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†

Improvement of corrosion resistance and biocompatibility of rare-earth WE43 magnesium alloy by neodymium self-ion implantation

Microstructure evolution and mechanical properties of a novel beta γ-TiAl alloy

New high-performance engine concepts for aerospace and automotive application enforce the development of lightweight intermetallic γ-TiAl based alloys with increased high-temperature capability above 750 °C. Besides an increased creep resistance, the alloy system must exhibit sufficient hot-workability. However, the majority of current high-creep resistant γ-TiAl based alloys suffer from poor workability, whereby grain refinement and microstructure control during hot-working are key factors to ensure a final microstructure with sufficient ductility and tolerance against brittle failure below the brittle-to-ductile transition temperature. Therefore, a new and advanced β-solidifying γ-TiAl based alloy, a so-called TNM alloy with a composition of Ti–43Al–4Nb–1Mo–0.1B (at%) and minor additions of C and Si, is investigated by means of uniaxial compressive hot-deformation tests performed with a Gleeble 3500 simulator within a temperature range of 1150–1300 °C and a strain rate regime of 0.005–0.5 s −1 up to a true deformation of 0.9. The occurring mechanisms during hot-working were decoded by ensuing constitutive modeling of the flow curves by a novel phase field region-specific surface fitting approach via a hyperbolic-sine law as well as by evaluation through processing maps combined with microstructural post-analysis to determine a safe hot-working window of the refined TNM alloy. Complementary, in situ high energy X-ray diffraction experiments in combination with an adapted quenching and deformation dilatometer were conducted for a deeper insight about the deformation behavior of the alloy, i.e. phase fractions and texture evolution as well as temperature uncertainties arising during isothermal and non-isothermal compression. It was found that the presence of β-phase and the contribution of particle stimulated nucleation of ζ-Ti 5 Si 3 silicides and h-type carbides Ti 2 AlC enhance the dynamic recrystallization behavior during deformation within the (α+β) phase field region, leading to refined and nearly texture-free α/α 2 -grains. In conclusion, robust deformation parameters for the refinement of critical microstructural defects could be defined for the investigated multi-phase γ-TiAl based alloy.

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Hot-working behavior of an advanced intermetallic multi-phase γ-TiAl based alloy

Effect of addition of rare earth elements, such as, Nd, Sm and Tb, was investigated on various properties of Ni–Mn–Ga ferromagnetic shape memory alloys (SMAs). The solubility of the rare earth in the L2 1 phase was found to be very low, most likely less than 0.1 mol%. Insolvable rare earth segregated into subgrain boundaries or grain boundaries to form precipitates. Phase transformation behavior and the structure of martensite phase exhibit a similar dependence on valence electron concentration to those in ternary Ni–Mn–Ga alloys. It was revealed that the addition of rare earth significantly improves the compressive ductility of the alloy.

Modification of Ni–Mn–Ga ferromagnetic shape memory alloy by addition of rare earth elements

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti52Al48-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti52Al48-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti52Al48-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti52Al48-1V, Ti52Al48-Cr, and Ti52Al48-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti52Al48-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.

Alloying mechanism of beta stabilizers in a TiAl alloy

Effect of Nd on the microstructure and properties of a TiAl alloy

A study of Ti5Si3/γ interface in TiAl alloys

The microstructures and the tensile properties and creep resistance of Ti–47.5Al–0.25Nd and Ti–46.5Al–2Cr–2Nb–0.25Nd alloys were studied. With the addition of Nd, neodymium oxides formed, and a uniform and homogeneous distribution of the neodymium oxides was observed in Ti–46.5Al–2Cr–2Nb–0.25Nd alloy. The cast microstructure of the Ti–46.5Al–2Cr–2Nb–0.25Nd alloy was remarkably refined after conventional heat treatment. The tensile strengths and ductility of the Ti–46.5Al–2Cr–2Nb–0.25Nd alloy at room temperature were increased. The colony refinement of the Ti–46.5Al–2Cr–2Nb–0.25Nd alloy by Nd was attributed to the accelerated recrystallization due to the decrease of the activation energy, and the impediment of the neodymium oxide particle to the grain growth. The strengthening of the Nd-bearing TiAl alloys was the superposition of the second phase strengthening by big neodymium oxide particles, the dispersion strengthening by fine precipitated neodymium oxides and fine colony grain size strengthening.

Microstructural refinement and strengthening of Nd-bearing TiAl alloys

The Al3Zr precipitation behaviours of as-quenched and heat-treated samples of a melt-spun Al-0.2Cu-l.2Mg-0.5Zr alloy have been studied by TEM. The results show that metastable f.c.c. Al3Zr can be formed during solidification, and that initial Al3Zr precipitation occurred in the subgrain boundaries. Fine spherical LI 2 Al3Zr particles precipitated within the grains after heat treatment at 450°C for 16 hours, and they were stable in this form even after holding at 450°C for 100 hours. The stable tetragonal DO23 Al3Zr phase was found after heat treatment at 500°C for 100 hours. 

Ming-Gao Yan

Papers

Alloying mechanism of beta stabilizers in a TiAl alloy

Effect of Nd on the microstructure and properties of a TiAl alloy

A study of Ti5Si3/γ interface in TiAl alloys

Microstructural refinement and strengthening of Nd-bearing TiAl alloys

Al3Zr Precipitation Behaviour in a Melt-Spun Al-0.2Cu-1.2Mg-0.5Zr Alloy