Instructor: Yuntian Zhu MSE 791: Mechanical Properties of Nanostructured Materials Module 3: Fundamental Physics and Materials Design Instructor: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 6 Deformation Mechanisms of Nanocrystalline bcc metals Text book, Office hour, by appointment Department of Materials Science and Engineering 1

Deformation mechanisms in nano bcc metals has not been well studied It is expected to be different from those in nano fcc metals

Strain rate sensitivity m is the strain rate sensitivity Higher m can lead to higher ductility For superplastic deformation: m > 0.3 Department of Materials Science and Engineering 3

Some Difference in Strain Rate Sensitivity between bcc and fcc metals What caused the difference? Wei, J. Mater. Sci. 42, 1709 (2007).

We need to check some fundamental backgrounds What caused the difference? We need to check some fundamental backgrounds

Difference in Strain Rate Sensitivity between bcc and fcc metals: Coarse grains The strain rate sensitivity of bcc metals are an order of magnitude higher than that of fcc metals This is caused by the difference in dislocation structure

Difference in dislocation structure between bcc and fcc metals: Coarse Grain Edge dislocations dissociate into two partials on the same slip plane Screw dislocations dissociate onto three slip planes fcc Both screw and 60o dislocations dissociate into two partials on the same slip plane

Dislocation structure is determined by crystal structure Could be (110), (011) and planes fcc bcc Difference in dislocation type caused the difference in strain rate sensitivity BC = Ba +aC BC = Bd + dC Screw dislocation structure in bcc

Screw dislocations moves by double-kink nucleation in bcc metals Edge dislocation kinks glide fast and disappear Screw dislocations move by double-kink nucleation and motions Deformation controlled by the double–kink nucleation, which is thermally activated This leads to high strain rate sensitivity in coarse-grained bcc metals Wei, J. Mater. Sci. 42, 1709 (2007).

What caused the strain rate sensitivity decrease in bcc metals What caused the strain rate sensitivity decrease in bcc metals? Current Theory For bcc metals, deformation is controlled by the nucleation double-kinks When the flow stress is very high, v* decreases to nearly constant Flow stress increases with decreasing grain size, hence smaller m Theory is simple and easy to understand. The implicit assumption: The deformation mechanisms remains controlled by double-kinks from coarse-grained bcc metals to nano bcc metals Is this correct?

HPT-processed Mo Distribution of dislocation density 1.0 x1014 m-2 The dislocation density was calculated as the number of dislocations per unit area in high-resolution TEM (HRTEM) micrographs. Grain size effect on the density of screw dislocations, which was calculated as the total length of screw dislocation segments per unit volume. Edge dislocation: the number of dislocations per unit area; Screw dislocation: the total length of screw dislocation segments per unit volume; Cheng et al, Mater. Res. Lett. http://www.tandfonline.com/loi/tmrl20 11 11

Grain Size Effects on SRS Dislocations distribution Cheng et al, Mater. Res. Lett. http://www.tandfonline.com/loi/tmrl20 12 12

Grain Size Effects on SRS SRS vs grain size Cheng et al, Mater. Res. Lett. http://www.tandfonline.com/loi/tmrl20 13 13

Some Difference in Strain Rate Sensitivity between bcc and fcc metals What caused the difference? Wei, J. Mater. Sci. 42, 1709 (2007).

Conclusion The high strain rate sensitivity in coarse grained bcc metals is caused by the double-kink mechanisms for screw dislocation slip The low strain rate sensitivity in nano bcc metals is caused by edge and mixed dislocations In very small grain sizes, strain rate sensitivity increases again due to grain boundary mediated processes 15 15