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Nanoscale Growth Twins in Sputtered Metal Films

October 7, 2008

By Zhang, X Anderoglu, O; Hoagland, R G; Misra, A

This article reviews recent studies on the mechanical properties of sputtered copper and 330 stainless-steel films with {111} nanoscale growth twins preferentially oriented perpendicular to growth direction. The mechanisms of formation of growth twins during sputtering, unusually high strengths, and excellent thermal stability of nanotwinned structures are highlighted. INTRODUCTION

Twin interfaces, which are low-energy planar defects, can be categorized into deformation, annealing, and growth twins. Although each type of twins, especially deformation twins, has been studied extensively in the past, the influence of a high density of nanoscale twins on mechanical properties has attracted renewed interest recently. Deformation twins have been observed in nanocrystalline pure metals (see the article by Y.T. Zhu et al. in this issue). Nanotwinned structures have also been reported in electrodeposited1-5 and sputter-deposited face-centered cubic (fcc) metals6-10 (i.e., in processing methods that do not involve severe plastic deformation). The focus of this article is on nanotwinned structures in sputter-deposited metals.

NANOSCALE GROWTH TWINS IN COPPER AND STAINLESS-STEEL FILMS

A high density of growth twins has been observed in sputtered 330 stainless-steel (330 SS) and copper films.6,7 As shown in Figure Ia and b, twin interfaces in both cases are the symmetric Sigma3{111} type, oriented perpendicular to the growth direction. The average twin lamellae thickness in both cases is less than 10 nm. The hardness of nanotwinned 330 SS and copper films is approximately 6.5 GPa and 3.5 GPa, respectively, much higher than their bulk counterparts. Sputtered nanotwinned copper foils possess 1-2% uniform elongation with a tensile strength of 1.2 GPa.7 Twin boundaries with spacing on the order of a few nanometers pose a significant barrier to the transmission of dislocations. Molecular dynamics simulations were performed to study the interaction of glide dislocations with a twin boundary in fee nickel modeled with embedded-atom-method potentials. Under applied pure shear (Figure 2b), a resolved shear stress of 1.7 GPa is needed to transmit a glide dislocation across the twin interface onto the complementary {111} glide plane in the neighboring crystal, leaving behind a residual dislocation at the twin boundary.6

In a different simulation (Figure 2c), the slip transmission was studied under tensile loading. For applied tension parallel to the interface, the force acting on a glide dislocation has the wrong sign to move it away from the boundary on the {111} slip plane in the adjacent crystal. Thus, slip is observed to transmit onto the {200} plane, a non-typical glide plane in fee, at a resolved shear stress of ~3 GPa.9 Molecular dynamics simulations thus indicate that the inherent strength of the twin interface to the transmission of single dislocations is rather high. A single dislocation was considered in this simulation since a dislocation pile-up is unlikely at a twin lamellae thickness of 5 nm or less. The simulations shown in Figure 2 compute an upper bound estimate of the barrier strength of twin boundaries to slip transmission in the absence of dislocation pile-ups.

Tailoring twin spacing in nanotwinned metals is still a scientific challenge. Understanding the formation mechanisms of growth twins is key to achieving a wide range of twin spacing in a reproducible way.

FORMATION OF GROWTH TWINS IN VAPOR-DEPOSITED METALS

During physical vapor deposition, atoms from the vapor phase condense on a substrate to form the solid film. The initial nuclei that form may be either perfect (i.e., free of planar defects) or have stacking faults and/or twins. The total free energy (DeltaG^sub 1^) of a discshaped perfect nucleus with radius r and height h is given in Equation 1, where gamma is the surface energy and DeltaG^sub v^ is the bulk free energy per unit volume driving the nucleation. (All equations are presented in the table.) For the nucleus with a twin interface, Equation 1 is modified to Equation 2, where gamma^sub t^ is the twin boundary energy. For gas-solid transformation (sputtering, in this case), the critical nucleus size r* for the perfect and twinned nuclei are given as Equation 3(11) and Equation 4, respectively, where k is the Boltzmann constant, T is the substrate temperature during deposition, [mu] is the atomic volume, J is the deposition flux, and P^sub s^ is the vapor pressure above solid. In comparing Equation 3 with Equation 4, we note that r*^sub perfect^

A series of experiments has been performed to test the predictions of the model described above. 330 SS films were deposited at varying rates from 0.5 [Angstrom]/s to 7.5 [Angstrom]/ s. The volume fraction of twinned grains in 330 SS is very small at deposition rates

The hardness of the 330 SS film increases monotonically with ithe ncreasing volume fraction of twinned grains, and approaches 6,5 GPa for films with approximately 50% twinned grains. X. W. Zhou and H.N.G. Wadley13 have modeled the formation of growth twins in copper during physical vapor deposition using MD simulations. Their simulation has shown that the influence of deposition rate and temperature on the formation of growth twins is less significant at a deposition rate of 400 [Angstrom]/s or higher, consistent with calculations that show that at 400 [Angstrom]/s or higher, the Deltar/r value at the tail of the plot varies only slightly with deposition rate, and will potentially have little influence on the formation of growth twins. A linear fit (shown as a dashed line in Figure 3b) to the plot of hardness as a function of volume fraction of twinned grains intercepts with the y axis at 4.2 GPa, which is an estimate of the hardness of twin-free, nanocrystalline 330 SS films, with an average column gram size of 25 nm.

For materials with relatively higher gamma^sub t^ (as compared to austenitic stainless steel) such as nickel (43 mJ/m^sup 3^),14 sputtering at a very high deposition rate of 125-300 [Angstrom]/s was needed to form growth twins. Spacing was [asymptotically =]170 nm,15 significantly coarser than the nanotwins reported here for low stacking fault energy metals (330 SS and copper). For aluminum with a much higher gamma^sub t^ of 75 mJ/m^sup 2^ than nickel,14 the formation of nanotwinned structures via sputter deposition may be even more difficult. The above analysis does not include the film growth stresses that may also influence the formation (e.g., as a strain relief mechanism) of the observed nanoscale twinning. A recent study shows that the formation of growth twins in 330 SS films depends very little on the sign and magnitude of residual growth stress.9

THERMAL STABILITY OF TWIN INTERFACES

Although both twin and high-angle grain boundaries are effective in strengthening metals, the low-energy twin interfaces may have certain advantages, such as better thermal stability at elevated temperatures. A recent study of thermal stability of nanotwins in sputtered copper films supports this hypothesis.10 A series of high vacuum annealing experiments were performed on freestanding sputtered copper foils. Annealing up to 800[degrees]C leads to grain coarsening, but the high density of growth twins is retained, as shown in Figure 4a and b. After annealing at 800[degrees]C, the average columnar grain size is increased by an order of magnitude to over 500 nm, whereas the twin lamellae thickness increased only moderately and remained below 20 nm.

A microstructure comprised of twin boundaries has better thermal stability than high-angle grain boundaries because of its low energy characteristics. The energy of a high angle grain boundary in copper is typically 625-710 ml/m2,14,16 whereas the twin boundary energy of copper is much lower, typically 24-39 mJ/m2,14,17 For a 1 [mu],m thick film with the microstructure shown in Figure 1b, the total grain boundary energy stored is approximately 27 J, whereas the total twin boundary energy stored is approximately 5 J. The driving force for reducing the total energy of the system via coarsening of column grains is much higher than for nanotwin coarsening. After annealing at 800[degrees]C, the total energy stored at column grain boundaries, with an average diameter of 500 nm, is approximately 2.5 J, similar to the energy stored in twin boundaries, 1,25 J, with an average twin spacing of ~20 nm. The activation energy for twin boundary migration during coarsening is estimated to be ~238 kj/ mol,10 a factor of three higher than the activation energy (83 kJ/ mol) for highangle grain boundary migration in pure copper.18 The thermal stability of columnar grains in nanotwinned copper seems to be better than nanocrystalline equiaxed grains. Besides the fact that accumulative grain boundary energy in columnar grain boundaries is lower than that in equiaxed grains of the same diameter, ~40 nm, faceted columnar grain boundaries observed in annealed nanotwinned copper may indicate a pinning effect on the migration of grain boundaries. Also, iron precipitates at the grain boundaries (-0.5 at.% iron in deposited copper films) may exert a Zener drag force on the high-angle grain boundary migration. Molecular dynamics simulation shows that the presence of 1% iron in copper could cause an order of magnitude increase in barrier strength for grain boundary migration.19 During annealing, the hardness of as- deposited copper thin films decreased gradually and continuously from -3.5 GPa to approximately 2.2 GPa at 800[degrees]C, as shown in Figure 5a. In comparison, the hardness of nanocrystalline or ultrafine grain (ufg) copper decreases rapidly to -1 GPa or lower at annealing temperatures of 400[degrees]C.20[degrees]22 A Hall-Petch plot of flow stress vs. t^sup -1/2^ (where t stands for the average twin lamella thickness) is shown in Figure 5b for nanotwinned copper and annealed coarse-grain copper.23 The Hall-Petch slope (k) of the bulk copper is approximately 0.15 MPa.m^sup 1/2^ 23 and its extrapolation to the nanometer range overestimates the peak strength of nanotwinned copper. A linear fit of the data for nanotwinned copper gives a slope of -0.06 MPa-m^sup 1/2^, almost a factor of three lower than the k for bulk copper, indicating a weaker dependence of flow strength on lamella thickness in twinned copper.

CONCLUSION

Vapor-deposition of low-stackingfault energy face-centered-cubic (fee) metals produces a nanotwinned structure such that the average twin lamellae thickness is below 10 nm and twin boundaries are preferentially aligned normal to the growth direction. These nanotwinned structures exhibit unusually high strengths: tensile strength [asymptotically =] 1.2 GPa for copper and hardness of [asymptotically =]7 GPa for austenitic stainless steels. In the absence of dislocation pile-ups, twin boundaries are strong obstacles to slip transmission. Due to the low energy of a twin boundary as compared to a high-angle grain boundary, the nanotwinned structures exhibit very high thermal stability and much better retention of strength after annealing, as compared to nanocrystalline metals.

With regard to future work, the mechanisms of twin nucleation during growth in sputter deposition need to be better understood to tailor the deposition conditions to produce nanotwinned structures over a broader range of length scales and material systems.

ACKNOWLEDGEMENT

X. Zhang acknowledges financial support by the National Science Foundation-Division of Materials Research under grant no. 0644835. X. Zhang also acknowledges access to the Center for Integrated Nanotechnologies at Los Alamos National Laboratory through the user program. A. Misra acknowledges support from the U.S. Department of Energy, Office of Basic Energy Sciences. Discussions with J.D. Embury, F. Spaepen, and J.P. Hirth are acknowledged.

How would you…

…describe the overall significance of this paper?

This paper presents an overview of the structure and mechanical properties of face-centered cubic metals and alloys such as copper and ausienilic stainless steels that can be magnetron-sputter deposited to produce growth twins preferentially oriented normal to the growth direction with spacing on the order of a few nanometers.

…describe this work to a materials science and engineering professional with no experience in your technical specialty?

Low stacking fault energy face-centered cubic metals and alloys can be magnetron sputtered to produce nanometer-scale twinned structures with a unique combination of properties such as high tensile strength, high electrical conductivity, and high thermal stability.

…describe this work to a layperson?

Certain metals can be synthesized via magnetron sputtering to contain growth twins with average lamellae thickness of a few nanometers leading to almost an order of magnitude increase in hardness with little change in electrical conductivity.

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X. Zhang, assistant professor, and O. Anderoglu, graduate research assistant, are with Texas A&M University, Department of Mechanical Engineering, 3132 TAMU, College Station, TX 77843; R.G. Hoagland and A. Mlsra, technical staff members, are with Los Alamos National Laboratory, Los Alamos, New Mexico. Dr. Zhang can be reached at zhangx @tamu.edu.

Copyright Minerals, Metals & Materials Society Sep 2008

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