چكيده فارسي :
In this project, the production of cobalt-based alloy powder according to ASTM F1357 was investigated using the mechanical alloying method. The powder materials used included pure cobalt, chromium, molybdenum, manganese, silicon, and silicon nitride in an Ar atmosphere. The amount of silicon nitride (Si3N4) used in this alloy is 0.425 wt.%. The purpose of adding silicon nitride is to provide the required nitrogen content in the chemical composition. For milling purposes, a dedicated planetary milling stainless-steel vial and steel balls were used. Initially, these powders were mixed together and placed inside the milling chamber along with the steel balls. In each milling operation, a total of 200 grams of steel balls and 10 grams of the powder mixture (with a specific weight percentage of each component, with a ball-to-powder weight ratio of 20:1) were poured into the vial. To investigate the effect of milling time on the powder properties, milling was conducted for durations of 12, 24, 30, 36, 42, and 48 hours at a speed of 500 rotations per minute using a planetary mill. Scanning Electron Microscopy (SEM) was used to evaluate morphology and particle size, X-ray Diffraction (XRD) for phase analysis, Transmission Electron Microscopy (TEM) for examining the microstructure, and microhardness testing for measuring the hardness of the milled powders. SEM results showed that initially, at time intervals of 12 hours, 24 hours, and 30 hours, the particle size of the powder increased with longer milling time, and irregular and asymmetric particles with a layered structure were observed. At this stage of milling, smaller particles were also present, causing the particle size distribution to broaden. As the milling process continued and the milling time increased to 36 hours, 42 hours, and 48 hours, the particle size decreased, and the particle size distribution became more uniform, with less variation in particle size. The particle shape became nearly axisymmetric and spherical. In the 48-hour milling time, the particle size distribution ranged from 3 microns to 20 microns, with the highest frequency occurring between 5 and 10 microns, accounting for approximately 63% of the particles within this range. XRD results indicated that after a milling time of 12 hours, the alloying process was not complete, and the mechanical activation for dissolving some elements in the cobalt structure was not well accomplished. However, at longer milling times, it was observed that no peaks corresponding to alloying elements were present, indicating that the mechanical activation for complete dissolution of elements into the matrix phase (cobalt structure) was effectively carried out, and the alloying process was successful. This was evident by the presence of peaks corresponding to cobalt with an HCP lattice. Additionally, with an increase in milling time, the crystal size decreased, and the lattice strain increased. The TEM results demonstrate that the microstructure of the milled powders consists of precipitates of secondary phases within the cobalt matrix with an HCP crystal lattice. Since nitrogen atoms have a strong affinity for chromium atoms, the presence of chromium nitride secondary phase precipitates was identified. Additionally, the Selected Area Electron Diffraction (SAED) pattern of the milled powders indicates the formation of a fine-grained polycrystalline phase. These findings are consistent with the XRD results mentioned in the previous section. The results of Vickers microhardness testing show that the hardness of the milled powders increases with longer milling time. In fact, after 48 hours of milling, the hardness has reached 976 Vickers. The significantly high hardness of the milled powders can be attributed to the intense plastic deformation that the powder particles undergo due to the mechanical alloying mechanism.
