Effects of lamellar boundary structural change on lamellar size hardening in TiAl alloy (original) (raw)
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Acta Materialia, 2017
Advanced intermetallic g-TiAl based alloys, which solidify via the disordered b phase, such as the TNM þ alloy, are considered as most promising candidates for structural applications at high temperatures in aero and automotive industries, where they are applied increasingly. Particularly creep resistant mi-crostructures required for high-temperature application, i.e. fine fully lamellar microstructures, can be attained via two-step heat-treatments. Thereby, an increasing creep resistance is observed with decreasing lamellar interface spacing. Once lamellar structures reach nano-scaled dimensions, deformation mechanisms are altered dramatically. Hence, this study deals with a detailed characterization of the elevated temperature deformation phenomena prevailing in nano-lamellar TiAl alloys by the use of tensile creep experiments and mechanical spectroscopy. Upon creep exposure, microstructural changes occur in the lamellar structure, which are analyzed by the comparative utilization of X-ray diffraction, scanning and transmission electron microscopy as well as atom probe tomography. Creep activation parameters determined by mechanical characterization suggest the dominance of dislocation climb by a jog-pair formation process. The dislocations involved in deformation are, in nano-lamellar TiAl alloys, situated at the lamellar interfaces. During creep exposure the precipitation of b o phase and z-silicide particles is observed emanating from the a 2 phase, which is due to the accumulation of Mo and Si at lamellar interfaces.
Effects of plastic deformation on lamellar structure formation in Ti–39at.% Al single crystals
Acta Materialia, 2010
The effects of plastic deformation on lamellar structure formation in solution-treated Ti-39 at.% Al single crystals were investigated, focusing on the role of dislocations of different slip systems. The dislocations were introduced by indentation on the surfaces of solutiontreated single crystals with different crystallographic orientations. Traces of basal and prism slips were observed, depending on the position relative to the indentation. During annealing at a 2 + c dual-phase temperatures, lamellar structures were formed faster where basal slip had occurred than where prism slip had occurred. After long annealing, the length scale of lamellar structures formed depends on the slip system operated during prior deformation: in the region where only one of either basal or prism slip had occurred the lamellar structure was coarser than in undeformed crystal, while in the region where both basal and prism slips occurred the lamellar structure was finer than those formed in undeformed crystal. The reasons for the differences in lamellar structures are discussed on the basis of the frequencies of stacking fault formation on (0 0 0 1) planes as precursors to c-precipitates. The results suggest that the cross-slip of dislocations between basal and prism planes, which gives rise to the formation of multiple stacking faults on many parallel (0 0 0 1) planes, is responsible for the refinement of lamellar structures.
International Journal of Plasticity
Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micromechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micromechanisms responsible for the highly anisotropic deformation response of TiAl on the intralamella and inter-lamella scales. This study focuses on two specific configurations of / 2 lamellar microstructure with the / 2 interfaces being aligned 25 and 55 to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 micropillar while the anisotropy of the 55 micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 micropillar less localised than that in the 55 micropillar. Moreover, the CP-FEM 2 / 42 model successfully predicted substantial build-up of internal stresses at / 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.
The effect of lamellar spacing on the creep behavior of a fully lamellar TiAl alloy
Intermetallics, 2000
The compression creep behavior of a cast alloy with the composition of Ti±46Al±2Cr±1.5Nb±1V was studied at 800 C under the stress level of 205 MPa. Dierent heat treatments were devised to produce fully lamellar microstructures with dierent lamellar spacings but remaining similar grain sizes, and the eects of the lamellar spacing on the primary creep strain and minimum creep rate of the alloys were investigated. The results indicated that the primary creep strain and minimum creep rate increase with the increase of the lamellar spacing. TEM observation reveals that the multiple generation of dislocation loops from lamellar interfaces contributes to the creep strain. Lamellar interfaces can also act as the barriers to the dislocation motion and multiple generation.
Intrinsic and extrinsic fracture resistance in lamellar TiAl alloys
Acta Materialia, 2004
The roles of colony boundary and crack orientations in the fracture resistance of two-phase lamellar TiAl alloys (Ti-46.5Al and Ti-47Al-2Nb-1.6Cr-1V, all in at.%) were investigated. In situ fracture testing of single-colony thick compact-tension specimens was performed at ambient temperature in a scanning electron microscope equipped with a loading stage. Near-tip micrographs of kink or twist cracks approaching a colony boundary were obtained as a function of applied loads and subsequently analyzed using a machine-vision-based stereoimaging technique to determine the displacement and strain fields. Metallographic and fractographic techniques were utilized to measure the kink and twist angles of the crack as well as the orientation of the colony boundary. The stress intensity factors of the kink and twist cracks were computed using a 2D boundary-integral-equation method and a 3D finiteelement method. Comparison of the computed stress intensity factors against those deduced from the near-tip strain field indicates that the intrinsic fracture resistance in the lamellar colonies of the TiAl alloys is relatively low (about 3 MPa p m, but its apparent value can be increased as the result of shear ligament bridging. The magnitude of shear ligament toughening is strongly influenced by the kink and twist angles of the crack, as well as the orientation of the colony boundary.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2018
In this work, the effects of lamellae orientation and the distribution of fine grains and fine lamellar colonies on the fracture toughness of orientation-controlled Ti-43mol%Al alloy were examined. Fracture toughness was determined by three-point bending tests following the introduction of a chevron notch, and thermomechanical processing to control lamellar orientation consisted of high temperature compression in the single-phase and (+two-phase regions. Materials processed via one-step compression in the single-phase region exhibited tilting of the lamellae interface by approximately 20° to 30° relative to the compression plane. Further compression in the (+two-phase region (termed two-step compression) arranged the lamellae interface nearly parallel to the compression plane. Fine grains were also observed around the orientation-controlled lamellar colonies, and were converted into fine lamellar colonies during heat treatment following processing in the () two-phase region. The proportion of fine grains decreased with an increase in the true strain rate during compression in the () two-phase region. Specimens processed by one-step compression had fracture toughness values lower than those of two-step compression specimens. In addition, specimens processed using two-step compression and exhibiting a crack arrester orientation had higher fracture toughness values than those with a crack divider orientation. The fine grains and colonies formed at the lamellar colony boundaries were found not to affect the fracture toughness of specimens with either of these crack orientations. However, decreased quantities of fine grains and colonies may increase the toughness of specimens for which crack propagation occurs in conjunction with crack arrester orientation.
Intermetallics, 2005
The structural transformations activated during the formation of the lamellar microstructure have been studied in a Ti 49 Al 47 Cr 2 Nb 2 alloy. By performing statistical analysis of the microstructure, a special attention has been paid to the orientations of the lamellae and to the interface relationships. Experiments were conducted on specimens in an as-HIPed condition as well in an isothermal forged plus annealed condition. In the annealed alloy, three mechanisms of the lamellar formation were evidenced depending upon the nucleation temperature: a heterogeneous transformation occurring at grain boundaries, a homogeneous nucleation in the a 2 matrix and an interfacial nucleation of twinrelated lamellae. The microstructural analysis of the as-HIPed alloy was aimed at determining the activated mechanisms among those evidenced in the annealed alloy. q
Plastic Behaviour of TiAl Crystals Containing a Single Set of Lamellae at High Temperatures
ISIJ International, 1992
Osaka-fu, Compressiontests were performed on TiAl crystals containing unidirectionally aligned lamellae based on the function of temperature and the angle (c) betweenthe loading axis and the lamellar planes in order to determine the effect of lamellar structure on plastic behaviour. For specimens with c=90', the yield stress increased anomalously with increasing temperature and reached a maximum peak around 500'C. This phenomenon was particularly noticeable in the specimens containing fine and homogeneouslydistributed lamellae. Thin oe~plates in lamellae act as an effective barrier to the motion of dislocations, and the plastic behaviour of 1 26>-type dislocations in the oe2 plates may be responsible for the anomalousstrengthening. In the cases of ip=45 and O', the weaktemperature dependenc.eof yield stress was mainly due to the mode of deformation in the y matrix. Cracks nucleate often at the interface between the c(. plate and y matrix and/or at thick ce2 plate. Lack of activation of pyramidal slip in the oe2 plate makesthe accommodationof the concentrated stress at the interface more difficult. Addition of vanadium, which is thought to activate the pyramidal slip, was effective in improving the ductility.