β Scribed by A Ascoli; E Germagnoli; G Guarini
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FIG. 1. Initial nucleation rate of isothermal martensitic transformation at -125Β°C in an Fe-24% Ni-3 % Mn alloy as a function of austenitizing temperature. Method A is based on equations (1) and (2); method B is based on equation (6). increases by over an order of magnitude with increasing austenitizing temperature, reaches a maximum at 850-9OO"C, and then decreases by three orders of magnitude with further increase in austenitizing temperature. The two methods of determination are in general agreement with each other, although method A gives larger nucleation rates than method B for the lower austenitizing temperatures, and vice versa for the higher austenitizing temperatures. It appears that method A suffers in reliability because of the uncertainty in determining the correct volume per martensitic plate from metallographic measurements. A further difficulty arises because the radii of martensitic discs, even those initially formed, are not necessarily defined by the austenitic grain size. For example, after austenitizing at the higher temperatures, some of the first-formed plates do not extend from boundary to boundary of the austenitic grains; equation (2) then over-estimates the volume per plate, and results in smaller values of $ via equation (1). This accounts for at least part of the discrepancy between methods A and B in the high austenitizing range.
Be that as it may, both methods lead to similar magnitudes for the absolute nucleation rates, and both show the same trends with respect to austenitizing temperature. This suggests that the numerical values for the solid-state nucleation rates reported here can be regarded with some confidence.
The striking effect of austenitizing temperature on the rate of martensite nucleation is now under study.
The authors wish to thank William M. Justusson of the Ford Scientific Laboratory for supplying the ironnickel-manganese alloy investigated here.
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