Oxidation Start Detection on Heating Parts of Thermogravimetry Curves for High Temperature Alloys Based on Nickel, Cobalt or Iron
The heating parts of thermogravimetry curves performed for cast alloys in air at high temperatures were analysed, after correction from the effects of air buoyancy variation, to characterize the transient oxidation before reaching the targeted temperature for the isothermal stage usually applied thereafter. This was done here for three binary M-30Cr alloys, six ternary M-30Cr-0.4 and 0.8%C alloys, and three M-30Cr-0.4C-6Ta quaternary alloys, heated in synthetic air at 20 K min-1 until 1000, 1100 or 1200°C. The cobalt alloys begin to be oxidized with a sufficient mass gain to be detected by thermogravimetry, earlier than the nickel or iron alloys (i.e. at lower temperatures). The temperature of oxidation start is lowered by the presence of tantalum for the three families of alloys. When the carbon content (or the carbides density) increases in the alloy, the temperature of oxidation start decreases for the cobalt alloys and remains almost constant for the nickel or iron alloys, while the total mass gain due to oxidation during heating increases for the nickel and cobalt alloys and decreases for the iron alloys.
Thermogravimetric Study of Oxide Spallation for Chromium-Rich Cast Cobalt-Based and Iron-Based Alloys Oxidized at High Temperature
During temperature cycles, metallic alloys for high temperature applications are usually oxidized with forma- tion of an external protective oxide scale, but they loose it during the cooling. This problem of oxide spallation can be studied by specific tests of cyclic oxidation but first indications can be provided by analyzing the cooling parts of thermo- gravimetry curves. This possibility was studied in this work for simple {Co or Fe} - 30 wt.% Cr alloys containing be- tween 0 and 0.8 wt.% of carbon, and for two of these alloys after addition of about 5 wt.% of tantalum. These alloys, elaborated by foundry process, were exposed to oxidation at 1000, 1100 or 1200°C for 50 hours, with recording of their mass gain during heating, isothermal dwell and cooling. The analysis of the cooling part of the thermogravimetry files al- lowed specifying several data characterizing the spallation of the external oxide, such as the temperature at which the phenomenon begins and the rate with which the mass decreases because of the loss of oxide. Metallographic data about oxidation and spallation were obtained by examination of cross-sections of the oxidized samples. Oxide spallation, char- acterized by its beginning's temperature and its corresponding mass loss rate, was obviously more severe for the cobalt al- loys than for the iron ones, for the carbon-free alloys than for carbides-containing ones and for tantalum-containing alloys than for the tantalum-free ones. Most of these effects were explained.
InTlP — a proposed infrared detector material
In1−xTlxP is proposed as a promising material for infrared detectors. A number of key optical and structural properties are studied within local density‐functional theory. In1−xTlxP at x=0.67 and In1−xTlxAs at x=0.15 are estimated to have a gap of 0.1 eV. Their binding energies are larger than that of InSb, and they are found to form stable zinc‐blende alloys for all x. In1−xTlxP nearly lattice matches to InP, and offers the potential to integrate detector array and read‐out circuit.