W. T. Wu

Theoretical study of the effects of alloying elements on the strength and modulus of β-type bio-titanium alloys

Titanium alloys are favorable implant materials for orthopedic applications, due to their desirable mechanical properties and biochemical compatibility (or bio-inertness). However, current bio-titanium alloys still possess too high an elastic modulus compared with that of the bone, which can lead to premature failure of the implant. Here, a theoretical methodology for the design and development of low modulus Ti alloys and/or structures is provided by means of electronic structural calculations using the discrete variational cluster method (DVM). The preliminary study concentrated on two β-Ti atomic clusters consisting of 15, and 27 atoms, respectively. The binding energies between titanium and various alloying atoms within the clusters were first calculated, from which strength and modulus were then estimated. The results of the calculation suggested that Nb, Mo, Zr and Ta were suitable alloying elements for β-type titanium alloys, capable of enhancing the strength and reducing the modulus of the materials.

Effect of Al2O3 and enamel coatings on 900°C oxidation and hot corrosion behaviors of gamma-TiAl

The effect of reactively sputtered Al2O3 and enamel coatings on oxidation and hot corrosion behaviors of TiAl was investigated at 900 degrees C. Isothermal oxidation testing indicated that both coatings were very effective in reducing the oxidation rate of gamma-TiAl. Cyclic oxidation testing indicated that reactive-sputtered Al2O3 coating had little effect in improving the cyclic oxidation resistance of TiAl due to the spallation of Al2O3 coating. However enamel coating exhibited an excellent positive effect on cyclic oxidation of TiAl. Enamel coating was very adherent to substrate and neither cracks nor spallation appeared in the coatings during cyclic oxidation. Furthermore, the hot corrosion behavior of TiAI with enamel coating was evaluated in (Na,K)(2)SO4 melts at 900 degrees C. The results showed that enamel coating was very stable in (Na,K)(2)SO4 melts and was very effective in protecting TiAI from hot corrosion attack

Effect of MCrAIY overlay coatings on oxidation resistance of TiAl intermetallics

Abstract The effect of Co-30Cr-6A1-0.5Y and Ni-30Cr-6A1-0.5Y overlay coatings on oxidation resistance of TiAl intermetallics were studied at 900–1000 °C in static air. The TiAl alloys showed poor oxidation resistance due to the formation of mixed Al 2 O 3 +TiO 2 scale which is easily spalled off. CoCrA1Y and NiCrA1Y coatings could remarkably improve the oxidation resistance of TiAl owing to the formation of protective Al 2 O 3 scale. Due to the inward diffusion of Co and Ni to the substrate, two brittle and hard diffusion layers formed between the CoCrAlY and NiCrAlY coatings and the substrate. TiO 2 appeared on both of CoCrAlY and NiCrAlY coatings, which may result in the acceleration of the oxidation rate at 1000 °C.

Accelerated corrosion of pure Fe, Ni, Cr and several Fe-based alloys induced by ZnCl2–KCl at 450 °C in oxidizing environment

Corrosion problems in waste incinerators are usually rather severe, also due to the presence of low-melting point heavy and alkali metal chlorides. In this paper, the corrosion behavior of several Fe-based alloys with different Cr content, as well as three pure metals Fe, Cr, and Ni, was studied at 450 degreesC beneath ZnCl2-KCl deposits in flowing pure oxygen. All the materials suffered from accelerated corrosion characterized by an important separation and spallation of the oxide scales. The Cr-containing alloys did not exhibit better corrosion resistance to this environment due to the poor adherence of the scales to the matrix, while higher Ni content could improve the corrosion resistance. Chlorine was always enriched on the substrate surface beneath the scale, and in some cases chlorine-rich corrosion products were also present within the alloy matrix. For the HP heat-resistant steel, Cr-rich Cr7C3 carbides were preferentially attacked in comparison to the matrix. The effect of Cr is discussed with respect to the nature of the scales

Development of oxidation resistant coatings for γ-TiAl based alloys

Abstract In this paper the suitability of various coating types for the oxidation protection of γ-TiAl based alloys is discussed. Conventional aluminide and MCrAlY coatings are not suitable as protective coatings for γ-TiAl due to the poor coating–substrate compatibility. A Ti–50Al–10Cr coating remarkably improves the oxidation resistance of γ-TiAl, and exhibits an excellent chemical and physical compatibility with the γ-TiAl substrate. Moreover, the potential suitability of Ti–Al–Cr–Ag alloys as coating system for γ-TiAl was investigated. Alloy Ti–48Al–7Cr–2Ag exhibits excellent oxidation resistance at 800 and 900xa0°C, and may become a new candidate coating material for γ-TiAl with improved coating toughness.

Effect of La2O3 particles on microstructure and cracking-resistance of NiO scale on electrodeposited nickel films

The effect of La2O3 particles on the microstructure and cracking-resistance of thermally grown NiO scale, formed at 900 and 1000 degreesC, on electrodeposited nickel films has been investigated through in situ acoustic emission (AE) testing in conjunction with SEM and TEM/EDX. NiO scale, formed on electrodeposited films of pure nickel, has a porous duplex layered structure, which exhibits a more brittle behavior than the scale on films with the addition of La2O3. particles. They played a profound role in the procrastination of the occurrence of scale cracking and reduction of the cracking events. Upon the comparison of the microstructure: of NiO scale on the nickel film with the NiO scale on the LaO-containing film, several points can be concluded. For example, the stresses in NiO scale on the film with La2O3 were partly relaxed by a nondestructive route, such as scale plastic deformation due to La2O1-induced formation of a single-layered and fine-grained NiO scale. Also, it is believed that the particles reduced the size of the defects at the scale-metal interface and probably enhanced the cohesion between NiO grains and the adhesion between the scale-metal interface by retardation of the inward diffusion of nickel vacancies

Effect of coatings on oxidation resistance and mechanical performance of Ti60 alloy

The effect of several sputtered coatings on oxidation resistance and mechanical performance of Ti60 alloy was investigated. Sputtered pure Al, Ti-36Al, Ni-16Cr-2.5Al(wt%) coatings and reactively-sputtered Al2O3 film were effective in inhibiting the oxidation and oxygen-embrittlement of Ti60 alloy at 600-700 degrees C, while Al2O3 and TiAl coatings exhibited a better effect than Al and NiCrAl coatings from the point of view of coating-substrate compatibility. The tensile test after 100h exposure at 600 degrees C in air also showed that Ti60 alloy with Al2O3 and TiAl coatings exhibited a higher ductility than that with Al and NiCrAl coatings, which exhibited a good correlation with coating-substrate compatibility

First-principles investigations of ordering in binary α-Ti solid solutions

The ordering tendency in binary f-Ti solid solution containing 3sp or 4sp simple-metal (SM) or 3d transition-metal (TM) solute is investigated systematically by the linear muffin-tin orbital (LMTO) method within the atomic sphere approximation (ASA). We demonstrated that the effective pairwise interaction (EPI) energy in a solid solution is equal to half the solute-solute interaction energies and can be evaluated by a supercell total energy approach. The calculations of EPI energy both with and without volume relaxation of the supercells and local density of states (LDOS) show that the EPI energies of Ti-SM and Ti-TM solutions are dominated by different factors. For Ti-SM solutions, the EPI energies are of large absolute values with a negative sign, indicating strong ordering tendency in these solutions. The volume relaxation does not alter the EPI energy substantially. The calculated LDOS shows that the ordering tendency in Ti-SM solutions may be related to the hybridization between the electrons of the SM atoms when they are close to each other. For most Ti-TM solutions, if calculated without relaxation, the absolute EPI energies are very small; however, if calculated with relaxation, they are of relatively large positive values, indicating a clustering tendency in these solutions. By combining the calculated EPI energy and Flinns model for short-range order (SRO) strengthening, the increase in critical shear stress sro due to SRO is estimated for Ti-SM alloys, and the results qualitatively agree with experiment.