| 1. |
- Jaladurgam, Nitesh Raj, 1993, et al.
(author)
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Microstructure-dependent deformation behaviour of a low γ′ volume fraction Ni-base superalloy studied by in-situ neutron diffraction
- 2020
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In: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 183, s. 182-195
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Journal article (peer-reviewed)abstract
- Ni-base superalloys are critical materials for numerous demanding applications in the energy and aerospace sectors. Their complex chemistry and microstructure require detailed understanding of the operating deformation mechanisms and interaction between the matrix and the hardening phase during plastic deformation. Here we use in-situ neutron diffraction to show that the dependence of the deformation mechanisms and load redistribution on $\gamma^\prime$ particle size in a Ni-base superalloy with a $\gamma^\prime$ volume fraction of around $20 \%$ can exhibit distinct differences compared to their high volume fraction counterparts. In particular, the load redistribution in the coarse microstructure occurs immediately upon yielding in the present case, whereas high $\gamma^\prime$ volume fractions have been observed to initially lead to shear mediated co-deformation before work hardening allows looping to dominate and cause load partitioning at higher stresses. The fine microstructure, on the other hand, behaved similar to high volume fraction alloys, exhibiting co-deformation of the phases due to particle shearing. A recently developed elasto-plastic self-consistent (EPSC) crystal plasticity model, specifically developed for the case of coherent multi-phase materials, could reproduce experimental data with good accuracy. Furthermore, the finite strain formulation of the EPSC model allowed deformation induced texture predictions. The correct trends were predicted by the simulations, but the rate of lattice rotation was slower than experimentally observed. The insights point towards necessary model developments and improvements in order to accurately predict e.g. texture evolution during processing and effect of texture and microstructure on component properties.
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| 2. |
- Pixner, Florian, et al.
(author)
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Wire-Based Additive Manufacturing of Ti-6Al-4V Using Electron Beam Technique
- 2020
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In: Materials. - : MDPI AG. - 1996-1944. ; 13:15
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Journal article (peer-reviewed)abstract
- Electron beam freeform fabrication is a wire feed direct energy deposition additive manufacturing process, where the vacuum condition ensures excellent shielding against the atmosphere and enables processing of highly reactive materials. In this work, this technique is applied for the α + β-titanium alloy Ti-6Al-4V to determine suitable process parameter for robust building. The correlation between dimensions and the dilution of single beads based on selected process parameters, leads to an overlapping distance in the range of 70-75% of the bead width, resulting in a multi-bead layer with a uniform height and with a linear build-up rate. Moreover, the stacking of layers with different numbers of tracks using an alternating symmetric welding sequence allows the manufacturing of simple structures like walls and blocks. Microscopy investigations reveal that the primary structure consists of epitaxial grown columnar prior β-grains, with some randomly scattered macro and micropores. The developed microstructure consists of a mixture of martensitic and finer α-lamellar structure with a moderate and uniform hardness of 334 HV, an ultimate tensile strength of 953 MPa and rather low fracture elongation of 4.5%. A subsequent stress relief heat treatment leads to a uniform hardness distribution and an extended fracture elongation of 9.5%, with a decrease of the ultimate strength to 881 MPa due to the fine α-lamellar structure produced during the heat treatment. Residual stresses measured by energy dispersive X-ray diffraction shows after deposition 200-450 MPa in tension in the longitudinal direction, while the stresses reach almost zero when the stress relief treatment is carried out.
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| 3. |
- Tsikayi, D., et al.
(author)
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Effect of Direct energy surface modification on fatigue life of rotary friction welded Ti6Al4V ELI fatigue coupons
- 2022
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In: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 163
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Journal article (peer-reviewed)abstract
- Fatigue crack initiation in engineering components is predominantly associated with tensile surface residual stresses or stress raisers caused by discontinuities or mechanical notches. During welding, surface residual stresses arising from plastic mismatch due to the temperature gradient can accelerate the initiation of cracks during cyclic fatigue testing. Direct energy surface treatment is currently considered a viable post-processing methodology for modifying surfaces within the weld zone to improve fatigue life. This paper discusses results obtained during experimental research, investigating the effect of direct energy surface treatment on surface residual stresses, microstructure, and fatigue life of cylindrical rotary friction welded Ti-6Al-4 V samples. Direct energy post-processing technique aided in improving the fatigue life of friction welded specimens by modifying the surface microstructure, introducing a uniform refined and homogenous structure in the welded region. This near-surface refined microstructure inhibited crack initiation in the welded region, shifting initiation sites to an area adjacent to the modified surface region. © 2022 Elsevier Ltd
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