Formation of nanocrystals by crystallisation of Zr-Al-Cu-Ni-Fe metallic glasses

The influence of iron addition on the crystallisation behaviour of Zr65Al7.5Cu17.5Ni10 bulk metallic glass was investigated by differential scanning calorimetry, (DSC), X-ray diffraction, (XRD), transmission electron microscopy, (TEM) and magnetisation measurements. The amorphous Zr65Al7.5Cu17.5Ni10 alloy crystallises eatectically into CuZr2 and Zr6NiAl2. Addition of iron in amorphous (Zr65Al7.5Cu17.5Ni10)(100-x)Fe-x (0 less than or equal to x less than or equal to 20) leads to a changed crystallisation sequence and to the formation of nanocrystals. The formation of a cubic NiTi2-type Phase (S.G. Fd (3) over barm, a(0) = 1.22 nm) is the first step of crystallisation in amorphous alloys with iron contents x greater than or equal to 1. Depending on the iron content the average crystallite size decreases to the nanometer regime. Ultrafine nanoclusters of down to 2 nm in size are formed as the first step of crystallisation for amorphous Zr52Al6Cu14Ni8Fe20 due to a high nucleation rate combined with a low growth velocity. The clusters grow by Ostwald ripening during isothermal annealing up to 5 nm average crystallite size. The magnetic behaviour of the (Zr65Al7.5Cu17.5Ni10)(100-x)Fe-x alloys is dominated by temperature-independent Pauli paramagnetism for x less than or equal to 15. For x = 20, a small contribution of magnetic clusters is observed. These ferromagnetic clusters are in accordance with statistic composition fluctuations within the homogeneous amorphous phase. With the formation of nanocrystals the size of the magnetic clusters increases to the same order of magnitude.