Temperature Oxidation Behavior Of Alloys Research Articles

Oxidation of Al-Co Alloys at High Temperatures.

In this work, the high temperature oxidation behavior of Al71Co29 and Al76Co24 alloys (concentration in at.%) is presented. The alloys were prepared by controlled arc-melting of Co and Al granules in high purity argon. The as-solidified alloys were found to consist of several different phases, including structurally complex m-Al13Co4 and Z-Al3Co phases. The high temperature oxidation behavior of the alloys was studied by simultaneous thermal analysis in flowing synthetic air at 773–1173 K. A protective Al2O3 scale was formed on the sample surface. A parabolic rate law was observed. The rate constants of the alloys have been found between 1.63 × 10−14 and 8.83 × 10−12 g cm−4 s−1. The experimental activation energies of oxidation are 90 and 123 kJ mol−1 for the Al71Co29 and Al76Co24 alloys, respectively. The oxidation mechanism of the Al-Co alloys is discussed and implications towards practical applications of these alloys at high temperatures are provided.

Oxidation Kinetics Researches under the Condition of Compressive Loading

High temperature oxidation behavior of alloys is mostly affected by an external loading. The growth of oxide scale, oxide products, interfacial reactions and other characteristics have be affirmed to be affected by the external loading. Here, it provided the oxidation kinetics of several metals under the compressive loading. And the growth of oxide scale was distinguished in the form of thin oxide layer and thick oxide layer. Different compressive effect on the growth of oxide scale was summarized. At the initial oxidation stage, the applied compressive stress induced more defects on the surface of alloy, supplying more oxide neclei, thereby increased the growth of the thin oxide layer. In the case of the growth of the thick oxide layer, it was attrbuted to the short circuit paths of the oxide grain boudary inherited from the initial oxidation stage and the cracks or other defects deduced by the applied compressive stress. Thus the oxide kinetics of alloys was promoted. However, considering the interfacial reaction at scale/metal, the coarsens of vacancies would occur after a long time of oxidation, decreasing the growth of oxide scale.

What did we learn on the reactive element effect in chromia scale since Pfeil's patent?

Materials able to form protective chromia, Cr2O3, are most of time good candidates to be used at high temperatures in oxidizing atmospheres. Minor element additions are necessary to increase their mechanical or chemical properties. Among them, the reactive elements (RE) are highly efficient to improve the high temperature oxidation behavior of alloys. Their addition in small quantity is enough to greatly decrease the oxidation rates, but above all to drastically increase the oxide scale adherence to the base materials. That was the key point of what observed Pfeil 75 years ago. Since that date, many results have been published, many theories have been proposed, and many discussions have been engaged. Until now, the reasons why RE are so efficient are not fully understood, several hypotheses have been proposed in order to explain their beneficial effect on the oxide scale formation. According to the large amounts of published papers on this topic, there is probably no single theory envisageable to explain their role, whatever their nature, the tested materials and then the native oxide scale, as well as the nature of the oxidizing atmosphere. The decrease of the oxidation rate and the improvement of the oxide layer adherence are the final results due to RE additions. Generally, RE additions favor the nucleation and the growth of scales. They decrease the oxide grain size, and consequently could change scale plasticity and creep, which can modify the growth and/or thermal stresses generated within the scale during its growth or its cooling to room temperature. They prevent the detrimental sulfur effect and suppress dislocation climb in the metal and, then, limit cation transport (poisoned interface model). 75th Pfeil's patent anniversary seems good to make a pause in order to analyze what we really understood on the reactive element effect in chromia‐forming materials.

Oxidation of Alloys 430 and 441 in SOFC Dual Atmospheres: Effects of Flow Rate and Humidity

The effects of flow rate and humidity on the high temperature oxidation behavior of alloys 430 and 441 in dual atmospheres (wet Ar-5vol% H2|dry or wet air) at 800°C were investigated. All the oxide scales were composed mainly of a surface (Mn,Cr,Fe)3O4 spinel layer and a Cr2O3 sublayer. Comparing to the air control samples, the air sides of alloy 430 and 441 exposed to a dual atmosphere showed different microstructures in some areas. For alloy 441, the higher flow rate of Ar-5vol% H2 resulted in thicker oxide scales and the formation of Fe-rich nodules on the air side of sample. In addition, particles with high concentrations of Ti and Nb were found near some of the Fe-rich nodules. The effects of high water vapor content in the air oxidation were different between alloys 430 and 441, which may be attributed to both differences in the alloy composition and alloy grain size.

High Temperature Oxidation Behavior of Alloy 617 and Haynes 230 in Impurity-Controlled Helium Environments

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 °C and 1000 °C in impure helium environments containing H2, H2O, CO, CO2, and CH4. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr2O3 layer, TiO2 ridges on the grain boundaries, and isolated MnCr2O4 grains on top of the Cr2O3 layer. On the other hand, those of Haynes 230 consisted of a Cr2O3 inner layer and a protective MnCr2O4 outer layer, which increased the oxidation resistance. The effect of small amounts of CH4 and H2 on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr2O3 oxides for alloy 617 and MnCr2O4 platelets for Haynes 230, was formed in the impure helium environment at 900 °C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr2O4 oxide crystals.

Oxidation behavior of Alloy 718 at a high temperature

The surface oxidation behavior of Alloy 718 is not well understood due to the chemical complexity of the alloy system. In this work, high temperature oxidation behavior of Alloy 718 in air at 1100 °C was investigated by varying the oxidation time up to 180 h. Three distinguished surface layers were observed depending on the oxidation time. Outside layer was single-phase Nb-rich Cr oxides with some voids and microcracks. The second layer had small amount of discontinuous phase (Al 2O 3). Both outside layer and second layer have often been observed in Ni-based superalloys. The third layer with complicate shapes of carbides was also observed in the present study. It was composed of the various (Ti x Nb y )C type carbides showing the different shape with the different ratio of x to y.

Improved high temperature oxidation behaviour of alloys by ion implantation

Numerous studies have been undertaken during the last decade examining the potential of ion implantation to improve the high temperature oxidation behaviour of a range of metals and alloys. Most of these investigations have focussed on improved mechanistic understanding of the role of specific elements and, in particular, of the effect of the so-called reactive elements, such as yttrium and the rare earths. It is now apparent that the ionimplantation studies have made a unique contribution and these will be reviewed.