The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED−9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED−9.4%specimens are similar, and they experience twinning exhaustion →
Role of deformation twinning in fatigue of CrCoNi medium-entropy alloy at room temperature
Cylindrical specimens of CrCoNi alloy with electropolished surfaces were subjected to constant total strain amplitude low cycle fatigue. The alloy exhibited an initial period of cyclic hardening followed by cyclic softening until failure occurred. At the end of hardening stage at the peak of cyclic stress, well-developed persistent slip markings (PSMs) consisting of extrusions and intrusions were associated with thin deformation twins. A sophisticated experimental workflow was designed to extract information from the surface and the bulk of tested material. A combination of SEM, EBSD, ECCI, FIB and HR-STEM was used to study the internal structure and the surface profiles around the deformation twins, which were produced during the initial period of cyclic loading. Furthermore, localized cyclic plastic strain and stress concentrations near deformation twins led not only to early, well-developed PSMs, but also to the activation of TWIP and TRIP plasticity even at low macroscopic stress amplitudes.
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- Award ID(s):
- 1905748
- NSF-PAR ID:
- 10358045
- Date Published:
- Journal Name:
- Scripta materialia
- Volume:
- 202
- ISSN:
- 1359-6462
- Page Range / eLocation ID:
- 113985
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED−9.4%specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens. -
null (Ed.)The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stress-shear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains; all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain.more » « less
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null (Ed.)Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR.more » « less
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Strain-transformable Ti-based alloys are known to display a superior combination of strength, ductility and strain-hardening and attracted considerable interest on recent years. They generally still display, however, a limited yield strength that can be possibly overcome by further precipitation strengthening of the developed systems. In that work, we developed a design strategy to reach a forged dual-phase (α+β) microstructure with TRIP/TWIP properties in a Ti–10V–2Fe–3Al alloy. The results showed an excellent combination of mechanical properties due to the strain-transformable deformed β-matrix. The investigation on the deformation mechanisms in the Ti–10V–2Fe–3Al alloy was accurately performed by means of both in-situ synchrotron XRD, mechanical testing followed by SEM/EBSD mapping and “post mortem” TEM microstructural analyses. Combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects were shown to be intensively activated in the alloy. The particular role of stain-induced martensite α″, acting as a relaxation mechanism at the α∕β interfaces, as well as the strong interactions between mechanical twins and primary α nodules were particularly highlighted.more » « less
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Abstract Propagating deformation bands are observed to accommodate the initial plasticity in an as-extruded Mg–1.5Nd alloy under tension using digital-image-correlation. The propagating bands cause an uncommon plateau in the stress–strain response of the alloy prior to restoring a common decreasing work hardening with further straining. Effects of the deformation banding and underlying plateau in the flow stress on small scale yielding are investigated during low cycle fatigue (LCF) and tension of notched specimens. Alternating formation/disappearance of deformation bands in the gauge section of as-extruded LCF specimens during testing is observed to reduce life compared to annealed specimens exhibiting no instabilities. In contrast, the bands deflect the plastic zone ahead of the notch from the principal plane orthogonal to the applied loading inducing positive effect on toughness of the alloy.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.scriptamat.2021.113985
