Loading...
Please wait, while we are loading the content...
Similar Documents
Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title Impurity effects on pore formation at Al 2 O 3 / Alloy interfaces Permalink
| Content Provider | Semantic Scholar |
|---|---|
| Author | Stringer, John Warren Hou, Peggy Y. |
| Copyright Year | 2002 |
| Abstract | Whatever the mechanism of pore formation, Grabke [12] suggested that interfacial voids are stabilized by the segregation of indigenous sulfur impurity from the alloy to the void surfaces. This segregation reduces the energy barrier for pore formation, hence facilitates the process. When sulfur is removed from the alloy by H2-annealling [13,14], or tied up by the presence of a reactive element in the alloy [15,16], interfacial void formation seems to be greatly reduced. However, even with a reactive element in the alloy, very small pores, nm in diameter, could still be found at the very initial stages [17] or after prolonged oxidation [18]. Either these pores formed by different mechanisms, or the effect of sulfur is not so much on enhancing pore nucleation, but on stabilizing them to allow for their continued growth. The adhesion of Al2O3 scales on commercial grade alloys that do not contain a reactive element is usually poor due to the presence of 10-50 wppm of sulfur impurity, and/or of pores that formed at the scale/alloy interface. Sulfur is usually believed to segregate to the interface to weaken the interfacial bonding and to stabilize interfacial pores. By using field emission scanning Auger microscopy, the distribution of sulfur on pores and on oxide imprinted areas at Al2O3/FeAl interfaces was precisely determined. Interfacial pore growth as a function of oxidation time was obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses. The effects of sulfur segregation, surface impurity and reactive elements on pore nucleation and growth are discussed. The purpose of this paper is to evaluate the effect of impurities on pore nucleation and growth. Sulfur in the alloy as well as possible surface contaminates are being considered. Introduction A characteristic feature of the oxidation process in several metals and alloys is the formation of pores at the oxide/metal interface or in the metal immediately below this interface. When the pore size and/or density reach a critical value, adhesion of the scale deteriorates resulting in scale spallation during cooling. The mechanism of pore formation has been a subject of extensive debates, but is still not understood. The flow of vacancies from the oxide has been suggested to condense at the oxide/metal interface to form interfacial voids [1,2], or to condense in the metal immediately below the scale to cause void formation there [3-5]. Harris [6], on the other hand, suggested that tensile stresses induced in the alloy by the compressive growth stress in the oxide caused voiding beneath the scale. Porosity in the substrate can also be due to an unequal diffusion of the alloying elements [7], i.e. the Kirkendall effect, or by the formation of gaseous species, such as CO2, as in the case of Ni [8]. Vacancy condensation to from voids at the scale/alloy interface depends on the effectiveness of this boundary as a vacancy sink [9]. Many oxide/alloy interfaces are believed to be good sinks where vacancies become annihilated [9,10]. Moreover, the oxide can often follow the retreating metal surface by plastic flow to avoid pore formation and scale separation [11]. Experimental A Fe-40at%Al alloy containing 27.6 wppm of sulfur impurity, from Oak Ridge National Laboratory, was used for the study. The alloy was received as a ~ 1 mm thick hot-rolled sheet from an arc melted casting. It was annealed in He at 1100°C for 50 hours before cutting into 15mm x 10mm x 1mm sized coupons. Specimens were polished to a 1 μm finish with diamond paste and cleaned ultrasonically in acetone prior to oxidation in flowing, dry oxygen at 1000°C. A Cahn TGA system was used for thermogravimetric analysis. Other specimens were placed in an alumina boat with a thermocouple attached at the back of the specimen and oxidized in a horizontal furnace. After the desired oxidation time, which varied from 1 min to 24 hours, the boat and specimen were quickly pulled out of the furnace and cooled in ambient air. In both systems, the specimen temperature took about 10 minutes to reach 1000°C. One specimen surface was doped with a few drops of about 4x10 mole/l NaNO3 solution to study its effect on pore formation. Interfacial pores were examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM) after the surface oxide was removed either by scratch |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://cloudfront.escholarship.org/dist/prd/content/qt4r87j3vt/qt4r87j3vt.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
