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    High Temperature Mechanical Properties and Microstructures of Thermally Stabilized Fe-Based Alloys Synthesized by Mechanical Alloying Followed by Hot Extrusion
    (Korean Inst Metals Materials, 2021) Kotan, Hasan; Darling, Kris A.; Luckenbaugh, Tom
    The key requirement to consolidate high-energy mechanically alloyed nanocrystalline powders is to achieve densification and particle bonding without impairment in the mechanical properties. Recent demonstrations of consolidation methods involving high shear, pressure and temperature have shown promising results for bonding high strength particulate materials produced by mechanical alloying. In this study, we report the ability of multi-pass high temperature equal channel angular extrusion to produce bulk ferritic alloys from nanocrystalline Fe-Ni-Zr powders. Subsequent microstructural characterizations indicate limited grain growth as the average grain sizes remain smaller than 100 nm after processing temperatures of 600 degrees C and 700 degrees C, above which grains reach micron sizes. The compression test results reveal that the alloys exhibit high mechanical strength at room and moderately high temperatures compared to the pure Fe and Fe-Ni alloys without Zr addition. Graphic
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    Influence of Zr and nano-Y2O3 additions on thermal stability and improved hardness in mechanically alloyed Fe base ferritic alloys
    (Elsevier Science Sa, 2014) Kotan, Hasan; Darling, Kris A.; Scattergood, Ronald O.; Koch, Carl C.
    The motivation of this work was driven to improve the thermal stability in systems where polymorphic transformations can result in an additional driving force, upsetting the expected thermodynamic stability. In this study, Fe92Ni8 alloys with Zr and nano-Y2O3 additions were produced by ball milling and then annealed at high temperatures. Emphasis was placed on understanding the effects of dispersed nano-Y2O3 particle additions and their effect on microstructural stability at and above the bcc-to-fcc transformation occurring at 700 degrees C in Fe-Ni systems. Results reveal that microstructural stability and hardness can be promoted by a combination of Zr and Y2O3 additions, that being mostly effective for stability before and after phase transition, respectively. The mechanical strength of these alloys is achieved by a unique microstructure comprised a ultra-fine grain Fe base matrix, which contains dispersions of both nano-scale in-situ formed Zr base intermetallics and ex-situ added Y2O3 secondary oxide phases. Both of these were found to be essential for a combination of high thermal stability and high mechanical strength properties. (C) 2014 Elsevier B.V. All rights reserved.
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    Isothermal Annealing of a Thermally Stabilized Fe-Based Ferritic Alloy
    (Springer, 2015) Kotan, Hasan; Darling, Kris A.
    In this study, the stability and microstructural evolution, including grain size and hardness of nanocrystalline Fe91Ni8Zr1 alloyed powders, produced by ball milling, were investigated after annealing at 900 and 1000 A degrees C for up to 24 h. Results indicate that rapid grain growth to the micron scale occurs within the first few minutes of exposure to the elevated annealing temperatures. However, despite the loss of nanocrystallinity, an extremely stable and efficient hardening effect persists, which has been found to be equal to that predicted by Hall-Petch strengthening even at the smallest grain sizes. The mechanical properties of the samples consolidated to bulk via equal channel angular extrusion at 900 A degrees C were evaluated by uniaxial compression at room and elevated temperatures. Results reveal high compressive yield stress as well as the appearance and disappearance of a yield drop indicating the presence of coherent (GP zone like) precipitates within the microstructure. Such a hardening mechanism has implications for developing new Fe-Ni-based alloys exhibiting a combination of high strength and ductility for high temperature applications.
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    Phase transformation and grain growth behavior of a nanocrystalline 18/8 stainless steel
    (Elsevier Science Sa, 2017) Kotan, Hasan; Darling, Kris A.
    Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y (at%) stainless steel powders were nanostructured by mechanical alloying from elemental powders and subjected to 90 min annealing treatments at various temperatures. The microstructural evolutions as a function of alloy compositions and temperatures were investigated by in-situ and ex-situ x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The dependence of hardness on the microstructure was utilized to study the mechanical changes. It was found that the resulting microstructures by mechanical alloying were bcc solid solution, the so-called alpha'-martensite structure. The high temperature in-situ x-ray diffraction experiments showed that the martensite-to-austenite reverse phase transformation was completed above 800 and 900 degrees C for Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y steels, respectively. A partial or complete retransformation to martensite was observed upon cooling to room temperature. Annealing of nanocrystalline Fe-18Cr-8Ni steel yielded grain growth reaching to micron sizes at 1100 degrees C while addition of 1 at% yttrium stabilized the microstructure around 160 nm grain size and 6 GPa hardness after 90 min annealing at 1200 degrees C.
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    A study of microstructural evolution of Fe-18Cr-8Ni, Fe-17Cr-12Ni, and Fe-20Cr-25Ni stainless steels after mechanical alloying and annealing
    (Elsevier Science Inc, 2018) Kotan, Hasan; Darling, Kris A.
    In this study, high-energy mechanical alloying technique was used to produce nanocrystalline stainless steels of three different compositions from elemental powders. The microstructural evolution (grain growth and phase transformation) as a function of alloy compositions and annealing temperatures were investigated by room and high temperature x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The results revealed that stainless steels with low nickel content (i.e., Fe-18Cr-8Ni) underwent a deformation-induced martensitic transformation during room temperature mechanical alloying. Deformation induced martensitic transformation with increasing nickel content (i.e., Fe-17Cr-12Ni and Fe-20Cr-25Ni) would not be possible by room temperature milling but was created by high strain rate cryogenic processing, the degree to which was compositional dependent. Post process annealing induced the reverse transformation from martensite-to-austenite the ratio of which was found to be a factor of alloy composition and annealing temperature. The real time in-situ x-ray studies showed that the martensite-to-austenite reverse transformation was completed at around 600 degrees C and 800 degrees C for Fe-18Cr-8Ni and Fe-20Cr-25Ni steels, respectively. Microscopy studies revealed a significant enhancement in the resistance to grain growth for Fe-17Cr-12Ni steel over other compositions at elevated temperatures as high as 1200 degrees C. As such, cryogenic processing following by reverse martensitic transformation of high Ni containing alloys provides a pathway for developing higher heat resistant stainless steel alloys.