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

Download
2014
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.

Suggestions

Production and development of aluminide coatings by chemical vapor deposition on nickel based superalloys for turbine engine applications
Ertürk, Umutcan; İmer, Muhsine Bilge; Department of Metallurgical and Materials Engineering (2017)
Diffusion coatings including aluminide coatings are utilized to improve the high temperature oxidation and corrosion resistance of turbine blades. Through the advancement in the aluminizing methods, higher purity and more homogeneously distributed coatings have been realized. The chemical vapor deposition (CVD) is the most advanced technique utilized for aluminizing process. The versatility of CVD provides a large possibility to control process variables such as temperature, pressure, gas flows and ratios, ...
Design and development of high temperature nickel-based superalloys
Eriş, Rasim; Akdeniz, Mahmut Vedat; Mekhrabov, Amdulla O.; Department of Metallurgical and Materials Engineering (2017)
Remarkable high temperature mechanical properties of nickel-based superalloys are related with the arrangement of ternary alloying elements in L12-type ordered γ'-Ni3Al intermetallics. Therefore, in this thesis, high temperature site occupancy preference and energetic-structural characteristics of atomic short range ordering of ternary alloying X elements (X = Co, Cr, Hf, Mo, Nb, Pt, Re, Ru, Ta and W, respectively) in Ni75Al21.875X3.125 alloy systems have been studied by combining the statistico-thermodynam...
Synthesis and characterization of Ti-based bulk amorphous/naocrystalline alloys for engineering applications
Abdelal, Ali; Akdeniz, Mahmut Vedat; Department of Metallurgical and Materials Engineering (2004)
Amorphous and bulk amorphous metallic alloys are an intriguing class of structural materials and possess a range of interesting properties, including near theoretical strength, high hardness, extremely low damping characteristics, excellent wear properties, high corrosion resistance, low shrinkage during cooling and almost perfect as-cast surfaces with good potential for forming and shaping. In this study, new Ti-based bulk amorphous alloys are tried to be modeled and synthesized. For that purpose, electron...
Development of high temperature creep resistant aluminium based sand cast magnesium alloys
Yalınız, Emrah; Kalkanlı, Ali; Department of Metallurgical and Materials Engineering (2018)
The aim of this thesis is to develop alternative sand casting aluminum based magnesium alloys to substitute current sand castable helicopter transmission housing magnesium alloys such as ZE41A and EV31A. AZ91 magnesium alloy has also been preferred for helicopter transmission housing material. However, the lower creep resistance above 120 oC due to the presence of -(Mg17Al12) phase in the microstructure and showing varying mechanical properties in different section thicknesses limit the usage of AZ91 in th...
Designing of Ti-Mg composites for various applications
Bütev Öcal, Ezgi; Dericioğlu, Arcan F.; Department of Metallurgical and Materials Engineering (2019)
In this study, galvanic corrosion of Ti-Mg based composites has been examined by combining different Mg-alloys with Ti6Al4V alloy and by addition of Na-rich layer between two dissimilar metals. After determining the wetting behavior of liquid unalloyed Mg, AZ91, and WE43 alloys on solid Ti6Al4V alloy, three different composites were manufactured via infiltration of liquid Mg/Mg-alloys into porous Ti6Al4V alloy skeletons. The same procedure was also repeated after coating the porous surfaces of Ti6Al4V alloy...
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.