Self-organized metal nanostructures through laser driven thermocapillary convection

When ultrathin metal films are subjected to multiple cycles of rapid melting and resolidification by a ns pulsed laser, spatially correlated interfacial nanostructures can result from a competition among several possible thin film self-organizing processes. Here we investigate self-organization and the ensuing length scales when Co films (1-8 nm thick) on SiO_{\text{2}} surfaces are repeatedly and rapidly melted by non-uniform (interference) laser irradiation. Pattern evolution produces nanowires, which eventually break-up into nanoparticles exhibiting spatial order in the nearest neighbor spacing, \lambda_{NN2}.The scaling behavior is consistent with pattern formation by thermocapillary flow and a Rayleigh-like instability. For h_{0}\leq2 nm, a hydrodynamic instability of a spinodally unstable film leads to the formation of nanoparticles.Comment: 10 pages, 3 figure

The effect of heat treatment on mechanical properties of pulsed Nd:YAG welded thin Ti6Al4V

Pulsed Nd:YAG has been adopted successfully in welding process of thin (0.7 mm) Ti6Al4V. Laser welding of such thin sheet requires a small focal spot, good laser beam quality and fast travel speed, since too much heat generation can cause distortion for thin sheet weld. The microstructures of Ti6Al4V were complex and strongly affected the mechanical properties. These structures include: a´ martensite, metastable ß, Widmanstätten, bimodal, lamellar and equiaxed microstructure. Bimodal and Widmanstätten structures exhibit a good-balance between strength and ductility. The microstructure of pulsed Nd:YAG welded Ti6Al4V was primarily a´ martensite, which showed the lowest ductility but not significantly high strength. A heat treatment at 950 followed by furnace cooling can transform the microstructure in the weld from a´ martensite structure into Widmanstätten structure

Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3

Adhesive transfer operates during galling

In order to reduce cobalt within the primary circuit of pressurised water reactors (PWR’s), wear-resistant steels are being researched and developed. In particular interest is the understanding of galling mechanisms, an adhesive wear mechanism which is particularly prevalent in PWR valves. Here we show that large shear stresses and adhesive transfer occur during galling by exploiting the 2 wt.% manganese difference between 304L and 316L stainless steels, even at relatively low compressive stresses of 50MPa. Through these findings, the galling mechanisms of stainless steels can be better understood, which may help with the development of galling resistant stainless steels

Understanding cycle tourism experiences at the Tour Down Under

Sport tourism experiences are subjective and emotional, laden with symbolic meaning. This study explores the experiences of participants who adopted the multiple roles of both an active participant and event spectator, within the parameters of one chosen sporting event. A professional cycling race event, the Tour Down Under in South Australia was chosen for this investigation, and 20 face-to-face individual interviews were conducted with cycle tourists. The three main themes emerging from the data were the interaction of people and temporary spaces on a sport tourism ‘stage’; the co-creation of authentic personal experiences and meanings; and identity reinforcement and the development of a sense of belonging. Consequently, a model for understanding sport event tourism experiences is proposed. The findings suggest that providing tourists with authentic and memorable experiences lies at the heart of what constitutes sport tourism. Whilst the results demonstrate that cycling events provide the individual with a sense of belonging or membership to a wider social group, they also illustrate that there is a continued need for more focused and nuanced approaches towards understanding sport tourism experiences that reflect the ever-increasing diversity and complexity of the interaction between sport, events and tourism

Measurement of friction in galling testing – An example of its use in characterising the galling behaviour of hardfacings at ambient and elevated temperature

© 2021 Elsevier B.V. Galling is a category of severe adhesive wear that is defined by surface damage arising between sliding solids, distinguished by macroscopic, usually localized, roughening and creation of protrusions above the original surface. The identification of galling is through a visual observation of the tested surface and is therefore inherently subjective. Due to the microscopic processes behind the onset of galling being poorly understood, further work is needed to understand the behaviour of different materials under galling conditions, both at room temperature and at elevated temperature. The current paper describes the development of a new galling testing apparatus, broadly under the ASTM G196 configuration with the addition of in-situ torque measurements, an automated worm drive for sample rotation as well as band heaters providing capability for testing at elevated temperatures up to a maximum applied stress of 950 MPa. Results from galling tests conducted at room temperate and 300 °C for both Stellite 6 (Co-based) and Tristelle 5183 (Fe-based) hardfacings are presented. The results show that the galling resistance of Tristelle 5183 is significantly reduced at elevated temperature

Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

Aerosol-cloud interactions are a major source of uncertainty in inferring the climate sensitivity from the observational record of temperature. The adjustment of clouds to aerosol is a poorly constrained aspect of these aerosol-cloud interactions. Here, we examine the response of midlatitude cyclone cloud properties to a change in cloud droplet number concentration (CDNC). Idealized experiments in high-resolution, convection-permitting global aquaplanet simulations with constant CDNC are compared to 13 years of remote-sensing observations. Observations and idealized aquaplanet simulations agree that increased warm conveyor belt (WCB) moisture flux into cyclones is consistent with higher cyclone liquid water path (CLWP). When CDNC is increased a larger LWP is needed to give the same rain rate. The LWP adjusts to allow the rain rate to be equal to the moisture flux into the cyclone along the WCB. This results in an increased CLWP for higher CDNC at a fixed WCB moisture flux in both observations and simulations. If observed cyclones in the top and bottom tercile of CDNC are contrasted it is found that they have not only higher CLWP but also cloud cover and albedo. The difference in cyclone albedo between the cyclones in the top and bottom third of CDNC is observed by CERES to be between 0.018 and 0.032, which is consistent with a 4.6-8.3gWmg‾² in-cyclone enhancement in upwelling shortwave when scaled by annual-mean insolation. Based on a regression model to observed cyclone properties, roughly 60g% of the observed variability in CLWP can be explained by CDNC and WCB moisture flux

The effect of temperature on sliding wear of self-mated HIPed Stellite 6 in a simulated PWR water environment

Cobalt-based Stellite alloys are widely used in the primary circuit of pressurized water reactors (PWR) to protect valve surfaces against wear and galling in a corrosive environment. In this study, self-mated sliding wear of HIP-consolidated (Hot Isostatically Pressed) Stellite 6 (Co − 27.1 Cr − 1.5 Si − 5.0 W − 0.96 C, in wt %) was investigated. A pin-on-disc apparatus was enclosed in an autoclave and wear was measured in water from room temperature up to 250 °C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The bulk HIPed alloy is predominantly two phase and comprises a cobalt-rich fcc matrix and an M7C3 carbide phase. However, surface grinding prior to wear testing causes a surface layer of the matrix to partially transform to hcp Co-rich phase. The wear (mass loss) is very low below 150 °C but increases by approximately an order of magnitude when the temperature is increased from 150 to 250 °C. SEM/EBSD reveals sub-surface damage and partial fcc to hcp transformation of the Co-rich matrix phase to a depth of ~ 15 μm in the disc. However, there is little change in transformation behavior and depth with temperature and this is not regarded as significant cause of the increased wear. The order of magnitude increase in wear is instead ascribed to a tribocorrosion mechanism associated with significantly higher corrosion rates at 250 °C than at 150 °C. As the material removal and factors affecting it are found to be so dependent on temperature, this work demonstrates the necessity of conducting assessments of materials for use in PWR environments under representative conditions