The non-cubic lattice of rapidly quenched packet martensite

Abstract

The positions of 200, 020 and 002 peaks of ferrous martensite have been measured individually by a precision X-ray diffractometer technique. Martensite with 18 wt pct Ni and 0.1, 0.2, 0.3, and 0.4 wt pct C was formed by salt-water quenching from preoriented austenite single-crystal slices about 0.5 mm thick. The variations in X-ray peak positions interpreted as changes in lattice parameter yield to the first approximation:Δa/a ₀ = −0.005 [wt pct C],Δb/a ₀ = −0.016 [wt pct C],Δc/a ₀ = −0.037 [wt pct C] for carbon contents between 0.1 and 0.4. For such carbon contents the martensite has packet or mixed packet-lenticular morphology; consequently packet martensite is not cubic as sometimes claimed. Instead its lattice is nearly like that of lenticular martensite with the"a " and"c " parameters varying with carbon content as previously observed for tetragonal martensite, and the" b " parameter unequal to the" a" . The significance of the inequality of the" a " and" b " parameters is unclear, but that inequality appears to be characteristic of some plate and lenticular martensite.