Thermomechanische Prozessführung der Molybdänbasislegierung MHC (Mo-Hf-C)

Translated title of the contribution: Thermo-mechanical processing of the molybdenum base alloy MHC (Mo-Hf-C)

David Lang

Research output: ThesisDoctoral Thesis


Precipitation hardened Mo-Hf-C alloys are of special interest for high temperature applications due to their excellent properties at elevated temperatures. Because of economic reasons the majority of these alloys are nowadays exclusively processed via a powder metallurgical (PM) approach followed by thermo-mechanical processes in order to adjust their mechanical properties, i.e. without arc-melting and solution annealing. Especially the Mo-Hf-C alloy, named MHC, possessing a nominal composition of 0.65 at.% Hf and 0.65 at.% C, reveals a good balance between its high strengthening potential and its processability. The major hardening contributions in MHC are strain-induced precipitation of nm-sized HfC particles (theoretically 1 vol.% HfC may be used for precipitation hardening) and dislocation hardening due to the occurrence of a sub-grain structure. A fundamental difference to arc-melted and solution annealed material is the introduction of a higher oxygen content during PM processing and different thermo-dynamically stable phases in the as-sintered MHC microstructure, which reduces the precipitation potential of fine HfC. In order to further improve the positive high temperature properties of MHC it is necessary to optimize the thermo-mechanical processing route with respect to the complex interactions between precipitation, recovery and recrystallization processes. Therefore, quantitative metallographic investigations with light and scanning electron microscopy in the ternary (quaternary) Mo-Hf-C-(O) system serve as a basis. These studies are supported by thermodynamic calculations via CALPHAD and MatCalc. The chemical compositions of the carbide phases in MHC were analyzed by means of atom probe tomography and relevant literature was assessed. Subsequently, these data were implemented in the new database for molybdenum base materials. The thermodynamic calculations show that small changes in the oxygen and carbon content have relevant consequences for the phase stability and volume fractions of the different phases. With targeted solution annealing procedures it was possible to increase the hafnium content in solid solution and to increase the volume fraction of Mo2C which serves as a carbon reservoir for strain induced precipitation of HfC. Furthermore, kinetic studies were done by means of small-angle neutron scattering which was complemented with conventional methods like two-stage deformation tests, electron back-scatter diffraction, scanning and transmission electron microscopy. For the first time the combined results of these investigation methods connect the timeline of strain-induced precipitation in MHC with the formed volume fraction of nm-sized HfC and the strength of the material.
Translated title of the contributionThermo-mechanical processing of the molybdenum base alloy MHC (Mo-Hf-C)
Original languageGerman
  • Buchmayr, Bruno, Assessor B (internal)
  • Clemens, Helmut, Assessor A (internal)
Publication statusPublished - 2016

Bibliographical note

embargoed until 28-10-2021


  • molybdenum base alloy
  • hafnium carbide
  • molybdenum carbide
  • thermo-mechanical processing
  • precipitation hardening
  • Mo-Hf-C

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