Long-Range Ordering Kinetics in B2-ordered Intermetallic Compounds

The main aim of the present project is an investigation of ordering kinetics in B2-ordered alloys by residual resistometry. Two model substances will be used: B2 FeA1 as an example for a directly ordering alloy and B2 beta-CuZn as an example for a sequential ordering alloy. The investigation should yield still missing but extremely needed ordering relaxation times and ordering actication energies and give insight into atomic jump mechanisms during ordering and disordering, which may be different to usual diffusion parameters. In addition, Monte Carlo simulations will be done starting from our recent procedure for ordered L12 -ordered structure. After optimizing the algorithm and extending the investigation to include activation barriers and separately chosen pair interaction energies the simulation will be applied to the ordering kinetics in B2-ordered alloys.

FeAl has been investigated with respect to the kinetics of B2 long-range ordering. In a FeAl alloy Fe and Al atoms are not distributed at random over the crystal lattice points. Instead, Fe atoms prefer the cube corners of the body centred cubic lattice and Al atoms prefer the body centres or vice versa (`long- range order`). In the present project changes of this preferential occupation of lattice sites during adequate temperature treatment was investigated by measuring electrical conductivity with very high accurycy. It turned out that the establishment of order occurs different in different temperature regions: At low temperatures (or correspondingly short annealing times) higher ordered, local regions are formed. At high temperatures (or correspondingly long annealing times) these regions begin to overlap and a long-range ordering in the total sample volume is observed. The reason for this observation is that all changes in a crystal occur by jumps of the crystal atoms into neighbouring vacant lattice sites (vacancies), always being present. As a consequence all structural changes at low temperatures or correspondingly short annealing times due to the restricted atom mobility will occur in restricted regions only. This result has far-reaching consequences for phase transformations. If the atom jumps to empty nearest neighbour sites are too slow for a long-distance diffusion, `particles` of the new structure are experimentally observed also for the case of a `homogeneous` phase transformation, when variations should occur in the total sample volume.