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- Article name
- Formation of volume nanostate in metastable austenitic steels as a result of γ-α-γ transformations
- Authors
- SAGARADZE V.V., , , Institute of Metal Physics, Ural Branch RAS, Ekaterinburg, Russia
- Keywords
- Year
- 2005 Issue 4 Pages 39 - 44
- Code EDN
- Code DOI
- Abstract
- Conditions have been determined for establishment of a volume nanostate in metastable nickel and chromium-nickel austenitic steel samples having virtually unlimited dimensions thanks to forward and reverse γ-α-γ martensitic transformations. This treatment usually does not lead to a considerable dispersion of the austenitic structure, because at medium and high heating rates the α-γ transformation causes the predominant recovery of the orientation of relatively coarse initial austenitic grains. However, if heating is realized at a very slow rate of 0.2-0.4 K/min, the α-γ transformation is followed by the multiplication of γ orientations (up to 576 in each initial grain after a γ-α-γ cycle). Consequently, the structure is refined to the nanosized level. The reason for this dispersion of the structure is the redistribution of nickel between the martensite and the retained austenite. As a result, boundaries of martensite crystals are covered with a low-nickel "buffer" α-layer (which is most resistant to the α-γ transformation) removing the orientation effect of the retained austenite. The establishment of the nanostate as the final or intermediate structure provides a profound change of functional properties of metastable austenitic steels and alloys. Specifically, when a nanocrystalline austenite is formed during a partial α-γ transformation, the martensite is strengthened considerably ("up quenching"), a hard magnetic state is established in high-strength maraging steels, stainless reactor steels improve their resistance to vacancy swelling, and the thermal expansion coefficient of the austenite in invar alloys can be adjusted over broad limits.
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