title: Scanning transmission electron microscopy characterisation of carbide precipitates in steel
Creators: name: Sala, Bianca
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Electron microscopy, EELS, electron energy loss spectroscopy, steel precipitates, DualEELS, STEM, scanning transmission electron microscopy.

description: High strength micro-alloyed steels are used extensively in a number of industries such as car manufacturing. The properties of these steels, including their high strength, are caused in part by the formation of nanoscale precipitates during thermomechanical processing. In the case of the steels examined in the work reported in this thesis, the microalloying elements added to the steel were titanium, vanadium and niobium. During hot rolling, these combined with carbon and nitrogen to form stable carbides, nitrides and carbonitrides. Many steel properties depend on the size, distribution and chemistry of these precipitates, which in turn depend on the exact thermomechanical processing steps used during manufacture. The overall aim of the project was to understand how the combination of Ti, V and Nb influences the properties of bainitic steels and characterise the precipitation at different stages during manufacture. Electron microscopy techniques have been used to characterise the distribution, size and chemistry of these precipitates. This involved two different approaches, both using electron energy loss spectroscopy (EELS). First, large areas of steels were mapped using low-loss EELS. This allowed us to identify areas rich in precipitates, image their distribution and obtain a measurement of their size distribution as absolute volume fractions. The results from this are presented in Chapter 4 of this thesis. To process this data and obtain results from precipitates smaller than 1nm in diameter, a method to analyse the low-loss datasets has been developed as part of this PhD. This is detailed in Chapter 3. Once areas containing precipitates had been found, some of these were picked for more in-depth quantitative chemical analysis using DualEELS. A series of results for different types of precipitates found in a range of steel samples is detailed in Chapter 6. Again, the method used to analyse these datasets was partly developed as part of this PhD and is described in Chapter 3. It uses a multiple linear least squares fitting approach where experimentally obtained cross-sections are fitted to the precipitate data. The results of this are then used to quantify the exact chemistry of the precipitates. The details of the measurement of these cross-sections, and the method used to obtain them, are presented in Chapter 5. For the first time, we have been able to perform large area volumetric analysis coupled with detailed analysis of the chemistry of individual precipitates. This gave us the ability to track the precipitate formation resulting from the manufacturing process while while obtaining both a statistically significant overview and fine chemical detail. This has clear applications to the future of steel processing and other reactions involving precipitation or formation of nanoscale chemical inhomogeneity in materials.