Transmission electron microscopy-investigation of the microstructure of rapidly quenched Co80B20 alloys

Transmission electron microscopy (TEM), high-resolution transmission electr on microscopy (HRTEM) and magneto-optical Kerr effect have been used to cha racterize different crystallization states of melt-spun Co80B20 alloys. The microstructures have been prepared by isothermal annealing at temperatures of T-A=325-600 degrees C for different annealing times. Samples annealed f or t(a)=30 min show an onset of the crystallization at annealing temperatur es T-A less than 325 degrees C with the primary precipitation of both face- centered-cubic Co and hexagonal close-packed Co embedded in the amorphous m atrix. By HRTEM it is revealed that Co grains with ordered as well as highl y distorted lattices are formed, whereby a characteristic feature of the la tter are stacking faults. The following step is the formation of orthorombi c Co3B with an average grain size of 7.6 mu m and of nanocrystalline Co gra ins, whereby still some amorphous phase is present. Fully crystallized samp les were obtained for annealing temperatures higher than 412 degrees C (t(a )=30 min). The decomposition of the large-grained Co3B in nanocrystalline, body-centered tetragonal Co2B and Co starts at temperatures of T-A>400 degr ees C. The volume fraction and average grain size of the remaining Co3B dec reases with increasing annealing temperature, whereas the Co2B grains show the opposite behavior. After a long time annealing for seven days at temper atures higher than 550 degrees C, the microstructure consists of fcc-Co gra ins with facet-like morphology and Co2B, but no Co3B have been detected. (C ) 1999 American Institute of Physics. [S0021-8979(99)04511-9].