Research progress in TiB<Subscript>2</Subscript> wettable cathode for aluminum reduction

Titanium diboride wettable cathodes are regarded as ideal for aluminum reduction because of their excellent wettability with molten aluminum. The TiB₂ inert wettable cathode materials for aluminum reduction may be divided into three groups: pure TiB₂ ceramic cathode, TiB₂ composite cathode, and TiB₂ coating. This paper briefly describes international research progress on TiB₂ inert wettable cathodes as well as problems faced, and concentrates on the activities of Central South University, Changsha, China, in researching the ambient-temperature solidified TiB₂ cathode coating. At the same time, the results of the coating applied in many aluminum smelters in China are presented, and the pattern of cathode surface of 160 kA cells coated with the ambient-temperature-solidified TiB₂ cathode coating after one year operation is discussed in comparison with the normal cells.     

Effect of the atomic packing density on the structural change rate of amorphous alloys under elastostatic stress

This study deals with the fundamental issue of whether the atomistic-scale structure of amorphous alloys can be altered by the application of elastic stress. The prolonged imposition of elastostatic stress on amorphous alloys induces permanent structural change. The rate of change largely depends on the atomic packing density, which is characterized by the fractions of various short-range ordered clusters. This structural change is accompanied by the formation of excess free volume, which in turn affects the mechanical properties in subsequent compression tests. The findings of the experiments and molecular dynamics simulations were analyzed in terms of atomistic-scale structural changes.     

Sintered Cu Alloyed Stainless Steels and Their Corrosion Behavior

Copper is an effective element to activate the sintering process of stainless steels and to enhance corrosion resistance of the sintered specimens. Ways of introducing Cu into stainless steel powders lead to different consequence in the microstructure and corrosion behavior of sintered Cu alloyed stainless steel. In the present work, two methods, mixing Cu with stainless steel powder and coating stainless steel powder with Cu by electroless plating, were introduced in order to investigate their influence on the sintered specimens. It was found that the sintered specimens from Cu-coated stainless steel powders (1-5 wt.%Cu) produce less porous surfaces with isolated pores than the specimens from mixed powders and the former have obviously high density and relatively even Cu distribution. Potentiodynamic polarization measurements indicate that Cu-electroless plating of 1-5 wt.%Cu improves the corrosion resistance of sintered stainless steel due to the lowering of passivation current density.     

Tensile Test Specimens with a Circumferential Precrack for Evaluation of Interfacial Toughness of Thermal-Sprayed Coatings

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K _Int at the tip of the precrack, which can be expressed as , where σ₀ is the apparent average stress, a the crack length, R the specimen radius, and F _I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F _I for various values of a/R. In the experiments, the flat surface of a pin was grit-blasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m^1/2. As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.     

Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD: A Review

The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.     

The design and production of Ti-6Al-4V ELI customized dental implants

This paper addresses the production of customized Ti-6Al-4V ELI dental implants via electron beam melting (EBM). The melting of Ti-6Al-4V ELI powder produces implants with great biocompatibility, fi ne mechanical performance, and a high bone ingrowth potential. The EBM technology is used to produce one-component dental implants that mimic the exact shape of the patient’s tooth, replacing the traditional, three-component, “screw-like” standardized dental implants currently used. The new generation of implants provides the possibility of simplifying pre-insertion procedures leading to faster healing time, and the potential of better and stronger osseointegration, specifi cally through incorporating lattice structure design.     

Effects of defects generated in ALD TiO<Subscript>2</Subscript> films on electrical properties and interfacial reaction in TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>/Si system upon annealing in vacuum

Thin TiO₂ layers grown at 130°C on SiO₂-coated Si substrates by atomic layer deposition (ALD) using TTIP and H₂O as precursors were annealed, and the effects of the annealing temperature on the resulting electrical properties of TiO₂ and the interface properties between a Pt electrode and TiO₂ were examined using transmission line model (TLM) structures. The as-deposited TiO₂ thin film had an amorphous structure with OH groups and a high resistivity of 6×10³Ω-cm. Vacuum annealing at 700 °C transformed the amorphous film into an anatase structure and reduced its resistivity to 0.04Ω-cm. In addition, the vacuum-annealing of the TiO₂/SiO₂ structure at 700°C produced free silicon at the TiO₂-SiO₂ interface as a result of the reaction between the Ti interstitials and SiO₂. The SiO₂ formed on the TiO₂ surface caused a Schottky contact, which was characterized by the TLM method. The use of the TLM method enabled the accurate measurement of the resistivity of the vacuum-annealed TiO₂ films and the characterization of the Schottky contacts of the metal electrode to the TiO₂.     

Morphology and chemistry of oxide inclusions after Al and Ti complex deoxidation

The morphology and chemistry of oxide inclusions after various Al and Ti complex deoxidation sequences were investigated at 1600°C. Depending on the addition sequence of Al and Ti deoxidants and the holding time, the morphology and the chemistry were changed. In particular, when Ti was added prior to Al, spherical Al₂O₃ oxide inclusions with hollow holes were frequently observed, which is reported for the first time. On the other hand, when Al was added prior to Ti, Al₂O₃ inclusion formed. When Al and Ti were added simultaneously, Al−Ti−O oxide inclusion formed in the beginning and was transformed to Al₂O₃ inclusion. Based on the experimental results, the formation mechanism of the oxide inclusions after Al−Ti deoxidation was proposed. The nozzle clogging of Ti bearing Al killed steel was also discussed based on the experimental results.     

Indirect evaluation of the long-term oxidation properties of Al−21Ti−23Cr and Al−37Ti−12Cr coating materials for TiAl alloy

The most pertinent coating materials in the Al−Ti−Cr alloy system to improve the high temperature oxidation resistance of a TiAl alloy, with respect to oxidation properties, resistance to thermal stress, and chemical compatibility, are the two-phase alloys of Al−21Ti−23Cr (L1₂+Cr₂Al) and Al−37Ti−12Cr (γ+TiAlCr). In this study, cyclic oxidation tests at 1000 °C and 1200 °C were performed for the specimens coated with both materials of 10 im thickness. Furthermore, breakaway oxidation caused by the formation of a rutile TiO₂ scale was observed, though both bulk alloys showed very stable oxidation behavior. This phenomenon was resulted from the depleted Al content in the coating layer due to Al₂O₃ oxide growth and interdiffusion with the substrate. Considering the decrease of Al content due to oxide growth, the Al−21Ti−23Cr coating with the initial higher Al content was more effective for long-term oxidation protection of the TiAl alloy. On the other hand, when the Al content changes due to the interdiffusion with the substrate, the Al−37Ti−12Cr coating with a smaller compositional gradient with the TiAl substrate was more effective than the Al−21Ti−23Cr coating. Cyclic oxidation tests at 1000 °C and 1200 °C confirmed that for the longer lifetime of coating materials the initial Al content was more important than the smaller compositional gradient with the substrate. Consequently, the Al−21Ti−23Cr coating was considered as more effective one for the long-term oxidation resistance of TiAl alloys.