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.

The Evolution of Microstructure, Mechanical Properties and Fracture Behavior with Increasing Lanthanum Content in AZ91 Alloy

AZ91 alloy is a widely applied commercial magnesium alloy due to its good castability, balanced mechanical properties and acceptable price, and lanthanum alloying has been proven to be one of the most effective methods to further improve its mechanical properties. Therefore, we reveal the evolution of microstructure, mechanical properties and fracture behavior with increasing lanthanum content in AZ91 alloy in this study. The magnesium matrix was significantly refined by lanthanum content, and this effect became more evident with increasing addition of lanthanum. The presence of Al3La precipitates significantly reduced the grain mobility and suppressed the formation of Mg17Al12 discontinuous precipitates along the grain boundaries. The rheo-cast alloys exhibited improved and balanced tensile strength and ductility after aging treatment. The fracture type of AZ91-La alloys could be classified as ductile fracture due to the presence of less quasi-cleavage planes and more dimples with a mixture of tear ridges and micropores. Due to the fully refined microstructure and the balanced mechanical properties, the AZ91–1.0La (mass%) alloy presented the greatest potential for industrial applications among the three studied AZ91-La alloys.

Effects of Rare Earth Lanthanum on the Solidification Structure and Hot Ductility of Fe-43Ni Expansion Alloy

Effects of lanthanum content on the solidification structure and the hot ductility of Fe-43Ni expansion alloy were investigated, the corresponding mechanisms were also discussed. The results showed that the macrostructures of the alloys were first significantly refined with increasing lanthanum content in the range of 0~0.025 % and then became coarse again with lanthanum content up to 0.04 %. La2O2S inclusions can serve as the effective inclusions sites promoting the refinement of the macrostructures. The changes of the macrostructures were influenced by the quantity density and the size distribution of La2O2S inclusions. The addition of 0.01–0.025 % lanthanum can improve the hot ductility over the whole testing temperature range, especially at 1000–1050 °C. The improvement of the hot ductility was mainly associated with the grain boundary strengthening and the acceleration of dynamic recrystallization by adding lanthanum. With addition of 0.04 % lanthanum, the hot ductility of the alloy became deteriorated, which was owed to the presence of brittle Fe-Ni-La intermetallic compounds.