The role of microstructure on strain localization in the near-alpha titanium alloy TIMETAL®834
2021
Online
Hochschulschrift
Near-α titanium alloys for applications at elevated temperatures like TIMETAL®834 can exhibit susceptibility to cold dwell fatigue (CDF), a type of failure due to cyclic loading with superimposed dwell or load-hold periods. The reduction in the number of cycles to failure observed is associated with room temperature creep, or cold creep, during the dwell cycles. In this study, the effect of the microstructure on the deformation mechanisms during initial loading to stresses below macroscopic yield and holds of up to 10 minutes were investigated. The selected loading conditions are similar to the dwell cycle of CDF tests and therefore results of these experiments are relevant to the mechanisms of cold-creep and CDF deformation. The material studied was TIMETAL834 with a bi-modal microstructure, and the microstructures investigated enabled comparisons between materials with large and small prior beta grain sizes, and with coarse and fine transformation products. The first part of the study was an in-depth microstructural characterisation of the material. Correlative microscopy, using Electron Probe Micro-Analysis (EPMA), Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM) characterisation, revealed that primary alpha-grains grew during cooling after the solution heat treatment, with element partitioning leading to the formation of a core-shell structure. Microchemical analysis has shown that the shell is depleted in alpha stabilising and enriched in beta stabilising elements compared to the core and, as a result of this partitioning effect, alpha2 precipitates are absent in the shell region. In the second part of the study, local slip mechanisms were investigated at the sub-grain length scale after loading to three different stress levels below the macroscopic proof stress. In-situ High-Resolution Digital Image Correlation (HRDIC), based on SEM imaging and coupled with Electron Back Scatter Diffraction data, was used to determine the strain distributions and local, quantifiable strain values. Slip Trace Analysis (STA) was used to determine the activated slip systems at each load step. To help explain the experimental observations and improve the mechanistic understanding for slip activation under the applied loading conditions, crystal plasticity modelling was used to determine the local stresses in the microstructures. The analysis of these experiments showed that, for all material conditions, slip is observed at stress levels as low as 70% of the 0.2% yield stress., Material with larger prior beta-grains exhibited higher strain values and higher deformation heterogeneity than material with finer prior beta-grains at stresses close to the 0.2% proof stress, but no difference could be measured at lower stress levels. Regarding the secondary alpha phase, produced by transformation on cooling, it was found that a coarser grain structure leads to higher strain values and local strain concentrations at all loading steps, which suggests that a coarser grain structure leads to a stronger susceptibility to cold creep deformation. Initial plastic deformation was localised in the primary alpha grains, suggesting that they have lower yield strength than the secondary alpha colonies. Activation of slip in secondary alpha colonies was more difficult for a finer transformation product, which explains its superior resistance against plastic deformation during initial loading, and during the 10-minute load hold periods. At the onset of plastic deformation, basal slip was the dominant slip mode for all microstructures. While the elastic anisotropy of titanium increases the RSS values for basal slip compared to prismatic slip, based on this observation the difference in elastic properties could not explain the early activation of basal slip, leading to the conclusion that CRSS values must be lower for basal slip. The findings are discussed in view of the proposed mechanisms for CDF, the correlation of microstructural features and CDF behaviour, and the implications for modelling techniques used to predict CDF performance.
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The role of microstructure on strain localization in the near-alpha titanium alloy TIMETAL®834
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Autor/in / Beteiligte Person: | Dichtl, Claudius |
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Veröffentlichung: | 2021 |
Medientyp: | Hochschulschrift |
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