Abstract

Nanostructured metals and alloys are known to behave differently during annealing comparing to their micrograined counterparts. They exhibit lower thermal stability and often abnormal grain growth occurs. In the present work, the combination of severe plastic deformation processing  followed by subsequent conventional annealing and annealing under high hydrostatic pressure was applied. Samples were subjected to the heat treatment at 700°C for 10 minutes either under atmospheric or hydrostatic pressures of 2 or 6 GPa. The recrystallization process is investigated in TEM observations using HD2700 Hitachi. In samples after deformation one can notice a high density of nanotwins inside nanograins (Fig 1 and 2). In samples after annealing nanograins appear, the smallest of about 50 nm in the diameter (Fig 1 and 3). The main question refers to the differences in the microstructure after conventional annealing and annealing under high pressure. At this stage, one can predict that diffusion processes are suppressed by high pressure. The question arises as to what information about the grain boundary migration mechanisms can be drawn from high pressure studies. There are only a few papers where influence of high pressure on the grain growth [1] and migration of individual grain boundaries [2] was studied. It was found, that the activation energy and activation volume for grain boundary migration in aluminium bicrystals are larger than that for grain growth in aluminium polycrystals and depend on the grain boundary crystallography. Moreover, the high pressure had greater impact on the movement of high-angle grain boundaries than low-angle grain boundaries [3]. These theories refer to microstructural materials and they will be verified in the case of nanostructured materials.

References

[1] Hahn, H. and Gleiter, H. (1979). Scripta Metall., 14, 3-6.

[2] Molodov, D.A., Swiderski, J., Gottstein, G., Lojkowski, W. and Shvindlerman, L.S. (1994). Acta Metall. Mater., 42, 3397.

[3] Sursaeva V., Protasova S., Lojkowski W., Jun J. (1999). Textures an Microstruct., 32, 175-185

Acknowledgements

This work was supported by Polish NSC project No.UMO-2014/15/D/ST8/00532

Figures:

Fig. 1 Microstructure of an austenitic stainless steel after severe plastic deformation

Fig. 2 Microstructure of an austenitic stainless steel after severe plastic deformation-nanotwins

Fig. 3 Microstructure of an austenitic stainless steel after severe plastic deformation and conventional annealing at 700°C/10 min

Fig. 4 Microstructure of an austenitic stainless steel after severe plastic deformation and conventional annealing at 700°C/10 min - nanograins

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