Morphology and crystallography of precipitates nucleated at grain boundary triple line in a two phase stainless steel and a β titanium alloy

Morphology and crystallography of precipitates nucleated at grain boundary triple line (T.L.) and/or in the vicinity of T.L. in Fe-24 mass%Cr-7 mass%Ni and Ti-22 mass%V-4 mass%Al alloys have been studied using TEM and SEM. The relative orientation relationships (O.R.) of the matrix/matrix and the matrix/precipitate were determined by Kikuchi pattern analysis. According to the crystallographical and morphological features, the microstructure in the vicinity of T.L. were classified into three types; Type I : T.L. nucleated precipitate with a globular morphology. Type II: grain boundary face nucleated precipitate with a film-like precipitate, and Type III: precipitation free T.L. In the case of Type II, the T.L. was eliminated by growth of the grain boundary precipitate. The probability of the appearance of these Types I, II and III were 20.1%, 71.9% and 8.0% in the Fe-24 mass%Cr-7 mass%Ni alloy and 26.5%, 64.7% and 8.8% in the Ti-22 mass%V-4 mass%Al alloy, respectively. In the Fe-24 mass%Cr-7 mass%Ni alloy, the crystallographical analysis revealed that the Type I gamma precipitate satisfies the {111}(gamma)/{110}(alpha) plane parallel O.R. with respect to at least two adjacent alpha grains at the same time. The {110}(alpha) plane, in which the {111}(gamma) plane of the precipitate was in parallel, had the smallest angle with the alpha grain boundary plane, since the precipitate was able to decrease its activation energy for the nucleation. Thus, the nucleation behavior of the Type I gamma precipitate was strongly dependent on at least two grain boundary plane orientation. The low probability of the appearance of Type I was due to such a restricted crystallographical condition. The T.L. orientation, therefore, plays a very important role in the nucleation behavior and the morphology of precipitates at the T.L. since the T.L. is junction of grain boundary planes.