Carbon in Nb-rich γ-TiAl based alloys
Intermetallic γ-TiAl based alloys are light-weight high-temperature structural materials of great technological interest. They have the potential to partly replace the heavier Ni based super alloys for applications as compressor and turbine blades in gas turbines and aero engines [1]. TiAl alloys are two-phase alloys consisting of tetragonal γ-TiAl and small amounts of hexagonal α2-Ti3Al. In order to extend their service range to higher temperatures research activities have been focused on high Nb bearing TiAl alloys. These so-called third generation or TNB alloys exhibit a significant increase in strength combined with improved creep properties and oxidation resistance [2].
Titanium and Titanium Alloys
The chapter discusses various methods used to treat titanium and titanium alloys for adhesive bonding. It is reported that both alkaline cleaning and phosphate-fluoride prebond treatments produce surfaces of good wettability, equal thickness and similar composition. Stabilized phosphate-fluoride process retards the conversion of anatase to rutile. It is also reported that the anodic process produces a porous oxide structure and is superior to the surface formed by the phosphate-fluoride process. Specific treatments for aluminum and aluminum alloys are presented in a process specification format including anodize process, Turco 5578, liquid hone/Pasa-Jell 107 process, dry hone/Pasa-Jell 107 process, alkaline peroxide process and stabilized phosphate-fluoride process. A study of the morphology and decomposition of the various surfaces revealed that different surface morphologies are obtained with the different treatments. The treatments were classified into three different groups. The Group I processes were characterized as producing surfaces which have thin oxide layers with little macro-roughness while Group II processes resulted in surfaces which had a high degree of macro-roughness. The Group III processes produced surfaces which were characterized by a primarily micro-rough porous oxide.
The Stability of Anodized Titanium Surfaces in Hot Water
Abstract Adhesive bonding of titanium structures for high performance aircraft has gained increased attention in recent years, but very little work has been done to determine the stability of bond strength in humid environments or the effect of higher operating temperatures envisioned in future aircraft. In this paper, the stability of an important bond component–the surface oxide– is considered. It is shown that an anodically formed amorphous TiO2 layer on Ti-6A1-4V is unstable in pure water at 85°C. Morphological and structural changes in the oxide and possible corrosion of the substrate lead to the formation of an anatase crystallite layer and an increase in the surface oxide mass. Crystallization does not occur when the oxide is heated in vacuum at typical adhesive curing temperatures. The relevance of oxide instability to bond failure is discussed.