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Title: Effect of grain size distribution and Nb addition on the recrystallisation Avrami exponent of Fe-30Ni model alloy
Author: Ji, Mo
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Recrystallisation is an important process for grain size control in steel, which is affected by the material grain size, alloy content, temperature, strain and strain rate. A crucial parameter for recrystallisation prediction is known as the Avrami exponent as this affects the recrystallisation rate. The Avrami exponent is independent of strain and strain rate, providing that the recrystallisation mechanism remains the same, but is affected by microstructure. In this thesis an austenitic Fe-30 Ni model alloy, with and without Nb addition, has been used to investigate the role of grain size distribution and Nb addition (through solute drag and strain induced precipitation) on the recrystallisation Avrami exponent. As-rolled Fe-30Ni with/without 0.044 wt.% Nb alloy was provided which was homogenised at 1150 °C for 4 hours to remove Nb segregation and the deformation bands present. To determine the influence of grain size distribution on recrystallisation Avrami exponent, two different grain size distributions, one with a mode grain size of 160μm and wider distribution and one with a mode grain size of 100μm and narrower distribution were generated by heat treatment. Both cold and hot deformation (to a range of strains from 0.2 to 0.45 and temperatures from 850 - 950 °C) have been used to examine the effect of grain size distribution. The cold deformation and annealing tests allowed the in-situ and ex-situ observation of recrystallisation nucleation and growth mechanism, also provided a more uniform macroscopic deformation. Hot deformation tests were used to validate the proposed recrystallisation Avrami exponent prediction model for a wider range of conditions. The hot and cold deformed microstructures were examined, and similar microband containing structures were observed, although the stored energy varied for the different deformation modes for the same applied strain, this was due to different amounts of recovery occurring (during reheating to the annealing temperature for cold deformed at annealed samples and during deformation (dynamic recovery) for hot deformed samples). Local strain inhomogeneity after both cold and hot deformation were also observed in the deformed microstructure of Fe-30Ni, attributed to the effect of grain size variation. Both intra- and inter-granularly inhomogeneity was seen: triple points and grain boundaries had a higher local misorientation than the grain interior; and smaller grains showed higher local misorientation than coarser grains. In-situ EBSD and interrupted recrystallisation characterisation showed that impingement of recrystallised grains along the grain boundary occurs at an early stage of recrystallisation due to the non-randomly distributed nuclei, and that finer grains recrystallise faster than coarser grains due to both their higher stored energy and larger number of potential nucleation sites. A reduced Avrami exponent was seen for the larger mode/wider grain size distribution and an equation to predict the Avrami exponent from the grain size distribution is proposed. A delay in the onset of recrystallisation due to the Nb addition was observed in the cold deformed and annealed as well as the hot deformed and annealed samples. In both conditions, the Avrami exponent was not affected by the Nb addition if the strain induced precipitates rapidly coarsened reducing the pinning force on recrystallisation. A decreased Avrami exponent was only seen for one deformation condition (cold deformation to 0.3 strain and annealing at 850°C) and this was attributed to the slow growth of the strain induced precipitates meaning that they remained fine in size and high in number density resulting in a larger pinning force reducing the recrystallisation rate.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council ; Tata Iron and Steel Company
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TN Mining engineering. Metallurgy ; TS Manufactures