Won-Wook Park

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al2O3. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.

Microstructural change and precipitation hardening in melt–spun Mg–X–Ca alloys

Abstract Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified bymelt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400%C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200%C in the Mg–Al–Si–Caalloys mainly due to the formation of Al2Ca and Mg2Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg2Ca. TEM results revealed that spherical Al2Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(Hv:180) after aging at 200%C for 1h.

Precipitation hardening and microstructures of rapidly solidified Mg−Zn−Ca−X alloys

Two quaternary magnesium alloys based on ternary Mg-5wt.%Zn-4wt.%Ca containing either Co(2wt.%) or Zr(0.5wt.%) were rapidly solidified using melt spinning. The nano-scale second phase distribution and thermal stability were studied using TEM, STEM and EDS system. Microhardness measurements conducted in the aged condition have identified that the Mg−Ca−Zn−Co alloy presented a peak value at 150°C followed by a hardness decrease. The Mg−Ca−Zn−Co alloy showed refined quaternary precipitates distributed along the cell boundary and a refined grain structure, yielding higher hardness values than the Mg−Ca−Zn−Zr alloy at all aging temperatures.

Effects of Ti-addition on the characteristics of wear resistant Al-Si-Fe base alloys processed via rapid solidification

Rapidly solidified Al-Si-Fe base alloys were prepared by gas atomization, hot pressing and extrusion. To optimize wear resistance and mechanical properties, Al-20 wt.%Si-5 wt.%Fe base alloys containing 1–3 wt.%Ti were newly designed and characterized in detail. The additions of Ti (especially, ~2 wt.%Ti) effectively increased the wear resistance and mechanical properties such as tensile strength and hardness; however, the addition of 3 wt.%Ti was not desirable because of the precipitation of the primary Ti7Al5Si12 phase in the as-quenched state. Based on TEM analyses, the improved properties in the Al-Si-Fe alloys containing Ti were found to be due to the formation of the (Al, Si)3Ti phase finely dispersed in the matrix.

Superplastic-like behavior of Al-high Si alloys produced from rapidly solidified powders and ribbons

The high temperature tensile properties of hyper-eutectic Al-Si alloys were studied at temperatures between 683 and 813 K at initial strain rates between 8.3X10−4 and 4.2X10−1s−1. The alloys were prepared from prealloyed powders and ribbons, which were respectively fabricated by the centrifugal atomization and melt spinning, through the hot extrusion process at an extrusion ratio of 110:1. The extruded alloy bars prepared from the powders and ribbons, i.e. the powder-extruded and ribbon-extruded bars, have homogenous micro-structures with the fine silicon particles dispersed in the aluminum matrices for the Al-25Si and Al-15Si alloys. The maximum elongation-to-failure of the powder-extruded bar and the ribbon-bar are almost equal, 150%, for the Al-25Si alloy, In the Al-15Si alloy, the ribbon-extruded bar has superior elongation compared to the powder-extruded bar, that is, these are respectively 520% and 400%. The maximum elongation was attained at the relatively high strain rate of 10−2s−1 independent of the silicon content and solidification process.