Influence of magnesium on high-temperature structural-phase stability of Al-Ni-La system alloys

The paper considers a relevance of the Al-Ni-La system cast alloys development as promising materials for application at elevated temperatures. The influence of magnesium on the structural-phase characteristics of alloys-representatives with a nickel content of about 2% wt. and lanthanum - about 5,5 and 11,5% wt. were studied in the cast condition and after annealing at 425 ° C for 5 hours. It is shown, that the addition of magnesium in the amount of 0,6 wt%. to alloys with a lanthanum content of 5,5 % wt. helps to increase the size of the lanthanum-containing eutectic component in the cast state, but stimulates its grinding after annealing. Since doubling the lanthanum content, magnesium has almost no effect on the structure of the eutectic in the cast state, but intensifies the process of changing its structure during annealing. In this case, the size of the eutectic components is almost unchanged and can be compared with an undoped alloy. Increasing the magnesium content twice to 1,2% wt. in the alloy with a lanthanum content of 11% wt. leads to a noticeable enlargement of Al11La3 intermetallics. After annealing, this structural component retains the characteristics of a fibrous structure and at the same time increases in size by about half. The magnesium content in the eutectic zones and in the solid solution hardly changes after annealing. The obtained data indicate the possibility of using magnesium as an additional alloying element of cast heat-resistant alloys of the Al-Ni-La system, which is able to simultaneously contribute to their strengthening both under normal conditions and at elevated temperatures. In this case, magnesium, in the amount of about 0,6% wt., also helps to preserve the fine structure of the eutectic components at high temperatures. Keywords: Al-Ni-La, Al-Ni-La-Mg, alloying, structural stability, heat resistance.

Effect of Composition, High Magnetic Field and Solidification Parameters on Eutectic Morphology in Cu-Ag Alloys

High magnetic field (HMF) and solidification processes were changed during the solidification of both Cu-28 mass %Ag and Cu-72 mass %Ag alloys. The results indicated that the eutectic morphology in Cu-Ag alloys was affected by HMF, composition and solidification parameters. The lamellar spacing of Cu-28 mass %Ag alloy solidified by furnace-cooling was refined by the application of HMF owing to the decreased diffusion coefficient in mushy zone. The lamellar spacing in both Cu-28 mass %Ag sample held at the eutectic temperature and Cu-72 mass %Ag sample by slow controlling cooling was increased by HMF, which might be attributed to the dominated thermolectromagnetic convection. The lamellar spacing in Cu-72 mass %Ag alloys was increased compared with that of Cu-28 mass %Ag alloys because of the decreased growth rates. In Cu-28 mass %Ag alloy, however, fluid transverse velocity gradient was dominate rather than the growth rate and the imposition of HMF had reverse influences on the lamellar spacing. The lamellar-rod transition of Cu phase was promoted by HMF because of the increased Cu volume fraction in eutectic component. These results shed light on the dependence of eutectic morphology in Cu-Ag alloys on composition, external high magnetic field and solidification parameters.

Formation of ultrafine eutectic-like microstructures of various rare earth oxide-Al2O3 systems by use of amorphous phases

Ultrafine eutectic-like microstructures of various rare earth (RE) oxide-Al2O3 systems were formed by use of amorphous phases. This new method uses a low migration rate in the amorphous phases. Mixtures of RE oxide (RE: Yb, Dy, Er, Ho, Gd, Sm, Eu) and Al2O3 powders with the eutectic compositions were melted and quenched rapidly to form the amorphous phases. A heat treatment of the amorphous phases of various eutectic systems at 1000 and 1300 °C, for 30 min, formed RE aluminum garnet (RE3Al5O12)/Al2O3 phases or RE aluminum perovskite (REAlO3)/Al2O3 phases. Scanning electron microscopy observation of these materials heat-treated at 1300 °C showed eutectic-like microstructures, in which crystals of eutectic component were entangled with each other. Furthermore, the microstructures were much finer than those of materials generally prepared from eutectic melts. In this study, it was confirmed that this method is useful for the formation of ultrafine eutectic-like microstructures for many eutectic systems.