The transition of 2 first-order facet-to-rough ridges into a single first-order rough-to-rough ridge, theoretically predicted for free surfaces, was observed in a slowly migrating grain boundary. The reversible faceting-roughening transition of the [10•0] tilt grain boundary in a Zn bicrystal was studied in situ. Above TR+ = 673K, the slowly migrating grain boundary semi-circle was discontinuously curved (2 rounded [rough] grain boundary portions intersected with a slope discontinuity). Above TR+, a single first-order facet-to-rough grain boundary ridge was observed. Below TR- = 668K, a facet appeared and intersected with rounded (rough) grain-boundary portions with a slope discontinuity. Below TR-, 2 first-order facet-to-rough grain boundary ridges existed. The grain boundary facet and tangents to the rounded (rough) grain boundary 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 5K, between the roughening temperatures measured on heating, TR+, and on cooling, TR-, was observed. The facet attained its steady-state length, not immediately after a change in sample temperature, but following a specific retardation.

Reversible Transformation of a Grain-Boundary Facet into a Rough-to-Rough Ridge in Zinc. B.B.Straumal, A.S.Gornakova, V.G.Sursaeva: Philosophical Magazine Letters, 2008, 88[1], 27-36