Kinetics of Ordering in Cu 3 Au
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H. LANG and W. PFEILER Institut fur Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria ABSTRACT L12-long-range ordering (LRO) was studied by residual resistometry in Cu 3Au. Variations of LRO-parameter were produced by isochronal and isothermal heat treatments for different initial states of order. By comparison of order-order and disorder-order relaxations, the contributions to ordering of atomic jumps within ordered domains and of antiphase boundaries were separated. Analysing isothermal annealings yielded ordering energies of vacancies involved. INTRODUCTION The evolution of LRO in ordering alloy systems is of great interest with respect to materials application as well as for fundamental reasons concerning order/disorder phase transitions. Cu3Au is a model system where the development of LRO is accompanied by the formation of an antiphase domain structure. Long-time annealing there leads to almost completely ordered domains separated by disordered antiphase boundaries [1]. Recent computer simulations generally show that part of the changes of LRO-parameter may be brought about by an increase or decrease in antiphase volume [2]. EXPERIMENTAL Samples (wet chemical analysis: 74.8±0.8at%Cu and 25.2±0.3at%Au) were prepared by coldrolling to a thickness d=O.35mm. Because of the brittleness of Cu 3 Au several intermediate
annealings above the critical temperature were applied. After homogenising the samples for 48h at 873K they were slowly cooled to 723K, annealed for lh at 723K and quenched. Annealings up to 573K were carried out in a bath of silicon oil (AT---_50mK). Above 573K an usual quartz tube furnace (AT=±_IK) with purified argon was used. Ordering was investigated by resistivity measurements inliquid N2 relative to a Cu3Au dummy specimen (residualresistometry). RESULTS Isochronal annealing treatment: LRO-kinetics overview In fig. 1 relative changes in resistivity with temperature during isochronal annealing (At=10min, AT=10K) after quenching from 723K are given (fully recrystallized state). In addition LRO-equilibrium values as obtained by isothermal order-order annealings are shown. Quenched from above the order/disorder temperature, the sample initially is in the disordered state. Increasing the temperature during isochronal annealing (0) the first decrease of resistivity is observed at about 420K. This decrease is explained by the onset of LRO due to the beginning atomic mobility caused by quenched-in vacancies. At about 470K all surplus vacancies are
annealed-out so that the ordering process is temporarily stopped. At about 550K ordering starts 297 Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
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Fig. 1: Relative change of resistivity during isochronal annealing ((1") temperature). In addition LRO-equilibrium values are given (+).
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