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

Microstructural changes and mechanical properties of IN617 superalloy aged at 900 °C for different durations from one hour to 2000 hours were investigated in the present work. The optical microscope (OM), scanning electron microscope (SEM), transition electron microscope (TEM), X-ray diffraction (XRD) and hardness and tensile tests were used to investigate the microstructure and mechanical properties of aged alloys. A significant amount of intergranular carbides were observed in the microstructure of aged alloys even in the microstructure of alloy which was aged for one hour. Block-shape carbides were observed in the sample which was aged for one hour. It was observed that with increasing the aging time the morphology of the carbides changed to quasi-spherical, plate and rod shaped. The carbides were first formed along the grain and twin boundaries and then within the grains, and continues carbide layer was observed along the grain boundaries for the sample which was aged for 2000 hours. Most of the carbides were M23C6 and a small percentage of them were determined to be M6C. Furthermore, it was observed that a small amount of Ti(C,N) phase which was present in the as received sample was converted to carbides after aging for 1500 hours. γ' phase was only observed in the microstructure of sample which was aged for one hour. Mechanical test results shown that the hardness, ultimate strength at room temperature and at 750°C increased with increasing the aging time, but after 2000 hours of aging these properties decreased to the values of as received sample. The impact energy of the sample which was aged for 2000 hours was equivalent to 25% of as received sample due to the formation of a continues carbide layer along the grain boundaries. The fracture surface of the impact samples were investigated and it was observed that fracture mode changed from ductile for as-received sample to brittle intergranular fracture for the samples which were aged for more than 100 hours.

The aim of this study was to evaluate the effect of aging on the stress rupture and hot pressure of novel cobalt base superalloy of Co-7Al-7W-4Ti-2Ta composition. After melting the superalloy by VIM method and remelting by VAR method, the ingot was homogenized at 1250 °C for 10 hours. Then, aging treatment was performed on homogenized samples at 800 °C for 8, 16 and 24 hours. After microstructural and phase analysis by SEM and X-ray diffraction pattern testing, the high temperature mechanical properties including hot pressure and stress rupture tests were performed. The results showed that the alloy in the cast condition contains ɣ, ɣ‘, a and β phases in which the ɣ‘, a and β phases are dissolved in ɣ phase by homogenization treatment. By aging of the studied alloy, ɣ‘ precipitates nucleate and grow, where in by increasing aging time the volume fraction and size of these precipitates increases. The results of pressure tests in the temperature range of 500 °C to 1000 °C showed that the strength of the alloy increased at a temperature of 700 °C, which indicates the occurrence of the stress anomaly phenomenon as a consequence of ɣ‘precipitation. Besides, the highest resistance to stress rupture at 770 °C under 300 MPa for 22 hours was achieved for the specimen aged at 800 °C for 16 hours.

Aging of a sheet explosive contain RDX explosive and HTPB binder is studied at temperatures of 50, 60 and 70 oC in times 60, 120 and 180 days. Dynamic-mechanical analysis (DMA), sol – gel analysis (SGA) and hardness measurements methods are used in this research.  The changes of tan_delta in DMA, hardness amounts and soluble fraction in SGA are measured for unaged and aged samples. The obtained results showed the samples are aged with increasing of temperature and time of aging process. The pseudo-first order kinetic equation and obtained data are used for calculation of kinetic rate constants of measured properties changes. The changes in tan_delta in DMA, hardness amounts and soluble fraction in SGA indicated kinetic rate constants, respectively, 0.9-2.3´10-3, 1.9-4.8´10-3 and 1.5-5.5´10-3 per day at temperatures of 50, 60 and 70 oC. The Arrhenius equation is used for calculation of activation energy of each changes and calculation of kinetic rate constants at temperature 25 oC. Kinetic rate constant at room temperature showed a reduction 50% in measured properties of the product after 3300 days (~9 years).

In this article aging behavior of composite solid propellant was investigated by determining its mechanical properties at elevated temperatures. For this purpose, three HTPB samples with different microstructures were selected and composite solid propellants were prepared based on them keeping similar other components and aging was studied by accelerated method. Mechanical properties were measured after accelerated aging using dynamic mechanical thermal analysis (DMTA) in order to investigate the effect of HTPB microstructure on composite solid propellant aging. Results showed that with increasing molecular weight, hydroxyl value, increasing vinyl and decreasing cis contents in resin microstructure, the mechanical behavior of the propellant becomes more rigid, which may be due to limiting the polymer chain mobility or facilitating post-curing crosslinking. Over aging time, the shelf life changed effectively with propellant binder microstructure. The obtained shelf life for the composite solid propellants based on D01, D02 and D03 resin samples were 6, 4.48 and 3.56 years. The results showed significant differences and show well the affect of the micro-structural properties of HTPB resin on the aging of its-based composite solid propellant. 

In the present study, cast IN718 superalloys solutionized at 1050◦C for 150 min. Afterwards, the samples were quenched with three various cooling rates including 240◦C/s, 5◦C/s and 0.1◦C/s. After solutionizing, the samples were aged at 750◦C for 30, 60, and 90 hrs. Microstructural investigation was carried out by optical microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM), and X-ray diffraction (XRD). The results indicated that as the cooling rate increased, the conditions for complete dissolution were provided and only a small amount of MC carbide (TiC) phase remains in the matrix. By reducing the cooling rate and increasing aging time, due to the increase in volume fraction and size of secondary precipitates, the shear transformation of the γ'' to δ increases, and consequently, the δ precipitates increased. After solutionizing, as the cooling rate increased, the hardness due to the decrease in the amount of cooling precipitates decreased. Besides, as the aging time increases, due to the abnormal growth of precipitates, hardness first increased and then experienced a decrease.

In this study, the effect of various heat treatment conditions on the microstructure, hardness and wear resistance of the Ti-6Al-4V alloy was investigated. For this purpose, solution anneal was done in temperature range of 800C to 1050C for 1 hour and then specimens were quenched in the air and the water. One of the specimens was aged in 550C for 4 hours. Microstructural investigations by optical microscope (OM) and scanning electron microscope (SEM) showed that the solution annealing treatment at 800C leads to limited growth of precipitates without changing the microstructure and increase of hardness compared to the control sample. Also, increasing the solution annealing temperature and quenching in water causes increase of hardness and creation of and acicular martensite dual microstructure; Solution annealing in 1050C leads to formation of martensite phase that changes to irregular phase by performing aging treatment and due to this phase transformation hardness increased by 47% compared to control sample. Wear test of pin-on-disk was performed under three forces of 20N, 30N and 40N and constant speed of 0.3m/s. Wear test results and microstructural investigation of worn surfaces showed that wear mechanism was controlled by microstructure and wear resistance is not dependent on hardness; Also, the highest wear resistance is related to the sample annealed in 950C and quenched in water. Wear mechanism in all samples in most of the loads was a mixture of abrasive mechanism and oxide wear.