Microanalysis investigations of transformation toughened Co-Ni steels

The evolution of the structural subsystems in secondary hardening cobalt-nickel steels was observed as function of tempering treatments with a special emphasis on the austenite dispersion in the AerMet100 alloy. The transformation toughening effectiveness was correlated with the austenite morphology and composition as a function of two-step nucleation and growth heat treatments. The co-location of the austenite precipitates with the M₂C carbide particles indicated a coupling by sequential nucleation of the two second phase dispersions. The necessity of the M₂C carbide dispersion to provide nucleation sites for austenite precipitation was shown in an investigation of a Fe-15Co-10Ni ternary alloy that did not produce an austenite dispersion. The optimal austenite dispersion for transformation toughening in AerMet100 consisted of 8 nm intralath precipitates with a 42 wt% nickel composition.; A technique to extract the precipitate composition information from a precipitate embedded in a matrix was developed using a scanning transmission electron microscope (STEM), an energy dispersive spectroscopy (EDS) detector, and a parallel electron energy loss (PEELS) detector. The validity of the microanalysis technique was confirmed by Monte Carlo modeling of x-ray emission from precipitate-matrix systems corresponding to those encountered in this investigation.; The experimental microanalytical data from AerMet100 was used to develop precipitation models for the interlath austenite films and the intralath austenite particle dispersion. The three phase unstable thermodynamic model incorporating capillary energy additions for the austenite and M₂C arbide particles successfully simulated the system evolution. The austenite interfacial energy was measured at 439 mJ/m2 and the capillary energy corresponding to the optimal austenite dispersion for transformation toughening was 1200 J/mol.; The knowledge gained from this investigation was used to design a new cobaltnickel secondary hardening steel optimized for precipitated austenite transformation toughening. The alloy composition was restricted to produce compatible thermodynamic driving forces and kinetic rates of the second phase dispersions to permit a practical heat treatment schedule while maintaining the desired strength and toughness properties.