Mechanical Properties of Nanocrystalline Alloys

The unusual properties exhibited by nanocrystalline metals have motivated extensive fundamental studies over recent years, with great emphasis on the scaling of mechanical properties with grain size reduction and the breakdown of the characteristic Hall-Petch relationship.  In our group, we use experiments as well as theory and simulations to study the unique properties that emerge at this scale.  Some of our recent work has probed hardness, rate sensitivity, and pressure sensitivity of deformation, all of which exhibit inflections at a finite nanocrystalline grain size.  The figure below compiles indentation curves from various nanocrystalline specimens and scanning electron micrographs of the associated residual impressions.  As evidenced by the discrete discontinuities in the load-displacement response and shear offsets in the pile-up of the d = 3 nm sample, inhomogeneous shear banding characteristic of metallic glasses appears as the grain size approaches the amorphous limit, thus signaling a shift to glass-like deformation behavior.  Experimental results such as this are also connected directly to simulations.

Typical load-displacement responses (a) for the finest grain size specimens performed at a nominal strain rate of 0.15 s-1.  As identified by the series of arrows, discrete discontinuities are evident in the d = 3 nm data, while the d = 6 and 12 nm curves (offset for clarity by 50 and 100 nm on the depth axis, respectively) are smooth and continuous.  SEM images of residual impressions on specimens with a grain size of (b) 12 nm, (c) 6 nm, and (d) 3 nm.  Note the shear offsets in the pile-up of the d = 3 nm sample shown in (d).

Published Articles:

The Hall-Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications
Trelewicz JR, Schuh CA; APPLIED PHYSICS LETTERS 93 (17): Art. No. 171916 OCT 2008 (PDF)

Mechanical properties of reticulated aluminium foams with electrodeposited Ni-W coatings
Boonyongmaneerat Y, Schuh CA, Dunand DC; SCRIPTA MATERIALIA 59 (3): 336-339 AUG 2008 (PDF)

The Hall-Petch breakdown in nanocrystalline metals: A crossover to glass-like deformation 
Trelewicz JR, Schuh CA; ACTA MATERIALIA 55 (17): 5948-5958 OCT 2007 (PDF)

Deforming nanocrystalline nickel at ultrahigh strain rates
Wang YM, Bringa EM, McNaney JM, Victoria M, Caro A, Hodge AM, Smith R, Torralva B, Remington BA, Schuh CA, Jamarkani H, Meyers MA; APPLIED PHYSICS LETTERS 88 (6): Art. No. 061917 FEB 2006 (PDF)

Strength asymmetry in nanocrystalline metals under multiaxial loading
Lund AC, Schuh CA; ACTA MATERIALIA 53 (11): 3193-3205 JUN 2005 (PDF)

Tension/compression strength asymmetry in a simulated nanocrystalline metal
Lund AC, Nieh TG, Schuh CA; PHYSICAL REVIEW B 69 (1): Art. No. 012101 JAN 2004 (PDF)