TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy.

The structure of Al3Zr precipitates in Al-1.0Mg-0.6Si-0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high-resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L12-Al3Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L12-structured Al3Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D023-type Al3Zr phase. The elongated precipitates (about 100 nm) were identified as D022-type Al3Zr. Energy-dispersive X-ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D022-Al3Zr as compared with D023-Al3Zr. [ABSTRACT FROM AUTHOR]