Design and development of iron aluminium intermetallic compounds for structural applications at high temperatures

Yıldırım, Mehmet
Fe-Al based intermetallic compounds are considered as suitable candidates for structural applications at high temperatures due to their outstanding oxidation and corrosion resistance, good intermediate temperature strength, low density, low cost and relatively high melting point. These outstanding physical and mechanical properties are mainly attributed to their long-range ordered superlattices, deviation from stochiometry and ternary alloying additions. However, poor ductility at ambient temperatures significantly restricts the fabricability and potential applications. Thus, further alloy design and development is needed in order to improve insufficient properties. This study focuses on two main topics: (i) investigation of the effect of ternary alloying additions on ordering characteristics especially B2↔A2 order-disorder transformation temperature and site occupancy characteristics of Fe50Al50-nXn intermetallics and confirmation of the validity of theoretical predictions proposed for single phase Fe50Al50-nXn intermetallics, (ii) improvement of room temperature mechanical properties and high temperature oxidation behavior of Fe50Al50-nXn intermetallics via alloying additions and/or proper heat-treatment. The effect of ternary alloying element additions on the ordering characteristics, room temperature mechanical properties and oxidation behavior of B2-type ordered FeAl intermetallic compounds were studied for as-cast and heat-treated states. It is shown that type and content of ternary alloying element together with heat-treatment have strong influences on these properties. It is also shown that type of ternary alloying element has also important effect on ordering characteristics such as order-disorder transition temperature and site occupancy behavior. In order to have better analyzing of order-disorder transition, experimentally measured transition temperatures were compared with theoretical predictions where excellent agreement was obtained. In this manner, the relative partial ordering energy parameter (RPOE), β, has been defined for the first time. The RPOE parameter takes into account both site occupancy behavior of alloying elements and variation of order-disorder transition temperature. The sign of this parameter implies the distribution of alloying elements over Fe or Al sublattices, while its magnitude provides useful information about order-disorder transition temperatures featuring the bond strengths of Al-X or Fe-X pairs relative to Fe-Al pair. Current predictions based on the RPOE parameter are consistent with the theoretical predictions. Mechanical characterization of the alloys reveals that room temperature mechanical properties strongly depend on ordering and microstructural features such as volume fraction, size, morphology and distribution of existent phases and solidification route, i.e, eutectic or liquidus + solidus. These microstructural parameters can be controlled by controlling the Al content and content of ternary alloying element. Al content dominates the order, whereas content of ternary alloying element dominates the volume fraction of present phases with regard to solid solubility of alloying element in FeAl intermetallic. The cyclic oxidation tests and structural characterization of oxide scale after oxidation demonstrated that protective, continuous and stable α-Al2O3 scale can easily be formed by proper selection of the type and content of alloying elements. Analyzing the results, formation of α-Al2O3 scale takes place in the first few cycles of oxidation indicating the fast growing of even, continuous and uniform scale without formation of any voids or cracks.
Citation Formats
M. Yıldırım, “Design and development of iron aluminium intermetallic compounds for structural applications at high temperatures,” Ph.D. - Doctoral Program, Middle East Technical University, 2014.