Multimodal Analysis of Phases Formed by Reaction Diffusion in Binary Fe-Al System

abstract

English

This work deals with identification possibilities for very fine microstructure phases formed by reaction diffusion in binary Fe-Al system. The Fe-Al samples were prepared by low-pressure cold spray technique using heated air (temperature 300 °C, pressure 1.5 MPa) as working gas. Pure Fe and Al powders mixtures were deposited on aluminium substrate. Prepared deposits were subsequently annealed isothermally at temperatures in range from 250 °C to 750 °C for two hours in protective argon atmosphere. Thereafter, phase composition and structure were examined on standard metallographic polished samples. The samples of FeAl elemental mixture in the form of solid cuboids 8x8x4mm were annealed at temperatures from 500 °C up to 750 °C. The general interest in these samples lies in the fact that by solid state or liquid-solid diffusion new intermetallic material with open porous structure is formed. However, the microstructure of the material is generally composed of fine grains or mixtures of phases, where identification of these phases by standard methods is difficult. First of all, the Energy Dispersive Spectroscopy - EDS (point analysis and line analysis of diffusion interfaces) and (bulk) X-ray diffraction - XRD were used. Because of too small size of the individual phase particles, the X-ray Diffraction was inconclusive. Very fine lamellar structure of two different intermetallic compounds begins to form at temperature of 650°C and a new problem with EDS chemical analysis for phase identification of the phases analysis arise too. The reason of this problem is that we face physical or/and spatial resolution limits of EDS. This problem can be solved by technique with better spatial resolution like Electron backscatter diffraction - EBSD and Low Voltage Backscatter Electron (LVBSE) imaging as additional tool. Both these methods have a spatial resolution of several nanometres and existing results from more conventional methods can be enriched with new details. The diffusion profiles between individual metal particles in samples annealed at temperature 500 °C and 600 °C were analysed by EDS line and point analysis. Newly formed particles exclusively of the Al5Fe2 phase were identified in the material after annealing at temperature of 500 °C. Annealing at 600 °C further created FeAl and FeAl2 phases [1]. For higher annealing temperatures where complex microstructures with very fine features had evolved, EDS line and point analysis for diffusion profiles and EBSD analysis for phase analysis of the fine lamellar structures were used. For this lamellar microstructural mixture of phases the stable phase diagram and XRD data were obtained. However the data were unsufficient to identify the phases. Here EBSD was used to identify and the phase types. Main result of this work is the proof of concept in the ability to identify very fine mixed microstructures and phases in annealed binary Fe-Al system by combination of several methods which allows overall improvements and overcomes the limits of individual analytical SEM methods.

EDS, EBSD, Cold spray, diffusion, Fe-Al, binary system

ČUPERA, J.; JAN, V.

11. 9. 2015

Gottingen

173

175

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