Papers by Keyword: Lamellar Microstructure

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Authors: Alice Chlupová, Milan Heczko, Karel Obrtlík, Přemysl Beran, Tomáš Kruml
Abstract: Two γ-based TiAl alloys with 7 at.% of Nb, alloyed with 2 at.% Mo and 0.5 at.% C, were studied. A heat treatment leading to very fine lamellar microstructure was applied on both alloys. Microstructure after the heat treatment was described and mechanical properties including fatigue behaviour were measured. The as-received material alloyed with C possesses high strength and very limited ductility, especially at RT. After application of selected heat treatment it becomes even more brittle; therefore, this process could be considered as not appropriate for this alloy. On the contrary, in the case of Mo alloyed material, both strength and ductility are improved by the heat treatment at RT and usual working temperature (~750 °C). Presence of the β phase is responsible for this effect. The selected heat treatment thus can be an alternative for this alloy to other thermomechanical treatments as high temperature forging.
Authors: Seong Woong Kim, Ho Nyun Lee, Myung Hoon Oh, Masaharu Yamaguchi, Dang Moon Wee
Authors: Jae Do Kwon, Yong Tak Bae, Sung Jong Choi, Young Suck Chai, Hitoshi Ishii
Abstract: Fretting is a potential degradation mechanism of structural components and equipments exposed to various environments and loading conditions. It is well known that the fatigue life under fretting condition decreases approximately 50-70% compared with that under non-fretting fatigue condition. The specific gravity of titanium alloy is 4.5 which is lighter than steel, however, its specific strength, heat and corrosion resistance are superior to steel. Ti-6Al-4V alloy is a kind of a+b phase titanium alloy, and mechanical properties are changed by alloy elements, shapes and distributions of microstructures. In this study, three different kinds of specimens are prepared under different heat treatments in order to produce different microstructures. Through various kinds of mechanical tests, the following conclusions are observed: 1) The microstructures are observed as equiaxed, bimodal and lamellar microstructures respectively. 2) The elongation percentage is superior for the equiaxed microstructure, and the hardness and tensile strength are superior for the lamellar microstructure. 3) The plain fatigue limit of lamellar structure shows higher value than that of the equiaxed and bimodal structures. 4) The fretting fatigue limit considerably decreases compared with the plain fatigue limit for all materials. 5) The fretting damage of contact surface increases with an increase of cyclic loading amplitude under the constant contact pressure.
Authors: Masuo Hagiwara, A. Araoka, Satoshi Emura
Abstract: The effect of the lamellar morphology on the high cycle fatigue (HCF) and low cycle fatigue (LCF) behavior of the Ti-22Al-27Nb alloy was investigated. The HCF tests were performed in air at an R ratio of 0.1 in the load-control mode, whereas the LCF tests were performed in vacuum at 923 K in the strain-controlled mode. The specimens with fine lamellar microstructure exhibited a better resistance to HCF than those with coarse lamellar microstructure. The microstructure-insensitive behavior was, however, observed in the LCF tests at 923 K. The fatigue mechanism was discussed based on the concurrent observation of the initiation facet and the underlying microstructure, and the TEM observations.
Authors: Koji Hagihara, Haruka Araki, Takaaki Ikenishi, Takayoshi Nakano
Abstract: The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11b lamellar-structured (Mo0.85Nb0.15)Si2 silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11b duplex-phase crystals. The variant-1-type C11b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.
Authors: Ayad Omran Abdalla, Astuty Amrin, Roslina Mohammad, M.A. Azmah Hanim
Abstract: Recently, iron (Fe) is introduced to substitute vanadium (V) in Ti-alloy. Therefore, new (α+β) titanium alloy, Ti-6Al-1Fe was designed through a complete replacement of V by Fe with major composition modifications of Ti-6Al-4V. This new alloy is believed could provide similar properties of Ti-6Al-4V through modification of its microstructures. Different heat treatments can lead to a diversity of microstructural permutations and combinations. Thus, it is very crucial to study in-depth understanding about the microstructure of Ti-6Al-1Fe. Results reveal that the microstructure of as-received alloy is a typical fine lamellar microstructure. The bi-modal microstructure can be obtained by hot rolling below beta-transus temperature (Tβ) followed by recrystallization treatment at 925°C. While cold rolling followed by recrystallization treatment at 925°C produce equiaxed microstructure.
Authors: Ayad Omran Abdalla, Astuty Amrin, Sallehuddin Muhammad, Mohd Ariff Azmah Hanim
Abstract: Many Ti-alloys were designed by introducing iron (Fe) as an alloying element to improve the mechanical properties and reduce the cost of the alloys. Therefore, new (α+β) titanium alloys, Ti-6Al-(1-3)Fe were developed through complete replacement of vanadium (V) by iron with major composition modifications of Ti–6Al–4V, which is commonly used for aerospace applications. Ti-Al-Fe alloys were melted through vacuum arc melting technique followed by hot rolling. This study aims to investigate the effect of Fe addition on the microstructure and hardness of emerging (Ti-Al-Fe) alloys in comparison with Ti-6Al-4V alloy. Results reveal that the microstructures are typical lamellar structures, and the hardness ranges from 32 to 40.7 HRC. The hardness of the investigated alloys increases with increasing Fe content.
Authors: Chun Yu Teng, Yun Fu, Zhan Yong Ren, Yong Hong Li, Yun Wang, Wen Li Ouyang
Abstract: The details of the lamellar microstructure in TiAl intermetallic alloys, such as the lath thickness and interfaces type governs the strength, ductility, creep properties and the long term microstructure stability of the alloy. The lamellar microstructure coarsening may induce property degradation of materials when the working temperature is high especially for the aero-engine turbine blades. At the same time, the reliability of the structure will decreases dramatically during long term working. In order to customize highly stable microstructure in high temperature, the phenomenon of lamellar formation during the solid-solid α→α2+γ phase transformation in fully lamellar TiAl alloys was investigated by phase field simulations. The lamellar structure morphology obtained with simulation is coincides with the experimental results. It is found that the independent nuclei and twin-related nuclei co-exist in the nucleation stage for the random noise nucleation. During growth stage, the independent nuclei grow slowly or disappear for the interfacial energy and elastic energy minimization. While most twin-related nuclei survived. During the following coarsening stage, big nuclei swallow small nuclei for interfacial energy minimization. The statistical character of twin area fraction changes complicated during these processes and it will be analyzed in detail. These findings could shed light on the understanding of the lamellar formation and coarsening mechanisms during phase transformation in TiAl alloys.
Authors: Wei Dong Zeng, Yi Gang Zhou, Jian Hua Zhou, Xiao Ying Wang
Abstract: The microstructural changes of alpha morphology in Ti-17 alloy with an initial lamellar microstructure were examined quantitatively using image analysis software. The alpha morphology changes at different locations (center, edge, 0.5R) of the cakes deformed to different strain level were measured by Feret ratio and related to the imposed strain estimated using finite element analysis. It was found that the modification of alpha morphology depended strongly on the forging strains and the locations in the cakes. The distribution curve of Feret ratio was characterized by a single peak at Feret ratio between 1.5-2.5. These peak values increased with the increase in the height reductions. Increasing forging strains caused the high-Feret-ratio lamellae to decrease but low-Feret-ratio alpha to increase noticeably. The average strains for initiation of globalization were of the order of 0.4 and those for completion were of the order of 1.0, which was lower than conventional α+β titanium alloys.
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