Phase transformations upon crystallization of amorphous Al-Ni-RE alloys

Changes in the structure and properties upon the crystallization of the amorphous phase in a group of Al-based alloys were studied. The crystallization in all samples occurs in several stages. In the alloys containing Y and Yb, a nanocrystalline structure consisting of fcc Al nanocrystals and the amorphous matrix is formed at the first stage. The size of nanocrystals depends on the chemical composition of the alloy. The formation of the nanocrystalline structure increases the microhardness of the Al88Ni10Y2 and Al(88)gNi(6)Y(6) samples from 2.2 and 2.1 in the amorphous structures to 2.8 and 2.3 GPa in the nanocrystalline structures, respectively. Further heating causes the formation of metastable crystalline phases and decreases the microhardness (and strength) of the samples. The crystallization of samples containing Ce starts with the formation of a metastable phase. After the completion of the phase transformations, the alloys contain equilibrium phases such as Al, Al3Ni, and Al16Ni3Y in the Al-Ni-Y alloys and Al, Al3Ni, and Al4Ce in the Al-Ni-Ce alloys. The microhardness of the crystallized samples is 1.6-1.7 GPa, which is substantially lower than that of the nanocrystalline samples.