Rate-Sensitive Deformation Characteristics
of Nanostructured Materials
Koteswararao V. Rajulapati, Sreedevi Varam and
K. Bhanu Sankara Rao
School of Engineering Sciences and Technology
University of Hyderabad, Hyderabad.
1
Driving force !
Significant literature on mechanical properties of nano pure metals but not on two phase nanomaterials (both matrix and second phase at nano scale)
Experimental investigations on deformation behavior of two-phase nanocrystalline materials and identifying the governing mechanics of plasticity in these materials as a function of grain size and second phase
2
Schematic representation of the variation of yield stress as a function of grain size.
(Ref: K. S. Kumar, H. Van Swygenhoven, S. Suresh, Acta materialia, 51 (2003) 5743.)
(Ref. M. A. Meyers, A. Mishra, D. J. Benson, Prog. Mat.Sci, 51 (2006) 427.)
Grain boundary sliding
Grain boundaries acting as sources/sinks for dislocations
Grain rotation and coalescence
Rate-sensitivity and activation volume
The plastic deformation characteristics of nc-FCC metals are much more sensitive to the rate of loading than those of mc FCC metals.
The SRS index is an order of magnitude higher for metals with nc microstructural features
The activation volume of engineering metals and alloys is some two orders of magnitude smaller in nc metals than mc metals
5
Ref: Q. Wei, S. Cheng, K. T. Ramesh, E. Ma, MSEA, 381 (2004) 71
Studies on rate sensitive deformation characteristics of electrodeposited nc-Ni
Deformation behavior of nc Copper
Ultra high strength, good ductility and dislocation based plasticity !
Ref: K. M. Youssef, R. O. Scattergood, K. L. Murty, J. A. Horton, C. C. Koch, APL, 87 (2005) 091904
Plastic deformation mechanisms that are responsible in
coarse grained materials can not simply be extrapolated to the nano grained
materials !
Dislocation based plasticity or grain boundary mediated plasticity?
SRS for bulk nanomaterials; in specific bulk two-phase nanocrystalline materials
Ƭ = Gb/L
L – interparticle distance
Conventional scale
Nano scale
Nano scale Two-phase alloys
40 µm
40 nm
Mechanics of plasticity depends on:
Grain size of the matrix
Second phase (size, distribution, amount etc.)
10
Minute additions of Pb result in precipitous decrease in hardness of nano Al !
Average grain size of Al ~ 25 nm
Effect of Pb on microhardness of nanocrystalline Al : Mechanical properties
10-2
0
21-3
0
31-4
0
41-5
0
51-6
0
61-7
0
71-8
0
81-9
0
91-1
00
101-1
10
111-1
20
121-1
30
0.0
0.1
0.2
0.3
0.4
Mean grain size of Al = 28nm.
Total 351 grains
Num
ber
fraction
Grain size of Al(nm.)
Nearly uniform grain size for Al in all the compositions;
5 nm5 nm
Broader distribution in Pb particle size
11
Effect of Pb on microhardness of nanocrystalline Al : Microstructure
Pb particles
Ref: K. V. Rajulapati et al, Scripta Materialia, 55 (2006) 155.
12
Pb segregates to the grain boundaries decreases the
critical resolved shear stress required to nucleate the
lattice defects from the nc Al grain boundary!
Ref: K. V. Rajulapati et al, Scripta Materialia, 55 (2006) 155.
S. Jang et al., Acta Materialia, 56 (2008) 4750.
XRD data of Al-Pb powders
formation of two-phase structure; no solid solution formation
average grain size of ~ 45 nm
Recent work
XRD data of spark plasma sintered Al-Pb samples
Bulk nanocrystalline samples are synthesized
2 cm X 1 cm
(dia. X thickness)
After SPS@300 oC
Grain boundary controlled plasticity in FCC metals
(based on MD simulations)
•It was suggested that the grain-boundary atoms as well as atoms upto 7-10
lattice parameters away from the grain boundary are heavily involved in
plastic deformation.
• Deformation was mostly found to be taken up by atoms at and nearby grain boundaries
•It was further suggested that the material near grain boundaries is easier to deform,
g.b.s are much softer compared to the grain interior
and that the associated deformation mechanisms are rate sensitive
•However the specific deformation mechanisms have not yet been identified and more
investigations are necessary
Ref: V. Yamakov, D. Wolf, S. R. Phillpot, H. Gleiter, Acta Materialia, 2002 (50) 61
H. Van Swygenhoven, M. Spaczer, A. Caro, D. Farkas, Physical review B, 60 (1999) 22
Summary and Conclusions
•Mechanics of plasticity in two-phase systems at nano scale are
complex; interplay between critical grain size Vs second phase
•Experimental evidence on SRS of two-phase bulk nanocrystalline
materials
•SRS is an order of magnitude higher than conventional coarse
grained materials
•The amount of nano-scale second phase enhances the SRS
•Contributions to higher SRS;
smaller grain size -> more grain boundary area;
second phase etc.
Nanocrystalline materials could be reasonably ductile!
Acknowledgements
Prof. K. Bhanu Sankara Rao
IISc nano facility
ARCI for SPS
Prof. Vikram Jayaram and Dr. Samir Kamat
DST for funding the work
Thank You !!!!!