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The effects of ultrasonic melt treatment (UST) and solution treatment on the microstructure and mechanical properties of a low-density multicomponent Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy at room and elevated temperatures were investigated. The as-cast Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy was composed of an fcc Al solid-solution, coarse secondary phases (5–30 μm), fine Cu-containing precipitates (<1 μm), and Zn clusters (<20 nm). The UST significantly reduced the porosity and size of primary Mg 2 Si particles and grains, resulting in an improvement in the strength and ductility of the as-cast alloy at 25 and 350 °C. The solution treatment at 440 °C caused the dissolution of hcp Zn and fine Cu-containing precipitates. It also resulted in the formation of Zn and Cu clusters in the supersaturated Al solid-solution at 25 °C, which led to an improved room-temperature strength. A phase transformation from Mg 2 Si to Q-Al 5 Cu 2 Mg 8 Si 6 occurred during the solution treatment along with the spheroidization of coarse secondary phases and grain growth, which deteriorated the ultimate compressive strengths at 25 and 350 °C and improved the ductility. Unlike the commercial Al alloys, the Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy showed excellent strengths at temperatures below 200 °C. This is attributed to the combined strengthening by coarse secondary phases, fine precipitates, and clusters. [ABSTRACT FROM AUTHOR]

Cr and Mo are known to be effective steel alloying elements for high-temperature applications, and they affect steel mechanical properties via carbide formation. Mechanical properties of Cr-Mo steels affected by the carbide evolution were investigated by performing heat treatments, and the treatment duration was determined by the Larson-Miller parameter fitting. The hardness value decreased after heat treatment at 690 °C for 10 h with the decomposition of the bainitic microstructure. The quantified chemical composition results for the carbides were plotted on the Fe-Cr-Mo ternary and Fe-Cr binary composition diagrams to classify the types of carbides formed. Most of the carbides were identified as Fe-rich M 3 C-type carbides at the initial stage of heat treatment. The Fe fraction in the carbides decreased with time while the Cr and Mo fractions increased. The Cr- and Mo-rich carbides were identified as M 23 C 6 - and M 2 C-type, respectively. An increase in the Cr and Mo fractions in the carbides resulted in the decrease in the alloying element concentration in the matrix, which in turn resulted in the deterioration of the solid solution strengthening. Further, it was determined that coarse carbides, observed on the grain boundaries after extended heat treatment, deteriorated the mechanical properties of the alloy. [ABSTRACT FROM AUTHOR]

This paper shows that the addition of transition elements significantly influences the microstructure and tensile properties of Al–12Si alloy cast under ultrasonic melt treatment (UST). The alloys contained primary Si, eutectic Si and a variety of intermetallic compounds (IMCs); the addition of Mn and Ni changed the solidification sequence by forming primary α-Al 15 (Fe,Mn) 4 Si 2 and ε-Al 3 Ni prior to primary Si formation. The primary Si and primary IMCs competitively consumed the nucleation sites introduced by UST, thereby influencing their refining efficiencies; the efficiency of primary Si refinement by UST increased with increasing the fraction of primary Si formed prior to IMC formation, whereas it deteriorated with increasing primary IMC fraction. The refining efficiency of eutectic Si and eutectic IMCs was not affected by the type and fraction of the primary phase. Using a Ti sonotrode for UST caused Ti contamination, resulting in grain refinement by forming Ti(Al,Si) 3 particles and increasing the amount of Ti solutes. Regardless of transition elements content, the application of UST improved the tensile strength at 25 °C and 350 °C. The refinement of IMCs caused by UST allowed the alloy to contain more IMCs without deterioration of room-temperature tensile strength. This enhanced the increment of high-temperature tensile strength by IMCs. Ductility of heavily alloyed systems was also improved by UST. [ABSTRACT FROM AUTHOR]