This manuscript explores the possibility of exploiting polymer morphology (thermal or flow-induced) as materials inherent template, and domain-selective plasma etching as pattern developer, to obtain nanopatterned surfaces with different and controlled geometries, with a particular focus on nanofibrillar patterns. Oxidative plasma treatment of PET films has rendered patterned surfaces with different geometries depending on the crystallinity and orientation of the PET sample and plasma treatment time (or etching ratio). Homogeneous patterns with either randomly distributed or aligned nanofibrils with diameters between 20 and 40 nm and lengths up to 1 μm (after extensive etching) were observed depending on the sample pretreatment. Our results demonstrate the potential of oxidative plasmas as templateless nanopatterning technique and reveal a complex interplay between plasma etching parameters and polymer microstructure driving the pattern formation mechanism. These results open the possibility of fabricating gecko-inspired surfaces in a cost-effective manner.

The deformation mechanisms under tensile loading in a 48 vol.% γ′ polycrystalline nickel-base superalloy (RR1000) have been studied in-situ using neutron diffraction at 20°C, 500°C and 750°C. In addition, post-mortem microstructural studies were carried out on deformed samples using an ultra high resolution field emission gun scanning electron microscope (FEGSEM). Deformation studies were carried out on three different model microstructures with a uni-modal γ′ mean particle size of 80 nm, 120 nm and 250 nm. The elastic response of γ and γ′ during in-situ loading was measured by neutron diffraction and load transfer from γ to γ′ was observed during plastic deformation at high temperature in samples with a coarse γ′ mean particle size. It was found that as the testing temperature increases, load transfer can be observed first only for the coarse γ′ microstructure and at 750°C for the medium and coarse γ′ microstructure showing that there is a combined particle size/temperature dependency for γ to γ′ load transfer. No significant load transfer was detectable in samples with a fine mean γ′ particle size at any temperature. In some cases a region of plastic deformation without load transfer was succeeded by γ to γ′ load transfer when a certain level of plastic straining had been exceeded. FEGSEM studies of the samples plastically deformed at 500 °C showed sheared particles only in the fine γ′ microstructure but not in samples with coarse γ′. The data recorded during the in-situ loading experiment demonstrate that such experiments are suitable for detecting changes of the deformation mode. But it is only in combination with post mortem electron microscopy studies that the load transfer observed can be related to a specific change of slip mode. So far, the experimental data suggest that fine γ′ is sheared during plastic deformation at room and high temperature up to 750°C whereas in coarse γ′ Orowan looping is the most likely deformation mechanism at high temperature although cutting by strongly coupled dislocation might also explain the observed load transfer.

Understanding the relationship between deformation mechanisms and microstructure is essential if one wants to fully exploit the potential of advanced nickel base superalloys and develop future alloys. In the present work, the influence of the lattice misfit between γ and γ' has been studied by means of in-situ loading experiments using neutron diffraction in combination with crystal plasticity modelling on RR1000 and Alloy 720Li. Both alloys were processed to generate three simplified uni-modal γ' microstructures to allow determination of γ' responses and experiments were carried out at 750°C. The results showed that a positive misfit strain increases the level of load partitioning from γ to γ' during plastic deformation introduced by uniaxial tensile loading. IntroductionHigh γ' volume fraction polycrystalline nickel superalloys (approximately 50% γ') are an important class of materials used by the gas turbine engine industry for disc applications. These alloys have been designed to perform at temperatures as high as 750°C, which is important for engine efficiency. A sound, fundamental understanding of the active deformation mechanisms is essential to determine the critical microstructural parameters in these alloys. In nickel-base superalloys, γ′ precipitates are coherent with the γ matrix with a cube-cube orientation relationship [1]. The unstressed lattice parameter of γ' is slightly different from that of the γ matrix and as a result internal, coherency stresses exist in both the precipitates and matrix. In addition, internal stresses at operation temperatures can be significantly different from those at room temperature. Currently there is no unambiguous understanding of how these coherency stresses might impact the dislocation mobility and therefore affect the materials performance. Neutron and synchrotron x-ray diffraction studies have shown that significant differences in coherency strain can be observed and that the level of misfit is dependent on the aging procedure and cooling rates [2][3][4][5]. The magnitude of the internal coherency stresses in these alloys is proportional to the misfit strain which is defined in Equation 1 [6], where δ is the misfit strain, a γ is the lattice parameter of γ and a γ' is the lattice parameter of γ'.