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The effect of the presence of a passivation layer on a metal rough surface during contact loading is investigated by means of dislocation dynamics simulations. The metal body is modeled as an FCC single crystal with a self-affine rough surface that is either bare, or covered by a thin coating, impenetrable to dislocations. This analysis permits to isolate the effect of surface roughening driven by dislocation motion: when the surface is bare the dislocations can glide out, leaving crystallographic steps at the surface that modify the local roughness; when the surface is passivated, dislocations are stopped by the interface. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 681813).

NiTi alloys are widely used in different industrial and medical applications. Due to the inherent difficulty in the machining of these alloys, the use of Additive Manufacturing (AM) methods has become a popular method for their production. When working with NiTi alloys, there is a requirement on the precise control of their chemical composition, as this determines the phase transition temperatures which are responsible for their shape memory or superelastic behaviour. The high energies used in AM to melt the NiTi alloy leads to nickel evaporation, resulting in a chemical change between the batch powder and the additively manufactured part. Therefore, in AM techniques applied to different NiTi alloys, understanding the relationship between the melting strategy and nickel evaporation is crucial during the developing the desired chemical composition of the final-fabricated material. In this study, three NiTi alloys were fabricated using laser powder bed fusion (LPBF) starting from elementally blended Ni and Ti powders. Different melting strategies, including single and multiple melting, were studied in this work. Remelting improved the density and reduced cracking of the AM part. Microscopic observations, using a Scanning Electron Microscope (SEM) with a Backscattered Electron (BSE) detector, showed that the chemical homogeneity of the materials was enhanced by multiple remelting. Pure Ni and Ti were not found in any of the samples, proving that the applied melting strategies ensured good alloying of both powders. Regardless of the number of melting runs, X-ray diffraction (XRD) analysis showed the presence of NiTi (B2) and (B19′ ) phases, as well as NiTi2, Ni4Ti3 and Ni3Ti precipitates in all samples. The research demonstrated that, during the AM process, and depending on the melting strategy, 1.6–3.0 wt% of nickel evaporates from the material. It was demonstrated that the amount of evaporated nickel increased with the increasing number of melt cycles., peer-reviewed