAl alloys in commercial aircraft engines

The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α
 2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.

Plastic deformation and fracture of binary TiAl-base alloys

The effects of Cr additions to y-base alloys have been investigated, using bulk materials consolidated from rapid solidification-processed ribbons. The composition ranges studied were 0 to 4 at. pet Cr and 44 to 54 at. pet Al. It was found that Cr additions do not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean duplex alloys which contain grains of single-phase γ and grains of lamellar γ/α2. Other Cr effects on microstructure, phase stability, site occupancy, and deformation sub-structures were characterized and correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr in duplex alloys is partially due to the tendency of Cr to occupy Al lattice sites. Ductilization is also partially due to the ability of Cr to modify the Al partitioning and, therefore, the thermal stability of transformed α2 laths.

The effects of Cr additions to binary TiAl-base alloys

IMPROVED STRENGTH AND DUCTILITY OF Ni3Al BY BORON MODIFICATION AND RAPID SOLIDIFICATION.

The room temperature deformation behavior and microstructure of Ti 48 Al 52 and Ti 52 Al 48 alloys are compared. The material was fabricated by rapid solidification melt spinning, and examined in both as-cast and consolidated forms. The Ti 52 Al 48 alloy exhibited enhanced strength and ductility in both forms in bend tests compared with the Ti 48 Al 52 alloy. The microstructure of the Ti 52 Al 48 alloy was two-phase γ–TiAl and α 2 –Ti 3 Al. The Ti 48 Al 52 alloy was single-phase γ–TiAl and had a larger grain size than the previous alloy. The microstructure of the Ti 52 Al 48 alloy after room temperature deformation consisted primarily of {111} mechanical twins and a /2〈110〉 perfect dislocations. The comparable Ti 48 Al 52 alloy microstructure contained fewer twins, and many more a 〈101〉 and a /2〈112〉 superdislocations were present in addition to a /2〈110〉 dislocations. The superdislocations had dissociated and formed sessile faulted dipoles. The possible reasons for the differences in microstructure and mechanical behavior between these two alloys are discussed.

Shyh-Chin Huang

Papers

Plastic deformation and fracture of binary TiAl-base alloys

The effects of Cr additions to binary TiAl-base alloys

IMPROVED STRENGTH AND DUCTILITY OF Ni3Al BY BORON MODIFICATION AND RAPID SOLIDIFICATION.

Stoichiometry effects on the deformation of binary TiAl alloys

Ductility in boron-doped, nickel-base L12 alloys processed by rapid solidification