Influence of the Thermo-Mechanical Treatment on the Microstructure and the Material Properties of Molybdenum
Project manager: Sophie Primig
Pure Molybdenum possesses a unique combination of physical properties including a melting point of 2620°C, excellent high temperature strength, high corrosion resistance except oxidation resistance, high thermal conductivity, a high elastic modulus and a low thermal-expansion coefficient. Molybdenum alloys as for example Titanium Zirkonium Molybdenum (TZM) and Molybdenum Hafnium Carbide (MHC) contain small dispersed second phase particles which improve the material´s creep resistance. Because of these outstanding properties, Molybdenum is used in a wide range of applications, including lighting technology, high performance electronics and high temperature furnace construction. Nowadays one of the most important products are sputter targets (Figure 1) for coating technology since they are needed for the production of liquid crystal display-thin film transistor (LCD-TFT) displays.
The material's poor room temperature ductility is a disadvantage for the use of Molybdenum as a structural material: Highly deformed Molybdenum is ductile at room temperature, but recrystallization leads to room temperature embrittlement, which is not an intrinsic property of Molybdenum itself, but is caused by the presence of interstitial impurities such as Carbon, Nitrogen and in particular Oxygen. That is the reason, why the recrystallization behavior of Molybdenum as well as particle strengthened Molybdenum alloys, which is strongly influenced by the process parameters of the thermo-mechanical treatment and the purity of the material, is decisive for the resulting mechanical properties.
The aim of this project is to increase the knowledge about deformation, recovery and recrystallization processes which take place during the thermo-mechanical treatment of pure Molybdenum as well as particle strengthened Molybdenum alloys. Furthermore, the influence of impurities as well as precipitates on the recrystallization behavior and the resulting mechanical properties has to be studied. Experimental methods, like scanning electron microscopy (SEM) (Figure 2), thermal analysis, atom probe tomography (APT) and small angle neutron scattering (SANS), which are applied in the Christian Doppler Laboratory “Early Stages of Precipitation”, offer a high potential for the establishment of a comprehensive microstructural understanding.
Figure 1: Sputter targets made of Molybdenum bonded to Copper backplates.
Figure 2: SEM back scattered electron (BSE) microsection of a partially recrystallized Molybdenum sheet.
Grains in their deformed or recovered state appear shaded, because the subcell structure can be
revealed by applying this method, whereas recrystallized grains appear of uniform contrast.