The development of ever more efficient gas turbines has always been paced by the results of research and development in the concurrent fields of design and materials technology. Improved structural design and airfoil cooling technology applied to higher strength-at-temperature alloys cast by increasingly complex methods, and coated with steadily improved coating systems, have led to remarkably efficient turbine engines for aircraft propulsion and power generation. For first stage turbine blades, nickel-based superalloys in various wrought and cast forms, and augmented by coatings since the 1960s, have been singularly successful materials systems for the past 50 years—and still no real world substitutes are on the horizon. This paper traces the history of protective coatings for superalloy airfoils beginning with the simple aluminides, followed by modifications with silicon, chromium and platinum, then MCrAlY overlay coatings, and finally the elegant electron beam vapor deposited ceramic thermal barrier coatings recently introduced to service. The publicly available results of several decades of research supporting these advances are highlighted. These include generic research on oxidation and hot corrosion mechanisms of superalloys and coatings, the intricacies of protective oxide adherence, mechanisms of low temperature (Type II) hot corrosion, and of aluminide coating formation and mechanical properties of alloy–coating systems. With no promising turbine materials beyond coated nickel-base superalloys apparent in the foreseeable future, continued progress will likely be made by further refinement of control of thermally grown oxide adherence, and by more cost effective manufacturing processes for contemporary types of protective coatings.

Progress in coatings for gas turbine airfoils

The aim of the present article is to review some of the common corrosion phenomena and describe the predominant corrosion mechanisms in high-temperature and supercritical water. Corrosion in aqueous systems up to supercritical temperatures is determined by several solution-dependent and material-dependent factors. Solution-depending factors are the density, the temperature, the pH value, and the electrochemical potential of the solution, and the aggressiveness of the attacking anions. Material-dependent parameters include alloy composition, surface condition, material purity, and heat treatment. Corrosion phenomena that are observed include intergranular corrosion, pitting, general corrosion, and stress corrosion cracking. The solubility and dissociation of both attacking species and corrosion products play the most important role for corrosion in high-temperature water. Both solubility and dissociation processes are strongly influenced by the density, or the ionic product, respectively, of the solvent. High values of both parameters favor ionic reactions, and thus, accelerate electrochemical forms of corrosion. At low densities, water behaves like a non-polar solvent, and thus, ions associate. In these cases, the concentation of e.g. aggressive H + drops down and thus, solutions containing species such as HCl become neutral and thus less aggressive. Further, corrosion products plug the surface and material loss stops. Materials parameters have influence especially on the initiation of corrosion. In the present article, these factors are linked with the physical and chemical properties of high-temperature and supercritical water. An outlook is also given for future research needs.

Corrosion in high-temperature and supercritical water and aqueous solutions: a review

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The influence of alloying elements on the development and maintenance of protective scales

Layered Machinable and Electrically Conductive Ti2AlC and Ti3AlC2 Ceramics: a Review

A review on fundamental of high entropy alloys with promising high–temperature properties

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO2 (rutile), while the inward-growing part is composed of a mixture of TiO2 and α-Al2O3. A barrier layer of Al2O3 on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al2O3 barrier dissolved and re-precipitated in the outer TiO2 layer. This “shift” leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al2O3 layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al2O3 barrier layer formed; large Al2O3 particles were part of the outward-growing layer. A brittle α2-Ti3Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti2AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al2O23 formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The paper critically reviews the state of the art on the oxidation of TiAl. At the beginning differences compared to the oxidation of conventional Fe, Ni and Co alloys are discussed. The Knowledge of the TiAlO phase diagram as a basis for an understanding of the processes occurring during oxidation is very incomplete. Three different scale structures can be distinguished as functions of time or scale thickness. In the sub-surface zone two phases with high oxygen solubilities can be identified after a certain time. These phases do not follow from the existing phase diagram. Nitrogen can have two different effects depending on the time it is added to the oxidation environment. The micro-structure also affects the oxidation behaviour significantly, so this effect can lead to a misinterpretation of the influence of other parameters such as alloying elements. The influence of alloying elements is not yet understood. Mechanical loading leads to scale cracking, however, the system has a significant scale repair capacity, though about twothirds of the scale grows by inwards transport.



Grundlagen der TiAl-Oxidation – Eine kritische Ubersicht



Die Arbeit gibt eine kritische Betrachtung der Kenntnisse uber die Oxidation von TiAl. Einleitend wird auf Unterschiede im Vergleich zur Oxidation konventioneller Fe. Ni und Co-Legierungen hingewiesen. Die derzeitigen Kenntnisse uber das Phasendiagramm TiAlO als Basis fur das Verstandnis der bei der Oxidation ablaufenden Prozesse sind sehr unvollstandig. Im Aufbau der Oxidschicht konnen drei Grenzfalle unterschieden werden, die als Funktion der Zit nacheinander durchlaufen werden. Stickstoff kann offensichtlich zwei Effekte auslosen, abhangig vom Zeitpunkt der Zugabe zum Reaktionsgas. Die Mikrostruktur kann einen starken Einflus haben und andere Einflusse wie die von Legierungselementen verdecken. Der Einflus von Legierungselementen wird derzeit nicht verstanden. Damit sind auch keine sinnvollen Vorhersagen moglich. Mechanische Belastungen verursachen ein Aufreisen der Oxidschicht, die jedoch eine beachtliche Ausheilfahigkeit fur Oxidschichtrisse hat.

Fundamentals of TiAl oxidation: a critical review

Thermodynamic data from the literature were used to calculate the activity variations of Ti and Al in the Ti-Al system at 700, 900, and 1100°C and of Ti and Si in the Ti-Si system at 800, 1000, and 1300°C. The activity data were then applied to estimate the stable oxide phase in equilibrium with the metal phase, assuming that the activity of the oxide phase was unity. It was found that in the Ti-Al-O system the stable oxide changed from TiO to Al
2O3
in the existence region of the TiAl phase, while in the Ti-Si-O system a comparable change from Ti-oxide to SiO
2
occurred in the existence region of the Ti
5Si3
phase. The significance of these results to the oxidation of intermetallics in the Ti-Al and Ti-Si systems is discussed briefly.

Thermodynamic aspects of TiAl and TiSi2 oxidation : the Al-Ti-O and Si-Ti-O phase diagrams

Ni-base superalloys contain beside other phases relatively large blocky MC carbides of the type (Ta, Nb, Ti, W)C, which oxidize much faster than the γ/γ′ matrix. The large volume increase during oxidation and the oxide formation at the carbide-oxide interface shift the corrosion products outward. High shear stresses between the Cr2O3 scale and the carbide oxidation products lead to scale cracking favoring internal corrosion processes in this area. The formation of Al2O3 in the subscale is accompanied by a volume increase and tensile stresses in the outer Cr2O3 scale. This causes scale cracking and gives nitrogen a chance to enter the metal and form the most stable nitride, TiN beneath the Al2O3 subscale.

Selective Carbide Oxidation and Internal Nitridation of the Ni-Base Superalloys IN 738 LC and IN 939 in Air

In order to gain a better understanding of the reactive-element effect (REE), the improvement of the oxidation behavior of chromia- or alumina-scale-forming alloys by the addition of small amounts of elements with higher affinity to oxygen than the scale-forming element, it is necessary to clearly distinguish between isothermal oxidation and the behavior of the metal/oxide composite system during cooling. An approach is presented based on fracture-mechanical considerations which correlates critical differential strain between scale and substrate, fracture toughness of the metal/scale interface, scale thickness, defect size and interfacial amplitude. This approach allows a quantitative assessment of the REE for scale adhesion, and although the necessary experimental data are yet lacking, it describes the reported REE in a qualitatively correct manner.

A. Rahmel

Papers

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

Fundamentals of TiAl oxidation: a critical review

Thermodynamic aspects of TiAl and TiSi2 oxidation : the Al-Ti-O and Si-Ti-O phase diagrams

Selective Carbide Oxidation and Internal Nitridation of the Ni-Base Superalloys IN 738 LC and IN 939 in Air

Mechanical aspects of the rare-earth effect