Abstract
The structure and morphology of Mo2C and VC resulting in secondary hardening of high purity alloys Fe-Mo-C and Fe-V-C during aging at 550°C are studied by electron diffraction, electron and field ion microscopy, X-ray measurements of α Fe matrix unit cell dimensions, and Mössbauer spectroscopy. It is stated that in the early stages of aging, when a sharp decrease in hardness occurs, the process takes place in two directions simultaneously, namely, the formation of cementite particles and the formation of globular clusters of alloying element atoms, the latter being mainly on dislocations and at grain boundaries. During the period of hardness increase, cementite particles are dissolving, and the content, shape, and location of alloying element clusters are changing. The alloying element clusters are mixed zones of M and C atoms, disclike in shape, the discs lying on {100}α planes. Special carbides having the fcc structure are observed at peak hardness in all the alloys investigated. The fcc structure is formed inside Mo2C and VC irrespective of the type of structure in the equilibrium phase. On overaging, the fcc structure of Mo2C changes into the equilibrium (hcp) structure, the particles of which are needlelike.
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