THE ROLE OF STRUCTURAL LEDGES AT PHASE BOUNDARIES .2. FCC-BCC INTERFACES IN NISHIYAMA-WASSERMAN ORIENTATION

The ledge mode of misfit accommodation was first postulated for bounda ries between b.c.c. and f.c.c. metal phases; the interfaces being {111 } f.c.c.-{110} b.c.c. planes and relative orientations varying from Ni shiyama-Wassermann (NW) to Kurdjumov-Sachs (KS). Here the geometrical quantities are uniquely related by the misfit ratio r of atomic diamet ers. We consider the so-called NW-x configuration in which the orienta tion is imposed by close matching along the [211BAR] f.c.c. and [110BA R] b.c.c. (taken as x-) directions. From the fact that no net shear pa ttern displacements are present with x-ledges (ledges normal to the x- direction) it is concluded that they are energetically preferable to y -ledges and justifies the approach of an energetic comparison between stepped interfaces with x-ledges and a planar interface containing con ventional misfit dislocations (MDs). The NW-x stepped configuration is at first subjected to a rigid model analysis, i.e. a model with rigid crystals and periodic (truncated Fourier representation) interfacial interaction. This analysis provides (i) energetic justification for a relation between terrace periodicity and misfit cancellation, (ii) val ues of upper and lower average energy bounds, (iii) a method for estim ating interfacial shear moduli, and (iv) a motivation for the suggesti on that a relative rigid translation of the crystals is needed for ene rgy minimization. The average energy per atom of a stepped interface ( epsilon(s)BAR) provides for misfit at the terrace patch and riser inte rface, as well as for misfit normal to the interface. The planar inter face energy epsilon(p)BAR accounts for interfacial misfit only. From a display of the excess energy DELTAepsilonBAR = epsilon(s)BAR - epsilo n(p)BAR plotted against r it is concluded that stepped interfaces are more stable than planar interfaces for all meaningful values of the mi sfit ratio r, i.e. within the range where the NW-x orientation is viab le.