Advancement of Compositional and Microstructural Design of Intermetallic γ-TiAl Based Alloys Determined by Atom Probe Tomography

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.

Intermetallic β‐Solidifying γ‐TiAl Based Alloys − From Fundamental Research to Application

“…[103] Nevertheless, both elements represents typical bstabilizing elements, [15] that is, the b o -phase surrounding the a 2 /g-colonies shows comparably higher concentration of Nb and Mo, which is particularly high in the case of Mo due to the stronger b-stabilizing effect in comparison to Nb. [92,103,165] An interesting feature that should additionally be mentioned here is that the chemical composition of a 2 has been determined to deviate severely from the ideal stoichiometric phase of Ti 3 Al. Especially, the Ti concentration is low with %61 at%.…”

“…Figure 10a depicts a SEM micrograph taken in BSE mode of the heat-treated specimen state of a TNM þ alloy. [92,96,103,162,165] In Figure 10c, a reconstruction of an APT measurement is depicted, whereby only the major alloying elements Ti (blue), Nb (red), and Mo (green) are shown for clarity. 9.…”

“…Schematic drawing of a designed NL þ g microstructure which can be adjusted by a two-step heat treatment in a forged TNM alloy (see text). [92,103,165] An interesting feature that should additionally be mentioned here is that the chemical composition of a 2 has been determined to deviate severely from the ideal stoichiometric phase of Ti 3 Al. (Reprinted from ref., [69] Copyright (2014), with permission from Trans Tech Publications Inc.).…”

“…[162][163][164] In recent years, APT has been extensively applied for the understanding of underlying phenomena occurring in intermetallic TiAl alloys such as phase formation, phase stability, elemental phase preference, and its correlation with mechanical properties. [92,96,103,162,165] In Figure 10c, a reconstruction of an APT measurement is depicted, whereby only the major alloying elements Ti (blue), Nb (red), and Mo (green) are shown for clarity. Besides, interfaces have been indicated using Ti:Al isoconcentration values.…”

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

“…by STEM-EDX [143], are difficult given the light elements present (Al, C, B) [144]. On the other hand, atom probe tomography (APT) studies of lamellar TiAl [145][146][147][148] have the potential to quantify the partitioning of elements to interfaces [149]. The accumulation of C in Ti-43Al-4Nb-1Mo-0.1 B-0.75C (TNM) at a β 0 /γ interface was recently measured [147,150] (Figure 16).…”