High-Temperature Oxidation Behavior of Fe-20Cr-4 Al Alloys with Small Additions of Cerium(Metallurgy)

The oxidation behavior of Fe-20Cr-4 Al alloys, some containing 0.01, 0.04 and 0.37%Ce, was studied in air at temperatures between 1273 and 1523K by weight-change measurements, X-ray diffraction, electron probe microanalysis and scanning electron microscopy. The surface oxide predominantly formed on all the alloys was α-Al_2O_3. The marker and kinetic studies suggested the α-Al_2O_3 scale grew into the alloys by inward diffusion of oxygen along the oxide grain boundaries. In the alloys without Ce and with 0.01% Ce, the α-Al_2O_3 Scale spalled during cooling from the oxidation temperature. Spalling of the surface oxide, probably caused by the formation of voids at the oxide-alloy interface, initially occurred at the intersection of the alloy grain boundaries, and subsequently extended to the interior of the grains. No spalling was observed on the surfaces of the alloys with 0.04 and 0.37% Ce. In both alloys, cerium primarily existed as a Ce-Fe inter-metallic compound throughout the alloy matrix, which mostly precipitated at the grain boundaries. Higher cerium additions resulted in good adherence of surface oxide due to the prevention of voids formation at the oxide-alloy interface, and pegging of intergranular oxides preferentially formed at the alloy grain boundaries

Effect of Heat Treatment on the Microstructure of Amorphous Si-N-C Fine Powders

Crystallization processes of four kinds of amorphous Si-N-C fine powders were studied by X-ray diffraction, IR spectroscopy, transmission electron microscopy, analytical electron microscopy and chemical analysis. The samples examined had varying contents of Si, N and C (Si : 53-58 mass%, N : 24-36 mass%, C : 5-20 mass%), and were prepared by pyrolysis of hexamethyldisilazane ((Me_3Si)_2NH, Me=CH_3). These samples were heat-treated in an argon atmosphere at a temperature between 1713-1803K for periods of up to 21.6ks. All of the fine powders examined were nearly spherical and were 100-500nm in size before heat treatment. The first sign of crystallization of the powders was observed after 21.6ks heat treatment at 1713K. X-ray diffraction showed that the crystallized phase consisted mainly of β-SiC and α-Si_3N_4 . The formation of β-SiC occurred during the early stage of heat treatment. The formation of α-Si_3N_4 then followed. After heat treatment for 21.6ks at 1743K, more than 80% of crystallinity was observed in all of the samples examined. The quantity of α-Si_3N_4 and β-SiC in the samples was related to the carbon content in the powders. The crystallized β-SiC was between 20-100nm in size. On the other hand, the size of α-Si_3N_4 was consistently larger than that of β-SiC and depended on the nitrogen content in the powders

An Organo-Metallic Polymer Used in Powder Metallurgy : The Effect of Polycarbosilane in Iron-Chromium Alloy(Metallurgy)

A new material was developed by introducing an organo-metallic polymer into powder metallurgy. In the uniform mixture of Fe-13Cr^* alloy powder and polycarbosilane (PC) using n-hexane, the Fe-13Cr particles were coated with PC. The product of Fe-13Cr+10wi%PC, obtained by hot-pressing the mixture, was subjected to an oxidation test, high-temperature hardness measurement and a wear resistance test, and found to be superior in all respects to that without the PC addition. The structure was observed by transmission electron microscope and it was found that grains of CrSi_2 and Cr_7C_3 about 0.1μm in size, dispersed uniformly in the Fe-13Cr+10%PC, contributed to improvement of the mechanical properties. Observation by scanning electron microscope showed some difference in the formation of the oxidation film between Fe-13Cr and Fe-13Cr+10%PC. This new alloy, while adding an organo-metallic polymer to powder metallurgy, has several outstanding features with the possibility of many applications in the future