Effects Of Chemical Short-range Order On The Formation And Thermal Stability Of Al-based Amorphous Alloys

Al-based amorphous alloys containing transition metals (TM) and rare earth elements (Re) have attracted considerable scientific and technological interests due to their good mechanical properties. Work on these materials has found that partially crystallised alloys via the precipitation ofα-Al nanocrystals within the amorphous matrix, also termed nanocomposites, may display outstanding strength and promising wear as well as corrosion resistance.We have noted that the Al-based metallic glasses have unique glass-forming ability and they do not follow the empirical rules developed for other glass-forming systems. However, the mechanisms underlying this process are not clearly understood and these materials have low thermal stability which limit their technological application. In this paper, we selected the Al-Fe system, the quasicrystals-forming system, as well as Al-Zn system, a typical decomposition system, as our model system. X-ray diffraction, DSC and HRTEM were used to investigate microstructure, glass-forming ability and crystallization behavior of the amorphous Al-Fe-Ce and Al-Zn-Ce alloys. The effects of chemical short-range order on the formation and thermal stability of Al-based amorphous alloys had been studied.During the rapid solidification of amorphous Al-Fe-Ce alloy, the icosahedral short-range order formed and homogenously distributed in the amorphous matrix. The presence of icosahedral short-range order seems to retard the growth of fcc-Al. It can be concluded that the grain growth of the fcc-Al needs the decomposition of the icosahedral short-range order and the redistribution of the solute elements. Strong interaction between Al and Fe make the icosahedral short-range order very stable. The stable icosahedral chemical ordering retards the redistribution of the solutes elements and causes the excess of Al forming the amorphous phase. The presence and homogeneous distribution of icosahedral chemical short-range order are attributed to the enhanced formability and stability of the amorphous phase.The prepeak of the Al-Zn-Ce alloys indicate the strong interaction of the atoms, which should be related to the formation of Al₂Zn₂Ce and Al₄Ce intermetallic compounds. Generally, as the concentration of Zn and Ce increases, intermetallic compounds appear, which prevent the original phase from separating into Al and Zn phases, and improves the stability of the single phase.There exists competitive nucleation and growth limitation behavior in the Al-Zn-Ce alloy. In Al₈₃Zn₁₀Ce₇ alloys, the precipitation of fcc-Al was accompanied with the Al₂Zn₂Ce and Al₄Ce phase. Moreover, the presence of fcc-Al appears to favour the nucleation and growth of the Al₂Zn₂Ce and Al₄Ce phase. However, it seems that the Al₂Zn₂Ce and Al₄Ce nucleat competitively with the fcc-Al phase in the Al-Zn-Ce alloys. But, the growth of fcc-Al and Al₂Zn₂Ce phase is more suitable than the Al₄Ce phase. The competitive nucleation and growth limitation of the various phases are critical for the glass formation of Al-based alloys.During the crystallization of Al86Zn5Ce9 alloy, there appear some non-equilibrium phases, such as Al92Ce8 phase and an unknown phase. Both of them have high stability. Compared the Al92Ce8 phase, the unknown phase have more higher stability. The appearance of shoulder peak in the DSC should be related to the decomposition of the unknowen phase.With the increase of Ce, a lot of Al-Ce clusters form in the quenched alloy, and the chemical short-range order increase, which make the packed structure more densed and improve the compound-coming tendency, then increased the stability of Al-Zn-Ce alloy. Compared the alloys as quenched, the weak exothermic peak at more lower temperature in the DSC curve of aged Al-Zn-Ce alloy could be related to the formation or coarsening of vacancy-rich clusters.We can suppose that the different transition metals have different kinds of chemical short-range order in the Al-based alloy. The chemical short-range order has important effect on the amorphous formation and thermal stability as well as microstructure of Al-based alloys. Compared to the chemical short-range order structure in Al-Zn-Ce amorphous alloy, the icosahedral chemical short-range order structure in Al-Fe-Ce amorphous alloy is more favorable to the formation and stability of the Al-Fe-Ce amorphous alloy.