Transmission Electron microscopy has been used to study the phases and microstructural ohanges produced by various heat treatments in an Fe : Cr : Hi alloy. Two types of martensite have been identified in this system. Needles of b.c.c. a - martensite are formed in slowly cooled or quenched specimens, the needles long dimension being parallel to < 111 >. A small number of plates of t - martensite are produced by deformation at room temperature. Tempering at temperatures up to 600 C produces a dispersion of the carbide M13C6 in the b.c.c. matrix. Some factors controlling the martensite and austenite in the microstructure are discussed.
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