Structure and Thermal Stability of Co60Ge40 Prepared by Mechanical Alloying

The phase composition and structure of Co60Ge40 prepared by mechanical alloying followed by heat treatment are studied by x-ray diffraction, x-ray microanalysis, differential scanning calorimetry, and scanning electron microscopy. The results indicate that milling a 60 : 40 mixture of Co and Ge for 2 h leads to the formation of phase-pure, nanocrystalline β-Co5Ge3 (B82 structure). This phase is chemically inhomogeneous and metastable. On heating to 720°C, it transforms into a homogeneous, equilibrium phase of β-Co5Ge3. The transformation into the stable β phase occurs through a two-phase state, involving the formation of the orthorhombic phase Co2Ge. At t ≥ 630°C, Co2Ge dissolves in β-Co5Ge3.