Fritz Appel

Recent Progress in the Development of Gamma Titanium Aluminide Alloys

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.

The compression behaviour of niobium alloyed γ-titanium alumindies

Abstract The underlying mechanisms behind the reported high strength of titanium aluminide alloys containing a large addition of niobium has been investigated by determining the flow stresses and activation parameters of plastic deformation. It has been found that alloys such as Tiue5f845Alue5f810Nb (at.%) and Tiue5f845Alue5f85Nb have 1.25% flow stress values in compression of > 800 MPa at room temperature and > 500 MPa at 1173 K. When compared with values from a more conventional alloy, Tiue5f847Alue5f82Crue5f80.2Si, they represent a considerable increase in strength. However, the activation volumes after 1.25% deformation are very similar to those of conventional alloys, particularly up to 973 K. This suggests that athermal dislocation mechanisms are responsible for the increased flow stress of the niobium containing alloys. By comparing the properties of the niobium containing alloys with different binary alloys it has been shown that the high strength is solely a result of the reduced aluminum content and that niobium plays no role in strengthening or work hardening.

Dislocation dynamics in carbon-doped titanium aluminide alloys

Abstract Different thermal treatments were conducted on carbon containing two-phase titanium aluminide alloys to obtain solid solution and precipitation hardening effects. The strengthening mechanisms were characterized by activation parameters of the glide processes and electron microscope observations. Carbon in solid solution was found to be less efficient than carbide precipitates for hardening the material. Fine dispersions of Ti3AlC perovskite precipitates form arrays of strong glide obstacles so that perfect and twinning partial dislocations were effectively pinned. This mechanism results in a high athermal contribution to the flow stress, which significantly improves the high temperature strength of the material.

Thermally activated deformation mechanisms in micro-alloyed two-phase titanium amminide alloys

The processes controlling the dislocation mobility in micro-alloyed two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining activation parameters of plastic deformation and TEM observations. The deformation behavior of the materials is characterized by a relatively high athermal stress component due to dislocation interactions with grain boundaries and lamellar interfaces. The glide resistance of the dislocations is controlled by several processes. At room temperature, the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from nonconservative processes at jogs in screw dislocations and leads to a thermal contribution to work hardening. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behavior.

Creep deformation in two-phase titanium aluminide alloys

Abstract The paper presents an electron microscope study of diffusion assisted creep processes in a two-phase (α 2 +γ) titanium aluminide alloy, which had been subjected to long-term creep. The results imply that the high primary creep rate of lamellar TiAl alloys is associated with the relaxation of mismatch structures and coherency stresses present at the interfaces. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. The mechanisms of these morphological changes are closely related to the atomic structure of the α 2 /γ phase boundaries and probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α 2 phase. After long-term creep the formation of precipitates was observed, which has been attributed to the α 2 →γ transformation, because α 2 has a significantly higher solubility for interstitial impurities than the γ phase.

Mechanical twins, their development and growth

Mechanical or deformation twinning is first explained as a shearing mechanism and compared to crystallographic slip. Then twinning in several metals is discussed. A twin can be represented as a thin layer, bound by an upper and lower plane. The average width of a disk-shaped twin is either given by the average diameter of a microregion (grain) or is calculated in the case of twin nuclei. An energy balance is outlined in detail for the untwinned and twinned status of a representative volume element. The elastic strain energy due to the twinning shear eigenstrain is studied in detail both analytically and numerically. An energy criterion for the stability of equilibrium allows to formulate a twinning condition which yields a minimum thickness in the case of a deformation twin. In the case of a twin nucleus Onsagers principle of maximum dissipation rate is engaged as a further criterion to find the dimensions of the twin nucleus. Comparisons with experimentally observed twins in TiAl intermetallics are reported finally. Further consequences of the study are listed, such as the estimation of the critical resolved shear stress for twinning.

Mechanical properties of submicron‐grained TiAl alloys prepared by mechanical alloying

Ti‐48 at.u2009% Al powders of the metastable hexagonal‐closed‐packed solid solution with a grain size of 15 nm were prepared by mechanical alloying. The powders were consolidated to a density of greater than 99.5% by hot isostatic pressing (HIP) at 800u2009°C. After HIP the material exhibits a globular microstructure of the equilibrium phases α2 and γ with a mean grain size of 150 nm. Microhardness measurements show a Hall–Petch type [E. O. Hall, Proc. Phys. Soc. B 64, 747 (1951); N. J. Petch, J. Iron Steel Inst. 174, 25 (1953)] dependence on grain size. Room temperature compression tests reveal low ductility, but high fracture strengths ≥1800 MPa. On increasing the test temperature the yield strength drops sharply in the temperature range 600–800u2009°C to very low values. The results indicate that submicron‐grained TiAl alloys can be deformed at much lower temperatures than coarse‐grained material, making them suitable as precursors for net shaping, in particular if high deformation ratios are required.

A thermodynamical model for the nucleation of mechanical twins in TiAl

Mechanical twinning has long been recognized as an important deformation mechanism in many intermetallics including γ(TiAl) based alloys. The generation of a twin can be triggered by dislocations at grain boundaries. The reduction of the stored strain energy in relation to a configuration without any twins can be shown by applying an established transformation criterion, considering twinning as a transformation shearing process. In addition, the application of a thermodynamical extremal principle allows one to predict a distinct twin nucleus in good agreement with experimental observations. The concept is rather general and can be adapted to any initial eigenstress field produced by dislocations present at misfitting interfaces.

Physical metallurgy of high Nb-containing TiAl alloys

Abstract Intermetallic titanium aluminides exhibit attractive thermo-physical properties, which give them the potential for extensive use as lightweight structural components. Novel design concepts are based on alloys with the general composition (in at.%) Ti-45Al-(5–10) Nb, which were subjected to precipitation hardening. Optimized compositions have been identified that are capable of carrying stresses in excess of 700 MPa at service temperatures of 700°C and have superior creep properties. The alloys exhibit at room temperature yield stresses in excess of 1GPa combined with plastic tensile elongations of about 2%. Wrought alloys of this type can be an attractive alternative to the nickel-base superalloys in certain ranges of stress and temperature. The future and promise of these new TiAl alloys lies in innovative processing methods designed to achieve better performance.

New approaches to designing alloys based on γ-TiAl + α2-Ti3Al phases

Based on the investigations of the microstructure of ingots depending on the content of aluminum and alloying additives and cooling rate, a new concept of alloying of γ-TiAl + α2-Ti3Al alloys has been developed, which is directed on the production of a chemically uniform cast material with a fine-grained structure. The results obtained open new opportunities in the designing of γ+α2 alloys with an improved processing plasticity.