An Analytical Framework for Predicting the Limit in Structural Refinement in Accumulative Roll Bonded Nickel
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THERE has been significant recent effort devoted to the field of nano-scale grain refinement for generating extraordinary property combinations in conventional metals and alloys.[1,2] Methods for generating extreme grain refinement by plastic deformation rather than by other processes are termed severe plastic deformation (SPD) processes. While there have been numerous SPD processes devised in recent years,[1,2] the most investigated processes are equal channel angular pressing (ECAP), high-pressure torsion (HPT), and accumulative roll bonding (ARB). In these processes, the material of interest is subjected to extreme strains (true strains>6 to 10) to generate a high density of dislocations in the form of a highly refined ( 10 and 8, respectively) of high purity nickel at ambient temperature generate an equiaxed grain structure of 300 and 100 nm, respectively.[5,6] However, deformation at cryogenic temperatures (e.g., cryo-rolling) yields further structural refinement.[7] As noted, ARB is a typical SPD process that generates a characteristic deformation microstructure. Briefly on ARB, two sheets of a material of similar thickness are stacked and roll bonded to ~50 pct reduction in thickness, followed by cutting and stacking the bonded sheets for subsequent rolling; this stacking, bonding, and cutting process can theoretically continue indefinitely under certain deformation conditions. Overall, ARB is a reasonably simple rolling process that retains the original sheet thickness regardless of the number of processing cycles. In this light, the process has certain technical and commercial advantages over other SPD techniques since conventional rolling mills in existing processing plants can be used. A notable aspect of ARB, which differs from other SPD methods, is the characteristic morphology of the deformation substructure that is generated within the bonded layers, as well as the through thickness structural inhomogeneities through the layers that are subsequently incorporated in further ARB cycles. There are two major structural features characteristic of ARB:
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Fig. 1—Bright-field TEM micrographs of the (a) bonding region and (b) layer interior showing nano-scale lamellar band formation at the bonding regions after a single ARB cycle. The corresponding selected area diffraction (SAD) patterns (insets) show (a) a high and (b) low misorientation spread, respectively.
(i)
(ii)
Each roll bonding cycle results in local strain inhomogeneities in the surface regions of the sheet adjacent to the rolls as well as introducing a new metal-metal interface at the center of the sheet thickness; these features of the process can introduce chemical (e.g., oxide debris) and strain inhomogeneities (e.g., shear strain components such as redundant shear[8]) into the material during the subsequent rolling cycles. Also, in-plane shear operates at the phase boundaries in ARBs that involve more than one phase.[9] A large number of roll bonding cycles generates highly elongated, plate-like structures with the
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