Wu Youshi

Origin of the prepeak in the structure factors of liquid and amorphous Al-Fe-Ce alloys

Distinct prepeaks have been found in the structure factors of the liquid and amorphous Al-Fe-Ce alloys. The structural size of the chemical short-range order corresponding to the prepeak in the liquid alloy increases with decreasing temperature, and, after glass forming, the structural size is much bigger, and the amount of the structure corresponding to the prepeak increases with decreasing quenching temperature, but the structural unit size remains constant. The position of the prepeak shifts to smaller Q-values as the concentration of Ce increases. The addition of Ce can improve the interaction between atoms, so it is favourable to Al-based glass formability. The structural unit corresponding to the prepeak is an icosahedral quasicrystalline structure with Fe as the interstitial atom, and the prepeak is diffraction peak broadening caused by fairly fine (about 0.5-2.0 nm) icosahedral clusters.

Formation of medium-range order clusters in Al-Ni-Ce amorphous and liquid alloys

The small-angle information in the total structure factors and x-ray diffraction pattern have been investigated. Prepeaks were found to exist in the total structure factors in the Al90Ni5Ce5 liquid and amorphous alloys. The prepeak position is almost the same in the liquid alloys at different temperatures and are nearly identical when the liquid alloys were quenched to the amorphous state. The medium-range order clusters are formed in the liquid state and the structural unit can be preserved to the amorphous state. The amorphous Al90Ni5Ce5 alloys crystallize in two stages. The first stage corresponds to the precipitation of face-centred-cubic Al and in the second crystallization stage intermetallic compounds are formed. The structure corresponding to the prepeak is preserved, and even strengthened, during rapid solidification, but is not stable and decomposes at room temperature. The structural unit is like a large atom during rapid solidification, which may be the reason for the high glass forming ability in Al-Ni-Ce systems.

Effects of the TM/Ce content on the stability of amorphous Al–TM–Ce alloys

X-ray diffraction (XRD) and differential scanning calorimetry were used to investigate the crystallization process of amorphous Al90TMxCe10−x alloys. Ageing effects were also examined by means of XRD. The structure corresponding to the prepeak for amorphous Al90Fe5Ce5 alloys is more stable than the amorphous matrix. However, amorphous Al90Ni5Ce5 alloys are not stable during the first crystallization stage, and even decompose at room temperature. The crystallization onset temperature for amorphous Al–Fe–Ce alloys is much higher than that for amorphous Al–Ni–Ce alloys. This is probably caused by the different stability of the structure corresponding to the prepeak. The crystallization onset temperature increases as the Ce/Ni ratio increases in amorphous Al90NixCe10−x alloys. However, the crystallization onset temperature decreases as the Ce/Fe ratio increases in amorphous Al90FexCe10−x alloys. A better atomic packing results as the Ce content increases due to size mismatch in Al–Ni–Ce systems and as the Fe content increases due to the increasing size of structural units in Al–Fe–Ce systems.