Formation study of the ball-milled Cr20Co80 alloy

The ball milling of blended chromium and cobalt powders was carried out in a planetary mill in order to obtain a nanostructured Cr20Co80 alloy. The structural modifications at different stages of the ball milling are investigated with X-ray diffraction. Several microstructure parameters such as the crystallite size, microstrains, stacking faults, dislocation density and phase fractions are determined. As the milling proceeded, the chromium peaks disappeared progressively indicating the dissolution of the chromium atoms into the cobalt matrix. Disordered hcp-Co(Cr) and fcc-Co(Cr) solid solutions were formed after 24 h of milling. The hcp solid solution has a lower value of the crystallite size and a higher degree of microstrains and dislocation density than the fcc solid solution. For prolonged milling (48 h), plastic deformations introduce large amounts of stacking faults in the hcp structure leading to the reverse hcp–fcc transformation of the Co(Cr) solid solution. The kinetic parameters n = 0.81 and k = 0.11, obtained using the Johnson–Mehl–Avrami formalism, correspond to diffusion mechanisms through interfaces and dislocations.