TY - BOOK

T1 - Processing – Structure – Property Relationships in Selected Iron-Based Nanostructures

AU - Müller, Timo

N1 - no embargo

PY - 2018

Y1 - 2018

N2 - Motivated by their extraordinary properties, many interdependencies between processing routes, microstructures and properties of nanostructured materials have been investigated in the past. Both top-down and bottom-up manufacturing techniques have been used to prepare unique nanostructures. In the present work, one aspect, that has only attracted little attention in the research so far, is investigated for one method from each approach. First, it is shown, that high-strength materials in multi-component systems can be obtained via HPT more efficiently, i.e. after reduced strains, when finer, more homogeneous starting materials are used. The two model systems – martensite and coated powder (compared to ferritic-pearlitic steel and powder mixtures) – demonstrate, that this holds on very different length scales. In the case of single-phase supersaturated solid solutions as starting materials, the process of mechanical mixing during HPT, which requires very high strains, is not necessary. Thus, high-strength materials can be prepared at moderate strains, as is demonstrated for a martensitic 0.1 wt.-% C steel reaching a tensile strength of 2.4 ± 0.1 GPa after a von Mises equivalent strain of only 7.5. For the coated powders, where the chemical distribution is homogenized only on a macroscopic length scale, intermixing still takes place during HPT, but is accelerated due to the shorter distances as compared to coarser and more inhomogeneous starting material. Therefore, high strength materials are obtained after moderate strains, too. On the other hand, oxides introduced by the coating process have significant influence on the results. Secondly, the mechanical properties and deformation behavior of nanostructured electrodeposited iron-based alloys, which have been mainly studied for their functional properties so far, are investigated. Microstructural anisotropy and alloying with light non-metallic elements have a large impact also in this case and are discussed in detail. In nanostructured Fe-C alloys prepared via electrodeposition, the microstructure with a grain size of about 20 nm might be the basis for a high-strength material, but the embrittlement due to oxygen codeposition results in brittle failure already at small loads. Whereas brittle behavior is present in Fe-P electrodeposits as well, it is shown that amorphous/crystalline multilayer structures can be deposited in this system from a single electrolyte and the hardness of the films can be adjusted via the sublayer thickness of the crystalline layer following a Hall-Petch behavior down to a sublayer thickness of 15 nm.

AB - Motivated by their extraordinary properties, many interdependencies between processing routes, microstructures and properties of nanostructured materials have been investigated in the past. Both top-down and bottom-up manufacturing techniques have been used to prepare unique nanostructures. In the present work, one aspect, that has only attracted little attention in the research so far, is investigated for one method from each approach. First, it is shown, that high-strength materials in multi-component systems can be obtained via HPT more efficiently, i.e. after reduced strains, when finer, more homogeneous starting materials are used. The two model systems – martensite and coated powder (compared to ferritic-pearlitic steel and powder mixtures) – demonstrate, that this holds on very different length scales. In the case of single-phase supersaturated solid solutions as starting materials, the process of mechanical mixing during HPT, which requires very high strains, is not necessary. Thus, high-strength materials can be prepared at moderate strains, as is demonstrated for a martensitic 0.1 wt.-% C steel reaching a tensile strength of 2.4 ± 0.1 GPa after a von Mises equivalent strain of only 7.5. For the coated powders, where the chemical distribution is homogenized only on a macroscopic length scale, intermixing still takes place during HPT, but is accelerated due to the shorter distances as compared to coarser and more inhomogeneous starting material. Therefore, high strength materials are obtained after moderate strains, too. On the other hand, oxides introduced by the coating process have significant influence on the results. Secondly, the mechanical properties and deformation behavior of nanostructured electrodeposited iron-based alloys, which have been mainly studied for their functional properties so far, are investigated. Microstructural anisotropy and alloying with light non-metallic elements have a large impact also in this case and are discussed in detail. In nanostructured Fe-C alloys prepared via electrodeposition, the microstructure with a grain size of about 20 nm might be the basis for a high-strength material, but the embrittlement due to oxygen codeposition results in brittle failure already at small loads. Whereas brittle behavior is present in Fe-P electrodeposits as well, it is shown that amorphous/crystalline multilayer structures can be deposited in this system from a single electrolyte and the hardness of the films can be adjusted via the sublayer thickness of the crystalline layer following a Hall-Petch behavior down to a sublayer thickness of 15 nm.

KW - nanokristallin

KW - Elektrodeposition

KW - Hochdrucktorsion

KW - mechanische Eigenschaften

KW - Eisenlegierungen

KW - Gefügeanisotropie

KW - metastabile Phasen

KW - nanocrystalline

KW - electrodeposition

KW - high pressure torsion

KW - mechanical properties

KW - iron alloys

KW - microstructural anisotropy

KW - metastable phases

M3 - Doctoral Thesis

ER -