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Nanoscale Analysis of the Growth Front of Pearlite.

Waters, Ryan. (2015) Nanoscale Analysis of the Growth Front of Pearlite. Doctoral thesis, University of Surrey (United Kingdom)..

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Abstract

The formation of pearlite and the growth mechanisms for the transformation are complex. This is due to a number of factors including, the ‘choice’ of lamellar spacing, multiple diffusion paths, the possibility of various distinct interfacial equilibrium criteria, variations in colony morphology and the disputed role of defects at the growth interface. To explain the growth mechanism of pearlite essentially two rival mechanisms have been proposed. One mechanism is generally referred to as the Zener-Hillert mechanism and assumes that atomic addition to an incoherent interface occurs. The second mechanism is referred to as the ‘ledge mechanism’, in which atomic addition occurs only at ledge risers on a semi-coherent interface. Neither of these approaches is able to explain all of the characteristics of the pearlite transformation. This study aims to to clarify the nature of the pearlite growth mechanism. More specifically, compositional changes at the nanoscale close to the growth front in a Fe-1.2C-12Mn alloy, have been studied for the first time using Scanning Transmission Electron Microscopy (STEM). Data relating to the kinetics of transformation and interface structures were also gathered. The eutectoid transformation was studied in this way over the temperature range of 640°C-500°C. In common with previous studies of a variety of alloys, in this alloy the interlamellar spacing decreases as the undercooling is increased and during the course of isothermal heat treatment the growth velocity of the colonies slows. The STEM composition profiles showed that full partitioning of Mn occurs over all of the temperature range investigated. This, and other evidence, strongly indicates that the diffusion path for Mn is along the growth interface and not via long range diffusion in the austenite. It was also observed that structural features that appear at the growth interface are most likely artefacts created by stacking fault intersection with the growth interface and are not growth ledges. A reassessment of the two models proposed to describe the growth mechanism of pearlite, using the experimental results of this study and the vast literature, concludes that neither of the classic models fits with the available data.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors : Waters, Ryan.
Date : 2015
Additional Information : Thesis (Ph.D.)--University of Surrey (United Kingdom), 2015.
Depositing User : EPrints Services
Date Deposited : 14 May 2020 14:56
Last Modified : 14 May 2020 15:06
URI: http://epubs.surrey.ac.uk/id/eprint/856781

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