Polysynthetically Twinned Crystals Research Articles

In-situ studies of TiAl polysynthetically twinned crystals: Critical fluctuations and microstructural evolution

Polysynthetically twinned (PST) crystals of TiAl alloys have attracted much attention due to its unique lamellar microstructure. Their microstructural evolution is essential for understanding the relationship between the microstructure and mechanical performance. Here, we report a comprehensive study of the microstructure and crystallography in Ti-47.5Al PST crystals via a combination of electron backscatter diffraction, in-situ neutron diffraction and high-temperature laser-scanning confocal microscopy during heating cycles. It was found that Σ3<111> twins of γ-phase are unlikely to form along the lath direction. The observable γ crystal orientation variants along the lath belong to the same twin family and are related to each other by a 120° rotation along the <111> direction. Further diffraction studies show that a short-range-ordered structure persists in the disordered α-phase matrix at high temperature and recrystallizes to form a poly-crystalline structure of fully lamellar colonies after heat treatment. The microstructural evolution during heating and cooling is revealed by in-situ high-temperature laser-scanning confocal microscopy, showing the detailed structural transformation and phase evolution from single grain PST crystals into a poly-crystalline structure.

Microstructure control and mechanical properties of directionally solidified TiAl–Nb alloys

The double directional solidification (DS) technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys. Polysynthetically twinned (PST) crystals with lamellar boundaries parallel to or inclined 45° to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface. The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions. Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase. Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys, the tensile plastic elongations of the alloys are only close to 2%.

Constrained Deformation of TiAl PST Crystals

AbstractTiAl polysynthetically twinned (PST) crystals were deformed under plane strain condition, in which the anisotropic macroscopic deformation of PST crystals is restricted with a channel die, in order to clarify the deformation behavior of TiAl/Ti3Al lamellar structure under constraint conditions. TEM analysis of deformation modes together with the Taylor analysis reveals that all TiAl orientation variants deform to yield the relaxed-constraint-type plastic strain, where three shear strain components are not zero for each TiAl variant but are macroscopically compensated to zero by the existence of twin-related TiAl lamellae at the early stage of deformation. The Taylor analysis assuming the relaxed constraint conditions is found to be adaptable for predicting the operative deformation modes in TiAl at the early stage of deformation and also for correlating quantitatively the stress-strain behavior of PST crystals under external constraint with those under the unconstraint condition.

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α 2–α 2 spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α 2–α 2 spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.

Fracture in lamellar TiAl simulated with the cohesive model

Fracture behaviour of lamellar TiAl is studied by means of experiments and numerical simulations using a cohesive model. The fracture process can be described by two cohesive parameters: traction, T 0, and separation work, Γ 0. These are identified for polysynthetically twinned crystals (PST) and a polycrystalline TiAl alloy by comparison of the numerical simulations with experimental fracture tests. The appropriate shape of the traction–separation law for quasi-brittle fracture was identified. The simulation of the PST crystal deformation is based on crystal plasticity including the specific lamellar orientation relations. In a PST crystal, fracture occurs by inter- and translamellar fracture. In an extruded polycrystal, the lamellae of the colonies are randomly aligned in radial direction but more uniformly in extrusion direction for which different fracture toughness values are obtained. For the polycrystalline γTiAl conventional J 2 plasticity is applied, which seems appropriate to predict macroscopic effects either for stable crack extension or unstable failure. The cohesive separation, δ 0, can be interpreted as a characteristic length for a representative microstructural volume.

AFM study of the plastic deformation of polysynthetically‐twinned (PST) TiAI crystals in soft orientation

PST TiAl samples with a nominal composition of Ti52Al48 were deformed at room temperature with compression axis inclined to the lamellar interfaces by 45 degrees and one of the side surface normal directions set to be (112). The deformation structures on the free surfaces of the deformed samples were investigated using Atomic Force Microscope (AFM). It was found that in-plane shear (shear in planes parallel to lamellar interfaces) is the dominant deformation mode in all gamma domains and most of the deformation traces on the free surfaces are parallel to lamellar interfaces. Out-of-plane shear (shear in planes inclined to lamellar interfaces) also occurs but contributes much less to the macroscopic strain. This selective activation of deformation modes leads to a highly anisotropic deformation behavior in PST crystals with this orientation.

Effect of stress axis orientation on the creep deformation behavior of Ti–48Al polysynthetically twinned (PST) crystals

The creep deformation behavior of PST crystals Ti–48Al was investigated at 1150 K under 100–400 MPa. Two hard orientations with the lamellar plates parallel or perpendicular to the compression axis and a soft orientation with the lamellar plates oriented 35° to compression axis were compared to investigate the effect of the orientation of lamellar interface on the creep behavior. The soft PST orientation exhibited the highest creep rates and larger primary creep strain than those of the hard PST orientations. The stress exponents were evaluated as 7.0 in the hard PST orientations for the stress dependence of the minimum creep rates. In the soft PST orientations, the stress exponent increased with decreasing stress from 4 at high stress regime to 8 at low stress regime. The primary creep strain increased with decreasing stress. The rotation of lamellar played an important role to accommodate the creep deformation in the soft PST orientation. The acceleration of creep rate after the minimum strain rates were attributed to the microstructural softening at strain concentrated regions. Those are kinking and dynamic recrystallization along the shear band in the hard PST orientations and recrystallization along the region at which angle of lamellar boundaries abruptly change in soft PST orientation.

Deformation structure during creep deformation in soft orientation PST crystals

Deformation microstructure of soft polysynthetically twinned (PST) crystals Ti–48A1 was investigated. The soft orientation with the lamellar plates oriented 35° to compression axis was deformed at 1150 K under applied stresses of 100–251 MPa. Macroscopically anisotropic deformation was observed after creep deformation. Deformation took place in the maximum shear stress direction in the soft orientation. Dislocation structures in γ domains of six different variants were examined by transmission electron microscopy. Operative slip and twinning systems were also analysed. Macroscopic plastic strain and strain compatibility at domain and lamellar boundaries were discussed in terms of the slip systems in each domain. Refinement of lamellar occurred by mechanical twinning during creep deformation.

On the microstructural stability of TiAl/Ti 3 Al polysynthetically twinned crystals under creep conditions

Abstract The lamellar structure of polysynthetically twinned (PST) specimens has been characterized before and after creep testing in terms of a quantitative description of the microstructural variables: lamellar spacing, α2 spacing and α2 volume fraction. An apparent α2 → γ phase transformation taking place when the samples were held at 1150k was encountered not only in creep-deformed specimens but also in a simply heat-treated specimen. The transformation of the α2 phase into the γ phase took place by partial dissolution of α2 lamellae. resulting in a substantial increase in the spacing of the α2 lamellae. The driving force for the α2 decomposition was identified as the chemical force to complete the thermodynamic equilibrium that was not reached in the as-grown PST crystals. Two processes of lamellar refinement during creep deformation were identified, namely mechanical twinning parallel to the lamellar interfaces and secondary precipitation of γ phase in α2 lamellae. Mechanical twinning parallel to the lamellar interfaces was much more pronounced in soft-oriented PST specimens than in hard-oriented PST specimens. Both refinement processes led to an increase in the total number of interfaces even though some α2–γ interfaces disappeared as a consequence of α2 dissolution. Both refinement processes may contribute to the self-strengthening of lamellar α2 + γ alloys in the course of creep and hence the superior creep resistance of the lamellar microstructure.

In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys

The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti–49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the main crack propagated discontinuously by nucleation, growth and linkage of microcracks formed ahead of crack tip. Slip band decohesion, fracture along prism plane of α 2 plates and twinning-induced microcracks are three modes of nucleation of microcracks. When the loading axis was perpendicular to the lamellae, the crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microcrack usually occurred ahead of the main crack tip. Sometimes, the main crack propagated along (111) cleavage plane within γ lamellae.

Deformation of PST crystals of a TiAl/Ti 3Al two-phase alloy at 1000°C

Polysynthetically twinned (PST) crystals of a TiAl(γ)/Ti 3Al(α 2) two-phase alloy (Ti–49.3 at% Al) with three different loading axis orientations were deformed in compression and tension at 1000°C and the deformation behavior and deformation modes in the γ and α 2 phases were investigated. The majority of dislocations observed in all six orientation variants in the γ phase were those with b= 1 2 <110] which often form dislocation networks or sub-boundaries. In addition to these ordinary dislocations, dislocations with b= 1 2 <112], [001] and <101] and twins on one or two 111 <112] systems were observed depending on orientation variant. However, <101] superlattice dislocations and deformation twins were observed much less frequently at 1000°C than at temperatures lower than 800°C. Ordinary dislocations were observed even in γ domains where the loading axis is parallel or perpendicular to the direction of their Burgers vector, indicating that they were generated from dislocation sources as well as through dislocation reactions. In α 2 lamellae, 1 6 <11 2 ̄ 0> dislocations as well as [0001] dislocations were observed for all loading axis orientations.

Auger Spectroscopy of TiAl Interface Diffusion Specimens

The utility of polysynthetically-twinned (PST) TiAl, which contains a high density of parallel, atomically-flat interfaces within a set of identical crystallographic orientations, as a potential model system for a detailed investigation of interface diffusion is explored. Macroscopic PST crystals were grown in an optical float zone fumace. Thin films were cut from oriented crystals and polished with and directions normal to the film. After sputter cleaning, Ag was deposited on one side of the TiAl thin films. Auger spectra were obtained from these films over a wide range of sputter/anneal conditions. The Al and Ti concentrations were analyzed as well as the important impurity elements, S, Ar, C, N and O. Ag diffusion on the interfaces was confirmed. Using the present data and existing knowledge of the microstructure and crystallography of PST TiAl, the potential of this material for providing a detailed understanding of the atomistic mechanisms of interface diffusion is analyzed.

Flow behavior of PST and fully lamellar polycrystals of Ti–48Al in the microstrain regime

The flow behavior of polysynthetically twinned (PST) crystals of a Ti–48Al alloy was studied as a function of orientation in the microstrain regime (from 10 −5 to 2×10 −2) at room temperature to understand the evolution of the anisotropy of flow stress with strain. After resolving the stresses on to the slip systems, the variation of flow stress with orientation was observed to be only 10–15 MPa at near-zero (10 −5) strain, but it increases rapidly to ∼70 MPa at 0.2% plastic strain. The stress–strain response had discrete steps, indicating the possible effects of the progressive deformation of lamellae of varying thicknesses within the distribution. The flow behavior of a polycrystalline fully lamellar (FL) alloy of the same composition was also studied in the same strain regime for comparison. The saturation engineering flow stress (0.2%) and the strain-rate sensitivity of the polycrystal were found to be close to those of the 0° orientation of the PST crystals. Combining the data of the hard orientations of the PST material and those of the polycrystal, an apparent Taylor factor for fully lamellar polycrystalline Ti–48Al was determined to be in the range of 3.2–3.8. The results on PST crystals were analyzed and rationalized using a mechanistic model that takes into account the distribution of the lamellar sizes ( γ/ γ and γ/ α 2). The work-hardening rate, the saturation flow stress of the hard orientations and the Hall–Petch slopes are all predicted to be sensitive to both the mean and the standard deviation of the lamellar thickness distribution, with dislocation sources from only a fraction of the distribution contributing to the deformation process even at 0.2% plastic strain. Narrower distributions of lamellar spacings are predicted to be beneficial in increasing 0.2% yield strength.

Deformation of lamellar structure in TiAl-Ti3Al two-phase alloys

Polysynthetically twinned (PST) crystals of TiAl(γ)-Ti3Al(α2) two-phase alloys with five different orientations were deformed in compression and in tension in the temperature range from- 196 to 1100°C. Deformation structures in gamma domains of six different orientation variants in each specimen were examined by transmission electron microscopy, and operative slip and/or twinning systems were determined. No significant change in the operative deformation modes was observed up to 800°C. On the basis of these results, macroscopic plastic strain tensors of PST crystals were derived. They were found to describe well the anisotropic deformation of PST crystals. The anisotropy of deformation is typically observed when the loading axis is parallel to the lamellar boundaries. The normal strain in the direction perpendicular to the lamellar boundaries is zero or much smaller than that in the direction parallel to the lamellar boundaries. Strain continuity at domain and lamellar boundaries is one of the most important factors in determining not only operative slip and/or twinning systems for each orientation variant in the gamma phase but also the yield stress and tensile elongation of PST crystals.

Role of Interfaces in Deformation and Fracture of Ordered Intermetallics

While sub- and grain-boundaries are the primary dislocation sources in Ll{sub 2} alloys, yield and flow stresses are strongly influenced by the multiplication and exhaustion of mobile dislocations from the secondary sources. The concept of enhanced microplasticity at grain boundaries due to chemical disordering is well supported by theoretical modeling, but no conclusive direct evidence exist for Ni{sub 3}Al bicrystals. The strong plastic anisotropy reported in TiAl PST (polysynthetically twinned) crystals is attributed in part to localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Calculations of work of adhesion suggest that, intrinsically, interfacial cracking is more likely to initiate on {gamma}/{gamma}-type interfaces than on the {alpha}{sub 2}/{gamma} boundary. 70 refs, 5 tabs, 5 figs.

Physical constants, deformation twinning, and microcracking of titanium aluminides

Physical properties that are relevant to mechanical behavior of single-phase TiAl and Ti3Al and two-phase TiAl/Ti3Al alloys are summarized. By using planar-fault energies and temperature-dependent elastic constants, dislocation dissociation reactions applicable to twin formation in TiAl are analyzed, and a pole mechanism based on a jogged [110]/2 ordinary dislocation is proposed to explain the available experimental data on deformation twinning in γ-TiAl single crystals. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed in part to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. The experimental findings reported on cleavage habit planes of PST crystals are discussed in terms of the calculated ideal work of adhesion and possible extrinsic factors.

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.

Fatigue, Cyclic Deformation and Microstructure. Effect of V and Nb Addition on the Fatigue Behaviour of TiAl Polysynthetically Twinned Crystals.

Effect of V and Nb addition on the fatigue behaviour of TiAl alloys was investigated using polysynthetically twinned (PST) crystals. The cyclic hardening and fatigue life of TiAl PST crystals depended strongly on the orientation and alloying elements. When the stress was applied parallel to the lamellar boundaries at ø=0° where ø is the angle between the loading axis and lamellar planes, slip occurred across the lamellar boundaries showing strong hardening. When slip occurred in the γ matrix parallel to the lamellar boundaries at ø=45°, cyclic behaviour and fatigue life were strongly influenced by the V and Nb addition: the fatigue life was prolonged by Nb addition, while V addition reduced the fatigue life. In the fatigued TiAl PST crystals, numerous deformation twins and 1/2 [110] ordinary dislocations were observed in the γ phase depending on the type of domain. Activation of twins and the frequency of cross-slip events for the 1/2 [110] ordinary dislocations were very sensitive to additional elements of V and Nb. Since V and Nb addition were expected to decrease and increase the stacking fault energy, respectively, V-doping accelerated twinning and suppressed the cross-slip of the ordinary dislocations. In contrast, a small number of twins were observed and the cross-slip of the ordinary dislocations frequently occurred in Nb-doped crystals. Since deformation twins induced in an early stage of cyclic deformation interrupted the motion of dislocations through γ/γ domain boundaries under cyclic loading and formed highly residual stress, the addition of V reduced the fatigue life.

Interface Diffusion in Polysynthetically-Twinned Tiai

ABSTRACTThe utility of polysynthetically-twinned (PST) TiAI, which contains a high density of parallel, atomically-flat interfaces within a set of identical crystallographic orientations, as a potential model system for a detailed investigation of interface diffusion is explored. Macroscopic PST crystals were grown in an optical float zone furnace. Thin films were cut from oriented crystals and polished with &lt;112&gt; directions normal to the film. After sputter cleaning, Ag was deposited on one side of the TiAI thin films. Auger spectra were obtained from these films over a wide range of sputter/anneal conditions. The Al and Ti concentrations were analyzed as well as the important impurity elements, S, Ar, C, N and O. Using the present data and existing knowledge of the microstructure and crystallography of PST TiAI, the potential of this material for providing a detailed understanding of the atomistic mechanisms of interface diffusion is analyzed.

Compositional variations at TiAl-TiAl lamellar boundaries in binary and some ternary polysynthetically twinned Ti-Al

Abstract Local chemical compositions as well as their atomistic structures of TiAl-TiAl lamellar boundaries in binary and some ternary polysynthetically twinned (PST) Ti-Al have been investigated using a transmission electron microscope equipped with a Schottky-type field emission gun. Ordered twin boundaries, on which two neighbouring TiAl lamellae are related by a 180° rotation about the (111) lamellar plane normal, are atomistically coherent and the local chemical composition at the boundaries is virtually the same as that of the bulk TiAl lamellae. On the other hand, lattice mismatches and compositional modulations are observed on pseudo-twin (60° rotation) and 120°-rotational boundaries. These two types of boundary in binary, Mn-bearing and Nb-bearing PST crystals are enriched with Ti with respect to the bulk of TiAl lamellae while those in Cr-, Mo- and Ta-bearing crystals are enriched not only with Ti but also with the corresponding ternary element. The occurrence of such compositional variations at...