Showing 9 results for Kinetic
A. Rasooli, H. R. Shahverdi, M. Divandari, M. A. Boutorabi,
Volume 29, Issue 1 (6-2010)
Abstract
In this research, the reaction kinetics of TiH2 powder in contact with pure aluminum melt at various temperatures on the basis of measuring the released hydrogen gas pressure was studied. To determine the mechanism the reaction, after Solidification of samples, interface of TiH2 powder in contact with melt was studied. The results showed that PH2-time curves had three regions. In the first and second regions, the rate of reaction conforms to zero and first order, respectively. In the third region, hydrogen gas pressure remains constant and the rate of reaction becomes zero order. In the first and second regions, the main factors controlling the rate of reaction are diffusion of hydrogen atoms within titanium lattice and chemical reaction of titanium with aluminum melt, respectively. Based on the main factors controlling the rate of reaction, three temperature ranges can be considered for reaction mechanism, a) 700-750ºC, b) 750-800ºC and c) 800-1000ºC. In the temperature range (a), the reaction is mostly chemical reaction control. In the temperature range (b), the reaction is diffusion and chemical reaction control, and in the temperature range (c), the reaction is mostly diffusion control.
A. Rasooli, M. Divandari, H. R. Shahverdi, M. A. Boutorabi,
Volume 30, Issue 1 (6-2011)
Abstract
In this research, DTA and TGA curves of titanium hydride powder in air with the heating rates of 5, 10, 20, 25, 30ºC/min were drawn, and XRD patterns of titanium hydride powder during heating rate 10ºC/min were prepared. Results showed that hydrogen comes out of titanium hydride in air during seven stages. And, by increasing heating rate, the mechanism of hydrogen emission from titanium hydride is almost fixed. Upon computation of activation energy of these stages, it was revealed that the mechanism does change at different temperatures. According to DTA curve at 10ºC/min, at temperatures lower than 460ºC, the mechanism is controlled by internal diffusion, at temperatures between 460-650ºC, it is controlled by physicochemical process, and at temperatures higher than 650ºC, it is controlled by chemical reaction. By increasing heating rate, the mechanism is changed at higher temperatures.
H. Omidvar, B. Fallah Ghanbary, M. Tamizifar,
Volume 30, Issue 2 (12-2011)
Abstract
In this paper, plastic flow behavior and micro structural evolution of Ti-6Al-4V alloy in temperature range of 750-1050 °C and strain rate range of 0.001-0.1 (S-1) in isotherm compression condition were investigated. The purpose was to estimate activation energy of globularization of lamellar structure and analyze this process kinetically. True Stress-strain curves obtained at the temperatures below 950 °C indicate a limited amount of flow softening imputed to a dynamic recrystallization occuring at about 950 ˚C. In contrast, at higher temperatures, the flow stress increases linearly with plastic strain until at temperatures about 1015°C where flow stress becomes nearly independent of the temperature. By analyzing flow stress data via Zener-Hol-lomon and sellars equation, Q activation energy of dynamic recrystallization was estimated and structural equation of plastic flow was obtained, whixh were comparable to results raeched by other investigators.
S. Mirzaei , A. Jazayeri Gharehbagh,
Volume 31, Issue 1 (6-2012)
Abstract
The soft magnetic nanocrystalline Fe73.5Si13.5B9Cu1Nb3 alloy (FINEMET®) is produced by heat treatment of amorphous precursor. Determining kinetic parameters of amorphous structure transformation to nanocrystalline allows the control of microstructure (e.g. size and volume fraction of nanocrystalline grains) in order to achieve desired soft magnetic properties by optimizing the heat treatment conditions. In this research, the nanocrystallization kinetics of amorphous FINEMET alloy were studied using isoconversional and isokinetic methods under non-isothermal conditions of various heating rates ranging from 5 to 20˚C/min. The changes in the microstructure and magnetic properties of amorphous ribbon during nanocrystallization process were studied using X-ray diffractometry and hysteresisgraph, respectively.
S. Otroj, F. Mohammadi, M.r. Nilforushan,
Volume 33, Issue 1 (7-2014)
Abstract
In this paper, the effect of MgCl2 addition on the kinetics of MA spinel formation was investigated. For this purpose, the stoichiometric mixture of MgCO3 and calcined aluminum was calcined at 1100 °C for 1 hr. Then, the calcined composition was wet-milled and after addition of 6% MgCl2 the compositions were pressed and fired at 1300 and 1500 °C for different times. Spinel phase content was determined using semi-quantitative phase analysis. With regard to Jander's equation, the rate constant was calculated, and the activation energy was obtained from Arrhenius equation. The results showed that the addition of MgCl2 leads to the acceleration of the spinel formation reaction. Besides, 55.71 Kcal/mol as the activation energy was calculated for the composition containing 6 wt.% MgCl2 compared with 93.06 Kcal/mol for the composition without MgCl2.
K. Sheybani, M.h. Abbasi, M. Shamanian ,
Volume 33, Issue 2 (3-2015)
Abstract
in this research, the kinetics of carbothermic reduction of molybdenite in the presence of sodium carbonate was studied. For this purpose, mixed powder of molybdenite, graphite, and sodium carbonate with 1:4:2 mole ratio was investigated using simultaneous thermal analysis (STA) at the heating rates of 10, 15 and 20 0C /min. The results of thermal analysis were evaluated through Friedman, Kissinger, Ozawa and Coats-Redfern methods. The activation energy of reduction reaction was determined 220 kj/mole, and it was found that the reaction was chemically controlled. To study the reaction mechanism, the mixed powder was heated to 400, 800 and 1100 0C in argon atmosphere at the heating rate of 10 0C/min. X- Ray diffraction
of the reaction products and thermodynamic analysis at these temperatures indicated that carbothermic reduction of molybdenite in the presence of sodium carbonate would advance through the formation of intermediate phases, Na2MoO4 and MoO2
B. Pourbahari, H. Mirzadeh, M. Emamy,
Volume 37, Issue 4 (3-2019)
Abstract
Microstructural evolutions during the high-temperature annealing of Mg alloys containing Al and Gd and after the extrusion process were evaluated and compared to those of the AZ61 alloy. It was revealed that during exposure at the elevated temperatures, the presence of (Mg,Al)3Gd phase, in the form of fine and dispersed particles in the matrix after the extrusion process, could be favorable for the inhibition of grain growth. It was also found that the Al2Gd particles could not effectively retard the coarsening of grains. On the other hand, the grain growth of AZ61 alloy was found to be problematic, which was related to the dissolution of the Mg17Al12 intermetallic phase at temperatures higher than 300°C. In the Mg alloys containing both Al and Gd elements, the increased thermal stability was observed, which was ascribed to the rise of the melting temperature. Finally, some abnormal grain growth was observed in the presence of Al2Gd phase, which was attributed to the nonuniform pinning of grain boundaries by this intermetallic compound.
H. Esfahani, M. Rasouli Samar, F. Dabir, A. Abdollahzadeh,
Volume 38, Issue 1 (6-2019)
Abstract
In this study, mechanism and kinetic of formation of boride layer on In-738 superalloy were investigated via diffusion pack cementation method. Boriding was carried out at 900 °C for several short times (5, 15, 45 and 60 min). Phase study by means of X Ray defragtion (XRD) indicated that in addition to Ni3B, other phases such as Cr5B3, AlB2, and W2B were formed at the first period of process, and other compounds such as MoB2, VB, TiB, Ni6Si2B, and Mo2NiB2 were generated in the more prolonged time. SEM study also showed that not only the thickness of boride coating was increased, but also an interdiffusion zone (IDZ) was formed under the coating and it was grown by the upward diffusion of alloy elements. The kinetic study was good according to diffusion theory, confirming the two diffusion steps for IDZ. Thickness and hardness of the boride coating over 60 min process were 27.8 µm and 853 HV, respectively.
M. Soltani, A. Seifoddini, S. Hasani,
Volume 39, Issue 1 (5-2020)
Abstract
In this research, the effect of heating rate on oxidation kinetics of magnesium powder particles under non-isothermal conditions was studied. For this purpose, differential thermal analysis (DTA) and thermogravimetry analysis (TGA) was done on magnesium powder particles at three heating rates of 5, 10 and 20 K min-1 up to 1000 °C under air atmosphere. Also, in order to better understand the oxidation process of magnesium powder, three temperatures were selected according to the DTA curve at a heating rate of 20 K min-1. Then, samples of magnesium powder were heated up to these three temperatures with heating rate of 20 K min-1 and were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) for phase and microstructural analysis. Then, kinetic studies were performed using some isoconversional methods such as Starink and Friedman as well as direct and indirect fitting methods. The activation energy (E) and pre-exponential factor (lnA) for oxidation of magnesium powder were in the range of 327-956 kJ mol-1 and 45-135 min-1, respectively. The reaction models for heating rates of 5, 10 and 20 K min-1 were obtained to be A3/2, R2 and D1, respectively.
