TY - THES

T1 - Characterization of the precipitation mechanism in the molybdenum based alloy MHC

AU - Lang, David

N1 - embargoed until 11-09-2017

PY - 2012

Y1 - 2012

N2 - In the course of this thesis the precipitation behavior of hafnium carbide (HfC) in the molybdenum based alloy MHC was analyzed after thermo-mechanical treatment. The material of interest was a powder metallurgical processed molybdenum based alloy with 0.65 at.% hafnium and 0.65 at.% carbon. The aim of this thesis was to reveal the mechanism of secondary HfC precipitation. The gained knowledge will be a basic work for further improvements of the high temperature strength of this alloy. The experiments were carried out with atom probe tomography (APT) and transmission electron microscopy (TEM). The microstructure of the sintered material consisted of primary hafnium carbides and oxides within the grain and intergranular molybdenum carbide. APT measurements of the as-sintered condition showed a residual dissolved content of 0.12 at.% hafnium, but no carbon. After deformation and subsequent aging in a deformation dilatometer, TEM studies revealed fine precipitates in a range of 10 to 100 nm preferentially at dislocations or dislocation networks. This indicates heterogeneous nucleation on dislocations which is assisted by pipe diffusion as mechanism for the secondary HfC precipitation in MHC. The carbon for this reaction is delivered by the intergranular molybdenum carbide which dissolves partially during aging. This had been elucidated with X-ray diffraction and optical light microscopy.

AB - In the course of this thesis the precipitation behavior of hafnium carbide (HfC) in the molybdenum based alloy MHC was analyzed after thermo-mechanical treatment. The material of interest was a powder metallurgical processed molybdenum based alloy with 0.65 at.% hafnium and 0.65 at.% carbon. The aim of this thesis was to reveal the mechanism of secondary HfC precipitation. The gained knowledge will be a basic work for further improvements of the high temperature strength of this alloy. The experiments were carried out with atom probe tomography (APT) and transmission electron microscopy (TEM). The microstructure of the sintered material consisted of primary hafnium carbides and oxides within the grain and intergranular molybdenum carbide. APT measurements of the as-sintered condition showed a residual dissolved content of 0.12 at.% hafnium, but no carbon. After deformation and subsequent aging in a deformation dilatometer, TEM studies revealed fine precipitates in a range of 10 to 100 nm preferentially at dislocations or dislocation networks. This indicates heterogeneous nucleation on dislocations which is assisted by pipe diffusion as mechanism for the secondary HfC precipitation in MHC. The carbon for this reaction is delivered by the intergranular molybdenum carbide which dissolves partially during aging. This had been elucidated with X-ray diffraction and optical light microscopy.

KW - Molybdenum- hafnium- carbon alloy (MHC)

KW - precipitation hardening

KW - precipitation mechanism

KW - transmission electron microscopy

KW - atom probe tomography

KW - XRD

KW - Molybdän- Hafnium- Kohlenstofflegierung (MHC)

KW - Ausscheidungshärtung

KW - Ausscheidungsmechanismus

KW - Transmissionselektronenmikroskop

KW - Atomsondentomographie

KW - XRD

M3 - Diploma Thesis

ER -