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Nanostructure formation in austenitic stainless steel

Agnieszka T. Krawczyńska ,  Malgorzata Lewandowska ,  Krzysztof J. Kurzydlowski 

Warsaw University of Technology, Faculty of Materials Science and Engineering (InMat), Wołoska 141, Warszawa 02-507, Poland


Nano-metals are prospective materials for applications in a wide range of industries due to their properties which are significantly improved in comparison to the conventional counterparts. One of the ways of producing nanomaterials is hydrostatic extrusion (HE). This process, carried out either at low or high temperatures, allows for severe plastic deformation of metals at very high strain rates, which result in profound grain refinement down to the nano-meters scale. The aim of this work is to show the possibilities of grain size reduction by HE in an austenitic stainless steel.

Samples of 316L stainless steels were hydrostatically extruded, in a multi-step process with the total true strain of 1.84, and subsequently treated at different annealing temperatures. The changes taking place as a result of HE and annealing were observed using transmission electron microscope. Microhardness was also measured under the load of 200g.

The results show that the process of hydrostatic extrusion results in uniform microstructure consisting of nanotwins. The subsequent annealing at 600°C results in the partial transformation into nano-grained structure with the average grain size of 54 nm. At slightly higher temperature, at 700°C, a totally recrystallized uniform microstructure with nanograins of 68 nm in diameter is formed. It should be noted, that the temperature 700°C, is much lower than that for micro-grained samples of the same chemical composition.

The observed microstructural changes during the annealing have significant effect on mechanical properties. The microhardness after HE is 421 HV0.2. It increases with the increase of the annealing temperature up to 484 at 500°C. Further temperature rise, up to 800°C, results in a drop of mechanical properties indicating a rapid grain growth.


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Related papers

Presentation: Poster at E-MRS Fall Meeting 2007, Acta Materialia Gold Medal Workshop, by Agnieszka T. Krawczyńska
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-10 16:25
Revised:   2009-06-07 00:44