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Dissertation
Structure and strain hardening of low stacking fault energy FCC alloys at large strains
Doctor of Philosophy (Ph.D.), Drexel University
1997
DOI:
https://doi.org/10.17918/00000456
Abstract
Structure and strain hardening of [alpha]-brass and two Co-Ni based superalloys, MP35N and AEREX350, were investigated. Constant true strain rate simple compression tests to large strains conducted on annealed, polycrystalline samples of [alpha]-brass, MP35N and AEREX350 alloys, revealed a four-stage hardening response in these materials, distinctly different from that observed in higher SFE fcc materials, such as copper. In each stage, microstructure was studied using optical microscopy and TEM. It was shown that deformation twinning played a decisive role in hardening response of the low SFE materials used in this study. At high strains, some localization in the form of grain-scale shear bands was observed. Although formation of these shear bands had no detectable effect on the macroscopic strain hardening rate, it did correlate with a marked change in texture evolution. Based on observations, a physical description of the microstructural phenomena responsible for the various strain hardening stages observed in low SFE fcc alloys is presented. A significant decrease in the hardening rate of ultra-fine MP35N alloy (1 [mu]m grain size) was observed at moderate to high strains. TEM studies showed that this behavior correlated with the lack of extensive twinning in the structure, confirming the critical role of deformation twinning in hardening response of the alloy of larger grain size. Deformed MP35N alloy could be further strengthened by heat treating at 593[degrees]C for four hours. This secondary hardening phenomenon together with the high temperature hardening response of MP35N was investigated. A model based on segregation of solute atoms to stacking faults was proposed to account for the observations. TEM studies showed that the major strengthening phase in the solution treated and aged AEREX350 alloy was [gamma]' (Ni3Al,Ti) with L12 structure. This phase was stable up to 1000[degrees]C and its growth kinetics followed the t1/3 law. The work hardening rate of the aged AEREX350 alloy in simple compression significantly increased in moderate strains, compared to that of the solution treated alloy. This behavior may be related to the reduction of the stacking fault energy of the aged material caused by precipitation of the [gamma]' phase.
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Details
- Title
- Structure and strain hardening of low stacking fault energy FCC alloys at large strains
- Creators
- Sirous Asgari
- Contributors
- Roger D. Doherty (Advisor)Surya Kalidindi (Advisor)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Number of pages
- xvi, 300 pages
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- Materials (Science and) Engineering (Metallurgical Engineering) [Historical]; College of Engineering (1970-2026); Drexel University
- Other Identifier
- 991014970339604721