Structure and properties of FeAl as influenced by processing
Doctoral thesis, 2007
Iron aluminides are candidate materials for a sustainable society since they are composed of abundant metals and have excellent corrosion properties in combination with high specific strength. However, there are some drawbacks that limit the practical use, e.g. low RT ductility and a drop in strength at high temperature.
Twelve different FeAl materials were produced with mechanical alloying and hot isostatic pressing, HIP, with differences based on both B-doping as well as on oxygen content and at which production step the oxygen was introduced to the milling system. Various aspects have been dealt with, i.e. powder characteristics, oxygen content, oxide formation during processing, microstructural development, mechanical properties, diffusion and, finally, vacancy densities attained at elevated temperatures. The analysis is done with following methods: OM, SEM, TEM, EDS, electron diffraction, EELS, XRD, AES, ESCA, chemical analysis of oxygen content, tensile tests, four-point bending, hardness, model analysis and heat treatments.
The main conclusions are as follows: The nature and thinness of the oxide scale on the alloyed powder is the most likely explanation for the successful use of HIP to consolidate FeAl, even with air exposed powder, a condition that could be compared to industrial production. Furthermore, the mechanical properties are not impaired by handling the powder in air, proved by the fact that a ductility of around 10% could be achieved. There seems to be an optimum oxygen content that can be much higher than the content after manufacturing in an inert gas atmosphere.
Grain boundaries are concluded to have a large effect on the global diffusion of Al in FeAl, even at 1150˚C, and the diffusion rate in a fine grained material can be 10 times as high as in a single crystal.
It was found that thermal vacancy hardening is dependent on Al-concentration (strongly), oxide dispersion strengthening and additional alloying, while grain size seems to have only a slight effect. Concluding, thermal vacancy hardening, which is commonly accepted to be the single hardening mechanism responsible for the anomalous stress peak for FeAl alloys in the vicinity of 40 at.% Al, can not be the single hardening mechanism for lower Al-concentrations.