فهرست مطالب y. mazaheri

Commercial pure titanium sheets were thermo-mechanically treated to investigate the effects of the treatment variables on the microstructure, mechanical properties, and recrystallization behavior. The as-received sheets were initially cold-rolled up to different reduction percentages of 60%, 75%, and 90%. Then, the cold-rolled samples were annealed at different temperatures of 500°C-700°C, for various time ranges of 5 to 60 minutes. The evolution of the microstructure of the samples was studied using X-ray diffraction analysis and optical microscopy. The hardness of the 90% cold-rolled sample was about 82% higher than that of the as-received sheet. Increasing the time and temperature of the annealing process caused a decrease in the microhardness values of the samples. The recrystallization activation energy and Avrami’s exponent of the 90% cold-rolled sample were calculated as about 179 kJ/mol and 0.75, respectively. The results of the uniaxial tensile tests revealed that the cold-rolling process significantly improved the yield strength (YS) and ultimate tensile strength (UTS) of the specimens. In the case of the 90% cold-rolled sample, these values improved by about 160% and 117% with respect to the as-received metal, respectively. However, the elongation of the cold-rolled samples dropped sharply. Moreover, annealing had a positive effect on the elongation of the cold-rolled samples. The UTS and elongation percentage of the as-received sheet were 415 MPa and 36.4%, respectively. These values were varied to 558 MPa and 29.15% for the 90% cold-rolled sample annealed at 700°C for 1 h. To study the fracture behavior of the different samples, scanning electron microscopy (SEM) was used.

Friction stir processing (FSP) in different pass number, accordingly one and four, was performed to AZ31 magnesium alloy. Optical and scanning electron microscopy (SEM) were used to investigate the effect of FSP and its pass number on the microstructure of FSPed samples. The hardness of thesamples was measured using microhardness measurement. Furthermore, wear behaviors of the samples, including wear rate and friction coefficient, were investigated using a reciprocal wear machine. To deduce the wear mechanism, SEM observations of the worn surface were carried out. Optical microscopy of FSPed samples showed grain refinement in the stir zone. Increasing FSP passnumber had a considerable effect on grain refinement. The average grain size of the as-received AZ31 base metal reduced from 11µm to about 4µm after four passes. Microhardness evaluations showed a substantial improvement by increasing FSP pass number, about 70% improvement. Wear tests results revealed enhanced tribological in FSPed samples. SEM observations of the worn surfaces indicated that the abrasion was the dominant wear mechanism governed in the samples.

In the current study, the recrystallization behavior of 75% cold-rolled Fe-22Mn-10Al-1.4C steel during annealing at 750, 770, 790, 810, and 830°C was studied. X-ray diffraction patterns and optical microscopy were used to characterize microstructures. The Vickers Micro-hardness test was used to characterize recrystallization kinetics during annealing. Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to evaluate the experimental data. The as-homogenized microstructure illustrated only austenite with a high fraction of annealing twins, and austenite to martensite phase transformation was not observed after quenching at a high temperature and also until high thickness reduction. Avrami exponent was decreased from 0.76 to 0.42, with increasing the annealing temperature from 750 to 830°C. The activation energy value was determined to be ~175 kJ/mol, which was slightly higher than the diffusion activation energy of carbon in austenite.