Bruno Xavier de Freitas

The influence of solution and aging heat-treatments on the corrosion behavior of a biodegradable Mg–Y–Gd–Zr alloy was investigated. Electrochemical behavior was assessed by potentiodynamic polarization curves and electrochemical impedance spectroscopy in an aerated 0.9 wt.% NaCl solution at 37 °C ± 0.1, complemented by SEM, EDS, and XRD analyses of corrosion-products. The as-cast condition exhibited the highest polarization resistance and impedance modulus, attributed to a thicker and more resistive corrosion-product layer, despite microstructural heterogeneity. Solution heat-treatment (525 °C/24 h) homogenized the microstructure and shifted the corrosion potential to nobler values, but produced a thinner, less protective film and reduced corrosion resistance. Among aged conditions, aging at 250 °C for 100 h yielded the best performance, with higher polarization resistance and impedance modulus values indicative of a more uniform and stable surface layer. Overall, the results demonstrate that corrosion behavior is primarily governed by the stability and resistive character of the corrosion-product layer, which is strongly controlled by the heat-treatment-induced microstructural state.

Mechanical alloying (MA) is widely applied in the synthesis of blended elemental or prealloyed powders. This work evaluates the effect of milling time on elemental nickel and titanium powders produced by oxi‐reduction and the hydrogenation–dehydrogenation process, respectively. The powders are characterized by scanning electron microscopy, X‐ray diffraction, particle size, powder yield, and differential scanning calorimetry. It is observed that increasing the milling time promotes the formation of a structure composed of thin lamellae of nickel and titanium, which results in the beneficial effect of lowering the temperature for the formation of the intermetallic of the Ni–Ti system and in the powder yield achieved. The reduction of milling time in the MA process of NiTi alloys enhances technological efficiency by decreasing their overall processing time.

Ceramic composites based on Ce-TZP/Al₂O₃/H6A were sintered in order to promote grain growth and to study the effects of ZrO₂ grain size on the properties of this material. A mixture of ZrO₂-CeO₂-Al₂O₃ powders was sintered at 1450 °C-2 h, following the manufacturer’s recommendations. Then, the samples were further treated at 1500 or 1600 °C, for 0, 2, 8, or 24 h. The sintered specimens were characterized by X-ray diffraction, relative density, and grain size distribution. Vickers nanohardness, Young’s modulus (E), and fracture toughness were measured. The materials showed complete densification for all sintering conditions studied. t-ZrO₂, α-Al₂O₃, and cerium hexaluminate (H6A) were observed. The alumina and hexaluminate grains had average grain sizes of 0.7 and 4.5 μm (AR>3), respectively, without significant variations during the additional heat treatments. However, significant growth took place for the ZrO₂ grains with increasing temperature and holding time, increasing the average grain size from 0.6~1.4 μm, when sintered at 1500 °C-2 h to 1600 °C-24 h, respectively. The materials exhibited Vickers nanohardness of 1800 HV and E=241±15 GPa. On the other hand, the considerable grain growth of the ZrO₂ grains as a function of holding time reflected in a reduction of the fracture toughness, which decreased from 8.5 to 5.7 MPa.m^1/2 for samples sintered at 1500 °C-2 h to 1600 °C-24 h, respectively.

In this work, mixtures of ceramic powders containing Al₂O₃ with different amounts of Y₂O₃ (1%, 3%, 5% and 10wt.%) along their cross-section were fabricated to obtain multilayer composites based on Al₂O₃ with different levels of Y₃Al₅O₁₂ (YAG) as reinforcement. Monolithic cylindrical multilayer Al₂O₃ blocks were compacted and subsequentially, sintered at 1610°C for 4h. Phase stability, microstructural aspects, and physical and mechanical properties of the specimens were acquired by relative density, X-ray diffraction, scanning electron microscopy, Vickers hardness, fracture toughness, Young’s modulus and biaxial flexural strength measurements. The results indicated that monolithic alumina specimens exhibited relative density of 98%, with average hardness of 1203 ±83 HV, fracture toughness of 2.1 ±0.8 MPa.m^1/2 and flexural strength of 187 ±64 MPa. Progressive incorporation of Y₂O₃ into the chemical composition of the specimens led to formation of the YAG phase by solid state reaction during sintering, which reduced hardness and increased densification, fracture toughness and flexural strength, with average values of HV=1023 ±28 HV, K_IC=3.5 ±0.3 MPa.m^1/2, σ_f=273 ±58 MPa and 14% Y₃Al₅O₁₂ for the specimens with 10wt.% Y₂O₃. This improvement is probably associated with toughness mechanisms, such as crack deflection and residual thermal stresses between the phases present in the composites.