A study of Ni-based refractory alloys via anomalous scattering techniques

Anomalous x-ray scattering methods provided means to probe the local interactions of specific chemical pairs in a Ni–Nb–Sn sequence. Data near and far from the absorption edges of individual constituent atoms were obtained to calculate differential distribution functions, revealing the atomic arrangements. The compositional fluctuations throughout a typical Ni60Nb40−xSnyNi60Nb40−xSny sample is described as alternating Ni-rich and Nb-rich clusters of ∼ 25 Å∼25Å dimensions. This nonrandom distribution of atomic species may partially explain the failure of previous modeling efforts of bulk metallic glasses to explain their mechanical behavior and thermal stability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87755/2/074906_1.pd

Combined transmission electron microscopy and x‐ray study of the microstructure and texture in sputtered Mo films

The microstructure and texture of thin Mo films sputtered onto the native oxide of Si(100) wafers were investigated with both conventional reflection x‐ray pole figures, and transmission electron microscopy and diffraction. Films were grown at two deposition rates (powers), 34 nm/min (1.5 kW) and 67 nm/min (3.9 kW), onto both moving and stationary substrates, under otherwise identical experimental conditions. The microstructure of the Mo films evolved into a zone 2 microstructure within the first 2 μm of growth. The development of both out‐of‐plane and in‐plane textures was found to be influenced by deposition rate and geometry. Films grown at the lower deposition rate exhibited predominantly {110} textures, while films grown at the higher rate exhibited predominantly {110} textures up to a film thickness of ∼0.5 μm and {111} textures above a film thickness of ∼1 μm. Films with the {110} textures developed grains with elongated footprints and faceted surfaces, while films with the {111} textures developed grains with elongated triangular footprints and faceted surfaces. In all of the films deposited onto moving substrates, an alignment of the grains normal to the tangent plane (defined by the substrate normal and the direction of platen rotation) was observed. In all of the films deposited onto stationary substrates, the development of an in‐plane texture was suppressed. These results suggest that a combination of geometric, energetic, and kinetic mechanisms are contributing to the evolution of the microstructure and texture in the Mo films.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70000/2/JAPIAU-76-8-4610-1.pd

Growth anisotropy and self-shadowing: A model for the development of in-plane texture during polycrystalline thin-film growth

The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e., oblique incidence deposition of anisotropic materials), an in-plane texture will develop. Grains that have “fast” crystallographic growth directions aligned with the “fast” geometric growth direction overgrow grains that do not exhibit this alignment. Furthermore, the rate of texturing increases with the degree of each anisotropy. This model was used to simulate in-plane texturing during thin-film deposition. The simulation results are in excellent quantitative agreement with recent experimental results concerning the development of in-plane texture in sputter deposited Mo films. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71031/2/JAPIAU-82-3-1397-1.pd

Surface roughness and in-plane texturing in sputtered thin films

Real surfaces are not flat on an atomic scale. Studying the effects of roughness on microstructural evolution is of relevance because films are sputtered onto nonideal surfaces in many applications. To this end, amorphous rough substrates of two different morphologies, either elongated mounds or facets, were fabricated. The microstructural development of films deposited onto these surfaces was examined. In particular, the development of a preferred crystallographic orientation in the plane of growth in 400 nm thick Mo films grown on the rough substrates was studied using scanning electron microscopy, transmission electron diffraction, and high resolution x-ray diffraction (using ϕ scans in the symmetric grazing incidence x-ray scattering geometry with a synchrotron light source). It was found that the degree of texturing was dependent upon the type of roughness and its orientation during deposition. By limiting the average oblique angle of incident adatom flux, rough surfaces slowed the development of in-plane texture. Comparison between experimental data and theoretical predictions showed that a recent analytical model is able to reasonably predict the degree of texturing in films grown onto these surfaces. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70129/2/JAPIAU-84-3-1346-1.pd

Growth textures of thick sputtered films and multilayers assessed via synchrotron transmission Laue

The growth textures of thick sputtered Mo metallizations and Mo/W multilayers, were characterized via a synchrotron white‐beam (WB) x‐ray transmission Laue technique. Transmission x‐ray diffraction studies of Mo specimens up to 61 μm thick were performed with WB synchrotron radiation; while the practical thickness limit for similar observations using a conventional laboratory Cu K(α) x‐ray source is ten times smaller. This unique approach used polychromatic x rays to simultaneously produce diffraction from a wide spread of orientations of many crystallographic planes for all the grains within a relatively large specimen volume (≊60×106 μm3). These patterns were obtained for polycrystalline 31‐ and 61‐μm‐thick Mo/W multilayer specimens, and a 35‐μm‐thick‐monolithic Mo foil specimen. In all three cases the alignment of specimen grains was similar to what would be expected for single‐crystal transmission patterns, except that the recorded intensity distributed was less localized. The WB transmission images were indexed using a reciprocal space construction for the Laue case. In the multilayers, the grains were oriented out‐of‐plane such that 〈110〉 crystallographic planes were aligned in the direction of sputter growth, while in the monolithic Mo specimen 〈111〉 crystallographic planes were so aligned, i.e., perpendicular to the deposition substrate. A spread in orientation of ∼5° was measured in the multilayer specimens, while the monolithic Mo specimen showed a spread of ∼30° when compared to a perfect single‐crystal orientation. Preferred orientation was also observed within the plane of growth to varying degrees for all three samples. © 1995 American Institute of Physics.  Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71326/2/JAPIAU-78-6-3812-1.pd

Evolution of anisotropic microstructure and residual stress in sputtered Cr films

A series of Cr films with varying thicknesses have been prepared using a multiple moving substrate deposition geometry. These films have been investigated with several experimental techniques, including synchrotron x-ray scattering, pole figures, electron microscope, and double crystal diffraction topography. It was found that the in-plane stresses are highly anisotropic in these Cr films. The anisotropic stresses, characterized by two principal stresses in two characteristic directions defined by the deposition geometry, are quantified based on a methodology given in the Appendix. The plan view transmission electron microscopy observations reveal that the Cr films develop well-organized microstructures. The grains, which are elongated along the radial direction, are crystallographically aligned as well. The development of crystallographic texture in the Cr films, further revealed by pole figures and azimuthal (ϕ) x-ray scans, depends on both the deposition geometry and the film thickness. The preferential orientation of film growth is [110] for thinner films (<1.6 μm), and then becomes [111] for thicker films. Correspondingly, the in-plane texture varies in a conformal manner. In the former case, [100] and [110] directions of grains preferentially align along the radial direction and the direction of platen rotation, respectively. In the latter case, the preferential orientation of grains in the radial direction becomes [112], while that in the direction of rotation remains to be [110]. The occurrence of the anisotropic stresses and their dependence on film thickness is related to the evolution of the anisotropic structure and in-plane texture. The correlation is discussed in terms of the modulus effect associated with in-plane texture, the stress relief at intercolumnar voids, and the texture transition. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69911/2/JAPIAU-92-12-7183-1.pd

Evolution of in-plane texture in reactively sputtered CrN films

The microstructure and texture of chromium nitride films reactively sputtered on silicon substrates were investigated using x-ray scattering, pole figures, transmission electron microscopy, and atomic force microscopy. Under the given deposition geometry, the CrN films were shown to develop a in-plane texture. The three preferred crystallographic orientations of the CrN films approximately coincided with the characteristic directions associated with the deposition geometry. There appear to be two regimes that govern the microstructural evolution and texture development for reactively sputtered chromium films. The first one involves the deposition conditions that lead to the formation of a single, stable phase such as stöichiometric CrN (above certain level of nitrogen partial pressure). In this regime, the film growth appears to be controlled by local epitaxy in individual columns, competitive grain growth, and kinetic roughening. The film characteristics resulted from this regime include the development of the in-plane texture, well-organized microstructures with relatively coarse grains, increased surface roughness, and large tensile stress. The second regime involves the transitional region prior to formation of the stable phase CrN in which significant microstructural refinements take place. This transitional region is associated with the thermodynamically metastable phase CrNxCrNx or the presence of multiple phases. The continuous renucleations during film growth disrupt the local epitaxy and impede kinetic roughening. This leads to film characteristics manifested by weakened or no texture, ultrafine microstructure (e.g., nanocrystalline structures), reduced surface roughness, and a tendency for residual stress to transit from tensile to compressive.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87655/2/023525_1.pd

Controlling strength and toughness of multilayer films: A new multiscalar approach

Multiscalar films are produced in order to combine both toughness and strength into a multilayer film. These structures incorporate both a strengthening phase and a toughening phase in a compositionally modulated microcomposite. The mechanical properties and microstructure for thick (∼50 μm) Mo/W multiscalar films have been characterized. A detailed microstructural analysis (including transmission electron microscopy, scanning electron microscopy, and x‐ray techniques) of Mo/W multiscalar films has shown that large single‐crystal columns of Mo interspersed with epitaxial layers of W extend for the entire film thickness. The microstructure is a zone‐II‐type microstructure, yet the temperatures during deposition are well below the lower limit (0.3 T/Tm) previously reported for such microstructures. Hardness and tensile tests have shown that a multiscalar approach is capable of tailoring a desired strength and toughness into a multilayered film.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70675/2/JAPIAU-74-2-1015-1.pd

Investigation of the fracture toughness of radio frequency magnetron sputtered Al–Cu–Fe films via white-beam synchrotron radiography/topography

A novel white-beam synchrotron radiography/topography substrate curvature technique has been used to study stress development in situ during annealing of Al–Cu–Fe quasicrystalline and approximant coatings, as well as to image their failure modes in real time. Single crystal Si and sapphire substrates were coated with a 2.55 µm precursor coating by RF sputtering from an Al65Cu23Fe12 powder composite target and subsequently annealed at 585°C while stress and imaging data were acquired. After the initial ramp to the annealing temperature, a stress plateau was reached for coatings on both Si and sapphire substrates, although the magnitude of the stress plateau was different in each case. A tensile stress developed in the coatings during cooling due to differential thermal expansion between the coating and substrate, allowing for calculation of both the coefficient of thermal expansion and elastic modulus of the film. During cooling, the films exhibited different stress evolution above and below 470°C, a temperature of interest in Al–Cu–Fe quasicrystal and approximant phase development. The Al–Cu–Fe coating on the Si substrate fractured at approximately 954 MPa, while the coating on the sapphire substrate fractured at approximately 431 MPa. From these values the fracture toughness was calculated to be 1.9 MPa m1/2 and 0.76 MPa m1/2 for the coatings on Si and sapphire, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48923/2/d5_10A_009.pd

The evolution of texture in thin films and multilayers via synchrotron transmission Laue and grazing-incidence X-ray scattering

Sputter-deposited films and multilayers are used for a wide variety of applications including protective coatings on turbine engine blades, magnetic recording heads, optical elements, electronic packaging, X-ray filters and monochromator components to name a few examples. This wide range of interest requires growth thicknesses from a few nanometres to tens of micrometres depending on the product. In many applications, specific film textures in the growth direction as well as in the plane of growth are required. The control and manipulation of these textures can be accomplished by using advanced characterization techniques to select particular processing conditions. A variety of X-ray methods including grazing-incidence X-ray scattering, conventional pole figure studies and synchrotron white-beam transmission Laue scattering were used to study texture evolution for the thinnest films up to the thickest multilayer coatings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48901/2/jd950d58.pd