Ishraq Shabib

Surface characterization and cytotoxicity analysis of plasma sprayed coatings on titanium alloys.

In the realm of biomaterials, metallic materials are widely used for load bearing joints due to their superior mechanical properties. Despite the necessity for long term metallic implants, there are limitations to their prolonged use. Naturally, oxides of titanium have low solubilities and form passive oxide film spontaneously. However, some inclusion and discontinuity spots in oxide film make implant to adopt the decisive nature. These defects heighten the dissolution of metal ions from the implant surface, which results in diminishing bio-integration of titanium implant. To increase the long-term metallic implant stability, surface modifications of titanium alloys are being carried out. In the present study, biomimetic coatings of plasma sprayed hydroxyapatite and titanium were applied to the surface of commercially pure titanium and Ti6Al4V. Surface morphology and surface chemistry were studied using scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. Cyclic potentiodynamic polarization and electrochemical impedance spectroscopy were carried out in order to study their electrochemical behavior. Moreover, cytotoxicity analysis was conducted for osteoblast cells by performing MTS assay. It is concluded that both hydroxyapatite and titanium coatings enhance corrosion resistance and improve cytocompatibility.

Evolution of displacement cascades in Fe–Cr structures with different [001] tilt grain boundaries

ABSTRACT Reduced-activation ferritic/martensitic steels of Cr concentration between 2.25 and 12 wt% are candidate structural materials for next-generation nuclear reactors. In this study, molecular dynamics (MD) simulation is used to generate the displacement cascades in Fe–Cr structures with different Cr concentrations by using different primary knock-on atom (PKA) energies between 2 and 10 keV. A concentration-dependent model potential has been used to describe the interactions between Fe and Cr. Single crystals (SCs) of three different coordinate bases (e.g. [310], [510], and [530]) and bi-crystal (BC) structures with three different [001] tilt grain boundaries (GBs) (e.g. Σ5, Σ13, and Σ17) have been simulated. The Wigner–Seitz cell criterion has been used to identify the produced Frenkel pairs. The results show a marked difference between collisions observed in SCs and those in BC structures. The numbers of vacancies and interstitials are found to be significantly higher in BC structures than those found in SCs. The number of point defects exhibits a power relationship with the PKA energies; however, the Cr concentration does not seem to have any influence on the number of survived point defects. In BC models, a large fraction of the total survived point defects (between 59% and 93%) tends accumulate at the GBs, which seem to trap the generated point defects. The BC structure with Σ17 GB is found to trap more defects than Σ5 and Σ13 GBs. The defect trapping is found to be dictated by the crystallographic parameters of the GBs. For all studied GBs, self-interstitial atoms (SIAs) are easily trapped within the GB region than vacancies. An analysis of defect composition reveals an enrichment of Cr in SIAs, and in BC cases, more than half of the Cr-SIAs are found to be located within the GB region.

Effect of dissolution of magnesium alloy AZ31 on the rheological properties of Phosphate Buffer Saline

The issue of long-term incompatible interactions associated with the permanent implants can be eliminated by using various biodegradable metal implants. The recent research is focusing on the use of degradable stents to restore most of the hindrances of capillaries, and coronary arteries by supplying instant blood flow with constant mechanical and structural support. However, internal endothelialization and infection due to the corrosion of implanted stents are not easy to diagnose in the long run. In the recent past, magnesium (Mg) has been widely investigated for the cardiovascular stent applications. Here we made an attempt to understand the biodegradation process of Mg alloy stent by studying the degradation of Mg alloy AZ31 (3 wt% Aluminum, 1 wt% Zn) powder at various time-intervals in simulated blood fluid using the Rheological methods. The degradability of the Mg stent in the arteries affects the stress-strain properties of blood plasma and the subsequent flow conditions. Blood and plasma viscosities alter due to the degradation of Mg resulting from the stress-strain experienced in the blood vessels, in which the stent is inserted. Here our objective was to explore the influence of Mg degradation on the blood plasma viscosity by studying the viscoelastic properties. In this work, the effect of dissolution of Mg alloy AZ31 on the rheological properties of Phosphate Buffer Saline (PBS) at various time intervals have been investigated. The viscosity of the PBS-AZ31 solution increased with the dissolution of both slurries and percolated clear solution. The only exception was day-7 of the percolated clear solution, where viscosity was decreased showing a reduction in viscosity at initial stages of dissolution. The frequency sweep showed the tendency of the PBS-AZ31 gelation up to 100 rad/s frequency.

Nanoindentation response of Fe-10%Cr bi-crystal structures with Σ5〈001〉 and Σ3〈110〉 tilt boundaries: An atomistic study

In this research, nanoindentation responses of Fe-10%Cr bi-crystal structures containing Σ5{310}〈001〉 and Σ3{111}〈110〉 tilt grain boundaries (GBs) have been investigated using atomistic simulation technique. Deformation analyses identify the nucleation of 1/2〈111〉, 〈001〉 and 1/6〈111〉 types of dislocations within the material. The {110} slip planes are found to be more active than the {123} and {100} slip planes. Load-displacement response and corresponding changes in contact area have been recorded and used to measure material hardness and reduced modulus. The lengths of the nucleated dislocations are measured and used to estimate dislocation density within the plastic zone beneath the indenter. Dislocation motion has been found to be much easier in model with Σ3 boundary and the early interaction of the dislocation with the boundary affects the shape of the load-displacement curve, contact area on the indented surface, and the volume of the plastic zone. The hardness of the material has been found to be affected primarily by the interaction of the dislocation with the boundary, rather than by the dislocation density within the plastic zone. Both the boundaries exhibit maximum resistance to slip transmission even at the maximum indentation depth.

Irradiation Induced Damage of Fe-10%Cr Under Uniaxial Pressure

Structural materials of next generation nuclear reactors are expected to experience severe operating conditions including intense heat, high irradiation doses, thermal and mechanical stresses, and corrosive environments, which would potentially degrade material properties and impose severe threat to structural integrity. For example, high irradiation doses cause the evolution of displacement cascades, consisting of point defects, which lead to void swelling, irradiation creep, irradiation assisted stress corrosion cracking, and embrittlement. Over the last several decades, extensive computational researches have been conducted to study displacement cascades and generate defect statistics over a wide range of irradiation doses and temperatures for pure materials, primarily Fe. However, very limited data can be found to determine cascade evolution and defect statistics of Fe-alloys under pressure. In this work, large-scale molecular dynamics simulations have been performed to study displacement cascade and generate defect statistics of Fe-10%Cr alloy under uniaxial pressure. The selection of the material is based on the fact that Fe and Cr are the two major alloying elements of Ferritic-martensitic steels, which have shown promise to be a candidate material for future generation reactors due to high temperature stability and reduced swelling under irradiation. The simulated material is built from a single crystal Fe model of [130], [310], and [001] orientation, and randomly substituting Fe atoms by Cr. Empirical EAM potential has been used to define interatomic interactions. Irradiation simulations are performed for doses between 2–15keV, and pressure ranges between −10,000 bars to +10,000 bars applied along the x-direction. Simulation temperature is kept at a minimum, e.g. 15K, to minimize thermal influences. Displacement cascades are generated by imparting kinetic energy to a lattice atom (i.e. primary-knock-on-atom, PKA) along an arbitrary crystallographic direction (i.e. the diagonal direction of the simulation cell). Point defects are identified using the Wigner-Seitz method. Upon collision, the PKA atom displaces the surrounding atoms from their perfect lattice cites and causes a rapid increase in defect numbers. As the imposed energy is dissipated through the crystal, the displaced atoms recombine with the vacancies and the defect numbers gradually decrease and become stable. The cascade structure shows the presence of the vacancies at the core of the cascades surrounded by the interstitials. The number of defects increases almost linearly with increasing the irradiation dose for any pressure. The effect of pressure is found to be more profound within the intermediate pressure range, e.g. between −100 to +1000 bar, within which the number of point defects continually decreases as the pressure changes from tension to compression. The trend is found to be consistent for the whole PKA energy range. Point defects are also found to form defect clusters. The common neighbor analyses haves been performed to determine the structure of the clustered defects. It has been revealed that the defect clusters are of cubic diamond type. Additional analyses are currently under progress to evaluate the influence of pressure on cascade volume, point defect composition, and cluster composition.Copyright

Lattice Thermal Conductivity of Fe-Cr Alloys With 〈001〉 Tilt Boundaries: An Atomistic Study

The objective of this study is to examine lattice thermal conductivity (κ) of Fe-Cr alloys containing different 〈001〉 tilt grain boundaries (GBs). The effects of Cr concentration (2 and 10%) and three different 〈001〉 tilt boundaries (Σ5{310}, Σ13{510}, and Σ17{530}) have been examined at 70K using the reverse non-equilibrium molecular dynamics (rNEMD) simulation technique. The results exhibit higher κ for Fe or Fe-Cr models with Σ5[310] GB. The values are 2–4% and 12–16% more than those of models with Σ13[510] and Σ17[530] GBs, respectively. Pure Fe single crystal models exhibit higher conductivities than Fe/Fe-Cr models with various Σ tilt boundaries. κ decreases 7–9% as GBs are introduced into the pure Fe single crystal models. On the other hand, the conductivities of Fe-Cr models are affected more by the Cr concentration than the presence of a particular GB. As 10% Cr is added into the system the conductivity decreases by 7.6–9.4% compared to the pure Fe models.Copyright