Role of mechanical activation in the dynamic transformation of austenite

Dynamic Transformation of an X70 Steel under Plate Rolling Conditions

“…24) These microstructures are similar to those that have been reported in other steels when dynamic transformation is taking place. 25,26) The presence of DT ferrite in the present samples was confirmed using EBSD techniques of phase identification. This is illustrated here in Fig.…”

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae3

“…For reference, the right-hand axis displays its corresponding value in J mol -1 . Note that the work/unit volume differences illustrated here are converted into thermodynamic quantities using the conversion factor 1 MPa = 7.2 J mol -1 [10]. For simplicity, the flow stresses at the melting point were considered here to go to zero, although they actually have small (and unknown) non-zero values at this temperature.…”

“…The formation of DT ferrite also involves a dilatation strain of about 3 %. The work of lattice dilatation per unit volume, (W/V) D , is then given by the equation [9,10]:…”

“…The other quantities are the work of dilatation and of shear accommodation [10]. Unlike the situation at ambient temperatures, at which these energies are stored elastically, the high temperature versions are not stored reversibly, but involve dissipation by means of plastic work.…”

“…In the dynamic case, however, the free energy difference is of opposite sign, so that it becomes a barrier to the transformation instead. Under these conditions, it is the flow stress difference between the work hardened austenite and the much softer ferrite that replaces it that provides the driving force [8,9].The other quantities are the work of dilatation and of shear accommodation [10]. Unlike the situation at ambient temperatures, at which these energies are stored elastically, the high temperature versions are not stored reversibly, but involve dissipation by means of plastic work.…”