Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel

• Published in: Materials characterization 2022-01, Vol.183, p.111639, Article 111639
• Main Authors: Biserova-Tahchieva, Alisiya, Chatterjee, Dipanwita, van Helvoort, Antonius T.J., Llorca-Isern, Núria, Cabrera, Jose Maria
• Format: Article
• Language: English
• Subjects: HPT; Phase precipitation; Severe plastic deformation; Superduplex stainless steel
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2021.111639

In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated. [Display omitted] •High-pressure torsion method carried out under non-restricted conditions up to 12 turns to two-phase stainless steel –UNS S32750 superduplex stainless steel.•Secondary phase precipitation after isothermal heat treatment has occured.•Scanning Electron Microscope (SEM) and Scanning Precession Electron Diffraction Mode (SPED) were used for the microstructure characterization.•Phase transformation was considered as an invariant transformation.•Sigma (σ) is a product from a solid-state parent phase with different crystal structure.