1. Deformation of nanocrystalline metals: The role of grain boundaries
Krystyn Van Vliet, Sedina Tsikata and Subra Suresh
Massachusetts Institute of Technology, Cambridge, MA USA
DEFORMATION MODES AND STRESSES
METHODS
PURPOSE
Polycrystalline bubble rafts of {111} in-plane orientation
were constructed with average grain size 4 – 37 nm
Polycrystals were deformed via nanocontact of a rigid
indenter
Variables included:
Grain size
GB orientation q
Proximity of GBs to the loading axis
Nanocrystalline metals have grain sizes < 100 nm, so the
volume fraction of grain boundaries (GBs) is much greater
than in conventional microcrystalline metals. Thus, GBs may
significantly affect strength and deformation modes in
nanocrystals.
The structure of GBs in nanocrystals is clean and atomically
sharp, but the role of GBs during plastic deformation is
unknown. For example, the Hall-Petch relationship predicts
dramatic increases in yield strength sy for decreasing grain
size d:
but a reverse Hall-Petch effect has been observed in several
nanocrystals. Why?
The Bragg-Nye bubble raft model allows visualization and
quantification of how GBs contribute to plastic deformation
in nanocrystals, and can estimate the strength of GBs for
input for computational models.
Dislocation activity always initiated at GBs or triple junctions,
NEVER from the grain interior, either by GB migration or
discrete emission of dislocations.
Local, continuum shear stress and far-field mean stress were
calculated at the onset of dislocation activity:
Critical shear stress (GRAIN BOUNDARY STRENGTH):
high speed/resolution camera
,where m(a, x, z) and n(a, x, z)
indenter
sy = so + kyd-1/2
Critical far-field mean stress (HARDNESS):
,where E*= Young’s modulus
R = indenter tip radius
a = contact half-width
5 nm
Side view
Plan view
EFFECT OF GRAIN SIZE
CONCLUSIONS
EFFECT OF GRAIN CONFIGURATION
d < 20 nm: emission from GB
d > 20 nm: GB migration
txz independent of d, and maximum for d = 13 nm
pm strong function of grain size:
For d > 7 nm, Hall-Petch strengthening
For d < 7 nm, reverse Hall-Petch effect
GB orientation
q < 55o: GB migration; q > 55o: emission from GB
txz independent of q, but maximal for q = 26o
GB proximity zp
Deformation mode independent of zp
txz a zp, but maximum txz 30% < tcrit for perfect crystal
Using the bubble raft as a 2-D analogue of close-packed metallic
nanocrystals, we find:
Nanocrystalline deformation originates from GBs and GB
triple junctions, and proceeds via GB migration or emission
of dislocations from the GB.
Orientation and proximity of the GB to points of nanocontact affect both the mode and critical stresses to induce dislocation activity.
A critical grain size exists below which a reverse Hall-Petch (softening) effect occurs, but a concurrent change in deformation mode is not observed.
Emission of dislocation from GB triple junction a x z q pm
pmmax
d-1/2
Hall-Petch
Reverse
Hall-Petch
PUBLICATIONS
d = 7 nm
This trend is in agreement with 3-D computational models of
Cu polycrystals in tension (Schiotz et al. Nature 391, 1998).