Discontinuous precipitation at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals

Abstract

The misorientation dependence of discontinuous precipitation (DP) at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals has been systematically investigated in the temperature range 523 to 723 K. A good correlation is found between both the incubation period, , to initiate DP and cell growth rate, v, and the energy of boundaries. The maximum of and the minimum of v occur where the cusps of the boundary energy exist. The formation and growth of DP are easier at higher-energy boundaries. The DP cells at [001] twist boundaries nucleate and grow less than those at [001] symmetric tilt boundaries in the same-alloy bicrystals. A kinetic analysis of DP using the models of Turnbull and Petermann and Hornbogen has yielded grain-boundary diffusion data. Although the activation energy, Q b , of boundary diffusion changes with the models, the values of Q b are smaller than the activation energy for volume diffusion of Be in Cu. The diffusivity in a boundary shows a close correlation with the energy of the boundary. A lower-energy boundary has a lower diffusivity with a larger activation energy and a larger pre-exponential factor.

Initiation and growth of the discontinuous precipitation reaction at [0 1 1] symmetric tilt boundaries in Cu–Be alloy bicrystals

“…11 revealed that, for the two models, the larger the Q b value, the larger is the (s D b ) 0 value. This result is in accordance with that of the previous studies [2,3].…”

“…However, the cell growth rate of DP was unchanged and linear in spite of the formation of ", ' or . This is in agreement with the result of our previous work [2,3]. That is, the DP cells at the [001] symmetric tilt boundaries nucleate more rapidly than those at the [011] tilt boundaries.…”

“…The interlamellar spacing was determined by the method previously reported by Tsubakino et al [8,14], as in previous studies [2,3]. The foils for TEM were thinned at 5 V in a twin-jet R e v i e w C o p y 4 electropolisher using a 20 % solution of phosphoric acid in water at about 5 FC.…”

“…Following our previous studies [2,3], kinetic analyses of DP in the present study have been confined to utilizing the models of Turnbull (T) [4] and Petermann and Hornbogen (P H) [5]. The equation of T on DP is written as…”

“…In previous articles, we have studied systematically the formation and growth of DP at various [001] symmetric tilt [2] and twist [3] boundaries in orientation-controlled bicrystals of a Cu 0.75wt% Be alloy. For both types of boundaries, we have found that the incubation period for the cell formation and the cell growth rate at a boundary show a good correlation with the energy of the boundary.…”