Evolution of nano-pores during annealing of technically pure molybdenum sheet produced from different sintered formats

Molybdenum is a refractory metal with no phase transformation in the solid state and a high melting point. It istherefore an excellent structural material for various high temperature applications. Especially in this field ofoperation, significant creep resistance is essential. To achieve this, a microstructure with grains in the range ofmillimeters is desired. However, as demonstrated in the present study, the onset temperature for secondaryrecrystallization, which would lead to a beneficial grain size, is among other things dependent on the initialdimensions of the sintered part. One possible reason for the different microstructural evolutions is the influenceof residual pores in sub-micrometer size. Sheets were thus fabricated via three different production routesemploying the same initial Mo powder to exclude chemical variation as an influencing factor. The samples wereinvestigated by in-situ small-angle X-ray scattering at a synchrotron radiation source with two different heatingrates. Additionally, selected annealed samples were studied ex-situ with high energy X-rays. The apparent volumefraction of pores is compared to a volatilization model for the vaporization of typical accompanying elementsand the induced thermal expansion.