Nanoscale twinning in thin films

Twinning can be obtained by different processing techniques, such as annealing, plastic deformation and growth process. According to their processing history, twins can be named as annealing twins, deformation twins and growth twins. It is known that twinning is a particularly important deformation mechanism in crystals with only a limited number of slip systems. However, research on the influence of twin interface on mechanical, electrical and other properties is still at a very early stage. One of the reasons is that it is difficult to predict or control the microstructure (e.g. coherent vs. incoherent twin interface) or geometry (twin spacing or the twin width) of twins. Twinning with twin spacing of less than 100 nm is rarely observed. Even after extremely high strain rate deformation, such as shock loading of a typical stainless steel, the twin spacing is over 100 nm.

Among the three types of twins, we are especially interested in growth twins. As the microstructure and geometry of growth twins could be precisely controlled by tailoring the growth process (such as physical vapor deposition). Studies on growth twins will provide important foundation for engineering design twin interface at small length scale with desired properties.

The concept of engineering design twin interface has recently been demonstrated in metallic thin films. We have recently observed high density {111} type growth twins with a few nanometer twin spacing in austenitic 330 stainless steel (330 SS) thin films. These thin films were prepared by magnetron sputtering (a widely used physical vapor deposition technique).

Bright field TEM of 330 SS films with an average columnar grain size of around 30 nm, showing high density twinning within the columnar grains.

HRTEM image showing nanoscale growth twins on {111} in sputtered 330 stainless steel films, with arrows indicating twin interfaces. The inset showing fast Fourier transform from the corresponding image.

Sputter-deposited 330 SS thin films, several micron thick, were found to have a hardness of around 7 GPa, about an order of magnitude higher than that of bulk 330 SS. Growth twins are also observed in sputter-deposited Cu/330 SS multilayer thin films. Molecular dynamics simulations show that, in the nanometer regime where plasticity is controlled by the motion of single rather than pile-ups of dislocations, twin boundaries are very strong obstacles to slip. These observations provide a new perspective to producing ultrahigh strength monolithic metals by utilizing growth twins with nanometer-scale spacing.

MD simulation showing the strength of symmetric (111) twin interface to block dislocation transmission.

(a) A perfect glide dislocation with b = 1/2 [101] resides in the upper layer. Unstrained.

(b) The model is subject to pure shear stresses such that the resolved shear stress on the dislocation is 1.77GPa. The dislocation is moving away from the twin interface. A Shockley partial with b = 1/6 [] remains at the interface.

Reference

1. “Nanoscale Twinning Induced Strengthening in Austenitic Stainless Steel”, Applied Physics Letter, 84 (2004) 1096. (PDF)

2. “Enhanced Hardening in Cu/330 Stainless Steel Multilayers by Nanoscale Twinning”, Acta Materialia, 52 (2004) 995. (PDF)

3. “Effects of Deposition Parameters on Residual Stresses, Hardness and Electrical Resistivity of Nanoscale Twinned 330 Stainless Steel Thin Films”, Journal of Applied Physics, 97 (2005) 094302. (PDF)

4. X. Zhang, A. Misra, H. Wang, X. H. Chen, L. Lu, K. Lu, and R. G. Hoagland, “High-strength Sputter-deposited Cu Foils with Preferred Orientation of Nanoscale Growth Twins”, Applied Physics Letter, 88 (2006) 173116. (PDF)

5. “High-strength twinned Nanolayer Structure”, US patent, Xinghang Zhang, Amit Misra, Michael A. Nastasi and Richard G. Hoagland, 2004.