The effects of mechanical alloying milling time and carbon concentration on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mechanical alloying and powder metallurgy methods were used to prepare the samples. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% pre-milled graphite were milled in a SPEX mill with tungsten milling media for up to 100h. The milled powders were then cold-compacted and pressure-less sintered between 900°C and 1200°C for 1h and 5h followed by furnace cooling. Milled powders and sintered samples were characterized using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, scanning and transmission electron microscopes. Density and micro-hardness were measured. The milled powders and sintered samples were studied as follows:
In the milled powders, the formation of Fe_3 C was observed through Mossbauer spectroscopy after 5h of milling and its presence increased with milling time and carbon concentration. The particle size of the milled powders decreased and tended to become more equi-axed after 100h of milling. Micro-hardness of the milled powders drastically increased with milling time as well as carbon concentration. A DSC endothermic peak around 600°C was detected in all milled powders, and its transformation temperature decreased with milling time. In the literature, no explanation was found. In this work, this peak was found to be due to the formation of Fe_3 C phase. A DSC exothermic peak around 300°C was observed in powders milled for 5h and longer; its transformation temperature decreased with milling time. This peak was due to the recrystallization and/or recovery α-Fe and growth of Fe_3 C .
In the sintered samples, almost 100% of pearlitic structure was observed in sintered samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time, contrary to what was found in the literature. The decrease in pearlite occurred at the same time as an increase in graphite-rich areas. With milling, carbon tended to form graphite instead of Fe_3 C. Longer milling time facilitated the nucleation of graphite during sintering. High mount of graphite-rich areas were observed in sintered samples prepared from powders milled for 40h and 100h. Nanoparticles of Fe_3 C were observed in a ferrite matrix and the graphite-rich areas in samples prepared from powders milled for 40h and 100h. Micro-hardness of the sintered samples decreased with milling time as Fe_3 C decreased. The green density of compacted milled powders decreased with milling time and the carbon concentration that affected the density of sintered samples. / Ph. D. / The effects of milling time and carbon composition of the alloy on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% nano graphite were milled, pressed and the sintered between 900°C and 1200°C for 1h and 5h. Milled powders and sintered samples were characterized. Density and hardness were measured. The milled powders and sintered samples were studied as follows:
In the milled powders, the formation of iron carbide was observed through Mossbauer spectroscopy after 5h of milling and its amount increased with milling time and carbon composition of the alloy. The particle size of the milled powders decreased with milling time. Hardness of milled powders increased with milling time as well as carbon composition of the alloy.
In the bulk samples, almost 100% of pearlitic structure was observed in samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time. The decrease in pearlite occurred at the same time as an increase in graphite with milling time. High mount of graphite areas were observed in samples prepared from powders milled for 40h and 100h. Hardness of the sintered samples decreased with milling time as iron carbide (hard phase) decreased. The density of bulk samples decreased with milling time and the carbon composition.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/80479 |
| Date | 22 November 2017 |
| Creators | Khalfallah, Ibrahim Youniss A. |
| Contributors | Materials Science and Engineering, Aning, Alexander O., Winkler, Christopher Reid, Reynolds, William T. Jr., Lu, Guo Quan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0021 seconds