Processing of Nanostructured High Nitrogen Stainless Steel by Mechanical Alloying

Author: Cisneros M.M.   López H.F.   Mancha H.   Rincón E.   Vázquez D.   Pérez M.J.   De La Torre S.D.  

Publisher: Springer Publishing Company

ISSN: 1543-1940

Source: Metallurgical and Materials Transactions A, Vol.36, Iss.5, 2005-05, pp. : 1309-1316

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Abstract

Mechanical alloying (MA) was employed to process high nitrogen nanostructural stainless steel from Fe-18 pct Cr-11 pct Mn powder mixtures. Two MA processing units were employed, a Szegvary attritor and a high energy Spex mill, with stainless steel balls under a nitrogen atmosphere. The level of nitrogen introduced by these means was relatively high (around 5 pct), which easily exceeded the equilibrium solubility limits in either the α or γ phase. The relatively high nitrogen concentration in solid solution was attributed to preferential accommodation at dislocation elastic stress fields, as well as at nanograin boundaries. Moreover, it was found that MA in the attritor produced a maximum of 29.4 pct austenite after 72 hours of milling time. In contrast, almost 100 pct austenite was obtained by MA for 120 hours in the Spex mill. The development of a fully austenitic structure was attributed to the high energy associated with the Spex mill during the MA of these powders. Apparently, under these conditions, the interfacial energy contributions associated with the nanograin structure thermodynamically favor the formation of the γ phase below a critical nanograin size. The resultant grain structure obtained from both MA processes was nanometric with grain sizes well below 10 nm. In the MA powder mixtures processed in the attritor, the powders were subsequently annealed to promote the α → γ transformation. In this case, it was found that almost 100 pct austenite was produced by annealing at 1000 °C to 1100 °C for 60 minutes in MA powder mixtures processed for 72 hours. Longer MA processing times gave rise to the development of either the CrN or Cr2N phase upon annealing. Alternatively, using spark plasma sintering at 800 °C and 1000 °C for 7 minutes led to almost full densification with maximum amounts of austenite of the order of 95 pct. It was found that either sintering or powder annealing at the mentioned temperatures did not give rise to significant nanograin growth, with the nanograin sizes remaining between 14.5 and 48.8 nm. Apparently, the nanograin boundaries are pinned by preferential nitrogen segregation, making grain boundary (GB) migration rather sluggish in this alloy system.