Search

Abstract: Adsorption of halogen on the metal surface has received much attention due to its technological applications and major relevance for material surface processing, corrosion protection and etching. In this work, first-principle approach was used to investigate the interaction of halogen molecules on Ti (110) surface. The present results revealed that adsorption of the halogen molecule is exothermic and occurs by dissociation bonding. The HF molecule was found to be more thermodynamically stable than the HI molecule. In addition, our results revealed that the adsorption of halogen ions on Ti (110) surface is energetically favourable than the adsorption of halogen molecule. The possible adsorption sites were tested, and the top site position was found to be the most favourable followed by the hollow and bridging site for both halogens. Furthermore, the results showed the linear relationship between adsorption energy strength and charge transfer. Also, the density of states and charge density difference was studied to investigate the electronic interaction. The charge redistribution showed an electron depletion on Ti atom and charge accumulation on the halogen region. Graphic Abstract:

Titanium and its alloy components are one of the most important technological materials, which has found extensive application in various industries. However, surface defects play a key role in the mechanical properties of these components. Currently, wet chemical etching is one of the most important procedure for surface processing due to the presence of HF since it can etch metal oxide. Therefore, there still a need to investigate the etching mechanism. In this work, adsorption of HF on TiO2(110) surface has been studied using density functional theory to investigate the fundamental process of etching. HF molecule is adsorbed on the TiO2 surface by dissociation to form Ti-F and O-H species. The interaction between HF and TiO2surface become more favorable at high HF coverage. The pre-adsorbed of water molecule is favorable for HF adsorption process, which is in good agreement with experimental results. Fluorination processes show the formation and desorption of water intermediate at 0.50 ML coverage. We also investigated the relation between the work function and Mulliken charge for HF adsorption. The results indicate that the interaction of F on the surface attracts electrons due to its higher electronegativity than oxygen. Our results suggest that adsorption of HF is considered chemisorption process.

Ti50Pd50 alloy exhibits thermoelastic martensitic phase transformation above 823 K and has the potential for high temperature shape memory material applications. Previous studies showed that this alloy is mechanically unstable with a negative C' at room temperature. A systematic investigation of the structural, thermodynamic, electronic and elastic properties of TiPdOs is conducted using a first-principles calculation based on density functional theory. The calculated heats of formation show that this compound is thermodynamically stable as Os content is increased. It is found that an increase in Os content stabilizes the TiPd with a positive Cx2032; observed above 18.25 at. %. An increase in Os content results in an increase of the Bulk modulus and Young's modulus above 18.25 at. %. Anisotropy factor and Vickers hardness are studied and hardness is found to increase with an increase in Os content. The results suggest that the stability of the B2 phase can be significantly enhanced by the addition of Os in TiPd alloy. These findings have important applications for future materials design in aerospace industries.

Shape memory alloys (SMAs) have been widely used in the fields of actuators and aerospace industry due to their pseudo-elasticity and shape memory effect which are displayed in phase transformations. The martensitic transformations (MT) of TiPt is much higher, at approximately 1273 K and this is considered to be of potential technological interest for elevated temperature SMA applications. TiPt based alloys exhibit very low shape memory effect due to low critical stress for slip deformation compared to the stress required for martensitic transformation, hence it is necessary to enhance the mechanical properties of the equiatomic alloy. The first principles approach was employed to study the effect of the third element (M = Ru, Cu, Zr, and Hf) on the TiPt shape memory alloy. The supercell approach in VASP was used to substitute Pt with Ru and Cu, Ti with Zr and Hf on the TiPt structure to evaluate their mechanical stability from elastic properties for actuators and higher temperature applications. The Ti50Pt50-xRux and Ti50Pt50-xCux decreases in density with increase in Ru and Cu concentration, whilst the Ti50-xZrxPt50 and Ti50-xHfxPt50 substitution increases with an increase in their concentration, which result in larger lattice parameters. The heats of formation suggest that Ti50Pt50-xRux substitution is more thermodynamically stable than Ti50Pt50-xCux substitution, and Ti50-xHfxPt50 substitution is more stable than Ti50-xZrxPt50. The elastic properties suggest that the ternary structures become mechanically stable with an increase of the third element. The Ti50Pt50-xRux and Ti50Pt50-xCux substitution became more ductile with the increase in concentration. Zr and Hf substitution became more ductile at higher compositions (31.75 - 43.75 at.%). The Ru and Hf substitutions have potential to be used for high-temperature applications.

Bimetallic FexPt1-x alloys with the L10 and L12 structures have recently gained a lot of consideration in practical applications for solid-state devices, storage of ultra-high density magnetic data and biomedicine. This is due to their high magnetic and magnetocrystalline anisotropy, density, and coercivity. In order to gain knowledge on the structural, electronic and mechanical properties of the cubic and tetragonal FexPt1-x alloys, we have calculated their equilibrium lattice constants, density of states, and elastic constants at 0 K, employing first-principles calculations. The calculated equilibrium lattice constants were found to be in good agreement with the experimental data to within 3 %. All independent elastic constants satisfy the necessary stability conditions for both cubic and tetragonal systems, suggesting mechanical stability. The shear anisotropic factors predict that the tetragonal FexPt1-x crystals are highly anisotropic along the {001} plane than {100}. Moreover, the percentage of bulk (AB) and shear (AG) anisotropies revealed completely isotropic systems in the bulk and slightly anisotropic in shear modulus.

Iron-aluminides based systems play a significant role in many industrial applications due to their excellent resistance to oxidation at high temperatures. In particular, these alloys are applicable as thermocouples. In this paper, we used first-principles density functional theory to investigate the stability of FeAl and ternary FeAl-X alloys. Their equilibrium lattice parameters are in better agreement to within 2% with the experimental data. We employed virtual crystal approximation and supercell approaches to model various atomic compositions at 0 [?] x [?]1 for Ag, Pt, Pd and Ru. Their thermodynamic, electronic and mechanical properties were deduced from their equilibrium lattice constants. It was found that the addition of Ag, Pt, Pd and Ru enhances the stability at lower atomic percentage composition. It was found that Ag and Pd stabilizes the FeAl-X to lower percentage compositions. Furthermore, their heats of formation, density of states and elastic constants gave essential agreement to describe the structural, thermodynamic and mechanical stability of these systems.

Shape memory alloys are smart materials that can remember their shape after being deformed. This is due to their unique properties, shape memory effect and super-elasticity, which occurs because of phase transformation. High temperature shape memory alloys are being developed in order to increase the application area of shape memory alloys. There is an increasing demand of shape memory alloys, which can be used in high temperatures environments such as in aerospace, actuators, sensors in the automotive industry. The first-principle approach was employed to investigate the effect of ternary alloying with Ru, Co, Cu, Zr and Hf on the equi-atomic B19 TiPt alloy. The supercell approach based on the B19 TiPt structure was used to construct various compositions containing a third element with reduced Pt content, and evaluate their thermodynamic and mechanical stability. The Ru, Co and Cu calculations were performed to compare their effects on the TiPt relative to Zr and Hf addition. It was found that partial substitution with Ru was the most stable structure as compared to the Hf, since it displayed the lowest heats of formation. Their martensitic transformation temperature increased due to their lowest C' shear modulus. However, the addition of Cu reduced the transformation temperature of the TiPt. All structures were found to be completely anisotropic.

The demand for shape memory alloys for high temperatures have increased in recent years and TiPt alloys are promising candidates. The martensitic transformation temperature of TiPt is higher with the value of approximately 1300 K as compared to the commercialised TiNi which transforms at 300 K. Titanium-based SMAs have been widely used in the fields of engineering and medicine due to their shape memory effect (SME) and super-plasticity which are displayed in martensitic transformations. Some of the applications include actuators and medical stents. Previous studies showed that the alloy is mechanically unstable with the negative C' modulus at 0 K. In this work, supercell approach is used to substitute Pt with vanadium on the B2 TiPt structure and evaluate the mechanical stability of the structures. The equilibrium lattice parameters, elastic properties and the phonon dispersions were calculated to investigate the stability of the Ti50Pt50-XVX. Interestingly the vanadium addition was found to be stabilising the TiPt with all the Cij's being positive.

Alkali metals and alkali earth metals can be used as reducing agents of titanium halide in titanium production. Despite South Africa's position as being the major raw titanium material producer, titanium production is low and expensive as a direct consequence of the outmoded technology that is used in its extraction from raw materials such as the Kroll process. In this study, computational modelling techniques were employed to simulate the conditions for LiCl that will be suitable for generating a large quantity of metallic titanium in pure and powder form. We used a combination of density functional theory and molecular dynamics, employing FHI-aims, DL_POLY and GULP to characterize LiCl in a solid and molten form. The derived potentials reproduced the LiCl structure to within 1% in agreement with experimental data. More importantly, the melting temperature was deduced from the diffusion coefficient as 800 K which is closer to the experimental melting point of 878 K. Furthermore, the interaction of Ti-Li, Ti-Cl and Li-Cl-Ti were tested and gave reasonable results to set an environment for titanium clusters. The new pair potentials were deduced as Ti-Cl: De = 0.400 a0 = 1.279 r0 = 2.680 and Ti-Li: De = 0.730 a0 = 1.717 r0 = 2.000. The findings of this work will contribute towards the development of alternative ways of titanium production in a continuous and less expensive processes.

First-principles density functional theory approach was employed to investigate the martensitic temperature of the equiatomic TiPt, TiZr and TiTa shape memory alloys. Firstly, their structural, thermodynamic, elastic and phonon dispersion have been deduced to mimic the relative stabilities. These alloys have potential to operate as high-temperature shape memory alloys since they possess a reversible B2 to B19 martensitic transformation. It was found that the lattice parameters are in good agreement with the experimental results to within 5%. Furthermore, the TiPt structure is thermodynamically stable displaying lowest heats of formation, while TiZr and TiTa are not. Our results show that the B2 phases of these binary alloys are vibrational unstable due to the presence of soft modes along high symmetry direction on the phonon dispersion curves. TiTa alloy is mechanically stable (C' > 0), which may be used to develop future high-temperature alloys.