Electron Microscopic Observations of Dislocations and Precipitates in Annealed and Deformed Chromium

Abstract

The microstructure of thin foils of chromium containing 40 ppm of nitrogen which were recrystallized, aged or deformed was examined by means of transmission electron microscopy. The results obtained are as follows:
(1) Nitrogen in those specimens, quenched or rapidly cooled after annealing to recrystallization, precipitates as minute spots or plate-like precipitates.
(2) Ageing at 500°C of the rapidly cooled specimens results in redissolution of the spots and growth of plate-like precipitates.
(3) In the specimens deformed more than 2% by rolling at 300∼700°C, many dislocations are generated which form tangles and cell boundaries. Dislocation tangles are observed around the precipitates.
(4) The plate-like precipitates which are found in aged or prestrained specimens are presumed to be circular discs about 1 μ in diameter, the surface of which develops parallel to the {100}-plane of the matrix.
(5) By annealing at 800∼1050°C after working, the redistribution of dislocation tangles proceeds into networks. In the specimens annealed above 900°C, the precipitates are observed on the dislocation networks.
In view of the above results, the process of precipitation and the morphology of precipitates are discussed in relation to the prestraining and heat-treatment.