High Voltage Electron Microscopy Study of the Metastable Iron Carbide in a Eutectoid Fe–C Alloy

Abstract

The crystallography and morphology of the metastable iron carbide in 0.79 wt%C alloy tempered at 150°C for 3 days have been investigated by means of 650 kV high voltage electron microscopy, especially by utilizing the selected microarea electron diffraction technique which enables us to take diffraction patterns from very small areas such as 500 Å in diameter. It was found that the crystal structure of the carbide is not of a hexagonal lattice, but of an orthorhombic lattice with lattice constants a_o=2.85₈, b_o=4.71₅ and c_o=4.32₉ Å (the suffix o means the orthorhombic lattice). The carbide precipitated coherently in the tetragonal martensite with orientation relationship (0\bar11)_o⁄⁄(001)_m, [100]_o⁄⁄[100]_m. High resolution bright and dark field micrographs showed that the carbide which appeared to be about 1000 Å in length is not truly a single particle but consists of smaller particles of about 50 Å size. The carbides precipitated in fine transformation twins of martensite as well as in the matrix with the same morphology and orientation relationship.