Two-phase TiAl Research Articles

Atomic-scale wear behavior of two-phase TiAl alloys by vibrational horizontal friction

To investigate the influence of vibrational horizontal friction on the microscopic distortion of γ/α2 biphasic TiAl alloys, a molecular dynamics approach was used for simulation by analyzing the mechanical properties, atomic displacements, shear strains, and defect distributions of the substrate in vibrational horizontal friction and conventional friction. The friction force and coefficient of friction in vibrational horizontal friction were found to be less than conventional friction. Vibrational horizontal friction has a shallower depth of abrasion marks and a larger wear area compared to conventional friction. At the same time, this γ/α2 phase boundary changes the direction of motion of the atom so that they move in the same orientation as parallel to the interfacial boundary. Vibrational horizontal friction has a larger strain concentration area than conventional friction. The existent γ/α2 interface impedes the strain transmission so that some of the stresses are concentrated near the interface, and the γ-phase accumulates a large proportion of dislocations, which leads to work-hardening of the material. The slip of the horizontal stacking faults in the vibrational horizontal friction releases internal stresses. The formation of structures such as stacking fault tetrahedrons has a significant strengthening effect on the material.

Characterization of primary, secondary and tertiary {20[formula omitted]1} successive internal twins in the D019 ordered hexagonal α2-Ti3Al phase

Due to the lack of independent slip systems and the difficulty in mechanical twinning, D019 ordered α2-Ti3Al phase is known to be of substantial mechanical anisotropy, which partly accounts for the low ductility of two-phase (α2+γ) TiAl alloys. However, recent studies have shown that in Al-rich α2 phase compression deformation twins can be activated, which could be beneficial for the ductility of the alloys. In the present study, for the first time, we have revealed a novel internal twinning mechanism of α2 phase in a TiAl alloy deformed at high temperature. Three twin generations of a common type {202¯1}<1¯014> were revealed: Secondary twins formed within the primary twin, and tertiary twins formed in the secondary twin. It is important to note that this internal twin structure can only be observed in the <21¯1¯6> direction but not the commonly used <112¯0> direction. The shear plane P of all twin variants is of type {12¯10}. Due to the variation of the twinning elements between the twin variants, the twinning shear is blunted. The interfaces between the twin variants are characterized by high-resolution electron microscopy. Special attention was paid to the structure of the coherent and incoherent twin boundaries. The effect of chemical ordering is discussed. Our results also shed light on the complex twinning mechanisms in hexagonal structures.

Temperature-dependent deformation processes in two-phase TiAl + Ti3Al nano-polycrystalline alloys

Although deformation processes in single-phase nano-polycrystalline alloys at different temperatures are well described, the deformation mechanism in two-phase nano-polycrystalline alloys at different temperatures is still unclear. Here, the deformation behaviour of the two-phase TiAl+Ti3Al nano-polycrystalline alloys is investigated at different temperatures by molecular dynamic simulation. For a comprehensive understanding of the processes, mechanical properties of the single-phase TiAl and Ti3Al nano-polycrystalline alloys are explored as well. The results of numerical simulations indicate that temperature has a strong influence on the deformation mechanism of the alloys. When temperature is below 800 K, the critical grain size (~8.3 nm) becomes a dominant factor controlling the deformation process in the single-phase TiAl. In the two-phase TiAl+Ti3Al nano-polycrystalline alloys, the motion of dislocations in the TiAl phase with the grain size smaller than the critical size dominates the deformation process. On the other hand, no obvious phenomena linked with the critical size occur in the Ti3Al phase. Once the strain exceeds 18.0%, the dislocation emission is observed in the Ti3Al grains. At high temperatures (≥800 K), in the two-phase nano-polycrystalline alloy, the deformation mechanism changes from the plastic deformation to the boundary slip and recrystallization.

Influence of lattice misfit on the deformation behaviour of [formula omitted]/[formula omitted] lamellae in TiAl alloys

Interfaces play a significant role in the deformation behaviour of lamellar two-phase TiAl alloys and contribute to their increased strength. We study the deformation behaviour of α2/γ bilayers with either coherent or semicoherent interfaces, using atomistic simulations. We identify the nucleation sites for dislocations and decouple the effects of the microstructural parameters volume fraction and layer thickness on the yield stress and strain. Uniaxial tensile tests are carried out on bi-layer specimens with α2 and γ phases along directions parallel and perpendicular to the interface. Coherent α2∕γ bi-layers show residual stresses due to lattice mismatch which are linearly related to the volume fractions of the phases. These residual stresses, superimposed with tensile stresses during loading, lead to early yielding of the γ phase. In contrast, a semi-coherent interface leads to negligible residual stresses, but contains misfit dislocations which create localized stresses within the γ layer and thus contributes to dislocation nucleation. We show that along loading directions parallel to the interface, the layer thickness does not affect the deformation behaviour, irrespective of the type of interface, instead volume fraction is the governing parameter. When loading perpendicular to the interface, the absolute layer thickness does not affect the deformation behaviour of a bi-layer with a coherent interface, but determines the yield stress and strain in case of a semi coherent interface.

Tensile behavior of γ/α2 interface system in lamellar TiAl alloy via molecular dynamics

The deformability and strength of lamellar two-phase (γ and α2) TiAl intermetallic based alloy strongly depends on the lamellar interfaces in such microstructure. Molecular dynamics simulations are conducted to investigate the mechanical response and the underlying deformation mechanism of two-phase TiAl intermetallic with γ/α2 interface. Uniaxial tensile loading is applied along three different directions under “free” and “constrained” boundary conditions respectively. The evolution of dislocations and cracks has been investigated in atomic scale. The results show that the stress-strain relation exhibits strong anisotropy. Meanwhile, the dominant failure mode is plastic deformation under free boundary condition while fracture under constrained boundary condition. Moreover, the phase interface can act as dislocation source or crack source under different boundary conditions.

Interface properties in lamellar TiAl microstructures from density functional theory

The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112¯] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well.

Nanostructured Dual Phase Ti-Al through Consolidation of Particles by Severe Plastic Deformation

A two-phase Ti-Al material was fabricated by severe plastic deformation. Particles of finely mixed elemental Ti and Al were mechanically milled and then consolidated by equal channel angular pressing. The bulk material has a unique interpenetrating structure of Ti and Al phases with multiple scales from micro to nano. Compared to its coarse structured counterpart, the multiscale structured material exhibited a significant increase in strength without compromising plasticity.

Properties and electronic structures of titanium aluminides–alumina composites from in-situ SHS process

Titanium aluminides–alumina composite was synthesized by in-situ self-propagating high-temperature synthesis (SHS) method, followed by hot-pressing process. To understand the fundamental differences between the composite and A1 2O 3 ceramic, a comparative study was carried out using first-principles plane-wave pseudopotential method based on density functional theory (DFT). XRD analysis of final products confirmed the formation of TiAl, A1 2O 3 and a small amount of Ti 3Al phases in the composites and the reaction mechanisms of the process were proposed. SEM observation revealed that a two-phase (γ + α 2) TiAl–Ti 3Al lamellar structure was formed, and the composites exhibited a homogeneous microstructure. Moreover, density of states (DOS), band structure, charge density difference and Mulliken population analysis showed that metallic, covalent and ionic bonding were produced at the interfaces of the composite. O–Al interface bonds showed more covalent character with respect to pure Al 2O 3. Therefore, interface combination of the composite was improved, making the composite tougher (a toughness as high as 7.9 MPa m 1/2) than monophase Al 2O 3 ceramic.

Frictional shear induced modification of the mechanical properties of TiAl(Nb) intermetallics near the surface

Mechanical properties of the two-phase TiAl(Nb) alloys have been studied after being subjected to mechanical as well as electrolytic polishing. Indentation tests at different maximum loads using different instrumentation allow probing the overall mechanical behavior of the alloys. Nanoindentation experiments have been carried out to study the local hardening of each phase during mechanical polishing. Since the deformation of γ-TiAl is facilitated by twin formation, the mechanical polishing associated with considerable frictional forces causes a significant work hardening of the γ-TiAl phase resulting in an enhancement of the overall surface hardness of the alloy. A similar hardening effect has been observed during a scratch test. The influence of local hardening owing to different crystal structures on the overall mechanical behavior in the mechanically polished alloys has also been investigated.

Slip Transfer Across Hetero-Interfaces in Two-Phase Titanium Aluminum Intermetallics

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned (PST) crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity in both the γ and the α2 phases. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study details of the hard mode of the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl.

Microstructural Characterization of Lamellar Features in TiAl by FIB Imaging

A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl-based two-phase alloys using the focused ion beam (FIB) technique. Two γ-titanium aluminide alloys with a fine-grained duplex and a nearly lamellar microstructure are examined. The special FIB-based preparation procedure results in high contrast ion beam-induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two-phase structures, e.g. lamellar colonies in γ-TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB-derived data, we compare them to results obtained with another method to determine phase fractions, electron back-scatter diffraction (EBSD). This direct comparison shows that the FIB-based technique generally provides slightly higher α2-fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB-based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two-phase TiAl alloys and other such materials with fine, lamellar microstructures.

TEM of C-component Dislocations Associated with Pyramidal Slip Activity in Hexagonal α2-Ti3Al

AbstractWhile only a minor phase constituent, the deformation behavior of the hexagonal α2-Ti3Al phase, significantly affects the mechanical properties of two-phase TiAl based alloys. We have used conventional and high-resolution transmission electron microscopy to investigate the fine structure of pyramidal plane glide dislocations, with Burgers vectors of the type b=&lt;2c+a&gt;, in α2-Ti3Al after room temperature compression of binary polysynthetically twinned TiAl normal to the lamellar interfaces to nominal plastic strains of 1%-7%. We report atomic resolution observations of non-co-planar dislocation core configurations for &lt;2c+a&gt;-dislocations and show that translamellar deformation twins active in the majority γ-TiAl phase play an important role in facilitating pyramidal plane slip in α2-Ti3Al in the lamellar two-phase alloys.

Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.

Simulation of quasi-brittle fracture of lamellar γTiAl using the cohesive model and a stochastic approach

Quasi-brittle fracture of fully lamellar two-phase (α2+γ)TiAl is simulated and analysed using the cohesive model. Applications of the model for predicting crack initiation, propagation and unstable fracture are discussed and validated by fracture tests. The experiments have been conducted at room temperature, where specimens exhibit small force drops (pop-in) during stable crack propagation and all end by unstable failure. The global behaviour of the specimens is simulated by adjusting the cohesive parameters and the shape of the cohesive law using stochastic considerations.A systematic analysis is carried out and various aspects of modelling and simulation are discussed including the contribution of the specific lamellar microstructure. To some extent, the cohesive model can explain and describe the physical phenomena of the two-phase material.

Isothermal oxidation behavior of two-phase TiAl–Mn–Mo–C–Y alloys fabricated by different processes

The isothermal oxidation behavior of two-phase Ti–46.6Al–1.4Mn–2Mo–0.3C–0.3Y based alloys in synthetic air at 800 and 900 °C was investigated. The main emphasis was focused on the effect of microstructures obtained by different processes of extrusion, melting and hot rolling, i.e. fully lamellar, nearly lamellar and duplex. On the exposure for 350 h at 800 °C, the growth rate and the spallation behavior of oxide scales of the alloys depended strongly on the fabrication process. The extruded alloy with a fine fully lamellar microstructure showed an excellent oxidation resistance due to the lowest mass gain and strong adhesion of the scale to the substrate, and Y-addition was effective in improving oxidation resistance because of its oxygen-scavenging effect in EPM processing. However, the overall oxidation process was dominantly controlled by the beneficial effect of the Mo-addition in the melted alloy due to the doping effect and formation of Mo-rich Al-depletion layer close to the substrate. The rolled alloy with the duplex microstructure greatly accelerated the oxidation rate because of a high volume fraction of α 2-Ti 3Al. At 900 °C, the melted alloy showed an excellent oxidation resistance during 350 h exposure. The oxidation rate and the spallation resistance of oxide scales did not depend strongly on the microstructure for the extruded or the rolled alloy. The poor spallation resistance of the oxide scales in the extruded alloy was probably related to thicker Mn-containing scales. Neither Mo-rich layer nor Y-rich zone in the oxide scales in the rolled alloy was observed at two exposure temperatures as a result of fast oxidation rate of the scale.

Slip Transfer across Hetero-Interfaces in two-phase Titanium Aluminum Intermetallics

AbstractThe role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.

Slip Transfer across Hetero-Interfaces in two-phase Titanium Aluminum Intermetallics

AbstractThe role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.

Development and elemental powder metallurgy of a Y-containing two-phase TiAl alloy

The Y modification of a two-phase (γ+α2) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method. The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge harmful oxygen. Deformation was found to be mainly provided by 1/2<110] ordinary dislocations and a much lesser amount of <011] superdislocations as compared to what has been reported in other (γ+α2) TiAl alloys. The oxidation resistance of the experimental alloy was evaluated by air-exposure tests at 800 °C, from which the oxidation kinetics and the morphological and phase characteristics of the oxide scales were analyzed. With a Y addition, the constituents of the oxide scale changed from those of the Y-free alloy. In the case of the Y-free alloy, the oxide scale which formed upon extended air exposure (350 hours) at 800 °C consisted of a mixture of TiO2 and α-Al2O3. In the case of the alloy modified with Y (0.6 at. pct), however, the oxide scale formed in an identical environment was considerably different: it consisted of a complex mixture of TiO2, α-Al2O3, Y2O3, and Al5Y3O12. The formation of the multiphase (Y,Al)O-rich oxide scale impedes the oxygen transport and the thermal-expansion stress in the Al2O3 layer. It is also suggested that a Y addition reduces the oxygen solubility and concentration of oxygen vacancices in the TiO2 layer.

Determination of γ/γ interface types in a γ-TiAl alloy using convergent beam electron diffraction

The domain orientations and lamellar interfaces play a critical role in the mechanical properties of TiAl based alloys. An umambiguous experimental determination of all six domain orientation in lamellar structure and the interfaces between neighboring γ laths in a (α 2+γ) two-phase TiAl alloy containing Mn and Nb were analyzed by means of convergent-beam electron diffraction. The frequency of different interface types are reported.

Effects of refractory metals on microstructure and mechanical properties of directionally-solidified TiAl alloys

By using an appropriately oriented seed from the TiAl–Si system, the TiAl/Ti 3Al lamellar structure was aligned parallel to the growth direction for ingots having composition of Ti–46Al–0.5Si–0.5X (X=Re, W and Mo) and Ti–47.5Al–0.5Re (at.%). The seeded and directionally solidified quaternary alloys containing either Re, W or Mo exhibit a nice balance of low- and high-temperature mechanical properties. Tensile elongation more than 20% is noted for the W- and Mo-containing alloys. The creep resistance for the three quaternary alloys is more than an order of magnitude better than other relevant TiAl alloys produced by conventional ingot metallurgy methods, with the best creep properties being obtained for the Re-containing alloy. The method to predict alloy compositions appropriate for aligning the lamellar structure of two-phase TiAl alloys of multi-component by directional solidification is proposed based on the assignment of ‘Al-equivalent’ for each of the alloying elements.