Microstructure Development and Precipitation Behavior of Laves Phase during Thermomechanical Process in High Chromium Ferritic Stainless Steel

  • 蕭 証文

Student thesis: Doctoral Thesis


Crofer? 22 H has been considered as the metallic interconnect material for SOFC applications due to the low thermal expansion coefficient and high oxidation resistance Thus it is very important in high temperature applications to tailor the grain size and precipitation behavior In this work we combined deformation and annealing thermomechanical process to induce the Laves phase precipitates in Crofer? 22 H First of all we investigated the effect of annealing temperature on the precipitation behavior of Crofer? 22 H at 600°C 700°C and 800°C Backscattered electron images (BEIs) showed that the Laves phase precipitated from the grain boundaries to the grain interiors when the temperature was increased However the average grain size (150 ?m) of the ferritic matrix did not significantly increase at 600°C 700°C and 800°C for 10 h Then for deformation analysis electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques were combined to investigate the deformation pattern of coarse ferrite grains after 3% 6% and 10% tensile deformation We identified two edge dislocations that occurred on the (110)[1-11] and (32-1)[1-11] slip systems for selected grains that is breaking down Schmid’s law Finally we investigated the influence of deformation on precipitation behavior and microstructure evolution during annealing process Here after the prior deformation of 3% 6% and 10% the specimens were annealed at 700?C for 10 hours Experimental results showed that fine Laves phase (Fe Cr Si)2(Nb W) precipitates occurred in grains and along subgrain boundaries as increasing the deformation Furthermore the volume fraction of Laves phase was increased but the average particle diameter of precipitate was reduced as the deformation was increased In addition considering the deformation microstructure the grains revealed no preferred orientation
Date of Award2017 Jul 7
Original languageEnglish
SupervisorJui-Chao Kuo (Supervisor)

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