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Momeni A.,Hamedan University of Technology | Abbasi S.M.,Metallic Materials Research Center
Journal of Alloys and Compounds | Year: 2015

The influence of chemical composition on the hot deformation behavior and dynamic recrystallization was studied by conducting hot compression tests on two low alloy steels, AISI 4135 and VCN200, over a temperatures range of 1000-1150 °C and at strain rates of 0.001 s-1 to 1 s-1. The activation energy of dynamic recrystallization in 4135 and VCN200 was determined as 374 kJ/mol and 435.3 kJ/mol, respectively. The different apparent activation energies (about 17%) were attributed to about 30% difference between the carbon equivalents of the steels. The results confirmed that the higher the alloying elements, the higher the peak stress and strain of DRX flow curves. The results were associated with the dragging force of solute atoms on the grain boundaries. A new formula was proposed as a chemical factor to quantify the influence of solute dragging on the dynamic recrystallization behavior. Simple power equations described how the peak stress and strain depends on the Zener-Hollomon parameter. The material constants of the developed equations were related to the proposed composition factor. Unlike to dynamic recrystallization, dynamic recovery did not delayed by solute dragging. © 2014 Elsevier B.V. All rights reserved. Source


Yazdani M.,Sharif University of Technology | Abbasi S.M.,Metallic Materials Research Center | Taheri A.K.,Sharif University of Technology | Momeni A.,Hamedan University of Technology
Transactions of Nonferrous Metals Society of China (English Edition) | Year: 2013

Hot compression tests were carried out on a Fe-29Ni-17Co alloy in the temperature range of 900 °C to 1200 °C and at strain rates of 0.001-1 s-1. Dynamic recrystallization was found responsible for flow softening during hot compression. The flow behavior was successfully analyzed by the hyperbolic sine equation and the corresponding material constants A, n and α were determined. The value of apparent activation energy was determined as 423 kJ/mol. The peak and steady state strains showed simple power-law dependence on the Zener-Hollomon parameter. The dynamic recrystallization kinetics was analyzed using Avrami equation and the corresponding exponent was determined to be about 2.7. This value, higher than 2 reported in the literatures, is associated with the mechanism of continuous dynamic recrystallization in the studied alloy. The flow curve up to the peak was modeled by the Cingara equation and the strain exponent, c, was determined about 0.85. The higher value of c compared with the value of 0.2 which has been reported for some stainless steels fortified the idea of extended dynamic recovery or continuous dynamic recrystallization in the studied alloy. © 2013 The Nonferrous Metals Society of China. Source


Seifollahi M.,Iran University of Science and Technology | Kheirandish S.,Iran University of Science and Technology | Razavi S.H.,Iran University of Science and Technology | Abbasi S.M.,Metallic Materials Research Center | Sahrapour P.,Sharif University of Technology
ISIJ International | Year: 2013

In this paper, the shear deformation behavior of A286 Iron-based superalloy was studied with an emphasis on the influence of η phase on shear strength. The η (Ni3Ti phase precipitates at high temperature heat treatment or during services at the expense of gamma prime phase. According to the microstructural features, no evidences of η phase were found at 650 and 720°C. η phase precipitated at 780 and 840°C and the amount of it increased with an increase the time and temperature. Because of using the alloy as fasteners, investigation of shear properties and the influence of η phase on it are indispensable. The shear strength of the alloy with different volume fractions of η was examined. It was found that, with an increase of η volume fraction, the ultimate shear strength decreases. The shear punch fracture surfaces were also examined by the scanning electron microscopy. The fracture surfaces of sheared samples indicated that low and high volume fraction of η phase result in interior cracks and grain boundary decohesion, respectively. In fact, the fracture of weak grain boundary films (η phase produces this kind of decohesive cracking. © 2013 ISIJ. Source


Seifollahi M.,Iran University of Science and Technology | Razavi S.H.,Iran University of Science and Technology | Kheirandish S.,Iran University of Science and Technology | Abbasi S.M.,Metallic Materials Research Center
Journal of Materials Engineering and Performance | Year: 2013

In this research, the mechanism of eta (η-Ni3Ti) phase precipitation in iron-nickel-based A286 superalloy was assessed during aging heat treatment in the temperature range between 650 and 900 C for the times of 1-30 h. Optical microscopy, scanning electron microscopy, differential thermal analysis, and x-ray diffractometry were used to describe the η phase transformation. The results showed that the major precipitates at temperatures below 840 C were γ′ and η. The η phase started to precipitate at the expense of the γ′ phase after prolonged aging. The η phase existed in the samples aged at temperature higher than 760 C with cellular morphology. The η volume fraction increased with increasing heat treatment time. In addition, when the aging temperature was increased from 760 to 820 C, the η volume fraction increased and then decreased after 840 C. The η phase morphology also changed from cellular to Widmanstätten- type during aging. The time-temperature-precipitation diagrams of these morphologies are presented. The results indicated the differences in precipitation mechanisms of η phase at 840 and 860 C. © 2013 ASM International. Source


Mohammadi Shore F.,Metallic Materials Research Center | Morakabati M.,Metallic Materials Research Center | Abbasi S.M.,Metallic Materials Research Center | Momeni A.,Hamedan University of Technology | Mahdavi R.,Metallic Materials Research Center
ISIJ International | Year: 2014

0.001 s-1-1 s-1 to study the hot ductility of Incoloy 901. Hot ductility of the material was optimized in range of 950°C-1 050°C and descended at either higher or lower temperatures. Dynamic recrystallization was the reason for the improvement of ductility at high temperatures. At lower temperatures, e.g. 850°C, dynamic precipitation of intermetallic phases could effectively inhibit dynamic recrystallization and resulted in poor hot ductility. At very high temperatures, e.g. 1 150°C, the hot ductility drop was due to the decohesion of particles/matrix interfaces. The insensitivity of material to flow localization was understood from the monotonic increase of the strain rate sensitivity over the studied temperature range. The peak strain of the material unexpectedly increased with increasing temperature up to 1 050°C and then decreased at higher temperatures. These results accounted for the possibility of dynamic precipitation of intermetallics at temperatures below 1 050°C and thereby delaying dynamic recrystallization. The hyperbolic sine constitutive equation was used to describe the dependence of tensile stress on deformation temperature and strain rate and the corresponding material constants were determined. The average apparent activation energy for the initiation of dynamic recrystallization was determined as 359 kJ mol-1. © 2014 ISIJ. Source

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