Plastic deformation and fracture behaviour of Ti3Al single crystals deformed at high temperatures under cyclic loading (original) (raw)
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Acta Materialia, 1998
ÐThe eect of prism slip on cyclic hardening and deformation substructure in Ti 3 Al single crystals cyclically deformed was examined. Fatigue tests were carried out in a symmetrical tension/compression mode at a ®xed total strain amplitude (De) of 20.2% to 20.4% at room temperature. As De and the number of cycles increased, the stress amplitude rose depending on crystal orientation. The cyclic hardening was more accelerated for specimens deformed by double prism slips than for those deformed by single prism slip. At the saturated stage, the edge dipoles and/or multipoles gathered at a localized region and formed bundles with a high residual stress ®eld in specimens deforming by double prism slips, while a high density of dislocations existed the bundled structure did not develop under operation of single prism slip. The peculiar structure containing bundles was called saturated bundled structure (SBS). The formation process of the SBS and the role of the bundled structure on the plastic behaviour are described.
Science and Technology of Advanced Materials
Plastic deformation behaviour in Ti±54.7 at.%Al and Ti±58.0 at.%Al single crystals was examined around and above the anomalous strengthening peak temperature (T p) focusing on the effect of Al 5 Ti 3 superstructure. The Al 5 Ti 3 superstructure developed in the L1 0 matrix of Ti±58.0 at.%Al, and the size of the Al 5 Ti 3 phase once increased during annealing at 8008C and then decreased with increasing temperature, while no signi®cant evidence of the Al 5 Ti 3 particles was obtained in Ti±54.7 at.%Al from TEM observation although diffuse scattering corresponding to the spots for the Al 5 Ti 3 superstructure was observed. The transition of slip plane for 1 2 k110 ordinary dislocations from {111} to {110) and/or (001) occurred at and above T p due to anisotropy of anti-phase boundary energies on {111}, {110) and (001) in the Al 5 Ti 3 superstructure. Anomalous strengthening is related to the development of this superstructure which may assist the cross-slip of some parts of 1 2 k110 ordinary dislocations onto {110) and/or (001) resulting in the formation of dragging points to the motion of the dislocations.
Dislocation dipoles in cyclically deformed Ti3Al single crystals
Intermetallics, 2000
Dislocation dipoles and debris in Ti 3 Al single crystals fatigued at 300 and 77 K were examined focusing on geometric con®guration and formation mechanism of superlattice dislocation dipole (superdipole). Longer and lower density debris were observed more at 77 than at 300 K. Although the superdipoles composed of S-type and Z-type con®gurations may exist, all superdipoles observed in present Ti 3 Al had Z-type stable con®guration. Geometric con®guration of superdipoles was calculated on the basis of elastic theory. The superdipole height was estimated from the relation between the height and the separation distance of each superpartial in the superdipole. Debris (short superdipoles) formation is triggered dominantly by jog formation due to the double cross slip and it is controlled by the thermally activated pinching o process.
Acta Materialia, 1998
ÐFormation and stability of deformation substructure in fatigued Ti 3 Al single crystals deformed by double prism slip were examined under dierent applied strain amplitude (Áe). The specimens were initially fatigued at De =20.2% or 20.3% for 1 Â 10 4 cycles and subsequently cyclically deformed after the change in De from 20.2% to 20.3% or from 20.3% to 20.2%. The saturated bundled structure (SBS) was composed of the bundles with high density of edge dislocations aligning along [0001] and screw dislocations between the bundles were formed in specimens fatigued at initial De to the saturated stage. The bundles in the SBS were developed and expanded after the increase in De to 20.3%, while the rearrangement of screw dislocations occurred and some parts of the saturated edge bundles were removed after the decrease in De to 20.2%. The thermal stability of the fatigued substructure was examined in order to understand the formation process of edge dipoles and debris in the SBS. The change in the maximum shear stress amplitude and the Bauschinger eect were also examined during cyclic deformation.
Metallurgical and Materials Transactions A, 1998
Randomly oriented single crystals of high-purity titanium were prepared by strain annealing and were subjected to multiple-step fatigue testing under strain-controlled conditions, in order to determine their cyclic stress-strain curves (CSSCs). These were found to fall into three groups, depending on orientation and the extent of slip and twinning. For those crystals oriented for single prismatic slip, a plateau was observed in the CSSCs, persistent slip bands (PSBs) occurred, and the plateau stress was 38 MPa. In a second group, oriented for prismatic slip but for which cross-slip and twinning was favored, the plateau was suppressed and the flow stresses were higher. In a third group, connected with orientations on the borders of the unit triangle, extensive hardening occurred, the CSSCs were steep, and there were multiple cases of slip and twinning. The results are interpreted in terms of maps in the stereographic projection recording the Schmid factors for the various deformation modes.
On the dislocation mechanisms leading to cracking in dwell fatigue of a near-alpha titanium alloy
2019
The dislocation structures appearing in highly mis-oriented soft/hard grain pairs in near-alpha titanium alloy Ti6242Si were examined with and without the application of load holds (dwell) during fatigue. Dislocation pile-up in a soft grain resulted in internal stresses in an adjacent hard grain which could be relaxed by dislocation multiplication at localized Frank-Read sources, a process assisted by the provision of a relaxation time during a load hold. The rate of this process is suggested to be controlled by <c+a> pyramidal cross-slip and <a> basal junction formation. A high density of <a> prism pile-ups is observed by dual slip, together with the nucleation of edge dislocations in the soft grain of a highly mis-oriented grain pair, increasing the possibility of cracking. The stress concentration developed by such pile-ups is found to be higher in dwell fatigue (single-ended pile-ups) than in LCF (double ended). Analytical modelling shows that the maximum norma...
The role of dwell hold on the dislocation mechanisms of fatigue in a near alpha titanium alloy
International Journal of Plasticity, 2020
The dislocation structures appearing in highly mis-oriented soft/hard grain pairs in near-alpha titanium alloy Ti6242Si were examined with and without the application of load holds (dwell) during fatigue. Dislocation pileup in a soft grain resulted in internal stresses in an adjacent hard grain which could be relaxed by dislocation multiplication at localized Frank-Read sources, a process assisted by the provision of a relaxation time during a load hold. The rate of this process is suggested to be controlled by c + a pyramidal cross-slip and a basal junction formation. A high density of a prism pileups was observed with dual slip on two prism planes, together with edge dislocations on the third prism plane in the soft grain of a highly mis-oriented grain pair, increasing the pileup stress. The stress concentration developed by such pileups is found to be higher in dwell fatigue (single-ended pileups) than in LCF (double ended). Analytical modelling shows that the maximum normal stress produced on the hard grain in dwell fatigue by this pileup would be near-basal, ≈ 2.5 • to (0002). This provides support for the dominant hypothesis for the rationalisation of dwell fatigue crack nucleation in Ti alloys, which derives from the Stroh pileup model, and elaboration of the underlying dislocation phenomena that result from load shedding and lead to basal faceting.
Deformation behavior of [001] Ti-56Al single crystals
Materials Science and Engineering: A, 1995
Specimens of an [OOl]-oriented Ti-.56at.%Al single crystal were tested at room temperature (RT) and 873 K. The elevated temperature sample displayed anomalous yield behavior, confirming the previous findings. Dislocation structures in the deformed samples at RT and 873 K, respectively, were observed by TEM. The micrographs for both RT and 873 K show long, partial dislocation loops of (1011 superdislocations. However, dislocation dissociation behavior at the two temperatures was quite different. At room temperature, the (1011 dislocations dissociated into a 1/6[ 1121 plus a 1/6[5 141 dislocation connected by an SISF, whereas at elevated temperature, a dissociation involving two l/2(101] dislocations linked by an anti-phase boundary (APB) was observed. These observations suggest that at elevated temperature, a Kear-Wilsdorf or a "roof-type" barrier is responsible for the anomalous flow stress. These results and their impact on the overall deformation behavior of gamma titanium aluminides will be discussed.
Crystal plasticity, fatigue crack initiation and fatigue performance of advanced titanium alloys
International Journal of Fatigue, 2007
The fatigue behaviour of a novel large grained variant of the near a titanium alloy Ti 685 is described. Load controlled low cycle fatigue fractures in plain cylindrical specimens demonstrate the highly crystallographic nature of the failure process. When compared to similar data for conventional grain size Ti 685 variants the LG685 material clearly offers reduced fatigue and static strength. An alternative flat plate specimen design was employed together with electronic speckle pattern interferometry and strain gauges to monitor the inhomogeneous strain accumulation in this large grained variant under load control conditions. Dwell fatigue tests indicated that the eventual location for crack initiation and subsequent failure could be identified as early as the first loading cycle. The precise crystallographic orientations of the surrounding microstructure were defined using electron back scattered diffraction. In contrast to previous models to describe facet formation and early fracture in this class of alloy, basal plane slip systems were not implicated. The use of EBSD to identify ''effective structural units'' of common orientation, which do not necessarily relate to colony size, will be demonstrated for both the large grained material and a conventional compressor disc alloy Timetal 834 in typical forged condition.