Influence of alloying elements on the mechanical properties, especially fracture toughness, of the WB2-z base system

Transition metal diborides are an emerging class of thin film materials with promising properties ranging from ultra-low compressibility, high thermal stability, super hardness to superconductivity. These properties allow an application as protective coating in harsh environments. Our recent ab initio calculations suggest an attractive combination of both, high hardness and relatively high fracture toughness, for WB2. This is enabled by a stabilization of the α-structure (space group 191, AlB2-prototype, P6/mmm) over the intrinsic more stable ω-structure due to omnipresent point defects in physical vapor deposited coatings (i.e. boron and metal vacancies) [1]. However, those point defects in turn lower the thermal stability as the are affected by recovery events, leading to phase transformation into the ω-type. Further calculations point towards a stabilization of the α-type with the addition of Ta (which diboride is stabilized in the α-structure without the need of vacancies) at—compared to other transition metals investigated—low cost on ductility. Within this study we deposited various W1-xMxB2-z solid solution coatings with different alloying element contents and examined them for mechanical properties and thermal stability. It was found for M=Ta that the hardness increases ~4 GPa (from 40.8±1.5 to 45.0±2.0 GPa) together with an improvement of the thermal stability (a change of the phase transformation temperature from ~800-1000 °C to over 1400 °C was observed) [2,3]. Besides these characteristics, in various applications a certain amount of damage tolerance (crack initiation and propagation) is required to prevent premature failure. To assess this behavior, we determined the fracture toughness of these coatings by performing micromechanical experiments by means of single cantilever bending tests within the framework of specifications given by Matoy et al. and Brinckmann et al. [4–6]. At the same time of the increase in hardness and thermal stability, we observe a decrease (in agreement with our DFT calculations) in fracture toughness (from 3.7±0.3 MPaÖm for to 3.0±0.2 MPaÖm) with the addition of tantalum up to a maximum content of 26 at% on the metal sublattice. [1] V. Moraes, H. Riedl, C. Fuger, P. Polcik, H. Bolvardi, D. Holec, P.H. Mayrhofer, Sci. Rep. (2018). [2] V. Moraes, C. Fuger, V. Paneta, D. Primetzhofer, P. Polcik, H. Bolvardi, M. Arndt, H. Riedl, P.H. Mayrhofer, Scr. Mater. 155 (2018) 5–10. [3] C. Fuger, V. Moraes, R. Hahn, H. Bolvardi, P. Polcik, H. Riedl, P.H. Mayrhofer, MRS Commun. (2019) 1–6. [4] K. Matoy, H. Schönherr, T. Detzel, T. Schöberl, R. Pippan, C. Motz, G. Dehm, Thin Solid Films 518 (2009) 247–256. [5] S. Brinckmann, C. Kirchlechner, G. Dehm, Scr. Mater. 127 (2017) 76–78. [6] S. Brinckmann, K. Matoy, C. Kirchlechner, G. Dehm, Acta Mater. 136 (2017) 281–287

Redescription of the Deep-Sea Cirrate Octopod Cirroteuthis magna Hoyle, 1885, and Considerations on the Genus Cirroteuthis (Mollusca: Cephalopoda)

31 pages, 29 figures, 4 tables, 1 appendix.The deep-living octopod Cirroteuthis magna Hoyle, 1885 is redescribed, based on the only three specimens known of the species: a mature female (holotype) captured in the south Indian Ocean between Prince Edward and Crozet islands at 2557 m and two specimens, one submature female and one mature male, recently captured in the central Atlantic at 1300 and 3351 m depth, respectively. Video images from the capture of the latter specimen were recorded. This species is characterized by its very great size (to 1300 mm TL), making it the largest known cirrate octopod; butterfly-like shell with open wings; very voluminous eyes with large lenses; arm length 73-79% of the total length; primary web inserted at different levels on the dorsal and ventral ends of the dorso- and ventrolateral arms on both sides, and at the same level on both ends of the dorsal and ventral arms; each arm is independent of the primary web, and is connected with it by a single vertical membrane or intermediate web that is attached along the dorsum of the arm; absence of nodule at the fusion point of both webs. Very large cirri, the first cirri commencing between the 4th and 5th suckers, with three types of suckers on all the arms; cylindro-conical form and those with the acetabulum highly deformable on the first 2/3 of arms and barrel-like on the rest of the arm; absence of particularly enlarged suckers. C. magna is compared with C. muelleri and other related species. Sperm sacs and spermatozoids from C. magna and C. muelleri are described and compared. The Cirroteuthis genus is reviewed and a diagnosis is proposed. This study confirms that the members of the Cirroteuthidae family show several unusual features of great interest.During this study one of the authors (R.V.) was supported by a post-doctoral fellowship from the Spanish Ministery of Education and Science.Peer reviewe

Strain and stress analyses on thermally annealed Ti-Al-N/Mo-Si-B multilayer coatings by synchrotron X-ray diffraction

In order to analyse the main failure mechanism of multilayered coating material in oxidative environments, we separately investigated the cross-sectional strain/stress evolution induced by thermal loads and oxidation for the Ti-Al-N/Mo-Si-B model system. The bilayer period (Λ) of the crystalline Ti-Al-N/amorphous Mo-Si-B layers was varied between 26, 130, 240, and 1085 nm. The stress state was characterised by synchrotron X-ray nano-diffraction, using monochromatic X-ray radiation with a beam size of around 200 × 300 nm$^2$. This allows for analysing the spatially resolved strain/stress evolution of the as-deposited state as well as after thermally treated coatings – either 1 h annealed in vacuum or ambient air at 900 °C. For small bilayer periods, the alteration of face centred cubic Ti-Al-N by amorphous Mo-Si-B layers effectively reduces the as-deposited compressive strain profile along the growth direction. Furthermore, for Λ ≤ 130 nm, the decomposition of Ti-Al-N to form hexagonal structured AlN as well as the crystallisation of the Mo-rich layers towards the intermetallics Mo$_5$SiB$_2$ and Mo$_5$Si$_3$ is significantly delayed. After oxidation, the oxide scale grows in the low-compressive stress regime, while the intact multilayer shows similar microstructural changes as the vacuum annealed coatings