A. M. P. Pinto

Retrogression and re-ageing of 7075 aluminium alloy: microstructural characterization

Abstract Industrial Summary: The 7075 aluminium alloy presents a low stress corrosion cracking strength when aged to achieve maximum mechanical strength, T6 temper; high stress corrosion cracking strength is attained with overageing, T7 temper; but with loss of mechanical strength. Retrogression and re-ageing treatments improves the stress corrosion behaviour of the alloy whilst maintaining the mechanical resistance of the T6 temper. The microstructures produced by the retrogression and re-ageing treatments were characterized in this study by transmission electron microscopy, electron diffraction and differential scanning calorimetry. The precipitation is extremely fine and distributed homogeneously inside the grains, being slightly denser and more stable than that resulting from the T6 temper; whilst the grain boundary precipitation is quite different from that resulting from T6 treatment, the particles being coarser, and much closer to the precipitation resulting from T7 temper. The retrogression temperature is the main property controlling factor; a higher retrogression temperature, increasing the dissolution degree, promotes the formation of more stable precipitates on re-ageing.

Multilayered interface in Ti/Macor® machinable glass-ceramic joints

Macor ® machinable glass-ceramic and commercially pure (cp.) titanium were joined by active metal brazing, using a 64Ag‐34.5Cu‐1.5Ti (wt.%) filler alloy. The influence of the brazing temperature and holding stage on the microstructure and hardness profile of the interface, as well as on the shear strength of the joint, were assessed. Brazing was performed in a high vacuum furnace at 850, 890 and 930°C for 10 and 30 min. The reaction between the braze alloy and both materials led to the formation of a multilayered interface. The interfacial microstructure was analysed in a scanning electronic microscope (SEM) and the composition of each reaction layer was investigated by energy dispersive X-ray scans (EDS). The interfacial hardness profile was determined by a series of microhardness tests on each reaction layer. The mechanical strength of the joint was assessed from shear tests conducted at room temperature. Brazing at 850°C with a 10 min holding stage produced stronger joints, with an average shear strength of more than 85% of the glass-ceramic bulk strength.

Joining Ti-47Al-2Cr-2Nb with a Ti/(Cu,Ni)/Ti clad-laminated braze alloy

The joining of Ti-47Al-2Cr-2Nb using Ti-15Cu-15Ni (wt%) as braze alloy was investigated. Experiments were conducted at 980 and 1000°C for 10 min. The microstructure and the chemical composition of the interfaces were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. For both processing conditions the reaction between the γ-TiAl alloy and the braze alloy produced layered interfaces, which are essentially composed of α2-Ti3Al and of Ti-Ni-Cu-Al and Ti-Ni-Cu intermetallic compounds. Microhardness tests showed that all reaction layers are harder than either the γ or the (α2 + γ) lamellar grains of the intermetallic alloy.

The effect of brazing temperature on the titanium/glass-ceramic bonding

Abstract The aim of this work is to study the effect of time and brazing temperature on the interfacial microstructure and mechanical properties of the joint obtained by active metal brazing between c.p. titanium and a fluorosilicate machinable ceramic–glass using a 64Ag–34.5Cu–1.5Ti (wt%) brazing alloy. The reaction between the brazing alloy and the two materials leads to the formation of several interfacial reaction layers with different compositions, morphologies and extensions. These layers are constituted by various reaction products that ensure chemical bonding between the two materials, their stability and capability to accommodate the discontinuity of properties across the interface determining the success of the joining. The interfacial microstructure was analysed by SEM and the composition of each reaction layer was investigated by EDS. Microhardness tests were performed across the interfacial zone and the global interfacial mechanical behaviour was evaluated by means of shear tests.

Tribocorrosion behavior of bio-functionalized highly porous titanium

Titanium and its alloys are widely used in orthopedic and dental implants, however, some major clinical concerns such as poor wear resistance, lack of bioactivity, and bone resorption due to stress shielding are yet to be overcome. In order to improve these drawbacks, highly porous Ti samples having functionalized surfaces were developed by powder metallurgy with space holder technique followed by anodic treatment. Tribocorrosion tests were performed in 9g/L NaCl solution using a unidirectional pin-on-disc tribometer under 3N normal load, 1Hz frequency and 4mm track diameter. Open circuit potential (OCP) was measured before, during and after sliding. Worn surfaces investigated by field emission gun scanning electron microscope (FEG-SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Results suggested bio-functionalized highly porous samples presented lower tendency to corrosion under sliding against zirconia pin, mainly due to the load carrying effect given by the hard protruded oxide surfaces formed by the anodic treatment.

Grain refinement of Al-Mg-Sc alloy by ultrasonic treatment

In foundry practice, ultrasonic treatment has been used as an efficient technique to achieve grain refinement in aluminium and magnesium alloys. This article shows the strong effect of pouring temperature and ultrasonic treatment at various temperatures on the grain refinement of Al-1 wt% Mg-0.3 wt% Sc alloy. Without ultrasonic treatment, a fine grain structure was obtained at the pouring temperature of 700 °C. The average grain size sharply decreases from 487 ± 20 to 103 ± 2 μm when the pouring temperature decreases from 800 to 700 °C. Ultrasonic vibration proved to be a potential grain refinement technique with a wide range of pouring temperature. A microstructure with very fine and homogeneous grains was obtained by applying ultrasonic treatment to the melt at the temperature range between 700 and 740 °C, before pouring. Cavitation-enhanced heterogeneous nucleation is the mechanism proposed to explain grain refinement by ultrasound in this alloy. Moreover, ultrasonic treatment of the melt was found to lead to cast samples with hardness values similar to those obtained in samples submitted to precipitation hardening, suggesting that ultrasonic treatment can avoid carrying out heat treatment of cast parts.

Tribocorrosion behaviour of hot pressed CoCrMo−Al2O3 composites for biomedical applications

Abstract Alumina/alumina wear couple can lower the wear rates and thus metallic ion releasing on load bearing metallic implant materials. However, the low fracture toughness of ceramics is still a major concern. Therefore, the present study aims to process and to triboelectrochemically characterise the 5 and 10 vol.-%Al2O3 reinforced CoCrMo matrix composites. Corrosion and tribocorrosion behaviour of the composites were investigated in 8 g L−1 NaCl solution at body temperature. Corroded and worn surfaces were investigated by a field emission gun scanning electron microscope equipped with energy dispersive X-ray spectroscopy. After tribocorrosion experiments, wear rates were calculated using a profilometer. Results suggest that Al2O3 particle addition decreased the tendency of CoCrMo alloy to corrosion under both static and tribocorrosion conditions. However, no significant influence on the corrosion and wear rates was observed in composites mainly due to increased porosity and insufficient matrix/reinforcement bonding.

Joining Ti-47Al-2Cr-2Nb with a Ti-Ni Braze Alloy

Ti-47Al-2Cr-2Nb was joined by diffusion brazing using a Ti-Ni clad-laminated filler alloy. Brazing was performed in the temperature range of 1000 to 1200oC for 10 min. The microstructure and the chemical composition of the interfaces were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. The reaction between the γ-TiAl alloy and the liquid produced interfaces essentially composed of α2-Ti 3Al and TiNiAl intermetallics. When joints are brazed at 1150 and 1200oC, the γ-TiAl phase is also detected at the interface.

Improved tribocorrosion performance of bio-functionalized TiO₂ nanotubes under two-cycle sliding actions in artificial saliva

After insertion into bone, dental implants may be subjected to tribocorrosive conditions resulting in the release of metallic ions and solid wear debris, which can induce to peri-implant inflammatory reactions accompanied by bone loss, and ultimately implant loosening. Despite the promising ability of TiO2 nanotubes (NTs) to improve osseointegration and avoid infection-related failures, the understanding of their degradation under the simultaneous action of wear and corrosion (tribocorrosion) is still very limited. This study aims, for the first time, to study the tribocorrosion behavior of bio-functionalized TiO2 NTs submitted to two-cycle sliding actions, and compare it with conventional TiO2 NTs. TiO2 NTs grown by anodization were doped with bioactive elements, namely calcium (Ca), phosphorous (P), and zinc (Zn), through reverse polarization anodization treatments. Characterization techniques such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and scanning transmission electron microscopy (STEM), were used to characterize the films. Tribocorrosion tests were carried out in artificial saliva (AS) by applying two cycles of reciprocating sliding actions. The open circuit potential (OCP) was monitored before, during, and after both cycles of sliding, during which the coefficient of friction (COF) was calculated. The resulting wear scars were analyzed by SEM and EDS, and wear volume measurements were performed by 2D profilometry. Finally, the mechanical features of TiO2 NTs were accessed by nanoindentation. The results show that bio-functionalized TiO2 NTs display an enhanced tribocorrosion performance, ascribed to the growth of a nano-thick oxide film at Ti/TiO2 NTs interface, which significantly increased their adhesion strength to the substrate and consequently their hardness. Furthermore, it was discovered that during tribo-electrochemical solicitations, the formation of a P-rich tribofilm takes place, which grants both electrochemical protection and resistance to mechanical wear. This study provides fundamental and new insights for the development of multifunctional TiO2 NTs with long-term biomechanical stability and improved clinical outcomes.

Diffusion Brazing of a γ-TiAl Alloy Using Tini 67 : Microstructural Evolution of the Interface

A detail study focussing the microstructural evolution of the interfacial zone in the course of the processing of Ti-47Al-2Cr-2Nb joints using Tini 67 as filler alloy was carried out in this investigation. Experiments, aiming the understanding of the mechanisms that promote the melting of the braze alloy, were performed below the solidus temperature of the filler, at 750 and 900°C. Diffusion brazed samples were joined at 1000 and 1100°C, with no dwelling stage and subsequently quenched in water in order to frozen the microstructure formed at the bonding temperature. The interfaces were analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. In the course of the heating stage, several single phase layers were formed within the filler alloy due to the solid state interdiffusion of Ti and Ni atoms. At 900°C, the microstructure of the filler evolved form the initial Ti (α)/(Ni)/Ti/ (α) layers to a Ti (β)/Ti2Ni/TiNi/TiNi3/TiNi/Ti2Ni/Ti (β) layered microstructure. The filler alloy begun to melt due to the eutectic reaction between the contiguous layers composed of Ti (β) and Ti2Ni. After joining, the main phases detected at the interfaces were α2-Ti3Al, Ti-Ni-Al and Ti-Ni intermetallics. For joining at 1000°C, a substantial amount of residual filler (Ti2Ni and Ti (α) particles) was also detected at the central zone of the interface. No marked evidences of residual filler zones were noticed for joining at 1100°C; instead, a mixture α2-Ti3Al with Ti-Ni-Al or Ti2Ni intermetallics was detected at the centre of the interface.