Heat treatment of a melt spun Fe 70Cr 18Mo 2B 10 alloy

Abstract

Microstructural changes during annealing of melt spun Fe 70Cr 18Mo 2B 10 ribbons have been studied by a combination of differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, and energy dispersive X-ray microanalysis. The as-melt spun microstructure consists of bcc ferrite primary crystals embedded in an amorphous matrix. The crystallization onset, peak and finish temperatures of the amorphous phase are 533, 552 and 573°C, respectively, during continuous heating at 10°C/min. Isothermal annealing of the as-solidified amorphous ribbons for 2 h at different temperatures in the range 500–1000°C leads to a variety of different microstructures depending on the annealing temperature. At 500 and 600°C, the amorphous alloy crystallizes to form a fine scale mixture of bcc ferrite grains and Cr rich primitive tetragonal M 3B boride particles. At 700 and 800°C, the alloy microstructure transforms into a mixture of faulted Cr rich orthorhombic M 2B and Mo rich fcc M 23B 6 boride particles embedded in a matrix of bcc ferrite. At 900°C, the bcc ferrite matrix is replaced by fcc austenite, which transforms to dislocated lath martensite on cooling to room temperature. At 950 and 1000°C, the Mo rich fcc M 23B 6 boride particles are replaced by Mo rich primitive tetragonal M 3B 2 boride particles.