Polysynthetic Twinning Research Articles

Structural characterization of high-pressure C-Na 2 Si 2 O 5 by single-crystal diffraction and 29 Si MAS NMR

Single crystals of C–Na2Si2O5 have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P21/c with Z= 8 and unit-cell parameters a= 4.8521 (4)A, b=23.9793(16)A, c=8.1410(6)A, β=90.15(1)° and V=947.2(2)A3. The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C–Na2Si2O5 is closely related to β-Na2Si2O5. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the 29Si MAS-NMR spectrum of C–Na2Si2O5 four well-resolved lines of equal intensity are observed at −86.0, −86.3, −87.4, and −88.2 ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.

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.

Microscopic and Quantitative Investigations on PST Ti-Al / Ti Reaction Diffusion Couples

ABSTRACTInterdiffusion in multi-phase diffusion couples of polycrystalline Ti and polysynthetically twinned (PST) Ti-49.3 at.% Al, with the diffusion direction parallel to the lamellar planes, is investigated in the temperature range 973 – 1173 K. A reaction zone (RZ) of the a2-Ti3Al phase forms between the end materials and exhibits deeper penetration in the a2 lamellae than in the primary g lamellae. The mass balance and the lamellar thickness across the RZ / PST interface are believed to be the major factors that lead to the different behaviors in the penetration depth of the RZ. Direct measurements of the RZ thickness reveal a parabolic growth of the RZ, indicating a diffusion-controlled growth macroscopically. Concentration profiles from the Ti, through the RZ, into the PST g and a2 lamellae are measured by x-ray spectroscopy in a transmission electron microscope. Deviations from a diffusion-controlled composition profile indicate some extent of interface-controlled growth. Plateaus are seen in the concentration profiles in the RZ adjacent to the RZ/PST interface, extending through most of the deeply penetrated well region. The interfacial energy and strain energy are possible reasons for the plateaus. The interdiffusion coefficients are found to be largely independent of composition with a temperature dependence that obeys the Arrhenius relationship.

Selection and change in deformation mode to maintain continuity of strains and slip/twinning planes at lamellar boundaries in fatigued TiAl polysynthetically twinned crystals

Change in deformation mode in six types of γ domain (AM–CT) and α2 plates in TiAl polysynthetically twinned (PST) crystals fatigued at a loading axis parallel to lamellar planes with stress amplitude (Δσ) of 420–450 MPa was examined by the transmission electron microscope focusing on continuity of macroscopic strains and slip/twinning planes at lamellar boundaries. At Δσ = 420 and 450 MPa, the strain continuity is always maintained at lamellar boundaries by activation of one of the symmetric twinning systems in A-type domain and selection of the dominant deformation mode between ordinary dislocations and twins in (B and C)-type γ domain. The (B and C)-type γ domains of BM,BT,CM and CT behave as two sets of (BM,CT) and (BT,CM) because each set selects either the deformation mode of ordinary dislocations or twins as a dominant system in order to keep macroscopic strain continuity. The set (BT,CM) which accounts for a larger volume fraction than the set (BM,CT) in TiAl-PST crystals used in this study selected a twinning system at Δσ = 450 MPa, while ordinary dislocations were selected at Δσ = 420 MPa. At Δσ = 450 MPa, twinning deformation prevented the further motion of ordinary dislocations with a Burgers vector parallel to lamellar boundaries, and rapid fatigue hardening occurred accompanied by reduction of the accumulative plastic strain energy. Anomalous change in strain energy during fatigue is infiuenced by the volume fraction of a set of (B and C)-type domain and the anomalous behavior in fatigued TiAl-PST crystals may disappear when each type of γ domain is equally distributed.

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.

Design-tool representations of strain compatibility and stress-strain relationships for lamellar gamma titanium aluminides

Engineering use of gamma titanium aluminides requires further development of design models of the material. Presently, modeling tools are limited by computational capability, uncertainty in experimental data, and physical accuracy. Lamellar Ti–Al alloys are plastically inhomogeneous and exhibit anisotropic flow. The origin of this behavior is that there are at least four different length scales in the microstructure: the grain size, the domain size, the lamellar thickness and the separation between either dislocations or twins. They range from mm to nm and give rise to strain incompatibilities and internal stresses over a similar range of lengths. Traditional engineering finite-element analysis of plastic deformation ignores all microstructural length-dependent aspects of deformation, but uses instead constitutive equations to describe plasticity. The gap between the scientific and engineering analyses of plasticity might be bridged by using Ashby's strain-gradient arguments. These capture most of the microstructural scale effects and may deliver descriptions of plasticity that are capable of being used in simulations. In this study, strain-gradient arguments are used to interpret experimental stress–strain measurements of both PST (poly-synthetically-twinned) and polycrystalline TiAl.

Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009

We describe the first known occurrence of primary melt inclusions in a ureilite. The ureilite is Hughes 009, one of a small number of ureilites whose primary mineralogy is olivine-augite-orthopyroxene, rather than olivine-pigeonite. Hughes 009 has a coarse-grained, equilibrated texture typical of ureilites, and homogeneous primary mineral compositions: olivine — mg 87.3; augite — mg 89.2, Wo 37.0, Al 2O 3 = 1.6 wt.%; orthopyroxene — mg 88.3, Wo 4.9. It shows only limited secondary reduction effects and no petrographically recognizable carbon phases, which indicates that its original carbon content was lower than in most ureilites. The melt inclusions occur in olivine crystals. They are concentrated in the central regions of their hosts, showing elongate (mostly 20–60 μm in maximum dimension), negative olivine crystal shapes and parallel alignment. These and other features indicate that they were trapped during initial growth of their hosts from a liquid, and are likely to be representative samples of that liquid. They consist of glass and single, subhedral crystals of high-Ca pyroxene, with minor Cr-rich spinel and metal-phosphide-sulfide spherules. They are surrounded by halos of olivine with rounded outlines defined by tiny bits of metal and thin arcs of glass. Pyroxenes within each inclusion show zonation patterns indicating that they nucleated at the olivine/ liquid interface with compositions close to that of the primary augite, and then grew inward with dramatically increasing Al 2O 3 (to 10.8 wt.%), Wo (to ≈50), TiO 2 and Cr 2O 3 contents. Glasses within each inclusion are relatively homogeneous. Glasses from all inclusions show well-defined trends of CaO, TiO 2, Cr 2O 3 , Na 2O and SiO 2 vs. Al 2O 3, (16–23 wt.%) that can be modelled as resulting principally from crystallization of various amounts of the pyroxene. The halos, which represent olivine that grew from the trapped melts, are zoned in Cr and Ca with concentrations decreasing inward, reflecting cocrystallization of pyroxene; they have homogeneous Fe/Mg identical to that of the primary olivine, indicating reequilibration with the host. We develop a petrologic model for the postentrapment history (crystallization, reaction and reequilibration) of the inclusions, based on which we reconstruct the composition of the primary trapped liquid (PTL). The PTL was saturated only with olivine. This result implies that Hughes 009 is a cumulate (consistent with the high Mn/Mg ratio of its olivine and a low abundance of graphite) and that the composition of the PTL is close to that of its parent magma. The low-pressure equilibrium crystallization sequence predicted by MAGPOX calculations for the PTL (olivine → augite → plagioclase → pigeonite) is not, however, consistent with the primary mineralogy of Hughes 009. If the conditions of these calculations are, indeed, appropriate, then complex processes such as magma mixing must have been involved in the petrogenesis of this ureilite. This conclusion is consistent with other evidence that the olivine-augite-orthopyroxene ureilites record a more complex magmatic evolution than is evident in the olivine-pigeonite ureilites. TEM investigations of microtextural features in all phases and XRD determination of Fe 2+-Mg site distribution in orthopyroxene have elucidated the cooling and shock history of this ureilite. Hughes 009 experienced an extremely high cooling rate (7 ± 5°C/h at the closure T of 630°C) late in its evolution, and two distinguishable shock events—the first at peak pressures of 5 to 10 GPa, resulting in mechanical polysynthetic twinning in augite and orthopyroxene and mild undulatory extinction in olivine; and the second at lower pressures, resulting only in brecciation and redistribution of metal. Its late history is similar to that of most ureilites, and probably reflects impact excavation.

Numerical simulation of plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl

Plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl have been simulated by using periodic unit cells representing the relaxed-constraint model recently proposed by Lebensohn et al. [Acta Mater. 46 (1998) 4701–4709] for the co-deformation of the lamellar compound of PST-TiAl. The unit cells contain both intermetallic phases, α 2-(Ti 3Al) and γ-(TiAl). Furthermore, the six orientation variants of the γ-phase are also considered. The constitutive behaviour of both phases is described by crystal plasticity, and the damage behaviour has been implemented by means of cohesive elements. The unit cells have been used as submodels for multi-scale finite element simulations of compression tests and fracture mechanics tests of notched micro-bend specimens. It is shown that the anisotropy of plastic deformation and damage in PST-TiAl can be well represented.

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.

On the microstructural stability of TiAl/Ti3Al polysynthetically twinned crystals under creep conditions

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: la...

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.

Interdiffusion and Phase Behavior in a Vanadium / Polysynthetically Twinned Tial Diffusion Couple

Abstract Polysynthetically twinned (PST) TiAl, which has a two phase lamellar structure composed of multiply oriented lamellae of the TiAl phase (γ, with ordered tetragonal L10 structure) with a low density of interspersed layers of the Ti3Al phase (α2, with ordered D019 hexagonal structure), has attracted a large amount of interest in the past decade for its potential as a high temperature structural material. The strongly crystallographic twins and pseudo-twins between the lamellae of the PST TiAl crystal provide an ideal system to study interface diffusion. In this paper, we describe early results on the interrelationships between phase and interface structure, bulk and interface diffusion and phase equilibrium in the diffusion- bonded system PST TiAl and vanadium. A PST TiAl “single crystal” was grown in an optical floating zone furnace by zone-melting and directional solidification of a polycrystalline as-cast TiAl rod. Within the “single crystal”, all the lamellar interfaces are parallel.

In-situ deformation of TiAl PST crystals in TEM

The deformation process of TiAl polysynthetically twinned (PST) crystals was in-situ observed in a transmission electron microscope (TEM) with a tensile holder. Microcracks, resulted from different reasons, were observed to nucleate at several different sites. In many cases microcracks were observed to nucleate on lamellar boundaries and on the boundary between the deformation twin and the matrix. The intersecting parts of two deformation twins within the same TiAl lamella have been found to be favorite nucleation sites of microcracks. Microcracks were also observed to nucleate on the domain boundary of two TiAl domains coexisting in a TiAl lamella and at the triple point among TiAl domain boundary and lamellar boundary. It has revealed that cracks tend to propagate along the lamellar boundaries when the angle between tensile axis and lamellar boundaries is about 78°. When crack propagation was hindered, one or more microcracks nucleated ahead of the main crack. Then the microcracks grew up, connected each other and connected with the main crack through shear rupture of the ledgement between them.

Interdiffusion and Phase Behavior in Polysynthetically Twinned (PST) TiAl / Ti Diffusion Couples

ABSTRACTDiffusion couples of pure Ti and polysynthetically twinned (PST) TiAl (49.3 at.% Al) were prepared by high vacuum hot-pressing, with the bonding interface perpendicular to the lamellar planes. Diffusion experiments were carried out by annealing the couples in the same furnace at 650, 700 and 850°C for various times. The cross-section of the couple was studied using scanning electron microscopy (SEM) and quantitative wavelength-dispersive x-ray spectroscopy (WDS). A reaction layer whose composition is close to that of the stoichiometric α2-Ti3Al phase formed along the PST TiAl / Ti bonding interface in PST TiAl side. Direct measurements of the thickness of the reaction zone were performed at different phase regions and various boundaries. By assuming the thickness of the reaction zone increases as (Dt)1/2, where D is the diffusion coefficient and t is the annealing time, the diffusion coefficients at these temperatures were calculated. Composition profiles in the reaction zone, along the lamellae and at the lamellar interfaces were obtained by WDS analyses.

Deformation characteristics of polysynthetically twinned (PST) crystals during creep at 1150 K

The creep deformation characteristics of a lamellar polysynthetically twinned (PST) crystal of the composition Ti–48 mol% Al was investigated as a function of the lamellar orientation with respect to the compression axis and the applied stress. The creep resistance of hard PST orientation with the lamellar plates parallel or perpendicular to the compression axis was substantially higher than that of soft orientations with their lamellar plates oriented at intermediate angles to the compression axis. This fact could be associated with the predominant deformation of the hard orientations by deformation modes with the slip plane inclined to the lamellar interfaces in contrast to the predominant deformation of the soft orientations by deformation modes with the slip plane parallel to the lamellar plates. In the soft orientations, mainly straight ordinary dislocations with the Burgers vector b=1/2[1-10] aligned parallel to the lamellar interfaces were encountered in domains with a high resolved shear stress as well as in domains with no resolved shear stress. In the hard orientations, ordinary dislocations b=1/2[110] and superdislocations with a Burgers vector of the type b=1/2〈112] were observed. Despite a high resolved shear stress in certain oriented domains, superdislocations of the type b=〈101] were not found to play a considerable role during creep deformation under the investigated conditions. Cross twinning contributed to the deformation in favourably oriented variants, but twinning parallel to the lamellar interfaces was much more pronounced in the soft orientations leading to a substantial lamellar refinement. This microstructural hardening during creep results in the observed stress exponents of the soft orientations near unity at high stresses. A change in stress exponent from near unity at high stresses to about 9 at low stresses occurs at about 200 MPa. A critical stress of about 200 MPa for parallel twinning is proposed as a reason for the change in stress exponent.

Superlattice dislocations in Ti-rich polysynthetically twinned crystals of TiAl

The behaviour of superlattice dislocations in polysynthetically twinned (PST) TiAl crystals deformed from room temperature to 850 degrees C is studied by transmission electron microscopy. The dissociation modes undergo significant changes with increasing deformation temperature. At room temperature, the [011] superlattice dislocations dissociate into superpartials including either an antiphase boundary and a superlattice intrinsic stacking fault (SISF), or superlattice extrinsic stacking faults (SESFs), forming faulted dipoles, and are sessile. The 1/2[112] superlattice dislocations dissociate into faulted dipoles and are also sessile. While the dislocation configurations in the crystals deformed at 400 degrees C and 600 degrees C are identical with those in the crystals deformed at room temperature, one important variation is that the density of faulted dipoles decreases with increasing deformation temperature. In-situ heating observations show that self-annihilation of the SESF-bounding hairpin superpartial occurs within a temperature range that compares with that of the brittle-ductile transition of PST TiAl crystals. At 850 degrees C, most [011] superlattice dislocations dissociate to form Kear-Wilsdorf locks. The 1/2[112] superlattice dislocations are edge in character, dissociated into superpartials bordering a SISF and are glissile. The experimental observations suggest that the behaviour of 1/2[112] superlattice dislocations is an important factor controlling the mechanism of brittle-ductile transition of PST TiAl crystals.

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.

Ultrastructure of lamellae, mineral and matrix components of fish otolith twinned aragonite crystals: implications for estimating age in fish

Atomic force microscopy (AFM) of the crystalline ultrastructure of otoliths fromPagrus major(Sparidae),Macruronus novaezelandiae(Merlucciidae),Oncorhynchus tshawytscha(Salmonidae),Sebastes alutus(Scorpaenidae), andHoplostethus atlanticus(Trachichthyidae) showed regular deposition of lamellae in the size range 13–490 nm. The orientation of lamellae in the {010} plane was the same as lamellae in crystals of mineral aragonite. Lamellae in mineral aragonite were in the size range 15–45 nm. Lamellae observed in the otolith ofM. novaezelandiaeby transmission electron microscopy showed a range of widths (25–225 nm) similar to lamellae observed by AFM. The observed lamella widths were in the size range that has been described for sub-daily and daily microincrements in otoliths. Observed lamellae widths were also in the size range of α-recoil trajectories of222Rn and provide a potential diffusion sink correction for the222Rn losses in radionuclide method of ageing otoliths. Comparison of the orientations of lamellae to templates based on the Bragg unit cell structure of twinned aragonite indicated that the lamellae resulted from polysynthetic twinning on the {010} aragonite crystal face. Additional cyclic twinning occurred on the {110} face of the aragonite crystal and sometimes led to pseudohexagonal crystals, whose sizes were orders of magnitude larger than lamellae. The organic matrix of the otolith was visible by atomic force and transmission electron microscopy at the nanometer level of resolution, but the organic matrix was confined to the {110} twinning plane of symmetry of the otolith crystal.

Dislocation Interactions with Lamellar Grain Boundaries in Titanium Aluminum Intermetallics

AbstractThe interactions of dislocations asscociated with deformation modes active in hard orientation compressed polysynthetically-twinned (PST) TiAl with lamellar boundaries have been studied by transmission electron microscopy (TEM). The deformation modes observed in the γ-phase involved soft superdislocation and hard ordinary dislocation slip, and hard twin systems. The transfer of these latter hard twin shears occurred across all types of lamellar (γ/γ)- and frequently across (γ/α2)-boundaries. Slip of superdislocations in the minority U2-phase lamellae has been shown to be associated with both direct transfer of piled-up twinning dislocation shears across the lamellar boundaries and the activation of interfacial sources due to pile-up stresses. The active deformation modes were consistent with macroscopic shape changes of the PST-TiAI when the various defect interactions with the lamellar boundaries and the proposed shear trasnfer processes were considered.

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.