Spröd-duktil-Übergang von ultra-feinkörnigem Eisen

Translated title of the contribution: Ductile to brittle transition of ultrafine-grained iron

Christoph Nikolaus Kammerhofer

Research output: ThesisDiploma Thesis

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Abstract

Due to its bcc lattice the toughness of iron is strongly temperature dependent. At low temperatures failure occurs almost without any deformation whereas at elevated temperatures significant amount of plasticity is involved before fracture occurs. Thus the expressions brittle and ductile fracture are well known, respectively. Consequently there is a characteristic temperature - the so-called ductile to brittle transition temperature (DBTT) - where below brittle and beyond ductile failure takes place. One way of decreasing the DBTT provides the method of grain refinement. Classical metallurgy, for instance controlled rolling allows reducing the mean grain size to a minimum value of about one micron. Recently developed deformation techniques like high pressure torsion (HPT) which was applied in this work are able to further reduce the mean grain size down to some hundred nanometers. By subsequent heat treatment it is possible to adjust different grain sizes. Because of the huge grain refinement an enormous strenghtening effect takes place. During the shear deformation a characteristic deformation structure is being established which causes an anisotropy of the mechanical properties. The questions which now arise is if the tremendous grain refinement due to HPT will shift the DBTT to even lower values and what effect the anisotropy will have. For this purpose fracture mechanics experiments were carried out over a wide temperature range which revealed that the DBTT is higher for the submicron state than for the lower micron regime. So the desirable effect of further reducing the DBTT by decreasing the grain size significantly below one micron did not come true. Fractography showed that the submicron samples solely failed by intergranular fracture whereas cleavage was the dominant failure mechanism for samples with grain sizes in the lower micron regime. The different crack paths are believed to be responsible for the increase of the DBTT of the submicron specimens. The mechanical anisotropy is only significant for the submicron state leading to large differences of the DBTT depending on the mutual arrangement of the proposed crack propagation plane and the shear plane of the HPT disc. Consequently it is possible to benefit from the huge strengthening effect due to grain refinement and obtain good toughness properties by taking advantage of the mechanical anisotropy.
Translated title of the contributionDuctile to brittle transition of ultrafine-grained iron
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Pippan, Reinhard, Supervisor (internal)
Award date26 Mar 2010
Publication statusPublished - 2010

Bibliographical note

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Keywords

  • Ductile to brittle transition DBTT iron cleavage HPT

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