Crystallization Of The Nanocrystals Forming Mechanism

The nanocrystalline alloys can be formed from some amorphous alloys that prepared using melt spinning method by annealing. Up to now,much attention has been paid to the formation mechanism of the nanocystalline microstructure. According to the literatures abroad,The formation mechanisms of the nanocrystalline microstructure have been studied on the influence of Cu atom and Nb atom during the process of nucleation and growth. We consider that the ability to form the nanocrystalline microstructure from the amorphous alloys is deeply related to their intrinsic dynamic characteristic. From the thermal expand coefficient and GPS (generalized phonon spectra) studies of amorphous Invar alloy in the past and the GPS studies of the nanocrystalline soft magnetic material at present,we find that the amorphous Invar alloys with the characteristic of phonon softening are favorable for the formation of nanocrystalline. The phonon softening phenomenon in the amorphous Invar alloys indicates that the interaction forces between atomics decrease and these alloys appear a characteristic of structural instability. So,we suggest that the structural instability of amorphous alloys increase the nucleation rate which is favorable for the formation of the homogeneous nanoscale grain structure.I .The thermal expand coefficient and the dynamic characteristic in the three series of Fe-Co-Zr,Fe-Cr-P-C and Fe-Si-B were investigated,their ability to form nanocrystalline microstructure also were tested.We investigated the thermal expand coefficient for the three series of amorphous alloys using the device for the thermal expand measurements and observed whether nanocrystalline microstructure can be formed in alloys by the transmission electron microscopy. The thermal expand measurements show that those amorphous alloys such as Fe Co Zr Fe Cr P C and Fe Cu Nb Si B have Invar80 10 10 76 4 13 7 73.5 1 3 13.5 9characteristic below the Curie temperature(Tc). Results from the transmission electron microscope show that these amorphous Invar alloys are very favorable to form the nanocrystalline microstructure,but those amorphous alloys without Invar characteristics are difficult for the formation of homogeneous nanoscale grains. Measurements of GPS for the three series of amorphous alloys were performed by means of inelastic neutron scattering(INS) below the Curie temperature. Compared with those amorphous alloys without Invar characteristic,all the three amorphous Invar alloys appear the softening of GPS,the more stronger in the Invar characteristic,the more softening in the phononspectra. Results are also show that the GPS in the low- energy region for the amorphous Feg6Co4Zrio alloybecome softening below the Curie temperature,compared with it's GPS above the Curie temperature.We propose that the phonon softening is one of the important characteristic of the amorphous Invar alloys,and the dynamic abnormal characteristic is deeply related with the ability for the formation of the nanocrystalline microstructure .II .The mechanisms for the formation of the nanocrystalline microstructure are investigated from the dynamics point of view.The amorphous Invar alloys are favorable for the formation of nanocrystalline alloy,but it is difficult for the amorphous alloy without Invar characteristic. It is suggested that the softening of the GPS for amorphous Invar alloys in the low-energy is closely related with the formation mechanism of nanocrystalline. The softening of the GPS for the amorphous Invar alloys predicates that the forces between atomics decrease,so these amorphous alloys appear a characteristic of structural instability. The structural instability of amorphous Invar alloys increase the nucleation rate which is favorable for the formation of the homogeneous nanoscale grain structure.