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Abstract
The transition of two first-order facet-to-rough ridges into a single first-order rough-to-rough ridge theoretically predicted for free surfaces has been observed in a slowly migrating grain boundary (GB). The reversible faceting-roughening transition of the tilt GB in a Zn bicrystal has been studied in situ. Above T
R+ = 673 K, the slowly migrating GB semi-circle was discontinuously curved, e.g. two rounded (rough) GB portions intersected with a slope discontinuity. Above T
R+, a single first-order facet-to-rough GB ridge has been observed. Below T
R− = 668 K, a facet appeared and intersected with rounded (rough) GB portions with a slope discontinuity. Below T
R−, two first-order facet-to-rough GB ridges existed. The GB facet and tangents to the rounded (rough) GB portions were parallel to the closely packed planes of the constrained coincidence sites lattice. The steady state length of the facet increased with decreasing temperature. A hysteresis of about 5 K between roughening temperatures measured on heating, T
R+, and on cooling, T
R−, was observed. The facet reaches its steady-state length not immediately after a change of sample temperature but with a specific retardation.
Acknowledgements
The authors thank Professor G. Gottstein and Professor L. Shvindlerman for stimulating discussions and the Russian Foundation for Basic Research (contracts 06-02-04015 and 05-02-16528), Deutsche Forschungsgemeinschaft (contract 436 RUS 113/842/0-1(R)) and INTAS (04-83-3659) for financial support.