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Iron base superalloy Fe13CrWTiY hot extrusion and the microstructure evolution

Author: ZhangNingYi
Tutor: HuangBoYun;LiuYong
School: Central South University
Course: Materials Science
Keywords: Fe-based superalloy internal oxidation hot extrusion ODS recrystallization phase transformation
CLC: TG376.2
Type: Master's thesis
Year: 2008
Downloads: 196
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Abstract

In rencent years,some mechanically alloyed（MA）,oxide dispersion strengthened（ODS）ferritc alloy have exhibited dramatically improved high temperature creep properties at high temperatures.This temperature is significantly higher than the traditionally accepted operating range of ferritic alloys.Therefore,there appears to be considerable potential for developing ferritic and possibly other alloys with significantly improved high temperature properties for a number of different applications.In the investigation,a Fe13CrWTiY alloy was prepared using powder metallurgy method.The effects of the oxide temperature and oxide time on the content and state of oxide atoms were researched.The possible forms of oxide atoms to exist in the alloy were analysed.The distribution of O and other elements was analysed using SEM,optical microscopy and EDS.The alloy powders were extrusioned under the different extrusion ratio of 3:1,6:1 and 9:1.The microstructures of the material were observed by optical microscopy and SEM.The granule interfaces,grain boundarys and voids in the as-processed alloy were observed by TEM.The recrystallization behaviour in the process of extrusion was discussed via the TEM observation of newly-birth trifurcate grain boundary and the metallurgical structure.The consolidated alloy bulks were heat treated.They experienced vacuum annealing for 2 hours under the annealing temperature from 700—1250℃respectively,then cooled in air.The change of microhardness under these different temperatures was researched.The change of the metallurgical microstures and the phases were researched by the analysis of XRD and optical microscopy.It is believed that under the annealing temperature of 700—1100℃,like the ODS alloy,the alloy showed an ability to resist recrystalization.But when annealed under a temperature higher than 1100℃,the alloy would come across a exceptional hardeness effect.The effect intensified as annealing temperature rised.A ferrite to austenite phase transformation was found when the annealing temperature arrived 1250℃.It is discussed that a kind of precipitate phase which has a high Cr content is the cause of the hardening effect.It also has a connection with the phase transformation at 1250℃.The major conclusions are listed as follows:（1）When the powder was oxided at 500℃,the oxigen content and oxidition time is showed a linear relationship.The shape of oxide is evidently influenced by the oxidization time.When the powder is oxidized longer than 8 hours,the oxygen in the alloy will exist in the form of Fe₂O₃ scale coating the surface of the particle.Increasing the oxidation temperature and shortening the oxidization time could inhibit the formation of Fe₂O₃ scale.Oxidization could lead to the segregation of the Ti and Y atom in the alloy matrix for these two kinds of atoms was apt to diffuse towards direction of oxygen where tiny oxide and compound ion formed.。（2）When the oxidized powder were extrusioned at the extrusion ratio of 3:1,the neighboring particles could get full contact through plastic deformation and the particle interface could be observed distinctly.It is observed that O,Ti and Y atoms segregated on the particle interface.The extrusion ratio has an important influence on the process of consolidation. When it reached 6:1 or higher,the particle interface would vanish.At the location of original particle interface,voids distributed like a chain could be observed.Most of their size are around 0.5μm.There appeared obvious recrystallizaion behaviour during the extrusion process（3）When annealed below 1100℃,the alloy could resist the recrystallizaion as well as hold a steady mechanical property and microstructure.When the annealing temperature exceeded 1100℃,the microhardness of alloy enhanced dramatically;and when the annealing temperature reached 1250℃,aα→γtransformation happened.The transformation is presented in the microstructure as equalization of the fibrous metallurgical structure.

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