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Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel

Journal of Materials Science & Technology 2012年第4期28卷 336-342页

作者： Khodamorad Abbaszadeh hassan saghafian Shahram Kheirandish School of Metallurgy and Materials Engineering Iran University of Science and Technology Tehran *** Iran

The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.

关键词： Upper bainite Lower bainite Microstructure D6AC Mechanical properties

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Surface oxidation study of molten Mg–Al alloys by oxide/metal/oxide sandwich method

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Journal of Magnesium and Alloys 2022年第6期10卷 1704-1717页

作者： Mohammad Mahdi Jalilvand hassan saghafian Mehdi Divandari Mehdi Akbarifar School of Materials Science and Engineering Iran University of Science and TechnologyNarmakTehran ***Iran

The dynamic oxidation of molten Mg–Al alloy was investigated via the oxide/metal/oxide(OMO)sandwich *** characteristics of sandwiches were explored using optical microscopy,scanning electron microscopy,X-ray energy dispersive spectroscopy,and X-ray diffraction *** results showed the formation of porous oxide films with varying thicknesses from 0.43 to 16.7 *** the measurements and calculations confirmed the literature findings that the oxidation product consists mainly of MgO and Mg Al_(2)O_(4)*** increase in thickness and amount of folds formed on the oxide films implies the significant effect of aluminum in reducing the oxidation resistance of magnesium.

关键词： OMO sandwich Dynamic oxidation Surface film Oxide morphology Thermodynamics Mg–Al alloys

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Diffusion mechanism in molten salt baths during the production of carbide coatings via thermal reactive diffusion

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International Journal of Minerals,Metallurgy and Materials 2017年第12期24卷 1448-1458页

作者： Aliakbar Ghadi hassan saghafian Mansour Soltanieh Zhi-gang Yang Center of Excellence for High Strength Alloys Technology School of Metallurgy and Materials EngineeringIran University of Science and Technology Key Laboratory of Advanced Materials of Ministry of Education School of Materials Science and EngineeringTsinghua University

The diffusion mechanism of carbide-forming elements from a molten salt bath to a substrate surface was studied in this research, with particular focus on the processes occurring in the molten bath at the time of coating. Metal, oxide, and metal-oxide baths were investigated, and the coating process was performed on H13 steel substrates. Scanning electron microscopy and electron-probe microanalysis were used to study the coated samples and the quenched salt bath. The thickness of the carbide coating layer was 6.5 ± 0.5, 5.2 ± 0.5, or 5.7 ± 0.5 μm depending on whether it was deposited in a metal, oxide, or metal-oxide bath, respectively. The phase distribution of vanadium-rich regions was 63%, 57%, and 74% of the total coating deposited in metal, oxide, and metal-oxide baths, respectively. The results obtained using the metal bath indicated that undissolved suspended metal particles deposited onto the substrate surface. Then, carbon subsequently diffused to the substrate surface and reacted with the metal particles to form the carbides. In the oxide bath, oxide powders dissolved in the bath with or without binding to the oxidative structure(Na_2O) of borax; they were then reduced by aluminum and converted into metal particles. We concluded that, in the metal and oxide baths, the deposition of metal particles onto the sample surface is an important step in the formation of the coating.

关键词： thermal reactive diffusion diffusion mechanism carbide coating films metal particles borax bath

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On the dynamically formed oxide films in molten Mg

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Journal of Magnesium and Alloys 2020年第1期8卷 219-230页

作者： Mohammad Mahdi Jalilvand Mehdi Akbarifar Mehdi Divandari hassan saghafian School of Metallurgy and Materials Engineering Iran University of Science and Technology(IUST)NarmakTehran16846-13114Iran

The so-called“Oxide/Metal/Oxide sandwich”method is one of the technique used to investigate the dynamic oxidation of metals which happens during the casting *** this study,characteristics of the oxide films formed on the molten magnesium in dynamic conditions have been investigated using the aforementioned *** air bubbles were released into the cast sample at the pressure of 0.2 atm through a quartz tube of 1 mm internal *** interface of two adjacent entrapped bubbles is considered as the Oxide/Metal/Oxide(OMO)*** sandwiches were characterized by the aid of the optical and scanning electron microscopy and also X-Ray diffraction *** different approaches,including measuring the width of the folds formed on the oxide films and the edge of the sandwiches,were used to estimate the thickness of the *** thicknesses were estimated to be in the range of 200 to 800 *** features such as fold,wrinkle,and crack were observed on the OMO *** the microscopic scale,the oxide films were rough and *** is attributed to the non-protective behavior of oxide *** XRD patterns indicated that the oxide films formed on the pure molten magnesium in dynamic conditions are crystallized MgO.

关键词： Dynamic oxidation Bifilm Magnesium Oxide films Casting defects

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Effect of melt-to-solid volume ratio and preheating temperature on Mg/Al bimetals interface by centrifugal casting

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China Foundry 2023年第3期20卷 234-240页

作者： Morteza Sarvari Mehdi Divandari hassan Saghafan Sina Ghaemi Khiavi School of Metallurgy and Materials Engineering Iran University of Science and TechnologyNarmakTehran 16844Iran

Compound casting is an efficient method for bonding dissimilar metals,in which a dramatic reaction can occur between the melt and *** centrifugal casting process,a type of compound casting,was applied to cast Al/Mg dissimilar *** melt was poured at 700 °C,with melt-to-solid volume ratios(Vm/Vs) of 1.5 and 3,into a preheated hollow aluminum *** preheating temperatures of the solid part were 320,400,and 450 °C,and the constant rotational speed was 1,600 *** cast parts were kept inside the casting machine until reaching the cooling temperature of 150 °*** result showed that an increase in preheating temperature from 320 to 450 °C led to an enhanced reaction layer *** addition,an increase in the Vm/Vs from 1.5 to 3 resulted in raising the interface thickness from 1.2 to 1.8 ***,the interface was not continuously formed when a Vm/Vs of 3 was *** this case,the force of contraction overcame the resultant acting force on the *** interface formed at the volume ratio of 1.5 was examined using scanning electron microscopy(SEM) equipped with energy-dispersive X-ray spectroscopy(EDS),and the results demonstrated the formation of Al_(3)Mg_(2),Al_(12)Mg_(17) and(δ+Al_(12)Mg_(17)) eutectic structures in the interface.

关键词： compound casting centrifugal casting Mg/Al bimetal preheating temperature melt-to-solid volume ratio interface

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Hydrological alteration and biodiversity change along the river network caused by anthropogenic activities and climate variability

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Ecological Processes 2022年第1期11卷 189-205页

作者： Farshid Zolfagharpour Bahram saghafian Majid Delavar Department of Civil Engineering Science and Research BranchIslamic Azad UniversityTehranIran Department of Water Resources Engineering Tarbiat Modares UniversityTehranIran

Background:Population growth and intensified human activities in conjunction with climate variability continue to affect the hydrologic cycle,aquatic and terrestrial flora and *** this regard,understanding interactions among ecosystem functions,impacts of anthropogenic interventions and those of climate variability is vital for projecting future ecosystem responses to human activities and climate *** objectives of this study are to determine the ecological flow state via eco-flow index based on discharge hydrograph,to model the ecological diversity through the Shannon diversity index,and to assess the degree of hydrologic alteration using indicators of hydrologic alteration and range of variability approach in six hydrometric stations along the Zayandeh-Rud River in central *** river drains into Gavkhuni ***,the streamflow-induced potential changes for Capoeta damascina(a cyprinid fish species of the genus Capoeta),Petroleuciscus esfahani(a small cyprinid fish)and Aphanius isfahanensis(a Cyprinodontid fish)are *** outcome is expected to assist managers with understanding the effects of anthropogenic activities and climate variability on Gavkhuni aquatic ecosystems so that management options that enhance species resilience and adaptability are ***:Human activities,a primary factor influencing the natural flow regime,caused a significant increase in the minimum flow,July to March streamflow,low pulse number,and the number of reversals in most studied *** the contrary,some other hydrologic indices declined in *** impoundment,the most prominent factor among human interventions,resulted in an overall alteration degree of 74.8%in *** variability impacted the natural flow regime in the range of low degree hydrologic alteration(27.3%).In addition,the biodiversity of the study basin,as modeled by the Shannon diversity index,had strong relevance to the annual eco-surplus and was more sensitive

关键词： Eco-flow Shannon diversity index Human activity Indicators of hydrologic alteration Natural flow regime Range of variability Zayandeh-Rud River

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Skill Assessment of Copernicus Climate Change Service Seasonal Ensemble Precipitation Forecasts over Iran

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Advances in Atmospheric Sciences 2021年第3期38卷 504-521页

作者： Masoud NOBAKHT Bahram saghafian Saleh AMINYAVARI Department of Civil Engineering Science and Research BranchIslamic Azad UniversityTehran ***Iran Department of Civil Engineering Chalous BranchIslamic Azad UniversityChalous 46615/397Iran

Medium to long-term precipitation forecasting plays a pivotal role in water resource management and development of warning ***,the Copernicus Climate Change Service(C3S)database has been releasing monthly forecasts for lead times of up to three months for public *** study evaluated the ensemble forecasts of three C3S models over the period 1993-2017 in Iran’s eight classified precipitation clusters for one-to three-month lead *** and non-probabilistic criteria were used for ***,the skill of selected models was analyzed in dry and wet periods in different precipitation *** results indicated that the models performed best in western precipitation clusters,while in the northern humid cluster the models had negative skill *** models were better at forecasting upper-tercile events in dry seasons and lower-tercile events in wet ***,with increasing lead time,the forecast skill of the models *** terms of forecasting in dry and wet years,the forecasts of the models were generally close to observations,albeit they underestimated several severe dry periods and overestimated a few wet ***,the multi-model forecasts generated via multivariate regression of the forecasts of the three models yielded better results compared with those of individual *** general,the ECMWF and UKMO models were found to be appropriate for one-month-ahead precipitation forecasting in most clusters of *** the clusters considered in Iran and for the long-range system versions considered,the Météo France model had lower skill than the other models.

关键词： ensemble forecasts Copernicus Climate Change Service long-term forecasting model evaluation Iran

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Dry Sliding Oxidative Wear in Plain Carbon Dual Phase Steel

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Journal of Iron and Steel Research International 2007年第4期14卷 43-48页

作者： V Abouei H saghafian Sh Kheirandish Department of Metallurgy and Materials Engineering Iran University of Science and TechnologyNarmak 16844TehranIran

To investigate the tribological potential of the dual phase （DP） steel as a wear resistant material, the wear and the friction characteristics of this steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, have been investigated and compared with those observed in plain carbon hardened （H） steel that has the same carbon content of 0.2%. Dry sliding wear tests have been carried out using a pin-on-disk wear testing machine at different normal loads of 21.3 N, 28. 5 N, 35.7 N, and 42.6 N and at a constant sliding velocity of 1.20 m/s. The analysis of surface and wear debris of samples showed that the wear mechanism was mainly mild oxidative. The friction and the wear rate of the H steel and the DP Steel have been explained with respect to the microstructure and the wear mechanism.

关键词： dual phase steel wear microstructure heat treatment

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Evaluation of TIGGE Ensemble Forecasts of Precipitation in Distinct Climate Regions in Iran

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Advances in Atmospheric Sciences 2018年第4期35卷 457-468页

作者： Saleh AMINYAVARI Bahram saghafian Majid DELAVAR Department of Technical and Engineering Science and Research Branch Islamic Azad University Department of Water Resources Engineering Tarbiat Modares University

The application of numerical weather prediction （NWP） products is increasing dramatically. Existing reports indicate that ensemble predictions have better skill than deterministic forecasts. In this study, numerical ensemble precipitation forecasts in the TIGGE database were evaluated using deterministic, dichotomous （yes/no）, and probabilistic techniques over Iran for the period 2008-16. Thirteen rain gauges spread over eight homogeneous precipitation regimes were selected for evaluation. The Inverse Distance Weighting and Kriging methods were adopted for interpolation of the prediction values, downscaled to the stations at lead times of one to three days. To enhance the forecast quality, NWP values were post-processed via Bayesian Model Averaging. The results showed that ECMWF had better scores than other products. However, products of all centers underestimated precipitation in high precipitation regions while overestimating precipitation in other regions. This points to a systematic bias in forecasts and demands application of bias correction techniques. Based on dichotomous evaluation, NCEP did better at most stations, although all centers overpredicted the number of precipitation events. Compared to those of ECMWF and NCER UKMO yielded higher scores in mountainous regions, but performed poorly at other selected stations. Furthermore, the evaluations showed that all centers had better skill in wet than in dry seasons. The quality of post-processed predictions was better than those of the raw predictions. In conclusion, the accuracy of the NWP predictions made by the selected centers could be classified as medium over Iran, while post-processing of predictions is recommended to improve the quality.

关键词： ensemble forecast NWP TIGGE evaluation post-processing

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Effects of Silicon Content and Intercritical Annealing on Manganese Partitioning in Dual Phase Steels

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Journal of Iron and Steel Research International 2010年第5期17卷 44-50页

作者： A Nouri H saghafian Sh Kheirandish Department of Metallurgy and Materials Engineering Iran University of Science and Technology

Steels of constant manganese and carbon contents with silicon content of 0.34 %- 2.26% were cast. The as-cast steels were then hot rolled at 1 100 ℃ in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm in thickness. Dual phase microstructures with different volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition to intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual phase steels. The partitioning of manganese in dual phase silicon steels was investigated using energy-dispersive spectrometer （EDS）. The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritieal annealing temperature and silicon content of steels. It was also found that the solubility of manganese in ferrite and austenite decreased with increasing intereritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite existed in dual phase silicon steels.

关键词： partitioning dual phase steel intercritical annealing temperature silicon manganese

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