Orientation and formation of atypical Widmanstaetten plates in the Gd5(SixGe1−x)4 system

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to examine the bulk microstructure of Gd5Si2Ge2 and Gd5Ge4 compounds, and specifically the Gd5(Ge,Si)3 thin plates seen in these systems. The orientation relationship between the matrix and the precipitate thin plates was determined as [1¯010](12¯11)p//[010](102¯)m. High-resolution TEM images of the Gd5Ge4 were used to study the crystallographic relationship between the parent and matrix that exists along the interface boundary. The observed planar alignment and microscopic structure of the interface, which consists of ledges and terraces, indicate the presence of invariant line strain. The results are also consistent with the Δg approach suggested by Zhang and Purdy. A displacive-diffusional mechanism is proposed to explain the rapid formation of the precipitate Gd5(Ge,Si)3 plates.