Microstructure and Microhardness Evolutions of High Fe Containing Near-Eutectic Al-Si Rapidly Solidified Alloy

Abstract

Al-11 wt.% Si-11 wt.% Fe (11.29 at.% Si-5.6 at.% Fe) melt was rapidly solidified into ribbons and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and microhardness technique. The Rietveld X-ray diffraction analysis was applied successfully to analyze microstructure and phase precipitations. On the basis of the aluminum peak shifts measured in the XRD scans, a solid solubility extension value of 1 at.% Si in α-Al was determined. SEM investigations confirmed presence of a spherical shape α-phase particles in addition to needle and spherical shape β-phase particles with contents of 1.1 wt.% and 10.1 wt.% as deduced by XRD analysis. During prolonged annealing process at 350°C/25 h, α-phase disappeared, β-phase content increased to 30 wt.%, and Si presence becomes more evident as deduced by XRD analysis. EDS analysis confirmed that these β particles observed in the as-melt spun alloy are of lower Fe content comparing to those usually observed in the as-cast counter-part alloy. Besides, the length distribution of needle shape β-particles has been shortened to be diverse from 1 to 5 μm. The as-melt spun ribbons exhibited enhancement of hardness to 277 HV and further increased during heat treatment (150°C/12 h) to 450 HV. This improvement of microstructure and hardness are the influence of microstructural refinement and modification obtained during the rapid solidification process.