Second-order faceting-roughening of the tilt grain boundary in zinc

The faceting of the almost stationary [10 (1) over bar0] tilt grain boundary with a misorientation angle theta of 84 degrees in the Zn bicrystal has been investigated. The shape of the very slow migrating grain boundary has been studied in situ between 350 and 400 C degrees using polarized light. Two intersecting facets lie in the closely-packed planes of the constrained coincidence sites lattice with coincidence parameter Sigma = 15. In the as-grown sample flat grain boundary facets form the sharp first-order ridge with the break of the first derivative partial derivative y/partial derivative x of the grain boundary shape similar to those observed in our previous works on GB faceting in Zn. However, above 350 degrees C the sharp first-order ridge is substituted by the smooth grain boundary portion without a partial derivative y/partial derivative x derivative break similar to that observed in twin grain boundaries in Mo. The transformation of the as-grown first-order grain boundary ridge into continuous one has been observed for the first time. The critical parameter alpha has been calculated using the grain boundary shape in the transition region. alpha increases from alpha = 1.7 +/- 0.07 for 350 degrees C to alpha = 1.9 +/- 0.07 for 385 degrees C approaching the alpha = 2 value predicted in the mean-field Andreev model for the continuous surface roughening.