TiAl Crystals Research Articles

Yield Stress Anomaly for 1/2〈112] {1111} Slip in y-Titanium Aluminide

ABSTRACTTransmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.

Deformation of Bi-PST Crystals of TiAl Produced by Diffusion Bonding

ABSTRACTDiffusion bonded bi-PST crystals of three different series were prepared and they were deformed in tension at room temperature. Yield stress and elongation exhibited by bi-PST crystals consisting of component crystals with the lamellar microstructure aligned along the tensile axis do not significantly differ from those of component crystals. Plastic strain incompatibility at the interface exerts a strong influence on the deformation behavior of bi-PST crystals when the incompatibility activates additional deformation modes which are much harder than deformation modes operative in each component crystal.

Deformation of Polysynthetically Twinned TiAl Single Crystals with Near-Hard Orientations

ABSTRACTCompression tests were performed on specially-oriented PST TiAl crystals to which strain gages were attached in order to precisely measure the three axial strains in the samples during deformation. It was found that all samples deform by plane strain except when the compression axis is nearly perpendicular to the lamellar planes. This measurement technique was used to determine the orientations in which the various slip systems are operative in PST samples, and this, in turn was employed to explain the shape of the flow stress vs. orientation curve. An important component of this explanation is the fact that slip of ordinary dislocations and twinning are closely coupled in this material, implying that the twinning must occur relatively slowly-at about the same rate that ordinary dislocations move. Atomistic calculations have shown that a slow twinning process is to be expected in this material at low temperatures, perhaps even by a thermally activated process.

Crystal growth of TiAl alloys

Abstract The growth of TiAl alloys over a range of Al concentrations has been considered for ingots processed by the floating zone technique. For Al-rich alloys, single crystals of γ-TiAl cannot be grown for compositions below Ti-54 at% Al since a banded microstructure forms due to the limitations imposed by the L + α → γ peritectic reaction. However, near stoichemetric TiAl crystals can be grown by the traveling solvent method when using a TiAlCo flux, although optimum processing conditions have not yet been realized. For Ti-rich alloys, evidence of a growth morphology consisting of β-phase dendrites embedded within a continuous matrix of the peritectic α-phase is found for ingots processed up to 200 mm h−1. The final lamellar orientation of the ingot is then determined by the orientation of the peritectic α-phase and not by that of the leading β dendrites.

Numerical simulation of yield loci for PST crystals of TiAl

The yield strength of polysynthetically twinned (PST) crystals of TiAl depends on the orientation angle between the lamellar boundaries and the load axis. It is the aim of this work to model this anisotropic behaviour. A micromechanical model is employed which considers the lamellar microstructure and the deformation mechanisms such as ordinary slip and deformation twinning in γ -TiAl. Since the calculated and measured yield strength data show good agreement in the case of uniaxial loading, there is a real motivation to study numerically the behaviour of a PST crystal under biaxial loading. Yield points are calculated for various proportional stress paths defined by the ratio of the principal stresses σ 1 / σ 2 in relation to the orientations of the load axes towards the lamellae. These predicted yield loci are compared with the von Mises and the Hill yield criteria.

Effects of Al-concentration and lamellar spacing on the room-temperature strength and ductility of PST crystals of TiAl

The tensile deformation behavior of PST crystals of TiAl with several different compositions (Ti–46–49.3 at.%Al) has been investigated in tension at room temperature as a function of crystal orientation (the angle φ between the lamellar boundaries and tensile axis) and crystal growth rate. The average thickness of TiAl ( γ ) lamellae as well as the average spacing of Ti 3 Al ( α 2 ) lamellae decreases with the decrease in Al-concentration and with the increase in growth rate. The yield stress for all orientations increases with decreasing Al concentration. While the value of tensile elongation for perpendicular ( φ =90°) and intermediate ( φ =31–68°) orientations does not significantly depend on Al concentration, the value for the parallel orientation ( φ =0°) increases with decreasing Al concentration. The observed tensile behavior of these TiAl PST crystals are discussed in terms of microstructural features such as the thickness of both γ and α 2 lamellae, the spacing of α 2 lamellae, the volume fraction of the α 2 phase and γ domain size.

On the dislocation pole mechanism for twinning in TiAl crystals

A dislocation model is proposed for twin formation in the L1 structure, under a compressive applied stress along the c axis, on the basis of the original pole mechanism for twinning in the fcc structure. Dissociation of a jog on an ordinary \\[110]/2 dislocation into a Shockley and a Frank partial in TiAl is energetically favourable according to anisotropic elasticity theory. While a pileup of \\[101] superdislocations assists a Shockley loop to spiral around two sessile nodes, the Frank partial expands under mechanical and chemical stresses and mitigates the high repulsive interaction between two Shockley partials. Available data on the apparent critical resolved shear stress for twinning in Ti-56 at.% Al single crystals are explained in terms of reasonable values of a local stress concentration factor and a vacancy supersaturation at elevated temperatures.

Experimental determination of domain orientations and domain orientation relationships across lamellar interfaces in polysynthetically twinned TiAl crystals

Six domain orientations in an as-grown polysynthetically twinned (PST) TiAl crystal are unambiguously determined. Fifteen domain orientation relationships across lamellar interfaces (not including those across order-translational lamellar interfaces) are uniquely identified. The atomic arrangements of these 15 domain orientation relationships are investigated. Domains within a lamellar lath are related to each other by a 120°-rotation: either domains [ I] to [ III] or domains [ IV] to [ VI] are within a lamellar lath. Diffraction patterns across the lamellar interfaces show that a < 1 ̄ 01] zone disordered twin diffraction pattern (the twin diffraction pattern without superlattice diffraction spots) taken parallel to the lamellar interfaces cannot unambiguously determine whether the interface is a true-twin related interface or a pseudo-twin related interface. A second different zone diffraction pattern is necessary to characterize the twin-related interfaces but < 2 ̄ 11> zone diffraction patterns can not be used to determine the twin relationships since the diffraction spots from both crystals are coincident. A diffraction criterion for an unambiguous determination of domain orientations and domain orientation relationships across lamellar interfaces is proposed.

The role of slip and twinning in the deformation behaviour of polysynthetically twinned crystals of TiAl: A micromechanical model

Abstract This micromechanical study investigates the deformation behaviour of TiAl based alloys with a two-phase microstructure composed of the γ-Ti3Al phase and a small volume fraction of the α2-Ti3Al phase at room temperature. The modelling is based on the unit cell technique. A three-dimensional unit cell represents the microstructure. The main deformation mechanisms in the γ-TiAl phase, ordinary slip and true twinning, are considerd in the model. Slip is modelled by crystal plasticity. Deformation twinning is implemented by a constitutive law analogous to Schmid's law for crystallographic slip. This micromechanical model is applied to simulate uniaxial tests of so called polysynthetically twinned (PST) crystals of TiAl which contain only a single set of parallel γ-TiAl and α2-Ti3Al lamellae with a specific orientation. Stress-strain curves are calculated numerically for various lamellae orientations. The computed yield stresses are compared with experimental ones. The model is able to explain the deformation mechanisms in detail. Specifically it predicts the twin volume fractions in the γ-TiAl phase and confirms the importance of twinning as a deformation mechanism.

Temperature dependence of yield stress, deformation mode and deformation structure in single crystals of TiAl (Ti−56 at.% Al)

Abstract The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at.% Al has been studied in compression as a function of crystal orientation in the temperature range from −196 to 1100°C. The profile of yield stress-temperature curves for all orientations studied can be divided into three temperature regions; the yield stress rapidly decreases with increasing temperature at both low and high temperatures and an anomalous increase in yield stress is observed at intermediate temperatures, exhibiting a peak at 700–1000°C depending on crystal orientation. Both ordinary and [101] superlattice slip exhibit an anomalous increase in critical resolved shear stress (CRSS). However, the extent of the anomaly associated with the former slip is much smaller than that for the latter slip. The CRSS for [101] slip depends on crystal orientation. In the anomalous temperature region, the CRSS exhibits very small strain-rate sensitivity and thermal reversibility24Jun1996. Superlattice disloc...

Deformation and fracture under compressive loading in lamellar TiAl microstructures

Abstract A physically based micromechanical model is applied to study finite compressive deformation behaviour and the development of failure modes in polycrystalline fully lamellar and nearly lamellar microstructures. Orientation-dependent yielding of lamellar TiAl single crystals is specifically modelled. Finite-element computations show that deformation is inherently non-uniform in the lamellar microstructure, in accordance with results presented earlier by Kad, Dao and Asaro. Intergranular fracture initiation is found to be expected at small aggregate strains (i.e. strains less than 5%), while fracture initiated by internal buckling is found to be increasingly likely to occur at larger aggregate strains (i.e. strains larger than 5–10%). Internal buckling is found in lamellar TiAl crystals whose lamellae are initially nearly parallel to the compressive loading. A weak basal texture, normal to the compression axis, is developed at only 20% aggregate strain. Subtle variations in microstructural constitue...

Fracture behavior and toughness of PST crystals of TiAl

Fracture behavior and toughness of PST crystals of TiAl have been investigated through three-point bend tests of specimens which are notched, fatigue-precracked and side-grooved. Bend tests have been conducted in air at room temperature. Fracture surface morphology and toughness of notched PST crystals of TiAl strongly depend on their notch orientation with respect to the lamellar boundaries and crystal orientation of γ lamellae. In this study, we have tested specimens with five different notch orientations, i.e. types I-〈110〉, I-〈112〉, II-〈110〉, II-〈112〉 and III, where I, II and III are crack arrester, crack divider and crack delamination orientations, respectively, and 〈 hkl〉 indicates the direction normal to the notch plane, which is given with respect to crystal orientation of γ lamellae. Specimens of type III failed by cleavage-like mode on a macroscopic habit plane parallel to the lamellar boundaries and exhibited a very low fracture toughness value. Specimens of types I and II exhibited considerably rougher fracture surfaces and much higher fracture toughness than type III. The five notch orientations give increasing fracture toughness values in the sequence: III, II-〈112〉, I-〈112〉, II-〈110〉, I-〈110〉. The relative magnitudes of fracture toughness and microstructural characteristics of fracture surfaces for these orientations are discussed in terms of crystallography and toughening mechanisms available in PST crystals.

Deformation of Polysynthetically Twinned (PST) TiAl Crystals at High Strain Rate and High Temperature

ABSTRACTDeformation microstructures in a 45°&lt;321&gt; oriented (the lamellar interface was tilted 45° from the loading axis about the &lt;321&gt; direction in the lamellar interface) poly synthetically twinned (PST) TiAl crystal deformed in compression at 3000 s-1 and 800 °C was studied. Deformation of this PST crystal is characterized as follows: 1) Deformation of domains [III] and [IV] is dominated by 1/6 [112] parallel twinning (twinning parallel to lamellar interfaces). Ordinary dislocations observed in these domains are found to be a complementary deformation mode. 2) Deformation of domains [II], [V] and [VI] is controlled by a 1/2&lt;110] ordinary dislocation slip. Complementary deformation modes in these domains are ordinary dislocation slip, superdislocation slip and cross-twinning. 3) Domain [I] is not deformed after the specimen deforms up to ∼7% strain.

Deformation of Diffusion-Bonded Bi-Pst and Directionally Solidified Crystals of TiAl

ABSTRACTWith a data base now available on the microstructural characteristics and the deformation, fracture and macroscopic flow behavior of polysynthetically twinned (PST) crystals of γ/α2 TiAl-base alloys, an approach to achieve a good combination of strength, ductility and toughness in γ/α2 TiAl-base alloys was proposed using directional solidification (DS) techniques to produce a columnar grain material with the lamellar orientation aligned parallel to the growth direction. Such alignment of the lamellar microstructure was recently accomplished in γ/α2 TiAl-base alloys of near equiatomic compositions using an appropriately oriented seed crystal from the Ti-Al-Si system.At the same time, bi-PST crystals, each containing a planar boundary parallel to the loading axis were prepared by directional solidification and diffusion bonding of two PST crystals. Such bi-PST crystals were deformed in tension at room temperature and their deformation behavior was examined in terms of the compatibility requirements imposed at the grain boundary and the interaction of the two component PST crystals.In this paper, (i) the current status of our DS processing efforts, (ii) some results of microscopic characterization of grain boundaries in diffusion bonded bi-PST crystals and (iii) results of deformation experiments of bi-PST crystals prepared by DS processing and diffusion bonding, will be reported.

Temperature dependence of yield stress, tensile elongation and deformation structures in polysynthetically twinned crystals of Ti-Al

Abstract The plastic deformation behaviour of polysynthetically twinned (PST) Ti-Al with three different orientations has been studied in tension and compression as a function of temperature in a range from −196 to 1100°C. With increase in temperature, the yield stress decreases rather rapidly at low temperatures and then decreases gradually at intermediate temperatures for all orientations studied although the temperature dependence at low temperatures is less significant for an orientation where shear deformation occurs parallel to the lamellar boundaries. When the loading axis is perpendicular to the lamellar boundaries, the yield stress again rapidly decreases with increasing temperature at high temperatures. This is also the case for PST Ti-Al whose lamellar boundaries are parallel to the loading axis, although a small and broad anomalous yield stress peak is observed at 800°C for this orientation. When the lamellar boundaries are inclined at an intermediate angle to the loading axis, the yield stress gradually decreases with increasing temperature without showing any anomalous peak or rapid decrease at high temperatures. For the parallel and intermediate orientations, the tensile elongation is constant up to 200°C and begins to increase with increasing temperature at 400°C and continues to increase until 600°C. Above 800°C, tensile deformation for these two orientations is accompanied by the occurrence of necking at the part of specimens where recrystallization starts. For the perpendicular orientation, the tensile elongation is essentially zero at all temperatures studied and fracture occurs always parallel to the lamellar boundaries, indicating the weakness of such boundaries under tensile stress even at high temperatures.

Deformation mechanisms of near-stoichiometric single phase TiAl single crystals: A combined experimental and atomistic modeling study

The purpose of this paper is to present the results of a combined experimental and theoretical investigation of the reason why the room temperature deformation of PST (polysynthetically twinned) crystals of TiAl occurs primarily by twinning while deformation of single phase TiAl occurs by dislocation slip. It is not clear at present whether this difference in deformation behavior is due to differences in microstructure or differences in composition. Since the stress needed to produce twinning in PST crystals is substantially less than the stress required to produce dislocation slip in the single phase material, it would appear that the twinning stress is dramatically increased by increasing the aluminum content. This suggest is indeed supported by the results of atomistic studies of dislocation dissociations and core structures in the L1{sub 0} TiAl presented in this paper. However, since it is impossible to grow single crystals of single phase material having less than 54 at% Al, this finding cannot be easily checked experimentally For this purpose the authors have developed a novel sample preparation technique, Employing solid state diffusion to reduce the Al content, which allows them to produce layers of 51 at% Al single phase TiAl sandwiched between Ti{sub 3}Al andmore » TiAl with a surplus of Al. These samples were then tested in compression and it was, indeed, found that the single phase TiAl with low aluminum content deforms by twinning, in the same way as the PST crystals with a similar aluminum content.« less

Plastic deformation of V- and Zr-alloyed PST TiAl in tension and compression at room temperature

The deformation behavior of polysynthetically twinned (PST) crystals of TiAl alloyed with V and Zr has been studied at room temperature both in compression and in tension as a function of the angle ( ø) between the lamellar boundaries and loading axis. The yield stress as well as tensile elongation strongly depends on the angle ø. When compared to binary PST crystals, both V- and Zr-alloyed PST crystals, in particular, Zr-alloyed ones show high yield stress for all orientations. Environmental loss in ductility of TiAl PST crystals is reduced by the addition of V without affecting their intrinsic ductility. Deformation structures of the two ternary TiAl PST crystals have been investigated by transmission electron microscopy. In addition to deformation twins and ordinary dislocations, numerous superlattice dislocations and faulted dipoles associated with them are observed for both V- and Zr-alloyed TiAl PST crystals. Which of these deformation structure constituents prevails in each of the TiAl domains depends on the crystal orientation of the domain. Based on the results obtained, the effects of V and Zr additions on the strength and ductility of two-phase TiAl alloys are discussed.

Numerical simulation of the plastic behavior of polysynthetically twinned TiAl crystals

A micromechanical model describes the deformation behavior of TiAl intermetallics with a γ-TiAl-α 2-Ti 3Al lamellar structure. The tension-compression load orientation angle θ varies with respect to the lamellae from 0° ⩽ θ ⩽ 90°. Extended crystal plasticity (ECP) for the γ phase considering the significantly different free paths for dislocations on the various slip systems predicts both the yield stress and the hardening behavior in good agreement with experiment. Neither Mises flow plasticity nor standard crystal plasticity reproduce these results satisfactorily. Near θ = 90°, application of the ECP model for the γ phase also underestimates the yield stress, mainly owing to the representation of the α 2 phase by a purely elastic material. Some improvements are suggested by introducing an ECP description also for the α 2 phase, distinguishing between pyramidal and prism slip systems.

Cyclic deformation behaviour of Ti-Al alloys containing oriented lamellae

Abstract Cyclic deformation and fatigue behaviour of Ti-Al crystals containing unidirectionally aligned lamellae have been investigated by focusing on the effects of total strain amplitude and the angle ⊘ between the loading axis and the lamellar planes. Cyclic tension-compression loading was carried out at room temperature at total strain-controlled amplitudes of \pm0.3, \pm 0.5 and \pm 1.0%. At ⊘ = 0° a high maximum stress was obtained and the cyclic hardening became strong with increasing number of cycles and total strain amplitude. The α2-phase acts as a barrier to the motion of dislocations in the γ-phase. Specimens with ⊘ = 45°, on the other hand, deformed at low stress and the maximum stress reached saturation after showing a slight cyclic hardening at an early stage of deformation. Extrusions due to the to-and-fro motion of dislocations were observed in both specimens but were formed markedly at ⊘ = 45°.

Directional solidification of TiAl alloys by vertical float zoning

In order to understand the deformation behavior and to improve the ductility, single crystals of [gamma]-TiAl and pseudo-single crystal with oriented lamellar structure of [alpha][sub 2]/[gamma] have been used to investigate the deformation mode of the intermetallics. However, little work has been published solely on the preparation and microstructural characterization of [gamma]-TiAl single crystals or single oriented lamellar crystals. In this work, the directionally solidified microstructure of TiAl alloys has been investigated by optical, scanning (SEM) and transmission electron microscopy (TEM) in order to study the solidification process of two phase peritectic alloys. EDX compositional analysis has been carried out in order to understand the segregation behavior during peritectic solidification. Master alloys of Ti-48, Ti-52, Ti-54, Ti-56Al were used.