Neutron Scattering-Based Characterisation of Early-Stage Phase Transformations
Applied to Duplex Stainless Steels and Related Systems
Time: Fri 2025-12-19 10.00
Location: U1, Brinellvägen 26, Stockholm
Language: English
Subject area: Materials Science and Engineering
Doctoral student: Monika Rolinska , Hultgren Laboratoriet för Materialkarakterisering, Neutron och röntgenvetenskap för industriella transformationer (NEXT)
Opponent: Dr. Helen Playford,
Supervisor: Professor Peter Hedström, Hultgren Laboratoriet för Materialkarakterisering; Professor Joakim Odqvist, Strukturer
Abstract
Stainless steels are employed in a wide range of applications, spanning everyday household items, such as kitchen appliances, to advanced aerospace technologies, including space rockets. This range of applications is connected to the intimate link between the structure and properties of a material, influenced by the processing conditions. Duplex stainless steels, which consist of a mixture of the body centred cubic ferrite phase and the face centred cubic austenite phase, are used in highly corrosive environments under high stress. These alloys are prone to changes in the microstructure during service due to phase separation of the ferrite at elevated temperatures which limits their service life. Strategies to mitigate these microstructural changes caused by decomposition and delay embrittlement are of great interest for both economic and sustainability reasons. In this thesis, the effect of intermediate heat treatments on phase separation kinetics has been explored using in situ small-angle neutron scattering (SANS) to follow the evolution of the nanostructure. SANS is an established method for studying phase separation, and is well-suited for characterization of phase separation in stainless steels. During phase separation, a characteristic correlation peak develops in the SANS signal. The peak position and intensity are related to the wavelength and amplitude of decomposition in the sample, and the peak profile is often extracted with the help of models or peak fitting. To capture the earliest stages of the decomposition, the methodology was refined to account for the change of the structure factor induced by the changes in morphology during the ageing treatment. Using this new approach, the change in phase transformation kinetics due to the applied intermediate heat treatments was quantified, showing up to 60 % decrease in the SANS correlation peak amplitude, directly related to the amplitude of decomposition in the alloy. During the intermediate heat treatments, sigma phase precipitation was observed during some of the conditions. Sigma phase is a brittle intermetallic phase that needs to be avoided during processing, in order to not compromise the mechanical properties of the alloy. To investigate the kinetics of sigma phase formation in order to avoid it during the proposed heat treatments, it was studied in a model alloy as well as in a commercial alloy using in situ neutron diffraction. The study highlighted the complexity of predictive modelling and the limitations of the precipitation theory approach, in the same time showing good agreement with equilibrium calculations regarding formation of sigma phase but not other intermetallics. To gain further understanding of the mechanism of the delay of embrittlement caused by the intermediate heat treatments, pair distribution function (PDF) analysis of neutron total scattering data on model binary alloys was utilised. Due to the bulk characteristics of the material, texture effects were found during the analysis. The effect of texture on PDF analysis was investigated, showing a large influence over the results that could be extracted from a large-box model, limiting the conclusions that could be drawn from the local structure investigation.