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.

A systematic survey of morphology of bainite and proeutectoid ferrite was carried out in order to validate some old thoughts of bainite transformation mechanism. It is confirmed that there is no morphological evidence supporting a sharp change neither between Widmanstätten ferrite and the ferritic component of upper bainite, nor between upper and lower bainite. Both Widmanstätten ferrite and upper bainite start with precipitation of ferrite plates at a grain boundary while lower bainite starts with intragranular nucleation. In case of grain boundary nucleation, a group of parallel plates with same crystallographic orientation to the parent austenite grain forms. This process is followed by a second stage of decomposition of the austenitic interspace, which remained in between the primary ferrite plates. At high temperature, the austenitic interspace would either retain as thin slabs or transform into pearlite through a nodule originated from a grain boundary. At lower temperature, cementite precipitation starts to be possible and initiates simultaneous growth of ferrite. Generally, there are two modes of such eutectoid reactions operating in the second stage, i.e. a degenerate and a cooperative mode, which would lead to typical upper and lower bainite, respectively, in definition of carbides morphology. Both upper and lower bainite according to this definition are observed in a wide temperature range. A sharp temperature between the upper and lower bainite structures thus exists only when the definition is based on their nucleation sites, i.e. grain boundary nucleation for upper bainite and intragranular nucleation for lower bainite. Supposing that the first stage is a diffusionless process it should have a high growth rate to prevent carbon diffusion. This is not supported by lengthening rate obtained in current study as well as data from literature for Fe-C alloys. Finally, it is shown that the “subunits” play no role in the lengthening process of bainite. / <p>QC 20170523</p>