Nickel aluminum shape memory alloys via molecular dynamics

Shape memory materials are an important class of active materials with a wide range of applications in the aerospace, biomedical, and automobile industries. These materials exhibit the two unique properties of shape memory and superelasticity. Shape memory is the ability to recover its original shape by applying heat after undergoing large deformations. Superelasticity is the ability to undergo large, reversible deformations (up to 10%) that revert back when the load is removed. These special properties originate from a reversible, diffusionless solid-solid phase transformation that occurs between a high temperature austenite phase and a low temperature martensite phase. The development of the martensite microstructure is not well understood; this is especially true in regards to the role of size and mechanical constraints that dominate the properties in nanoscale samples. The goals of this research are to use molecular dynamics (MD) to (1) study the effects of simulation size on the martensite transformation to determine the ultimate limit of miniaturization, (2) to investigate the effects of mechanical constraints on the martensite transformation and resulting microstructure, and (3) to explore the effects of grain size in polycrystalline shape memory alloys. MD is well suited to study the transformation, as it shares a similar time scale with the extremely fast, diffusionless transformation.^ An extensive set of cooling and heating simulations were performed on Ni63Al37 disordered shape memory alloys (SMAs) to determine the effect of system size on the transformation. Simulation cell sizes in the range of 4.2 to 20 nm were studied. We discovered that decreasing system size only resulted in a slight increase of both transformation temperatures. However, the variability of the austenite transformation temperature increased considerably with decreasing simulation cell size, reaching 10% of the mean value for a system size of 10 nm. This variability can impose a fundamental limit on the miniaturization of this class of materials, as the reliability of device performance comes into question. Also, mechanical constraints were applied to force the cell angles to remain 90° in order to emulate the environment of a partially transformed polycrystal where grains are constricted by their neighbors. The mechanical constraints caused the austenite transformation temperature to decrease with decreasing size by up to 50%, and resulted in a two-domain microstructure for system sizes above 4.2 nm in order to accommodate the internal stresses. Finally, large scale MD simulations were done on polycrystalline samples with grain sizes ranging from 2.5 to 20 nm. We found that a critical grain size of 7.5 nm resulted in a minimum in the percent transformation to martensite. Below this critical size, martensite forms at the grain boundaries and the grains are able to rotate via grain boundary sliding to relieve internal stresses. In larger grains, martensite can nucleate and grow within the grains more easily. A uniaxial strain of up to 10% was applied to investigate the stress induced martensite transformation. Larger grains showed considerable work hardening when strained beyond about 2%. Plastic recovery was also calculated by unloading and relaxing at 4 and 10% strain. Samples strained to 10% were generally able to recover about 20-30% of the plastic strain, while samples strained to 4% showed varying amounts of recovery that peaked at 66% for a grain size of 7.5 nm

Explaining anomalies in SAR and scatterometer soil moisture retrievals from dry soils with sub-surface scattering

This paper presents the results of a laboratory investigation to explain anomalously-high soil moisture estimates observed in retrievals from SAR and scatterometer backscatter, affecting extensive areas of the world associated with arid climates. High resolution C-band tomographic profiling was applied in experiments to understand the mechanisms underlying these anomalous retrievals. The imagery captured unique high-resolution profiles of the variations in the vertical backscattering patterns though a sandy soil with moisture change. The relative strengths of the surface and sub-surface returns were dependent upon both soil moisture and soil structure, incidence-angle, and polarization. Co-polarised returns could be dominated by both surface and sub-surface returns at times, whereas cross-polarised returns were strongly associated with sub-surface features. The work confirms suspicions that anomalous moisture estimates can arise from the presence of sub-surface features. Diversity in polarization and incidence angle may provide sufficient diagnostics to flag and correct these erroneous estimates, allowing their incorporation into global soil moisture productsMorriso

The dependence of the hydrogen sorption capacity of single-walled carbon nanotubes on the concentration of catalyst

The adsorption of hydrogen on single-walled carbon nanotubes was measured using micro-gravimetric nitrogen and hydrogen adsorption isotherms at 77 K for gas pressures of up to 1 bar (nitrogen) and 12 bar (hydrogen). Results show that surface area and hydrogen uptake depend on the concentration of the iron catalyst used for making the nanotubes. Langmuir fits to the hydrogen uptake curves clearly show two adsorption energies for each sample which we attribute to the groove site for the higher adsorption energy and to the convex tube surface for the lower energy. We also present calculations of the binding energy of hydrogen on these same sites on SWCNTs and confirm that the groove site has a significantly higher (radius-dependent) binding energy than the surface site, consistent with the experimental values. This suggests that the use of the Langmuir model is appropriate to the adsorption of H2 on activated carbons for the temperature and pressure range investigated and could be used as a rapid way of estimating the average tube radius in the sample

Low cost carriers in the Middle East and North Africa: Prospects and strategies

We examine socio-economic indicators relevant to ‘low cost carriers’ (LCCs) in the Middle East and North Africa (MENA) and review the evolution of air transport liberalization and air service agreements in the region. We analyse the business strategies of MENA-based LCCs and using a benchmarking methodology, we compare the business strategy of Air Arabia Group with those of dominant European LCCs EasyJet and Ryanair. Our economic development indicators suggest future potential for LCC growth in Iran and Saudi Arabia while other MENA countries continue to face challenges. The lack of success in regional liberalization in air transport is restricting LCC growth although individual MENA countries have or will benefit from ‘open skies’ agreements. MENA-based LCCs while retaining some characteristics of the LCC model also deviate in significant ways. Benchmarking analysis shows that Air Arabia's business strategy represents a departure from the business strategies that have been most successful in Europe

A spectroscopic and photometric investigation of some extremely hydrogen-deficient stars

A photometric study of the extreme helium stars has detected three new variables (BD+1°4381, BD-1°3438, LSIV-1°2) and confirmed the suspected variability of two others (LSII+33°5, BD-9°4395). The timescale of the variations in BD+1°4381 (~"21 days), BD-1°3438 (5-8 days), LSIV-1°2 (~11 days) and LSII+33°5 (3-4 days), and the presence of colour changes concomitant with the luminosity variations, indicates that they are radial pulsators. In contrast, BD-9°4395 is believed to be pulsating non-radially. Photometric investigations of the extremely hydrogen-deficient binaries Upsilon Sagittarii, CPD-58°2721 and KS Persei are presented. KS Persei is suspected of having a ~5-day periodicity in addition to the ~30 day previously reported. A frequency analysis of the complex light curve of CPD-58°2721 shows it may be reconstructed from two sine-waves with periods of 9.3 and 14.1 days. For Upsilon Sagittarii, the results indicate that the variability previously ascribed to eclipses can instead be understood in terms of radial pulsation of the visible component, with a period of ~20 days. Additional rapid, ~0.02 mag luminosity variations with a period of 239 seconds are reported. Their origin is uncertain, and they are the shortest form of variability reported for this type of object. Theoretical modelling of the light curve indicates an early B-type main-sequence secondary with a mass of ~11M[sub] ⊙ , much higher than previously thought. A possible future merging of the binary components would result in a Type I supernova. There is no evidence of eclipses in any of the stars. A fine abundance analysis of CPD-58°2721 yielded T[sub]eff=14,000K, log g=1.25, n[sub]H:n[sub]He=0.005, n[sub]N:n[sub]c=40 and suggests an overabundance of heavy metals when compared to related objects. Its spectrum shows marked changes in the strengths of low excitation metallic lines which are evidence of temperature changes during pulsation. Spectroscopy of the surrounding nebulosity shows it to have an emission spectrum typical of an HII region. Radial-velocity measurements indicate it is probably part of the surrounding Eta Carina nebula, whilst the star is more distant

Effects of grain size on the martensitic phase transformation of nanocrystalline Ni/Al shape memory alloys

Shape memory alloys (SMAs) owe their distinct properties to a diffusion less martensitic phase transformation from a high temperature, high symmetry phase (austenite) to a low temperature (martensite) phase upon cooling or strain. Their shape memory and pseudoelastic properties make SMAs useful as active components in microdevices, medical implants and for vibrational damping. Despite their widespread application, the miniaturization limit of SMAs is not known. In this study, we use large-scale molecular dynamics simulations (up to ~40 million atoms) to characterize the martensitic transformation in nanocrystalline Ni/Al disordered alloys. We quantify how mechanical constraints affect both the transformation temperature and the resulting martensitic domain structure. We find that decreasing the grain size makes the transformation more difficult, and this results in a reduction of the transformed volume fraction at a given temperature. Interestingly, we find a minimum in the transformed fraction as a function of decreasing grain size, with extremely fine-grained samples showing a greater tendency to transform

National Competency Framework for Safeguarding Adults - concise version

Sets out in concise form capabilities expected of various professionals