The importance of creep strain in linking together the Wilshire equations for minimum creep rates and times to various strains (including the rupture strain): An illustration using 1CrMoV rotor steel

This paper highlights the observation that the Wilshire equations for failure times and times to various strains, as reported in the original literature, may not be the most appropriate ones for all materials—including the one selected in this study. Further, such appropriateness can be determined by looking at the consistencies between the parameter estimates obtained using minimum creep rates in comparison to using failure times. It is shown, using 1CrMoV steel as an illustration, that the parameter consistency can be achieved by generalising the Monkman–Grant relation so that it contains a temperature correction. Indeed, the ability of the Wilshire equations to produce meaningful physical parameters, such as the activation energy, is shown to be highly dependent upon a valid specification for the Monkman–Grant relation. It is shown that variations in the measured values for some of the Wilshire parameters (w and k 3) with strain indicate that the causes of deformation are different at different strains and different stresses. Finally, the measured variations in the parameters of the Monkman–Grant relation with strain enable accurate interpolated and extrapolated creep curves to be calculated for any test condition

Corrosion behaviour of porous Ti intended for biomedical applications

Porous Ti implants are being developed inorder to reduce the biomechanical mismatch between theimplant and the bone, as well as increasing the osseointegrationby improving the bone in-growth. Most of the focusin the literature has been on the structural, biological andmechanical characterization of porous Ti whereas there islimited information on the electrochemical characterization.Therefore, the present work aims to study the corrosionbehaviour of porous Ti having 30 and 50 % ofnominal porosity, produced by powder metallurgy routeusing the space holder technique. The percentage, size anddistribution of the pores were determined by image analysis.Electrochemical tests consisting of potentiodynamicpolarization and electrochemical impedance spectroscopywere performed in 9 g/L NaCl solution at body temperature.Electrochemical studies revealed that samples presenteda less stable oxide film at increased porosity, morespecifically, the complex geometry and the interconnectivityof the pores resulted in formation of less protectiveoxide film in the pores.This study was supported by FCT with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalizac¸a˜o (POCI) with the reference project POCI-01-0145- FEDER-006941, Programa de Acc¸o˜es Universita´rias Integradas LusoFrancesas’ (PAUILF TC-12_14), and The Calouste Gulbenkian Foundation through ‘‘Programa de Mobilidade Acade´mica para Professores’’. The authors would also like to acknowledge Prof. Ana Senos (University of Aveiro) and Prof. Jose´ Carlos Teixeira (University of Minho) for the provision of the characterization facilities.info:eu-repo/semantics/publishedVersio

Fatigue crack-growth in shape-memory NiTi and NiTi-TiC composites

An experimental study was conducted to examine the room-temperature Fatigue crack-growth characteristics of shape-memory NiTi matrix composites reinforced with 10 and 20 vol.% of TIC particles. Microstructural characterization of these hot-isostatically-pressed materials shows that the TiC particles do not react with the NiTi matrix and that they lack any texture. Overall fatigue crack-growth characteristics were found to be similar for the unreinforced and reinforced materials. However, a slight increase in the threshold for fatigue crack initiation was noted for the composites. The fracture toughness, as indicated by the failure stress intensity factor range, was found to be similar for all materials. Neutron diffraction studies near the crack-tip of the loaded fracture NiTi specimen detected no significant development of texture at the crack-tip. These results are explained by recourse to fractographic observations. Finally, a comparison is made between the micromechanisms of fracture of metal matrix composites, which deform by dislocation plasticity, and those of the present NiTi-TiC composites, which deform additionally by twinning

Lattice strain evolution and load partitioning during creep of a Ni-base superalloy single crystal with rafted gamma prime microstructure

In-situ neutron diffraction measurements were performed on monocrystalline samples of the Ni-based superalloy CMSX-4 during N-type γ′ raft formation under the tensile creep conditions of 1150 °C/100 MPa, and subsequently on a rafted sample under the low temperature/high stress creep conditions of 715 °C/825 MPa. During 1150 °C/100 MPa creep, the γ′ volume fraction decreased from ∼70% to ∼50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping γ matrix to the γ′ precipitates. On cooling back to room temperature, a fine distribution of γ′ precipitates formed in the γ channels, and these precipitates were present in the 715 °C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted γ′ microstructure exhibited superior creep strength to the cuboidal γ′ microstructure produced following a standard heat-treatment. A lengthy creep incubation period was observed, believed to be associated with {111} dislocations hindering propagation of {111} dislocations. Following the creep incubation period, extensive macroscopic creep strain accumulated during primary creep as the γ phase yielded. Finally, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated

Pre-existing immunity to influenza virus hemagglutinin stalk might drive selection for antibody-escape mutant viruses in a human challenge model

The conserved region of influenza hemagglutinin (HA) stalk (or stem) has gained attention as a potent target for universal influenza vaccines1-5. Although the HA stalk region is relatively well conserved, the evolutionarily dynamic nature of influenza viruses6 raises concerns about the possible emergence of viruses carrying stalk escape mutation(s) under sufficient immune pressure. Here we show that immune pressure on the HA stalk can lead to expansion of escape mutant viruses in study participants challenged with a 2009 H1N1 pandemic influenza virus inoculum containing an A388V polymorphism in the HA stalk (45% wild type and 55% mutant). High level of stalk antibody titers was associated with the selection of the mutant virus both in humans and in vitro. Although the mutant virus showed slightly decreased replication in mice, it was not observed in cell culture, ferrets or human challenge participants. The A388V mutation conferred resistance to some of the potent HA stalk broadly neutralizing monoclonal antibodies (bNAbs). Co-culture of wild-type and mutant viruses in the presence of either a bNAb or human serum resulted in rapid expansion of the mutant. These data shed light on a potential obstacle for the success of HA-stalk-targeting universal influenza vaccines-viral escape from vaccine-induced stalk immunity