ON THE MICROSTRUCTURE, HARDNESS AND IMPACT TOUGHNESS OF 356 AND 413 ALLOYS

摘要

The present study was performed on 356 and 413 alloys containing various amounts of hydrogen (H-2), titanium diboride (TiB2) and strontium (Sr). The results show that the eutectic silicon particles in non-modified alloys undergo coarsening during the solutionizing treatment. A new mechanism, based on the Ostwald theory, has been proposed to explain the observed phenomenon. Although the variation in melt treatment parameters has no significant influence on the alloy hardness, it does have a great effect on the alloy impact toughness. Best results are obtained when the alloy is degassed, Sr-modified and grain refined. Also, Sr-modified, degassed 413 alloys have been found to possess good impact toughness. Precipitation of Mg2Si phase particles in 356 alloy is more effective as a hardening agent than the Al2Cu phase precipitated in the 413 alloy. The increased Si content in non-modified 413 alloys accelerates crack initiation and propagation, leading to low toughness values. Modifying the 413 alloys with 200 ppm Sr results in a more even distribution of the spheroidized eutectic Si particles and hence improves the alloy toughness.