Atomic displacements due to C in Fe Ni C martensite

Abstract

The ratio of the X-ray intensity in the 002 to that in the 200, 020 in martensite formed in fine-grained 18 pct Ni ∼ 1 pct C austenite provides a measure of the strength of the carbon displacement field. Theoretical calculation of the spacial variation of displacement due to carbon in the c-oriented octahedral site coupled with the assumption that such carbon is dispersed randomly then allows computation of the displacement of the two nearest neighbors as 0.6Å in the c-direction, corresponding to a carbon-iron distance of 2.0Å. Aging the martensite for one hour at temperatures above −40 °C, the temperature of the above measurements, results in a decrease in the 002 intensity with little or no change in the 200 or 020 intensity, and little change in the lattice parameters until +40 °C when precipitation starts. This behavior is expected if clustering of carbon occurs and is contrary to expectations if carbon changes site during aging. If the absence of aging at —40 °C is assumed, then aging corresponds to formation of clusters of two to four carbon atoms.