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Recent Developments on Discontinuous Precipitation

Zięba P.

Archives of Metallurgy and Materials

2017

Vol. 62, iss. 2A

955--968

The discontinuous precipitation (DP) belongs to a group of diffusive solid state phase transformations during which the formation of a new phase is heterogeneous and limited to a migrating reaction front (RF). The use of analytical electron microscopy provided reliable information that there is no differences in the diffusion rate at the stationary grain boundary and moving RF of DP reaction. On the other hand, the use of “in situ” transmission electron microscopy observations indicated the importance of stop- go motion or oscillatory movement of the RF. During 2004-2016 period more or less 280 papers were published in which the terms “discontinuous precipitation, “cellular precipitation”, discontinuous coarsening” appeared either in the abstract or in key-words. In the present contribution, the research on the DP reaction will be reviewed taking into account new aspects of theories and modelling, new evidences and findings, effect of various factors including third element, external stresses, plastic deformation and GB orientation, occurrence in less known systems and alloys like: superalloys, nitrided Fe-based alloys and Cu-based alloys. Finally, some suggestions for the future research will be formulated.

Kształtowanie mikrostruktury i właściwości stopu Ti-48Al-2Cr-2Nb w procesie wyżarzania niezupełnego

Szkliniarz W.

Rudy i Metale Nieżelazne

2006

R. 51, nr 6

350-355

W artykule pokazano praktyczne możliwości wykorzystania reakcji „pogrubiania nieciągłego” do rozdrabniania ziarna i kształtowania właściwości stopów na osnowie fazy międzymetalicznej TiAl w procesie wyżarzania niezupełnego. Stwierdzono, że jest to możliwe tylko wówczas, kiedy wyżarzanie to jest poprzedzone zabiegami cyklicznej obróbki cieplnej. Pokazano, że poprzez zastosowanie zabiegów cyklicznej obróbki cieplnej i wyżarzania niezupełnego możliwe jest kilkakrotne zmniejszenie średniej średnicy ziarna stopu Ti-48Al-2Cr-2Nb. Pokazano przebieg zmian mikrostruktury kształtowanej w warunkach wyżarzania niezupełnego z udziałem reakcji „pogrubiania nieciągłego” oraz oddziaływanie temperatury i czasu wyżarzania niezupełnego na przebieg tej reakcji.

The paper presents practical possibilities of using the discontinuous coarsening reaction for grain refinement and forming of TiAl intermetallics phase based alloys properties in under-annealing process. It was established, that it is possible only when under-annealing is preceded by cyclic heat treatment. The discontinuous coarsening reaction starts on the former grain boundary and depends on nucleation and the growth in range of grain with lamellar structure, the new fine grains with considerably larger thickness plates. The process stops when products of discontinuous coarsening reaction take over the whole area of former grain. It was presented changes of microstructure formed in conditions of under-annealing with discontinuous coarsening reaction and influence of temperature and time of under-annealing on this reaction. It was established, that progress of discontinuous coarsening reaction and connect with effect of grain size refining is determined by temperature and time of under-annealing process. The time, after discontinuous coarsening reaction takes over area of all grains grows shorter with increasing of annealing temperature. It was showed that using of connected both cyclic heat treatment and under-annealing operations results in eight times decrease of average Ti-48Al-2Cr-2Nb alloy grain diameter and unexpected decrease of alloy hardness to level similar to state after homogenization annealing, which is resultant of opposed interaction of grain size refining and increase of lamellar microstructure thickness as a result of annealing.