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RISS 인기검색어
- 검색키워드
- 저자 : Mahmoud Nili‑Ahmadabadi (검색결과 5 건)
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Shahmir, Hamed,Nili-Ahmadabadi, Mahmoud,Huang, Yi,Jung, Jai Myun,Kim, Hyoung Seop,Langdon, Terence G. Elsevier 2018 Materials science & engineering. properties, micro Vol.734 No.-
<P><B>Abstract</B></P> <P>A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19' phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19' phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition.</P>
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Development and Microstructural Characterization of a New Wrought High Entropy Superalloy
Ahad Shafiee,Mahmoud Nili‑Ahmadabadi,Hyoung Seop Kim,Mohammad Jahazi 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.5
In this study, a newly developed γ′ precipitation hardened high entropy superalloy (designated as HES-A1 hereafter) wasproduced and its microstructural features were characterized. The new alloy composition is based on the major elementsof conventional IN718 alloy, however other elements were added or removed to minimize possible presence of δ (or γʺ)phase and increase the volume fraction of very fine γ′ precipitates instead. Broadening the hot working window, lowerdensity, and avoiding the use of expensive elements were other considerations taken into account in the design of the newalloy. CALPHAD (CALculation of PHAse Diagrams) and PhaComp methods were used for the prediction of phases andtheir evolutions. The microstructure of the HES-A1 alloy in as-cast, homogenized, hot rolled, annealed and aged conditionswere characterized using optical, scanning, and transmission electron microscopes as well as X-ray diffraction technique. The as-cast microstructure of HES-A1 contained 5.5% of Laves phase, which was reduced to less than 0.3% through thedevelopment of a homogenization treatment. Hot rolling with reductions up to 36% at 1015 °C did not produce any cracking,indicating a good forming potential for the new alloy. The application of double aging treatment, similar to the one forIN718 alloy, showed no presence of γʺ or formation of δ phase in the microstructure. HES-A1 has been designed with ahigher Al/(Ti + Nb) ratio and higher proportion of Al + Ti + Nb so that it could be mainly strengthened by γ′ precipitation. The average size of monomodal γ′ precipitates was smaller than that observed in conventional alloys after similar treatments.
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Hadi Ghasemi-Nanesa,Mahmoud Nili-Ahmadabadi,Arya Mirsepasi,Cyrus Zamani 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.2
Severe cold-rolling was applied on solution annealed Fe-Ni-Mn steel with fully lath martensite structure toobtain ultrafine-grained structure. Field emission scanning electron microscopy and high resolution transmissionelectron microscopy (HRTEM) were employed to investigate the microstructural evolution aftersevere cold-rolling. HRTEM images showed the typical deformed structure consisting of lamellar dislocationcell blocks. HRTEM study also revealed strain-induced reverse martensitic transformation (activated duringgrain refinement). It was assumed that severe plastic deformation route and related deformationmode were responsible for microstructural evolutions. X-ray diffraction (XRD) diagram revealed 7%(volume fraction) reverted austenite after final deformation pass. Moreover, HRTEM images revealednano-void nucleation at the interface of severely deformed martensite and reverted austenite presumablydue to high strain energy of misfit and molar volume difference between the austenite and themartensite. It seems that the coalescence of nano-voids could lead to the formation of microvoids in themicrostructure.
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On the control of structural/compositional ratio of coherent order-disorder interfaces
Forghani, Farsad,Han, Jong Chan,Moon, Jongun,Abbaschian, Reza,Park, Chan Gyung,Kim, Hyoung Seop,Nili-Ahmadabadi, Mahmoud Elsevier 2019 JOURNAL OF ALLOYS AND COMPOUNDS Vol.777 No.-
<P><B>Abstract</B></P> <P>Order-disorder coherent interfaces determine the microstructure and mechanical properties of precipitation-hardened high-temperature alloys. The characteristics of these interfaces can be defined by a compositional width, δ, and structural width, δ′. The latter, which can be considered as the width of the ordered part of the interface, can play an important role in high-temperature mechanical behavior of precipitation-hardened alloys. This is due to the fact that diffusion in the ordered part of the interface is generally much slower than diffusion in the disordered phase, thus hindering the solid-state diffusion-based phenomena. Here, we investigate the order-disorder interface in a Ni-19Al (at.%) alloy as a model alloy for Ni-based superalloys using atomic-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. Then, we employ thermodynamic modeling to describe the interplay between the structural and compositional interface widths in binary Ni-Al and in ternary Ni-Al-Cr and Co-Al-W systems. We introduce the δ′/δ ratio as a critical parameter that varies significantly in different alloys. Our findings offer a general pathway to control the δ′/δ ratio of interfaces, which in turn affect the high-temperature properties of precipitation-hardened alloys.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Characterization of the order-disorder coherent interface in a Ni-19Al model alloy. </LI> <LI> Describing the dual nature of order-disorder coherent interfaces. </LI> <LI> Prediction of the structural interface width δ′ based on the thermodynamic modeling. </LI> <LI> Introducing the δ′/δ ratio as a critical parameter that varies in different alloys. </LI> </UL> </P>
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