The structural transformation in pure cobalt and in powder mixture Co - 3, 10, 20 and 50 at. % Al during mechanical alloying (MA) were studied by X-ray diffraction and differential scanning calorimetry (DSC). Additionally, the influence of deformation under ball milling on cobalt modifications and on arc melted alloys with similar compositions were investigated.
The solid solutions Cofcc(Al) were formed during MA by diffusion mechanism. The CoAl of Co-50 at. % Al (type CsCl) composition was formed by explosive regime without the intermediate stage of chemical interactions.
Two cobalt modifications, i.e., face-center-cubic (fcc) and hexagonal close-packed (hcp) phases usually coexist at room temperature. It was found that initial stage of MA was accompanied by structural transformation of fcc cobalt into equilibrium hcp modification. The long milling lead to the solution of aluminum in cobalt and structural transformation of hcp Co to fcc Co (Al). The fcc Co structure of after following annealing in DSC up to '720^o'C is conserved.
The substructure parameters changes (block size and microstrein) and lattice parameters of fcc and hcp cobalt modifications were studied. The milling of CoAl + Al mixture leads to amorphous phase formation. The milling of mixture CoAl + Co results in the formation of fcc Co(Al) solid solution.
Besides, the evaluation of probability of stacking faults formation in Co and in Co(Al) solid solution were monitored. The phase transformation of Co (hcp to fcc) induced by ball milling was caused by the accumulation of structure defects (stacking fault).
Similar investigations of substructure evolution were carried out for equilibrium alloys with the same compositions during transformation into nanocrystalline state under the ball milling.
The temperature stability of MA alloys was studied by DSC method and annealing.
The work was supported by RFBR (projects 03-02-06043 and 02-02-16154).

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