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- 저자 : Shahram Raygan (검색결과 2 건)
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Pegah Dalvand,Shahram Raygan,Gabriel A. López,Mariano B. Meléndez,Volodymyr A. Chernenko 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.9
In this study, the effect of isothermal aging on the thermoelastic martensitic transformation and microstructure of the asquenchedCu–12Al–3.5Ni–0.7Ti–0.05RE (RE = Ce, La) high temperature shape memory alloy was studied. The resultsshowed that the alloy microstructure and martensitic transformation are strongly influenced by the aging temperature ratherthan aging time. During aging at 350 °C the alloy was prone to both the precipitation of the γ2 phase and the bainitic transformation,resulting in a loss of martensitic transformation and damping capacity. The prolonged aging at 350 °C caused adecomposition of parent phase into the equilibrium γ2 phase alongside the α phase which produced a significant hardnessincrement. On the other hand, aging at 250 °C affected the microstructure only slightly producing insignificant shift in thetransformation temperatures. It was found that, the secondary phases including Ti-rich X-phase and the RE-rich phase werenot influenced by the aging process. The results prescribe a high temperature order of the stability of martensitic transformationfor this new alloy which is important for its high temperature shape memory applications.
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Hadi Torkamani,Shahram Raygan,Carlos Garcia Mateo,Jafar Rassizadehghani,Yahya Palizdar,David San-Martin 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.4
In this research Rare Earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel todisclose their infl uence on the microstructure and impact toughness. It is suggested that RE are able to change the interactionbetween the inclusions and matrix during the Solidification process (comprising peritectic transformation), which couldaff ect the microstructural features and consequently the impact property; compared to the base steel a clear evolution wasobserved in nature and morphology of the inclusions present in the RE-added steel i.e. (1) they changed from MnS-based to(RE,Al)(S,O) and RE(S)-based; (2) they obtained an aspect ratio closer to 1 with a lower area fraction as well as a smalleraverage size. Besides, the microstructural examination of the matrix phases showed that a bimodal type of ferrite grain sizedistribution exists in both base and RE-added steels, while the mean ferrite grain size was reduced from 12 to 7 μm andthe bimodality was redressed in the RE-added steel. It was found that pearlite nodule size decreases from 9 to 6 μm in theRE-added steel; however, microalloying with RE caused only a slight decrease in pearlite volume fraction. After detailedfractography analyses, it was found that, compared to the based steel, the signifi cant enhancement of the impact toughnessin RE-added steel (from 63 to 100 J) can be mainly attributed to the Differences observed in the nature of the inclusions, theferrite grain size distribution, and the pearlite nodule size. The presence of carbides (cementite) at ferrite grain boundariesand probable change in distribution of Nb-nanoprecipitation (promoted by RE addition) can be considered as other reasonsaff ecting the impact toughness of steels under investigation.
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