Stress-assisted isothermal martensitic transformation: Application to TRIP steels
- PDF / 696,826 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 8 Downloads / 195 Views
I.
INTRODUCTION
THE unusual
combinations of strength, ductility, and toughness exhibited by the metastable austenitic steels, termed TRIP steels, ~ have called attention to the potential benefits of deformation-induced martensitic transformations. These effects have been exploited in lower-strength austenitic steels for some time, 2-5 and dispersions of metastable austenite now show promise for the improvement of ductility in low-alloy s t e e l s 6'7 and for the toughening of some ceramic materials) However, some undesirable consequences, most notably inhomogeneous yielding and extreme temperature sensitivity,9 have hampered the practical development of the high-strength TRIP steels. In an effort to elucidate the interrelation of the transformation kinetics and the plastic flow behavior of these steels, Olson and Azrin ~~measured the true-stress, true-strain, and martensite content during both uniform and localized (Ltiders-band) deformation; the observations extended from low temperatures, where the transformation kinetics are dominated by stress-assisted nucleation on the same sites responsible for the spontaneous cooling transformation, to a higher temperature regime dominated by strain-induced nucleation on new sites produced by plastic deformation, u In addition to the well-known "static" hardening contribution of the transformation product, comparison of the transformation and flow behaviors revealed a "dynamic" softening associated with the operation of the transformation itself as a deformation mechanism. The latter effect was most pronounced at low temperatures where stress-assisted isothermal transformation was found to control the plastic flow. For these conditions, it is now of interest to compare the observed transformation and flow characteristics with those predicted from knowledge of the kinetics of isothermal martensitic transformations.
II. KINETICS O F I S O T H E R M A L MARTENSITIC NUCLEATION
strate that the activation energy (Q) for isothermal nucleation is linearly related to the transformation free-energy change per mole (AG) according to the relation: Q = A + BAG
with A and B being constants. While AG is usually varied in such experiments by changing the temperature, Eq. [1] has also been verified by experiments in which AG is varied independently by other driving forces such as magnetic fields. 16This linear relation has been interpreted in terms of rate-control by interracial dislocation motion,17'~8 supported by quantitative agreement with the kinetics of slip. When the nucleation activation energy obeys Eq. [ 1], the rate of transformation can be expressed by: 12 (
~f = n, Vu exp\
METALLURGICALTRANSACTIONS A
A + BA% ~ /
[2]
w h e r e f is the volume fraction of martensite, n, the density of nucleation sites, V the instantaneous mean martensitic plate volume, and u is the nucleat!on-attempt frequency. The critical AG to achieve a g i v e n f is then given by: AGc~,Q) = - ~ 1 ( A + RT in--.--;-n,vu~f)
[3]
indicating a linear temperature dependence. Straight lines AGentfor di
Data Loading...
