The Effect of an Inert Oxide Particle Dispersion on the Morphology of Martensite in Fe-27Ni-0.025C Alloys

Abstract

A series of dispersion strengthened Fe-Ni alloys has been prepared by powder metallurgical techniques. This series was designed to permit evaluation of the relative effects of M_s temperature, chemical driving force, and austenite yield strength on resultant martensite morphology without altering matrix chemistry. Using a carefully selected lath-forming Fe-27Ni-.025C base alloy, incremental additions of an inert oxide dispersion resulted in a decrease in M_s temperature, an increase in the thermodynamic driving force at M_s, and an increase in the austenite yield strength at M_s to values beyond those previously associated with the lath-to-plate morphology transition. As the M_s temperature dropped below about 0 °C, martensite morphology shifted from lath to an intermediate “twinned lath” to plate, while holding constant both matrix chemistry and thermal history. Previous correlations of thermodynamic driving force and austenite yield strength with martensite morphology have been shown to break down. It is concluded that the observed transition from lath to plate martensite in the present alloy series was induced primarily by the depression of M_s temperature into the plate-forming temperature regime of the Fe-Ni system.