FIGURE 51 Calculated radial-growth velocities of disk-shaped martensitic particles in two Fe-Ni alloys at their respective martensite-start temperatures; thermal changes due to heat-transfer effects are taken into account. From Haezebrouck.94

and phonon-drag controlled at the higher velocities. The marked differences in martensitic growth characteristics between the two iron-nickel alloys in Figure 51 originate from differences in the respective dynamic flow stresses and drag coefficients.94

However, not yet incorporated into these calculations is the quantitative effect of local plastic accommodation, including the associated defect formation and strain hardening, on the resistance to interfacial motion. This is a crucial issue, not only because of the combination of dynamic phenomena at play here but because it may be forcing a general study of the no-man’s-land between continuum and discrete-lattice solid-state science. It is somewhat reminiscent of the crack-tip/plastic-zone problem in fracture mechanics. Furthermore, the plastic accommodation process may hold the key to a deeper understanding of the autocatalytic nucleation that enters so significantly into the overall kinetics of nonthermoelastic martensitic transformations.

Transformation Plasticity and Toughening

The shear-like displacive nature of martensitic transformations allows these phase changes to operate as a deformation mechanism in parallel with ordinary slip processes.95 Applied stress can promote the formation of martensite by contributing to the transformational driving force and by introducing new nucleation sites through accompanying plastic flow. At the same time,



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