Mechanical Alloying (MA) is a process that involves repeated cold welding, fracturing and rewelding of powder particles in a high-energy ball mill and has been used extensively to synthesize both stable (equilibrium) and metastable phases in a number of alloy systems. This is due to its ability to achieve many effects simultaneously, viz., reduction in grain size, introduction of a variety of crystal defects, disordering of the lattice, and modifying the crystal structures of materials; all these allowing alloying and phase transformations to occur in powders. In this Dissertation, we have synthesized a number of different alloy phases in Ni- and Fe-based alloy systems using MA. The as-received, blended, and milled powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy techniques to obtain information about the overall microstructure and chemical compositions. The NiX2 (X = Ge and Si) phases were synthesized in the Ni-Ge and Ni-Si systems. MA of Ni-Ge powder blends was investigated to study phase evolution as a function of milling time. On milling the powders for 5 h, the equilibrium NiGe phase started to form, and its amount in the powder increased with increasing milling time. On milling for about 60 h, the equilibrium intermetallic NiGe and Ge powder particles reacted to form the metastable NiGe2 phase. However, on milling for a longer time (75 h), the metastable phase transformed back to the equilibrium NiGe phase. Synthesis of the NiSi2 intermetallic phase depended on the Si content in the initial powder blend. For example, while in the Ni-60 at.% Si powder blend, only the NiSi phase was present homogeneously, the powder blend of the Ni-67 at.% Si composition contained the NiSi phase along with a small amount of unreacted Si. But in the Ni-75 at.% Si and Ni-80 at% compositions, the NiSi phase that had formed earlier (after 2 h of milling) and the remaining free Si powder reacted to form the equilibrium intermetallic NiSi2 phase. This constitution in the milled powder has been attributed to a partial loss of Si content during MA. Formation of Ni(Si) solid solutions with a solubility of about 18.2 at.% and 20.6 at.% for the Ni-75 at.% Si and Ni-80 at.% Si powder blends, respectively, was also achieved in the early stages of MA. In the Fe-C system, we were able to synthesize ferrite, cementite, and mixtures of the two phases. We were able to obtain the Fe-C solid solution phase (ferrite) with a BCC structure and the cementite phase with an orthorhombic structure in the eutectoid Fe-0.8 wt. % C composition, while a homogeneous cementite phase had formed at the higher carbon content of Fe-7.0 wt. % C after 30 h of milling time. In the case of the Fe-18Cr-xNi (x = 8, 12, and 20) system, the current investigation showed that the phase constitution depended significantly on the Ni content in the powder blend. Whereas mostly the martensite or the ferrite and austenite phase mixture was present at lower Ni contents, a completely homogeneous austenite phase was present in the alloy with 20% Ni.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Materials Science Engineering
Materials Science & Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Al-Joubori, Ahmed, "Synthesis and Characterization of Stable and Metastable Phases in Ni- and Fe-Based Alloy Systems by Mechanical Alloying" (2016). Electronic Theses and Dissertations. 5092.