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 !!!!!