Precipitation of quasicrystalline phase in rapidly solidified AlMnZr alloys

Abstract

It is well known that the limiting equilibrium concentration of manganese in aluminum can be extended markedly by rapid solidification. Various metastable precipitates have been also found in this alloy, namely G (2), G', G'',. In addition to these phases, others have reported recently the presence of a less stable phase, designated T, in Al-5 wt% Mn foils following short annealing periods. T phase is now of interest because of the striking resemblance of electron diffraction patterns to those obtained from a quasicrystalline phase having icosahedral point group symmetry, which is formed in the rapidly solidified Al-15approx. =35 wt% Mn alloys. In recent work on the precipitation characteristics of rapidly solidified Al-7 wt% Mn- 1 wt% Zr alloys, it has been shown that appreciable precipitation hardening takes place during annealing at temperatures between 350approx. =450/sup 0/C. In these cases, the Mn enhanced the precipitation hardening since the increase in hardening was about two times as much as that of the Al-1 wt% Zr alloys under the optimum conditions of temperature and Mn content. The results of X-ray diffraction analyses and TEM observations suggested that principal precipitate attributing to the hardening was a pseudomorphous phase of the equilibrium Al/sub 6/Mn. However,more » there remain to be explored many problems including more exact structural information about the phase. In the present paper the authors report preliminary studies of the structure of the precipitate phase and propose the age hardening of the Al-7 wt% Mn-1 wt% Zr alloys to be induced by the precipitate similar to a quasicrystalline phase that appears in rapidly solidified Al-Mn alloys containing a large amount of Mn. The study was mainly performed using conventional transmission electron microscopy and selected area diffraction.« less