Heat-Resistant Al-Alloys with Quasicrystalline and L1₂- Precipitates

Abstract

We have been developing Al-Mn-Cu based alloys alloyed with minor additions of different elements. Small additions of beryllium enhance the formation of the icosahedral quasicrystalline phase (IQC) during solidification, especially during ageing. Upon solidification, primary IQC-particles may form, with sizes, ranging from 5 to 50 μm. IQC is also present as a part of binary eutectic in the interdendritic regions. More importantly, nanosized quasicrystalline precipitates can form during T5-treatment at temperatures ranging from about 250−450 °C. They are, in fact, metastable precipitates transforming to ternary T-precipitates (Al20Mn3Cu2) phase above 450 °C. The heat resistance can be increased considerably by the addition of Sc and Zr by forming L12-precipitates in spaces between quasicrystalline precipitates. In this paper, we studied three alloys, two Al-Mn-Cu-Be alloys and an Al-Mn-Cu-Be-Sc-Zr alloy. The alloys were produced by vacuum induction melting and casting into a copper mould. We investigated the response of the alloys to different heat treatments and their heat resistance at higher temperatures. It was shown that the alloys could be precipitation strengthened by ageing at 300 °C and 400 °C. The hardness of the alloy stayed at relatively high levels even at 500 °C, while more substantial softening occurred at 600 °C.