Duduzile Nkomo

By Duduzile Nkomo and Nomsombuluko Masia

Corrosion resistance is an important requirement in the study of biomedical implants. Implant surface can be modified to provide good adherence and/or optimum biocompatibility with the human body at the bone-implant interface. Titanium alloys are typically used because of their excellent corrosion resistance and biocompatibility. However, to improve these properties, the alloy surface is roughened using alumina (Al2O3). More details on the corrosion resistance of these alloys can be obtained by using electrochemical impedance spectroscopy (EIS) method. EIS is the most suitable method for monitoring corrosion rate values due to its reproducibility, it is non-destructive and has reliable determination of small corrosion rates, much lower than those measured by other techniques. It can also study high-impedance systems, such as coatings and linings, high-purity water, and organic coating/metal systems or corrosion in a low-conductive solution. This method has been used to evaluate electrochemical properties of modified surfaces. This chapter will explore the effectiveness of EIS in studying the corrosion behaviour of machined and surface-modified Pure Ti grade 4 for dental implant applications. The basic EIS concepts are discussed and their derivation thereof to provide information about the corrosion resistance of biomedical implants is explored.

Part of the book: Corrosion

By Duduzile Nkomo and Nomsombuluko Masia

Austenitic stainless steels require approximately 8% Ni to maintain austenitic microstructure at room temperature for alloys such as 304 stainless steel (304SS). Ni contributes approximately 60% of the total material cost and its price fluctuates, making the cost of austenitic stainless steel unpredictable. The use of low-nickel austenitic stainless steels as a substitute has been considered in order to remedy costs associated with Ni price fluctuations. Alloying elements such as Mn and N have been considered, however they have been found to reduce corrosion resistance. A new alloy namely Hercules™ has been developed with reduced Ni content (1.8–2% Ni). This chapter presents a comparative study of the corrosion behavior of Hercules™ and 304SS in different solutions. The alloys were evaluated using cyclic polarisation technique and immersion tests. The results demonstrated that the corrosion resistance of Hercules™ is comparable to that of 304SS. This presents the alloys as potential industrial substitutes of each other.

Part of the book: Stainless Steels

By Duduzile Nkomo, Maje Phasha and Hein Moller

Ruthenium (Ru) is one of the platinum group metals (PGMs). These metals belong to the transition metals group of the periodic table. They have excellent properties such as high melting point and are inert with variety of substances, thus also called noble metals. Currently, Ru is the cheapest of the PGMs, thus it is readily available compared to other PGMs. Recently, incorporating PGMs in shape memory alloys (SMAs) has been extensively explored, with titanium-nickel (TiNi) used as a bench-mark material. TiRu is amongst the compounds that are currently explored for various potential applications. This compound has an ordered B2 (CsCl-type) crystal structure. It is hard and brittle, thus some shape memory (SM) properties are difficult to induce in this compound. However, due to Ru possessing some good biomedical properties such as biocompatibility, corrosion resistance, improved radiopacity and ultra-low magnetic susceptibility for MRI diagnostics, the mechanical properties of TiRu must be improved for biomedical applications. Since niobium (Nb) is known to be biocompatible and is usually studied in biomedical alloys, a systematic substitution of Ti with niobium (Nb) was performed in an effort to reduce the stiffness (Young’s modulus). This chapter gives an insight on the structural and mechanical properties of biocompatible Ru-rich alloy compositions.

Part of the book: Ruthenium

By Ramogohlo Diale, Duduzile Nkomo, Bongani Ngobe and Maje Phasha

In this chapter, the density functional theory (DFT) based first-principles approach is used to predict the underlying lattice properties associated with the phase transformation and stability of B2 phase in titanium-platinum group metal (Ti-PGM) compounds. This ab- initio technique provides a good platform to accurately explore phase stability variation between the successful Ti-PGM shape memory alloys (SMAs) (Ti50M50, M = Rh, Pd, Ir, Pt) and other B2 Ti-PGM compounds that do not show any shape memory effect (SME), such as Ti50Os50 and Ti50Ru50. The B2 TiFe, TiNi and TiAu have also been considered in this chapter in order to draw similarities and differences. Amongst the predicted results, the heat of formation was calculated to determine the thermodynamic stability, whereas the total densities of states were used to evaluate the electronic stability of these compounds. Insights on the mechanical stability of the B2 crystals were derived from the calculated elastic constants. Mechanical instability was revealed in some compounds, indicative of a possible phase transition responsible for the intrinsic shape memory character. Although an attempt to correlate this mechanical instability with imaginary frequencies established from the phonon dispersion curves is made, the correlation is not yet conclusive due to some discrepancies observed in TiNi.

Part of the book: Density Functional Theory