Two-phase Titanium Aluminide Alloys Research Articles

Analysis of the plastic anisotropy and pre-yielding of (γ/α 2)-phase titanium aluminide microstructures by crystal plasticity simulation

The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α 2-Ti 3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively.

Effects of Hf on the Microstructure and Mechanical Properties of TiAl Base Alloys

The effect of Hf additions to single- and two-phase titanium aluminide alloys was investigated by X-ray diffraction, differential scanning calorimetry, metallographic characterization by scanning electron microscopy, and mechanical testing. Hf was chosen as an alloying element because it exhibits a significant solubility in γ(TiAl) and α2 (Ti 3 Al) and substitutes for Ti. There is a relatively large size misfit between Hf and Ti atoms, which provides a good potential for solid solution hardening. Hf additions in the amount of 2 and 5 at.% significantly strengthen α 2 +γ alloys, whereas the effect in single-phase y alloys is almost negligible. This observation, together with structural characterization, gives rise to the assumption that Hf additions do not lead to significant solid solution hardening. Thus, the strengthening of (α 2 +γ) alloys by Hf additions was mainly attributed to structural changes, which are probable associated with a shift of the phase boundaries in the Ti-Al system. This interpretation is essentially consistent with thermodynamic glide parameters determined for the binary and ternary alloy system.

SANS investigation of precipitation hardening of two-phase ?-TiAl alloys

Two-phase titanium aluminide alloys are being considered as lightweight materials to replace nickel-based superalloys in some high-temperature aero-engine applications. However, at an intended service temperature of 700 °C the strength and creep resistance of the titanium aluminides are inferior to those shown by superalloys. Therefore, precipitation-hardening treatments using carbon additions were tested for an improvement of the strength. Ageing of a Ti-48.5 at. % Al-0.4 at. % C alloy for 24 h at 750 °C after a solution treatment for 4 h at 1250 °C, followed by quenching, leads to the formation of perovskite-type precipitates, significantly improving the high-temperature strength. The size distribution and volume fraction of the perovskite precipitates were measured using small-angle neutron scattering (SANS). The results after extended ageing periods show that although the precipitate size distribution changes significantly, the effect on the yield strength is relatively small.

Creep deformation in two-phase titanium aluminide alloys

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.

Precipitation Phenomena and Strain Hardening of Intermetallic Titanium Aluminides

ABSTRACTIn two-phase titanium aluminide alloys, the implementation of precipitation reactions is a widely utilized concept to control the microstructure and strengthen the material. A study has been made on the influence of carbide and boride precipitates on dislocation mobility and strengthening at 300 K. Compression tests were carried out for characterizing the mechanisms determining flow stress and dislocation glide resistance. The interaction mechanisms between the precipitates and dislocations were assessed by thermodynamic glide parameters and transmission electron microscopy. It has been shown that small titanium boride precipitates and carbide precipitates of perovskite type act as long-range dislocation glide obstacles. The interaction between the dislocations and the borides and carbides mainly leads to an athermal stress contribution. However, the dislocation-particle interactions are quite different. Small groups of borides are encircled by dislocations. This gives rise to the formation of loop structures the density of which increases with strain. On the contrary, the carbide precipitates are shearable and can be overcome without Orowan looping. This different behaviour is also reflected in the work hardening characteristics. Whereas the work hardening coefficient of the boron doped material increases with increasing B-concentration, it is independent of concentration in the case of the carbon-doped material.

Point Defect Mechanisms in Deformation and Phase Transformation of Titanium Aluminide Alloys

ABSTRACTPoint defects in intermetallic compounds are subjected to significant constraints due to the directional bonding, long-range order and off-stoichiometric deviation of the materials. This leads to a variety of defects with distinct differences in morphology, concentration and mobility. In the present study the implications of these defect characteristics on the mechanical properties of two-phase titanium aluminide alloys will be investigated. The major areas of the study are: (i) deformation induced point defects that contribute to work hardening; (ii) dislocation locking due to the formation of defect atmospheres; (iii) transport processes involved in phase transformation and recrystallization occurring during long-term creep. The applied methods include mechanical testing, static strain aging and structural characterisation by high resolution electron microscopy.

Mechanistic understanding of creep in gamma-base titanium aluminide alloys

The paper describes an experimental study of creep processes in two-phase titanium aluminide alloys. The investigations involve long-term creep test at relatively low stresses and temperatures. These test conditions lead to very low strain rates, which are characteristic of the intended service conditions. The creep tests were coupled with detailed electron microscope observations involving high-resolution imaging techniques and in situ heating studies. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. Climb velocities were analyzed in terms of the critical vacancy supersaturation necessary for the operation of diffusion assisted dislocation climb sources. The mechanisms are closely related to the atomic structure of the interfaces and are probably driven by a non-equilibrium phase composition.

Novel design concepts for gamma-base titanium aluminide alloys

Two-phase titanium aluminide alloys are being considered as light-weight materials to replace nickel-base superalloys for some high temperature applications in energy conversion systems. Thus, their mechanical properties have to be assessed against the high standard set by the superalloys currently in use. In this respect most titanium aluminides are particularly inferior in high temperature strength and creep resistance even if these properties are related to density. In an attempt to overcome these problems several studies have been performed on titanium aluminides which have been subjected to solid solution and precipitation hardening. The intention of the present study is to examine more closely these strengthening processes in order to assess their potential for extending the service range of the titanium aluminides towards higher temperatures. There is growing evidence that two-phase titanium aluminides, microalloyed with carbon or niobium, can provide the necessary performance. Particular emphasis will be placed on processing routes acceptable for these materials.

Stress Driven Phase Transformations and Recrystallization Processes in Two-Phase Titanium Aluminide Alloys

ABSTRACTThe paper presents an electron microscope study of phase transformation and recrystallization in an intermetallic α2(Ti3Al) + γ(TiAl) titanium aluminide alloy, after long-term creep. The mechanisms are closely related to the atomic structure of the α2/γ phase boundaries and are probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α2 phase. The α2 /γ transformation is accompanied by the formation of precipitates, because the γ(TiAl)phase has a significantly lower solubility for interstitial impurities than the α2(Ti3Al) phase.

Static and Dynamic Strain Ageing in Two-Phase Gamma Titanium Aluminides

ABSTRACTThe deformation behaviour of two-phase titanium aluminides was investigated in the intermediate temperature interval 450–750 K where the Portevin-LeChatelier effect occurs. The effect has been studied by static strain ageing experiments. A wide range of alloy compositions was investigated to identify the relevant defect species. Accordingly, dislocation pinning occurs with fast kinetics and is characterized by a relatively small activation energy of 0.7 eV, which is not consistent with a conventional diffusion process. Furthermore, the strain ageing phenomena are most pronounced in Ti-rich alloys. This gives rise to the speculation that antisite defects are involved in the pinning process. The implications of the ageing processes on the deformation behaviour of two-phase titanium aluminide alloys will be discussed.

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.

Dislocation dynamics and fracture processes in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminides was characterized by fracture toughness tests performed on chevron-notched three point bending bars. The alloys investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The reliability of the materials was assessed by ranking toughness data in Weibull plots and considering the anisotropy of the crack resistance.

Solution and Precipitation Hardening in Carbon-Doped Two-Phase γ-Titanium Aluminides

ABSTRACTA two-phase titanium aluminide alloy was systematically doped with carbon to improve its high temperature strength. Solid solutions and precipitates of carbon were formed by different thermal treatments. A fine dispersion of perovskite precipitates was found to be very effective for improving the high temperature strength and creep resistance of the material. The strengthening mechanisms were characterized by flow stresses and activation parameters. The investigations were accompanied by electron microscope observation of the defect structure which was generated during deformation. Special attention was paid on the interaction mechanisms of perfect and twinning dislocations with the carbide precipitates.

Interactions of Dislocations and Deformation Twins with Interfacial Boundaries in Titanium Aluminides

AbstractThe deformation behaviour of two-phase titanium aluminide alloys with a lamellar microstructure of the intermetallic phases α2(Ti3Al) and y(TiAl) was studied. The interaction processes of dislocations and deformation twins, respectively, with the lamellar interfaces are investigated by conventional and high-resolution electron microscopy. The mechanisms of translation of shear deformation across, the lamellar boundaries depend on their structure. Semicoherent interfaces are very effective barriers limiting the propagation of shear deformation. The misfit dislocations present at these interfaces support, on the other hand, the generation of dislocations and deformation twins. The observed processes are discussed regarding plastic deformation and crack propagation in the material.

Twinning in TiAl

The results of this investigation are in agreement with the findings of Richards and Cahn which show that the formation of twin-related TiAl regions, TiAl T, and the Ti 3Al phase, rather than TiAl deformation twins, will be favored energetically in a two-phase titanium aluminide alloy. Due to the formation of Ti 3Al at the TiAl T interface, all possible {111}<112> TiAl T, as opposed to only {111}[112] deformation twins, can be formed within TiAl lamellae without the destruction of the symmetry of L1 0 structure.