Cross-correlative Microscopy Study of Five-fold Twinning at the Surface of Ni-based Films

The structure and phase stability of a thin film is influenced by the high surface area-to-volume dimensions. In this work we identify the formation of five-fold twins emerging at the surface of Ni-based films. This peculiar twin arrangement is very rare in nature [1], with 5-fold twin junctions most notably observed in nanoparticles or under annealing and deformation in a range of face centered cubic (FCC) bulk metals nicely summarized in reviews by Hofmeister [1][2]. Here, we use cross-correlative transmission electron microscopy (TEM) and atom probe tomography (APT) to ascertain how intentional solutes as well as impurity solutes stabilize this peculiar growth behavior. A series of Ni-1P-xCo, where x = 0, 5, 8, and 16 (at.%) thin films were magnetron sputter-deposited to a thickness of ∼2.4 μm onto Si wafers with a native oxide surface. During the deposition, the Ar gas flow served as the plasma gas source and was maintained at 10 standard cubic centimeters per minute keeping the working pressure at a constant level of 0.67 Pa. NiP and Co target gun powers were regulated to a ∼ 0.2 nm/s total growth rate for Ni-1P-xCo (at.%) films. Prior work has shown that the phosphorous solute segregates to the grain boundaries pinning the boundaries [3] whereas cobalt remains in solution lowering the {111} stacking fault energy of Ni [4]. Surface topology of the films was imaged using Tescan Lyra FIB-SEM with a secondary electron detector. Samples for TEM and APT study were prepared in plane-view and cross-section orientations by standard lift-out procedure using Tescan Lyra FIB-SEM. The microstructures films were characterized in a FEI Tecnai F20 TEM using conventional bright field imaging, selected area electron diffraction (SAED), and precession electron diffraction (PED). PED quantified the grain boundaries as well as grain texture and was facilitated by automated crystallography orientation mapping (ACOM) by means of the NanoMEGAS platform [5]. APT was used to study the elemental distributions within the Ni-1P-6Co (at.%) sample. The APT study was performed on a CAMECA LEAP-5000XS and CAMECA LEAP-6000XR in the laser-pulsed mode with 50 pJ for laser energy and 500 kHz pulse rate at 40 K with the AP Suite 6 software employed to reconstruct and analyze the APT datasets. Microstructural examination using SEM revealed that ideal condition for 5-fold twinning was with the Ni-1P-5Co alloy, where microstructure predominantly consisted of columnar grains, emerging at the surface as array of pentagonal pyramids. Plan view TEM-PED analysis confirmed that these columnar grains were fragmented by the 5-fold twin arrangements having {110} texture normal to the film growth direction (Fig. 1a). By matching regions with contrast differences collected by high angle annular dark field (HAADF) imaging along with APT, Fig. 2, it was found that oxygen and phosphorous are segregated to the interfaces that have a dendritic microstructure along the columnar grains. Since the angle between <111> planes is 70.5° in a FCC metal, five twins equally spaced around the axis leave a gap of 7.3°; according to Bringa, et al. [6], such a gap would be accommodated by elastic strain. In this case oxygen and phosphorous segregate to these regions and stabilize the 5-fold twin structures by lowering the elastic strain [7]. (a) SEM image of Ni-1P-5Co sample surface; (b) TEM-PED plan view orientation map of Ni-1P-5Co sample. (a) TEM-PED cross section view virtual bright field (VBF) image of Ni-1P-5Co sample; (b) NiO, PO and O APT reconstruction maps showing segregations in corresponding regions of (a).