PSI - Issue 23

Petr Haušild et al. / Procedia Structural Integrity 23 (2019) 179–184 Haušild et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. XRD patterns of FeAl20Si20 alloy prepared from different feedstock in the beginning (1h) and at the end (8h) of milling.

XRD patterns in the beginning and at the end of milling are shown in Fig. 3. First intermetallic phases were detected by X-ray diffraction after about 2 h of milling. Regardless the initial feedstock material, the progressive change of initial Fe, Al, FeAl or FeSi to Fe 3 Si and FeSi phases occurred as can be seen in Fig. 3. Final phase composition in all cases corresponds to predominantly Fe 3 Si with some content of FeSi. The fastest change of Fe (110) peak at 2θ = 44.68° to Fe 3 Si (220) peak at 2θ = 45.35° seemed to be in the sample Fe_AlSi (prepared from pre-alloyed AlSi30 powder (already after 2 h of milling). However, no substantial change in the phase composition occurred after milling times ≥ 6 h comparing powders prepared from different feedstock, which is in agreement with the microstructural observation and nanohardness measurement. Faster kinetics in powder mixture with pre-alloyed AlSi30 powders (comparing to the powder prepared from the elemental powders only) in the beginning of mechanical alloying, can probably be attributed to the effect of Si fine dispersion in the Al-Si eutectic. The same positive effect of Al-Si eutectic mixture on reactions between transition metals (titanium and iron) with aluminium and silicon has also been proved previously during their production by reactive sintering (Novák et al. (2010)). Moreover, the friction between ball-powder-mould vessel plays an important role in the kinetics of the process. Friction between ball, powder and mould vessel increases locally the temperature (it was shown in NiTi system that temperature increase exceeded 600 °C - Novák et al. (2018)), which significantly contributes to the mechanical alloying process. The need of the temperature increase as a result of the friction during mechanical alloying was demonstrated on Ni-Ti and Fe-Al systems, where the addition of ethanol as a lubrication agent or cooling of the mould vessel completely suppressed the reactions leading to the formation of intermetallics during 24 h of milling (Novák et al. (2018)). The dispersion of fine Si-particles in highly thermally conductive Al matrix in Al-Si eutectics favors therefore the rapid reaction with Fe powder particles and accelerate the onset of the solid solution formation. However, the content of fine Si particles in the Al-Si eutectic is limited. When the fine Si particles in the Al-Si eutectic are depleted, the coarse primary Si-phase in pre-alloyed Al-Si powder acts similarly as added Si elemental powder so that the kinetics of mechanical alloying is nearly the same as in the case of elemental powders. In the case of initially hard feedstock powders, the kinetics of mechanical alloying is delayed. The combination of two hard powders, i.e. FeAl and Si, led to the slowest increase of hardness during milling as the prevailing mechanism of mechanical alloying was fracturing and cold welding of fragments. The combination of (very) hard FeSi with soft Al led firstly to the decrease of the (average) hardness of particles as the convoluted lamellae with FeSi and Al altering were formed, then the similar evolution (increase) of hardness follows with subsequent formation of solid solution.