Evolution of strain-induced hafnium carbides in a molybdenum base Mo–Hf–C alloy studied by small-angle neutron scattering and complementary methods

Abstract

The powder metallurgically processed molybdenum base alloy MHC (Mo–Hf–C), with a nominal content of 0.65 at.% Hf and 0.65 at.% C, is of considerable interest for high temperature applications. After uniaxial deformation and subsequent aging of the as-sintered material, plate-like, nm-sized hafnium carbides are formed by heterogeneous nucleation at dislocations. The main focus of this work was to study the evolution of the size distribution of these small hafnium carbides and to gain statistically reliable data from a large volume regarding the morphology and the volume fractions using small-angle neutron scattering (SANS) experiments for samples aged at 1600 °C for different times. Additionally, complementary methods were used to support the SANS results. Hardness testing, two-stage interrupted compression tests, optical light microscopy, electron backscatter diffraction, scanning and transmission electron microscopy were used for further characterization of the aged MHC microstructures and as complementary methods to the SANS measurements. The small-angle neutron scattering investigations revealed anisotropic 2D patterns in relation to the loading direction of the deformed samples which were assigned to not fully compressed pores. It was possible to analyze the thickness distribution of the fine hafnium carbides due to streak formation of the scattering patterns caused by an orientation relationship between face centered cubic hafnium carbides and the body centered cubic molybdenum matrix. The evolution of the microstructure and hardness are in good agreement with the results of the small-angle neutron scattering measurements. The highest hardness was revealed after aging for 1 min at 1600 °C exhibiting small hafnium carbides with a mean diameter of ∼13 nm, a mean plate-thickness of ∼3 nm and a volume fraction of 0.05 vol%. Although, the volume fraction of these carbides was increasing with progressing aging time the hardness dropped due to recovery, recrystallization as well as increasing particle size.