جستجوی مقالات مرتبط با کلیدواژه "نرخ گرمایش" در نشریات گروه "مواد و متالورژی"

In this paper, effect of solution heat treatment media of IN738LC superalloy on the microstructure and hardness was investigated. Two solution heat treatment medias were selected, solubilizing in the normal or conventional furnace, and solubilizing in the salt base. For this purpose, the samples were solubilized in a temperature range of 1090-1200°C for 20-30 minutes in normal condition and 10-12 minutes in the salt bath, and then aged at 248°C in 850°C. The results of the scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) showed that increasing the heating rate improves the solution process, and with increasing the temperature and time, the volume fraction of 'γ precipitates decreased by 26% in normal condition and 29% in salt bath. The size of the residual 'γ precipitates after solution in the salt bath is smaller than normal state, and at high temperatures, due to the activation of the aggregation mechanisms, the size and volume fraction of residual precipitates in the salt bath increased compared to the normal condition. Solution in the salt bath reduced the size of the primary'γ compared to the normal condition and increased the volume fraction of the secondary 'γ after the aging. After aging, the total volume fraction of precipitates increased by about 30% and 40%, respectively, in the normal condition and in salt bath. The results showed that the secondary 'γ precipitates are about 16nm smaller than usual and the hardness of the alloy is strongly influenced by the characteristics of 'γ precipitates and the variables of the solution treatment. Increasing the heating rate at the solution resulted in a further increase in hardness after aging and a further reduction in hardness after solution, and in both conditions increasing temperature and time of solution reduced the hardness of the samples after solution and increased after aging.

The paper investigates influence of mechanical activation and heating rate (20°C/min and 40°C/min) on reactions occurrence in combustion synthesis process of nanostructured NiAl-Al2O3 composites by heating of Ni, NiO and Al powder mixture from 20°C to 1300°C. To study reactions occurrence, differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized. By heating the sample without mechanical activation, exothermic reaction of NiO reduction by Al and Ni-Al intermetallic phases production happened in the presence of molten Al; while mechanical activation caused occurrence of these reactions at lower temperatures and separately. By decrement in heating rate, reactions happened separately at lower temperatures. A mechanism for reactions occurrence during heating of mechanically activated sample is proposed.

Potassium perchlorate is an important energetic material. In this study, the thermal properties of this material were investigated by differential thermal analysis (DTA). The results show that the transition phase and the melting point of KClO4 occur at 305 and 588°C respectively and KClO4 decomposes at a higher temperature the activation energy of KClO4 decomposition was measured by Kissinger type isoconversional method. Activation energy obtains 215kj/mol that has a good agreement with that measured via other methods. It was seen that at a high heating rate of 15°C/min critical temperatures decreased with particle size from nearly 300μm and reached equilibrium temperatures when the particle size was~ 50nm.