Alloy design strategies for promoting protective oxide-scale formation

Abstract

Figure 1. A scanning electron microscopy (backscatter mode) cross-section micrograph of Nb-33Ti-40Al (in at.%) after 15 minutes at 1,400°C in air. The continuous Al 2 O 3 scale cuts off the formation of more rapidly growing niobium-rich transient oxides. 3 5 µm with the thermochemistry of the environment and the temperature of the reaction. Oxidation can lead to a loss of load-bearing capacity in a component by the reduction of metallic cross section and, for many high-temperature applications, is the primary factor limiting service life. Interstitial dissolution of oxygen, nitrogen, etc. into an alloy during elevated-temperature exposure, which may accompany oxidation, can also degrade mechanical properties and result in component failure.In practice, the oxidation of metals in many applications is not prevented, rather it is managed. Protection of the underlying metal is most often accomplished by the formation of a continuous scale over the surface such that it serves as a barrier between the remaining underlying, unoxidized metal and the environment. Long-term protection is generally associated with a scale that grows by a diffusion-controlled process or processes because the rate of scale growth slows with reaction time as the diffusion distance (scale thickness) increases. Most high-temperature environments of technological interest contain at least some oxygen. Given the fact that metal oxides are nearly always more stable thermoEditor's Note: Compositions are given in weight percent unless otherwise noted.This article discusses general strategies for designing alloys to form protective oxide scales. Approaches based on classical alloyoxidation theories work reasonably well for single-phase alloys. However, high-temperature alloy development has been and will increasingly be based on multiphase microstructures in order to achieve many of the needed, but usually opposing properties, such as high-temperature strength and room-temperature toughness. No theoretical-based, well-defined strategies exist for the design of oxidation-resistant multiphase alloys. Still, key factors are beginning to emerge, which can provide guidance for promoting the formation of protective scales on multiphase alloys and for taking advantage of some unique mechanisms that are operative in multiphase alloys but not in single-phase alloys.

Alloys developed for high temperature applications

“…Effect of Ag and Cr addition on the oxidation behavior of two phase α2-Ti3Al + γ-TiAl in TiAl based alloys have been investigated 34 . These two alloying elements were expected to have reservoir-effect that supports the formation of continuous stable Al2O3 scale as reported previously in single phase TiAl intermetallic compounds 35,36,37 . In this alloy systems, chromium can act as Laves phase-Ti(Cr,Al)2 stabilizer while silver stabilizes Z phase-Ti5Al3O2.…”

“…In the middle, intermixed of majorly TiN and minor of TiO2 and Al2O3 were formed. This TiO2 occurred when some TiN oxidized 36 . Longer times as well as higher oxidation temperatures might change the scale compositions.…”

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

“…The oxidation resistance will depend on the ability of the alloys to form, and maintain, a compact and adherent scale that will protect the alloy from further oxidation attack. 1 Loss of the protective oxide scale becomes problematic as it exposes the parent alloy to further oxidation attack. Stresses in the oxide scale due to intrinsic growth stresses, thermal stresses, or externally applied stresses all increase the likelihood of scale failure.…”

Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

“…These coatings change their structure and properties in response to various operating conditions [125][126][127][128]. (b) Smart coatings, which are functionally graded systems that may be characterized by the variation in composition and structure gradually over volume, resulting in corresponding changes in the properties depending on the stages of wear process [99,[129][130][131][132][133][134][135][136]. (c) Adaptive coatings that can change their structure and properties as a result of the engineered surface dynamic response to the environmental impact due to formation of surface tribofilms.…”

“…It was shown that addition of Cr to the TiAlN-based coating strongly promotes its oxidation resistance owing to favorable changes in electronic and crystal structure [63,252]. Many studies aimed to improve oxidation resistance of various heat-resistant metallurgical systems such as alloys based on MCrAl (M = Ni, Fe, Co) [253] or TiAl [132] by incorporating various foreign elements such as Y [68,[254][255][256], Hf, Nb, B [257,258], V [259] or Si [64,109,111,217,260,261]. Among these dopants, Si and Y seem to be the most promising for improving the oxidation resistance of hard nitride coatings at relatively small concentrations.…”