Deformation Behavior of FCC Crystalline Metallic Nanowires

Under High Strain Rates

 

Yue Qi *, Hideyuki Ikeda **, Tahir Cagin *, Konrad Samwer **, William L. Johnson **, William A. Goddard III*

*Materials and Process Simulation Center, California Institute of Technology

**Keck Laboratory of Engineering Materials, California Institute of Technology

 

ABSTRACT

 

We used molecular dynamics (MD) with the Q-SC many-body force field to study the deformation behavior of metallic alloy crystal nanowires of pure Cu, NiCu alloy and NiAu alloy, subject to uniaxial tensile strains at 300K. The strains were applied uniformly in time at rates ranging from 0.05%/ps to 5%/ps (5*108/s to 5*1010/s).

These nanowires start with a thickness of 2 nm (and infinite length) and hence cannot sustain dislocations. For Ni and NiCu at lower strain rates we find that deformation proceeds through twinning and coherent slipping mechanisms. This leads to a stress-strain curve exhibiting an initial yield strain of 7% (at a yield stress of 4.2GPa) at strain rate of 0.05%/ps, which then follows by stress dropping/loading cycles. From the (011) projection of the structure, we found the relative displacements for the atoms across the twin boundary is about on (111) planes, consistent with the well known (111)[112] twin system in FCC metals.

For all systems there is a critical strain rate above which the crystalline phase transforms continuously to an amorphous phase. This phase change exhibiting a dramatic change in atomic short-range. This amorphization (which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities) exhibits a new mode of amorphization. For NiAu (which has a 13% size mismatch) we find that the critical strain rate at which the "nanowire crystals" flow like a "liquid" is 100 times smaller than for NiCu (with a 2.5% size mismatch). Thus, strain-rate amorphization is favored by size mismatch just as for amorphization by rapid quenching

 Show a trwinned sample and discuss the nature

 

ACKNOWLEDGMENTS

Financial support was provided by the Japanese Ministry of Education for HI, by DOE (DEFOG3 86ER45242) for KS and WLJ, by ARO (DAAH04-95-1-0233) for WLJ and WAG, and by NSF (CHE 95-22179 and ACR-92-17368) and DOE-ASCI for YQ, TC, and WAG. In addition, the facilities of the MSC are supported by grants from ARO-DURIP, BP Chemical, Exxon, Avery-Dennison, Owens-Corning, ARO-MURI, Asahi Chemical, Chevron, and Beckman Institute.

 

Figure-1, Snapshots of NiCu nanowire at 100%strain deformed at strain rate of
    a) 0.05%/ps shows twinning and local melting in bulk
    b) 1%/ps shows maller fcc grains
    c) 2%/ps shows extensive disorder
    d) 5%/ps shows full amorphization to glass.

Figure-2. Stress strain curve for NiCu at different strains.

    At strain rate of 0.05%/ps, 0.5%/ps and 1%/ps, the stress drop is due to twin deformation.
    At strain rate of 5%/ps, after 50% strain, a constant stress is reached, which gives the viscosity of 0.7poise for the flowing glass.

Figure-3 (011) Projection for NiCu during tensile tests.

    (a) strain rate = 0.05%/ps strain = 7% before yielding
    (b) strain rate = 0.05%/ps strain=8%, two twins formed
    (c) strain rate = 0.05%/ps strain = 14%, twins grow one more layer strain rate = 0.5%/ps,
    (d) strain = 12.5%/ps, three twins formed

Table-1 Deformation under different strain rates in different system

 

Ni

NiCu

NiAu

RA/RB

1.0

1.025

1.156

0.05%/ps

C

C

A

0.5%/ps

C

C

A

1%/ps

C

C

A

2%/ps

C

C&A

A

5%/ps

A

A

A

C - keep cystalline, but twinning

A - Amorphization