Understanding the effects of rapid solidification and ternary alloying additions on chemical order-disorder transformation in FeNi 3

Owing to their unusual thermal expansion behavior and good magnetic permeability characteristics, iron-nickel alloys are often at the cutting-edge of technology. Understanding the chemical order-disorder phase transformation and its effects on the physical properties of these alloy systems have been a longstanding research challenge in materials science and engineering. In keeping with current research, the motivation of this thesis is to study the effect of rapid solidification and ternary alloying additions on the chemical order-disorder phase transformation in the FeNi3 compound. Results obtained from this study will inform factors affecting chemical ordering in specific compositions of iron-nickel alloys which undergo this type of phase transformation.;Chemical ordering in FeNi3 occurs at TOD = 490 +/- 10 °C and changes the crystal structure of this compound through short-range diffusion from the A1 structure (chemically-disordered phase in which the iron and nickel atoms can occupy any atomic site in an fcc structure) to the L12 structure (chemically-ordered fcc phase with specific atomic sites for iron and nickel atoms). Understanding the effect of rapid solidification through the melt-spinning process conducted prior to isothermal annealing as well as the influence of ternary alloying elements on the variation of the degree of chemical ordering in pure FeNi3 are the objectives of this thesis.;Coupled calorimetry and magnetic measurements show that rapid solidification prior to isothermal annealing at T = 470 °C does not influence the progression of L12 phase formation in pure FeNi 3. These same studies reveal that addition of both Mn and Cu affects the formation of chemically ordered L12 phase in FeNi3 during isothermal annealing at T = 470 °C; however, this effect is different for each of these elemental substitutions. A larger increase in the degree of chemical ordering, the saturation magnetization, and the saturating filed values as well as a greater decrease in the susceptibility values of Mn-substituted FeNi3 relative to those of the pure FeNi 3 sample upon annealing have been measured in this study. Therefore, it is hypothesized that Mn addition promotes the A1-to-L12 phase transformation in FeNi3 through replacing the Fe atoms in the lattice structure of this compound and producing partially formed MnNi3-L1 2 phase. On the other hand, a smaller increase in the degree of chemical ordering, the saturation magnetization, and the saturating field values as well as a smaller decrease in the susceptibility values measured for Cu-substituted FeNi3 upon annealing compared to those measured for pure FeNi 3 ingot sample have been detected. Thus, Cu addition delays the progression of chemical ordering in FeNi3. This effect is tentatively attributed to Cu atoms leaving the FeNi3 lattice during isothermal annealing due to immiscibility of Fe and Cu atoms.