D. Hu

Effect of boron addition on tensile ductility in lamellar TiAl alloys

Abstract Various cast or wrought fully lamellar TiAl-based alloys with and without boron addition have been assessed. It has been found that titanium boride precipitates are the predominant factor influencing the room temperature tensile ductility. Large sized titanium boride precipitates often observed in high-alloyed TiAl alloys (such as Ti–44Al–8Nb–1B) cause premature failure in as-cast samples through promoting crack propagation via debonding between boride-matrix interfaces or cracking through boride precipitates themselves, giving rise to a typical tensile ductility of 0.3%. Refinement in titanium boride precipitates, via hot working or fast cooling during casting, will significantly improve the tensile ductility. In low-alloyed alloys (such as Ti–48Al–2Cr–2Nb–1B) the effect of boride precipitates is not as significant as it is in the high-alloyed alloys mainly because of their small sizes.

Effect of composition on grain refinement in TiAl-based alloys

Abstract Grain refinement through boron addition has been investigated in a range of cast TiAl-based alloys. The alloys have 44–50 at.% Al, up to 8 at.% alloying elements, and 1 at.% boron. The observed grain size ranges from 40 to 300 μm. The grain size in as-cast TiAl-based alloys is strongly composition dependent with aluminium having the strongest effect. Decreasing Al concentration from 50 to 44% the grain size can be reduced to 50 from 300 μm with similar casting conditions. Alloying element species and concentration also have strong effect on grain size. The strong boride formers like W, Ta and Nb increase the grain size and change the prevalent titanium boride form from TiB 2 to TiB. It is elucidated that the compositional effect on the grain size could be through affecting the local boron concentration in the solidification front during casting.

Thermal stability of a fully lamellar Ti-48Al-2Cr-2Nb-1B alloy

Abstract Ti-48Al-2Cr-2Nb-1B with a fully lamellar microstructure has been exposed at 700 °C for 3000 h in air and the effects of this exposure on microstructures and tensile properties examined. It has been found that during exposure the α 2 lamellae decomposed to γ phase by a mechanism that involves the formation of thin γ lamellae within the original α 2 lamellae. There is very little α 2 phase left after exposure. A hexagonal phase with a composition of (Ti:Al:Cr = 68:23:9) was also found to be a product decomposition of α 2 . The yield strength and the ultimate tensile strength decreased by about 20% during the high temperature exposure.

The role of the α2 phase in the transmission of slip in lamellar TiAl-based alloys

Abstract Samples of a range of polycrystalline alloys based on TiAl have been examined using optical microscopy, analytical scanning and transmission electron microscopy in order to understand the role of the α2 phase in controlling plastic deformation in alloys with a lamellar structure. It has been found that for grains in the hard orientation, that is when the stress axis is either parallel or perpendicular to [0001]α2, fracture occurs preferentially along α2—γ interfaces. Transmission electron microscopy has shown that twinning in the γ phase can be transmitted into the α2 phase by the generation of dislocations with Burgers vector b along 〈1126〉 on {2021} planes. Slip along 〈1120〉 directions has also been observed in α2 in grains where the stress axis is perpendicular to [0001]α2. When crystals are stressed parallel with [0001]α2, the only dislocations observed in α2 have b along 〈1126〉. These observations are discussed in terms of the deformation of polycrystalline samples and of polysynthetically t...

Slip transfer between lamellae in fully lamellar TiAl alloys

Abstract TEM observations have shown that not only does slip transfer take place from one gamma lamella to adjacent gamma lamellae, but slip transfer also takes place at gamma/alpha-2 interfaces during deformation of lamellar TiAl alloys. The various possible slip transfer mechanisms between gamma/alpha-2 lamellae and gamma/gamma lamellae have been analysed. The results show that unit dislocations and twinning in the gamma lamellae can be transferred into the alpha-2 lamellae by actuating 1/6 〈11 2 6〉 or 1/6 〈11 2 0〉 dislocations. It is also shown that the gamma/alpha-2 interface is less transparent than the gamma/gamma because the CRSS of the slip systems in the alpha-2 phase is high and the geometry makes the reactions concerning twinning dislocations in the gamma phase more difficult. The significance of these mechanisms is briefly discussed in terms of the limited ductility of lamellar TiAl alloys.

The significance of acoustic emission during stressing of TiAl-based alloys. Part I: Detection of cracking during loading up in tension

Abstract Samples of fully lamellar Ti44Al8Nb1B and Ti44Al4Nb4Hf0.2Si have been tested in tension at room temperature while they were interfaced to an acoustic emission system. The generation of acoustic signals, at stresses well below the 0.2% yield stress and at higher stresses, has been shown to be due to the formation of cracks within individual lamellar colonies. Data obtained in the micro-strain region in loading–unloading measurements confirmed that there is significant plastic flow at stresses 200 MPa below the 0.2% proof stress of 625 MPa, and it is inferred that this flow gives rise to the cracking. The mechanism whereby such cracking occurs and the possible influence of these cracks on subsequent fatigue behaviour are discussed.

Microstructure and tensile properties of investment cast Ti-46Al-8Nb-1B alloy

Abstract The size of Nb-enriched TiB precipitates in the investment cast Ti–46Al–8Nb–1B was strongly affected by solidification conditions. The existence of long TiB ribbons was the cause of premature failure of this alloy at room temperature during tensile test. Refining TiB precipitates by fast cooling may improve the room temperature tensile ductility markedly.

The influence of thermal processing route on the microstructure of some TiAl-based alloys

Abstract The influence of a range of different continuous cooling rates from the alpha phase field has been investigated in large-grain-sized (∼1500 μm) Ti48Al2Cr2Nb and in grain-refined Ti48Al2Cr2Nb1B alloys. In addition the microstructure of direct laser-fabricated samples of Ti48Al2Nb2Mn has been examined in order to assess the influence of the complex heating cycle and high cooling rate on this alloy. It has been found that the critical cooling rates to obtain a particular structure are higher in grain-refined Ti48Al2Cr2Nb1B than in the alloy without boron (B). The increase in the amount of the lamellar structure during cooling takes place in different manners in the two alloys. The formation of the lamellar structure in laser-treated samples has been found to be very different with the formation of Al-rich gamma regions separating regions with a lamellar structure. The possible factors affecting phase transformations are discussed in terms of the role of grain boundaries and B in solution in the solid state transformations and, in the case of the laser-treated samples, of melting and remelting.

The influence of composition and processing on the structure and properties of TiAl-based alloys

Abstract Alloys based on TiAl, with Al contents between 44 and 48 at %, have been further alloyed and processed to produce a range of microstructures and the influence of these different compositions and structures on the mechanical properties have been assessed. In addition, the microstructural stabilities of some of these alloys, against exposure at likely operating temperatures, have been determined and any changes in microstructure correlated with changes in properties. The observations are discussed in terms of the factors which influence the strength of these alloys and in terms of the balance between obtaining alloys and structures which have the desired combination of properties, but also have acceptable stability at temperatures at which they are likely to be used. Finally, in the light of the current knowledge, some questions are addressed concerning the development of commercially successful TiAl-based alloys.

Deformation Behavior of α2-Lamellae in Fully Lamellar Ti-48Al-2Mn-2Nb at Room Temperature

The alloy Ti-48at.% Al-2at.% Mn-2at.% Nb (Ti-48Al-2Mn-2Nb) is a member of a new class of generic two-phase TiAl based alloys which will be referred to as {gamma}-alloys in the following. In general, polycrystalline {gamma}-alloys will always contain a number of lamellar grains which exhibit hard orientation behavior during loading. As a result, it may be predicted that the plastic properties of the minority {alpha}{sub 2}-Ti{sub 3}Al phase critically affect the deformation and fracture behavior of {gamma}-alloys containing lamellar colonies. Hence, in order to understand the plasticity and strength of {gamma}-alloys it is important to elucidate the deformation behavior of the {alpha}{sub 2}-Ti{sub 3}Al lamellae during loading in the hard orientations. In the present work the deformation modes in hard oriented grains of room temperature compressed fully lamellar Ti-48Al-2Mn-2Nb have been studied by transmission electron microscopy (TEM). Particular attention has been paid to the deformation characteristics exhibited by the {alpha}{sub 2}-lamellae and to deformation transfer processes between the two constituent phases. In this paper the results obtained for lamellar grains loaded approximately parallel to the interface normal, [0001]{alpha}{sub 2}//[111]{gamma}, are presented and discussed with respect to mechanical properties of {gamma}-alloys.