Development and microstructure of the Al‐depleted layer of oxidized TiAl

Abstract

The microstructure and development of the scale of oxidized y-TiAl were studicd in cross section by electron microscopy and metallographic techniques. Ti-SO(at.%)Al samples wcre oxidized at 900°C in air for various times and especially the evolution of the scale/metal interface was investigated. The formation of the aluminium depleted subsurface layer could be observed after 1 h and develops from a single phase to a two phase region with longer exposure times. The single phase region is visible up to I00 h and consists of a simple cubic phase with lattice parameter a = 0.692 nm. After an oxidation time of 140 h the formation of a two phase region consisting of az-Ti3Al and the cubic phase occurred. Both phases are depleted in aluminium compared to the base metal and contain some amount of oxygen whereby the cuhic phase contains more Al and 0 than a2-Ti3Al. The metallographic cross sections suggest that the U2-Ti3AI phase forms at the y-TiAli cubic-phase interface and grows into the cubic phase.Die Mikrostruktur und Entwicklung der bei der Oxidaiton von y-TiAl entstehenden Oxidschicht is1 mit Hilfe rnetallographischer und elektronenmikroskopischer Querschnitte untersucht worden. Ti-50(at.%)Al wurde bei 900°C in Luft fur verschiedene Zeilen isotherm oxidiert und speziell die Entwicklung der MetalUOxid-Grenzflache untersucht. Eine an Al verarmte Metallrandzone hat sich nach 1 h Oxidation gebildet und entwickelt sich rnit zunchmender Oxidationszeit von einer einphasigen in eine zweiphasige Schicht. Die einphasige Schicht ist bis 100 h vorhanden und besteht aus einer primitiv kubischen Phase mit Gitterparamcter a = 0,692 nm. Nach 140 h Oxidation liegt a2-Ti3Al in der Verarmungszone neben der kubischen Phase vor. Beide Phasen besitzcn einen geringeren Al-Gehalt als die Ausgangslegierung. Ebenso konnte Sauerstoff in beiden Phasen nachgewiesen werden. wobei die kubische Phase mehr Al und 0 enlhalt als a2-Ti3A1. Die metallographischen Querschliffe zeigen die Bildung von az-Ti3AI an der Grenzflache y-TiAUVerarmungszone mit einem anschlieflenden Wachslum in die kubische Phase der Verarmungszone.

Thermal oxidation of amorphous Al0.44Zr0.56 alloys

“…The significant solubility and diffusivity of oxygen in the am-Al 0.44 Zr 0.56 alloy substrate relates to the intrinsically high solubility and diffusivity of oxygen in pure crystalline (a-)Zr, 2 which has an oxygen solubility of as high as 28.6 at.% in its pure form at 500°C. A similar high oxygen solubility ($7 at.% at 900°C [45]) was found in a 2 -Ti 3 Al, which relates to the high oxygen solubility in a-Ti (32 at.% O at 900°C) [28,46]. The large O solu- Table 1.…”

“…The measured AES sputter-depth profiles after thermal oxidation of the am-Al 0.44 Zr 0.56 alloy for 15,30,45,180 and 240 min at 560°C are shown in Fig. 7a-f, respectively.…”

Long Term Oxidation of Model and Engineering TiAl Alloys

“…It should be noted that such extensive nitride formation and Al enrichment at the metal/scale interface does not appear to occur under shorter term, higher temperature exposure conditions. For example, Dettenwanger et al [14] report Al depletion and Z/X phase formation at the metal/scale interface for oxidation of Ti-50Al at 900°C in air, which suggests a change in oxidation mechanism between 900°C and 700°C for binary γ-TiAl alloys. On the other hand, Magnan et al, on nitridation of TiAl at 1000ºC reported the formation of a surface nitride scale and an Al-enriched subscale [21].…”

“…This intermixed region is followed by a bright Al-depleted (Ti-rich)/O-rich layer right at the metal/scale interface; its average microprobe chemistry is 58 Ti, 28 Al, 10.8 O and 2.3 Cr. This layer is speculated to be the Z/X phase described in [12][13][14]. No N-enrichment was detected at the subscale, although Haanappel et al [8,22] reported the presence of Ti-nitrides after 150h at 700°C in Ti48Al-2Cr.…”

“…The present investigation was undertaken to characterize the long-term oxidation behavior of various model and advanced engineering titanium aluminides at 704°C in air. Some engineering alloys were also exposed to 800°C in air for 1000 h. Of particular interest for this study was the formation of nitrides, which have been linked to disruption of alumina scale formation in air [12,14], and the formation of brittle oxygen-and titanium-enriched, aluminum-depleted phase(s) at the metal scale interface, termed by various researchers as Z, X or NCP phase (new cubic phase) [14][15][16][17][18].…”