Abstract
Advanced intermetallic alloys based on the γ-TiAl phase have become widely regarded as most promising candidates to replace heavier Ni-base superalloys as materials for high-temperature structural components, due to their facilitating properties of high creep and oxidation resistance in combination with a low density. Particularly, recently developed alloying concepts based on a β-solidification pathway, such as the so-called TNM alloy, which are already incorporated in aircraft engines, have emerged offering the advantage of being processible using near-conventional methods and the option to attain balanced mechanical properties via subsequent heat-treatment. Development trends for the improvement of alloying concepts, especially dealing with issues regarding alloying element distribution, nano-scale phase characterization, phase stability, and phase formation mechanisms demand the utilization of high-resolution techniques, mainly due to the multi-phase nature of advanced TiAl alloys. Atom probe tomography (APT) offers unique possibilities of characterizing chemical compositions with a high spatial resolution and has, therefore, been widely used in recent years with the aim of understanding the materials constitution and appearing basic phenomena on the atomic scale and applying these findings to alloy development. This review, thus, aims at summarizing scientific works regarding the application of atom probe tomography towards the understanding and further development of intermetallic TiAl alloys.
Highlights
Intermetallic titanium aluminides belong to the most promising materials to meet today’s most prevalent demands for structural high-temperature materials of combining high strength with low density [1,2,3]
Specimens need to be needle-shaped with tip radii of ≈20–100 nm in order to reach sufficient local electric fields of ≈10–60 V/nm resulting in ionization and emission of surface atoms at cryogenic temperatures and ultra-high vacuum
Considering Ti and Al, it is of particular interest to note that the composition of the phases α2 and βo deviates significantly from the compositions of the ideal binary phases (Ti3 Al and TiAl, respectively). This strong deviation of both phases is due to differing site occupation behavior of the transition metal atoms that are alloyed in the TNM system
Summary
Intermetallic titanium aluminides belong to the most promising materials to meet today’s most prevalent demands for structural high-temperature materials of combining high strength with low density (next to other important characteristics like high creep and oxidation resistance as well as high modulus and strength retention at elevated temperatures) [1,2,3]. The combination of thermo-mechanical processing and multiple heat-treatments, which exploit the occurrence of several elapsing phase transformations, has been extensively investigated with the result of enabling the adjustment of different types of microstructures [14] These in turn allow for tuning of mechanical properties over wide ranges toward the prerequisites of the particular area of operation mainly by the control of morphological parameters, e.g., grain size, colony size and aspect ratio or lamellar interface spacing. O much concentrations, to the contamination, it is argued that even in high purity single-phase γ-TiAl alloys, the excess leads to maximum solubility of this phase As this solubility level is much lower than the alloy’s impurity fine-scaled oxide precipitates or a local enrichment, which in turn result in embrittlement. In particular fundamental questions as well as issues pertaining to the enhancement of TNM alloys are contemplated in the frame of complimentary methods
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