Severe plastic deformation Compositionally complex alloys 3 dimensional atom probe tomography Microstructure a b s t r a c t An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to 520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of 630 HV at about 450°C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450°C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites.

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.

No abstract

The research and development of c-TiAl based alloys for aero-engine and automotive components have been the target of several R & D projects since more than 20 years. [1][2][3] Titanium aluminides are considered for future advanced aero-engines due to their potential of significant component weight savings. Although, remarkable progress has been made, today, titanium aluminides have not been applied for aeroengine parts. Both fundamental materials research and design as well as production technologies have achieved an advanced state of maturity. But overall, the limited tensile ductility, poor crack propagation resistance and detrimental effects of defects, damage and long term cycling loads as well as exposure to hot oxidizing atmospheres on the fatigue life are the mayor concerns in the area of aero-engine components reliability and lifetime issues. There are further needs of understanding the source and effect of the different relevant damages and defects on the life-prediction for a particular titanium aluminide alloy and aero engine component. The attempts of scaling up the production of ingot materials, castings and forgings, have not yet met the required targets of reproducibility and affordability. Large-scale production of titanium aluminides ingots and parts requires further alloy and process development to become a reliable technology. Current titanium and nickel alloys exhibit balanced properties and achieve all requirements of the current design practices.Intermetallic c-TiAl based alloys are certainly among the most promising candidates to fulfill the required thermal and mechanical specifications. Especially, TiAl alloys with high Nb-contents showing a baseline composition of Ti-(42-45)Al-(5-10)Nb-(0-0.5)B (all compositions are stated in at%), termed TNB alloys, have attracted much attention because of their high creep strength, good ductility at room temperature, good fatigue properties, and excellent oxidation resistance. [1][2][3][4][5][6][7] Nb reduces the stacking fault energy in c-TiAl, retards diffusion processes and modifies the structure of the oxidation layer. [4,6,8] Cast alloys based on Ti-(42-45)Al, which solidify via the body-centered cubic b-phase, exhibit an isotropic, equiaxed and texture-free microstructure with modest micro-segregation, whereas peritectic alloys (solidification via the hexagonal a-phase) show anisotropic microstructures as well as significant texture and segregation. [9] Alloy design concepts for c-TiAl based alloys showing refined cast microstructures were recently reported by Imayev et al. [10] An alloy design strategy to improve the hot-workability of TiAl alloys is to exploit a combination of thermo-mechanical processing and additional alloying elements to induce the disordered b-phase at elevated temperatures as ductile phase. [11][12][13][14][15][16][17] The disordered b-phase with bcc lattice provides a sufficient number of independent slip systems. Thus, it may improve the deformability at elevated temperature, where, for example, processes such as rollin...

No abstract

Intermetallic γ‐TiAl‐based alloys represent a new class of light‐weight structural materials for use at high temperatures. Because of their unique properties these alloys are considered for applications in aerospace and automotive industries. During the last decade both, alloy development and materials processing progressed significantly. New materials and powder metallurgy (PM) have formed a symbiosis since many decades. In fact, PM is an ancient technology which has been used for the processing for almost every metal or ceramic material. Therefore, it is hardly surprising that PM plays an important role in research and development of γ‐TiAl‐based alloys.

REVIEWSDevelopment and processing of high-temperature materials is the key to technological advancements in engineering areas where materials have to meet extreme requirements. Examples for such areas are the aerospace and spacecraft industry or the automotive industry. New structural materials have to be ªstronger, stiffer, hotter, and lighterº to withstand the extremely demanding conditions in the next generation of aircraft engines, space vehicles, and automotive engines. Intermetallic c-TiAl-based alloys show a great potential to fulfill these demands.

The present paper summarizes our progress in establishing a novel production technology for -TiAl components to be used in advanced aircraft engines. In the beginning the main emphasis is put on the design of a -TiAl based alloy which exhibits excellent hot-workability. Then, the development of a “near conventional” hot-die forging route for this type of intermetallic material is described. Finally, the effect of two-step heat-treatments on the microstructure and the mechanical properties is discussed. Because of the small “deformation window” hot-working of -TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favoured. In order to increase the deformation window a novel Nb and Mo containing -TiAl based alloy (TNMTM alloy) was developed, which solidifies via the β-phase and exhibits an adjustable β/B2-phase volume fraction. Due to high volume fractions of -phase at elevated temperatures the alloy can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. Examples for the fabrication of -TiAl components employing a near conventional forging route are given. With subsequent heat-treatments balanced mechanical properties can be achieved. The results of tensile and creep tests conducted on forged and subsequently heat-treated TNMTM material are presented.