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

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.

Extended solubility of CoO in ZnO and effects on magnetic properties

The metastable solid solubility extension of CoO in ZnO (wurtzite) was investigated in precursor-derived powders as well as in thin films grown on sapphire substrates by pulsed laser deposition. A maximum solubility of 30% Co2+ in ZnO was achieved in the powders. Transmission electron microscopy (TEM) of the films revealed them to have grown epitaxially and retained up to nearly 40% CoO in solid solution, but some Co2+ precipitated as rock-salt. The temperature dependence of the metastable solubility limit in the ZnO–CoO system was assessed and is discussed in terms of the relevant thermodynamic factors. The magnetic properties of n-type conductive Zn0.79Co0.2Al0.01Ofilms were studied, yielding evidence of a ferromagnetic phase with a TC of 25 K and a second, magnetically ordered, phase with positive exchange and arguably a TC of ∼250 K. Connections between the properties and microstructural observations in high resolution TEM are proposed.