Role of oxygen in enhanced fatigue cracking in a PM Ni-based superalloy : stress assisted grain boundary oxidation or dynamic embrittlment?

The role of oxygen in enhanced fatigue cracking in an advanced Ni-based superalloy for turbine disc application has been evaluated in fatigue crack initiation and propagation stages along with static oxidation tests. It is found that the grain boundary oxide intrusion has a layered structure. The microstructure- and deformation-dependent grain boundary oxidation dominates the fatigue crack initiation and early propagation processes. As the crack propagates, this contribution arising from oxidation damage may gradually be overtaken by dynamic embrittlement processes until the mechanical damage outstrips the oxygen-related damage, resulting in a transition from intergranular to transgranular crack propagation

Effects of Cu on the microstructural and mechanical properties of sputter deposited Ni-Ti thin films

The microstructure of sputter deposited Ti-rich Ni-Ti thin films doped with Cu in the range 0–20.4 at.% and annealed for 1 h at 500 and 600 °C has been investigated and correlated with the mechanical properties of the films measured by depth-sensing nanoindentation. X-ray diffraction analysis showed the microstructural evolution of Ni-Ti thin films when doped with Cu and annealed at different temperatures. Heat treatments promoted the nucleation and growth of Ti2Ni precipitates in Ti-rich Ni-Ti thin films, which affected the stability of austenitic and martensitic phases at ambient temperature. Doping with Cu caused the formation of Ti(Ni, Cu)2 plate precipitates, which became more finely and densely dispersed in the grains with increasing Cu content. TEM analysis showed a columnar grain morphology extended through the whole films thickness, while with increasing Cu content a noticeable lateral grain refinement was induced by segregation of a (Ni, Cu)-rich phase to grain boundaries. The nano-hardness increased almost linearly with increasing Cu content owing to this grain refinement, though differences between samples annealed at different temperatures were found which could be related to the evolution of Ti(Ni, Cu)2 plate precipitates with annealing temperature and Cu content. The Young's modulus exhibited a similar dependence on Cu content as nano-hardness, though no significant differences were observed with increasing annealing temperatures

Microstructural and nanomechanical characterisation of Ni-Ti(-Cu) shape memory alloy thin films for tribology

Protective and functional coatings have been undergoing development for decades and further improvement to their mechanical and tribological properties are more and more challenging. Nowadays most research is aimed at improving the tribological behaviour of hard and functional coatings through the optimisation of their microstructure on the nanoscale. Over time significant breakthroughs have been achieved, however improving material performance is becoming harder. Combining different layers is another possible way of improving the tribological performance of functional coatings. The use of bonding layers can mitigate the differences in mechanical and thermal properties between coating and substrate and might change the behaviour of the coating system in a tribological scenario. As a consequence, the nature of this bonding interlayer plays an important role in the response of the coatings to the complex stress conditions taking place in a tribological system. Among the possible interlayer candidates a layer with the capability of accommodating large deformation, thus protecting the substrate from plastic deformation as well as improving the adhesion of the top layer to the substrate, could represent a suitable choice. One of the potential classes of materials satisfying the above requirements are the Ni-Ti based alloys which are known to exhibit superelastic properties also when sputter deposited. In this study we first focus our investigation on the characterisation of sputter-deposited Ni-Ti based thin films. In particular, the Ni-Ti system is doped by a third element, Cu through which mechanical and microstructural properties can be changed without detrimental effects on the typical functional properties of Ni-Ti alloys. The effects of Cu, in the range 0 – 20 at.%, and of the post-deposition heat treatments, with particular regard to annealing temperature, on mechanical and microstructural properties of sputter-deposited Ni-Ti(-Cu) thin films are investigated by nanoindentation, X-ray diffraction and transmission electron microscopy. Based on the objective of using Ni-Ti(-Cu) thin films as the interlayer in tribological coatings, some of the Ni-Ti(-Cu) films are selected and integrated in a bilayer design. Among the tribological coatings self-lubricant W-S-C coatings are known for their excellent non-Amonton frictional behaviour with friction decreasing with increasing contact pressure. The low friction of W-S-C coatings is associated with the formation of a WS2 tribolayer on the sliding surface. When the basal plane of the WS2 tribolayer is aligned with the sliding direction, the friction coefficient drops to very low values owing to the weak bonding between chalcogenide atomic planes. The formation of the low-shear surface layer is directly related to contact pressure; therefore, a W-S-C coating is an ideal functional layer with which to study the effect of Ni-Ti(-Cu) interlayers on sliding properties. W-S-C/Ni-Ti(-Cu) bilayer coatings are fabricated following a three-step process consisting of deposition and annealing of the Ni-Ti(-Cu) layers and subsequent deposition of the top functional layer. Mechanical and nano scratch behaviour of these bilayers is investigated in order to study the functional role of different Ni-Ti(-Cu) interlayers on the response of the bilayers to nanoindentation and nano-scratch tests.The tribological performance of W-S-C single layer and selected W-S-C/Ni-Ti(-Cu) bilayers are investigated by sliding tests in humid air under different test conditions in order to assess the potential beneficial effects of the interlayer on the tribological properties of W-S-C. Correlation between the tribological properties measured and microstructural changes induced by sliding is achieved by investigating the tested coatings by focused ion beam and transmission electron microscopy. Chemical changes on the sliding surfaces are also investigated by Raman spectroscopy in order to highlight possible differences in the tribolayer formation under different test conditions and for different Ni-Ti(-Cu) interlayers. The study is also aimed at understanding how the stress-induced martensitic transformation is activated in the interlayers during sliding.<br/

Ni-Ti-Cu shape memory alloy interlayers supporting low-friction W-S-C coatings

This work is aimed at assessing the capability of Ni-Ti(-Cu) interlayers, integrated in a bilayer design (tribological top layer/Ni-Ti(-Cu) layer/substrate), to improve the resistance against adhesion damage and the tribological performance of W-S-C self-lubricant coatings, when these bilayers are subjected to different sliding conditions. Interesting differences on adhesion and tribological performance are observed in relation to the grain size of the interlayer materia

Structural and mechanical properties of irradiated multilayer nanocomposites

Radiation damage processes in ion-irradiated metals have been thoroughly studied in the last decade revealing complexity and multiscale nature of material damage. Conversely, very little attention has been paid regarding gamma rays damage, which, in reactor pressure vessels, was found to be comparable to that produced by fast neutrons. Nanoscale structural control of nuclear materials through design of interfaces is a promising way of limiting radiation damage. Here we report two case studies regarding: (i) the role of interfaces and of He-ion radiation doses on the structural and mechanical properties of a sputter-deposited Cu/W multilayer, and (ii) the role of interface density distribution on the structural and mechanical properties of gamma-irradiated Zr/Nb multilayers. Transmission electron microscopy and X-ray diffraction were employed to investigate radiation damage, while mechanical properties were explored by nanoindentation. We propose correlations between radiation experiments and materials properties for each case