Mg-7Gd-3Y-0.4Zr Alloy Microstructure And Properties

The microstructure, mechanical property and corrosion behavior of Mg-7Gd-3Y-0.4Zr (GW73K) were investigated by optical microscopy(OM), transmission electron microscopy (TEM), field emission scanning electron microscope (FE-SEM), EDS, XRD, immersion test and potentiodynamic polarization. Meanwhile, the microstructure and corrosion resistance of oxide films formed in different temperatures were also discussed. The main results show that:(1) The precipitation sequence for GW73K was super-saturated solid solution (S.S.S.S.)â†'β"(DOi9)â†'β'(BCO)â†'β(Mg5Gd,FCC), and the main precipitate wasβwhen the alloy was peak-aged at 200℃,225℃and 250℃.(2) When GW73K was aged at 225℃and 250℃, the solubility of Gd and Y in Mg and coarseβ'largely reduced the density number ofβ'. The hardness improvement of peak-aged alloy was mainly ascribed to the presence ofβ'phase. The volume fraction ofβ'phase decreased with increasing aging temperature while the average size of the precipitate particles increased. This alloy, when peak-aged at 200℃for 120h, displayed the highest hardness owing to the existence of a great manyβ' precipitate particles with a quite small size and uniform distribution in the matrix.(3) Heat treatment influenced the morphologies of corrosion products of GW73K alloy. The corrosion product was identified as Mg(OH)2 by XRD. For F sample, the corrosion product was composed of fluffy particles. For T4 sample, the product was compact and uniform with triangle shape particles. Two totally different morphologies appeared in T6 sample because of the presence ofβ'precipitations with a discontinuous distribution. The corrosion rates were T4> T6>F due to different morphologies of corrosion products.(4) The corrosion rates of the peak-aged samples increased as the isothermal aging temperature rise. The higher corrosion rates can be ascribed to the micro-galvanic corrosion acceleration byβ'phases. Theβ'precipitates were larger and appeared to be more active as galvanic cathodes for the alloy peak-aged at the higher aging temperatures. The potentiodynamic polarization curves showed that the corrosion current densities for the samples peak-aged at 225℃and 250℃were higher than for the sample peak-aged at 200℃. The results for the polarization curves were consistent with the microstructures of all the samples.(5) The film formed at solution temperature was more compact than the films formed in room temperature and aging temperature, and the thickness of the solution film was the thickest. Hence, the film can be easier to protect Mg matrix from NaCl solutions. In addition, during solution, Gd and Y were also oxidized. Therefore, the improvement of corrosion resistance of the alloy had strong connection with the formed oxide.