ABSTRACT This work assessed the fracture behavior of the AA7075-T6 aluminum alloy subjected to fatigue loading. Both the notched and unnotched specimens underwent a retrogression heat treatment and re-aging (RRA) and plasma nitriding before the mechanical tests. Plasma nitriding, which has been a method chosen to increase the surface hardness and the fatigue, wear and corrosion resistances of various alloys was carried out with an active screen with a 20 Hz DC source and a 40 Hz duty cycle. The RRA treatment, which is used to increase the strength and hardness of AA7075 aluminum alloy and obtain improvements in the fatigue and corrosion behaviors, was performed at 200 °C for 20 minutes with water cooling (resolubilization), followed by 24 hours at 120 °C (re-aging). The axial fatigue tests of notched and unnotched specimens were carried out with R = 0.1, frequency of 14 Hz, and at stress level of 150 MPa for notched specimens, and 500 MPa for unnotched specimens. The fatigue fracture was then performed via stereomicroscopy and scanning electron microscopy. For the unnotched specimens, AA7075-T6 and AA7075-RRA conditions presented similar fracture features, with good definition of the three stages of fatigue failure; the AA7075 nitrited condition presented no evidence of typical fatigue fracture. The notched specimens presented fractographies similar to the severe stress concentration condition, with surface marks convex to the nucleation region.

RESUMO Este trabalho avaliou o comportamento de fratura da liga de alumínio AA7075-T6 submetida a carregamento de fadiga. Ambos os corpos de prova entalhados e não entalhados foram submetidos a um tratamento térmico de retrogression and re-aging (RRA) e nitretação a plasma antes dos ensaios mecânicos. A nitretação a plasma, que tem sido um método escolhido para aumentar a dureza superficial e as resistências à fadiga, desgaste e corrosão de diversas ligas, foi realizada com tela ativa com fonte DC de 20 Hz e ciclo de trabalho de 40 Hz. O tratamento RRA, que é utilizado para aumentar a resistência e a dureza da liga de alumínio AA7075 e obter melhorias nos comportamentos de fadiga e corrosão, foi realizado a 200 °C por 20 minutos com resfriamento a água (resolubilização), seguido de 24 horas a 120 °C (reenvelhecimento). Os ensaios de fadiga axial de corpos de prova entalhados e não entalhados foram realizados com R = 0,1, frequência de 14 Hz, e em nível de tensão de 150 MPa para corpos de prova entalhados e 500 MPa para corpos de prova não entalhados. A fratura por fadiga foi então realizada via estereomicroscopia e microscopia eletrônica de varredura. Para os corpos de prova não entalhados, as condições AA7075-T6 e AA7075-RRA apresentaram características de fratura semelhantes, com boa definição dos três estágios de falha por fadiga; a condição nitretada AA7075 não apresentou evidência de fratura por fadiga típica. Os corpos de prova entalhados apresentaram fractografias semelhantes à condição de concentração de tensão severa, com marcas superficiais convexas à região de nucleação.

Maraging steel is a special class of high-strength steels that are hardened by the precipitation of intermetallic compounds at temperatures close to 480 °C, without carbon transformation. Since its development, it has been used for strategic applications (nuclear energy, defense, and aerospace areas) and the mastery of its process and application has been constantly improved. This study aimed to characterize the weld regions of a maraging 300 steel used to manufacture a rocket motor case. Welded samples of maraging 300 steel were subjected to metallurgical and mechanical characterization. Microstructural analysis showed three different zones in the hardness range of 291 to 465 HV and reduced toughness. X-ray diffraction showed metallic phases in each region, evidencing the increase of the austenite phase. Based on the results obtained, maraging steel has great potential for aerospace applications, with excellent strength even when subjected to welding.

The current materials research field demands sophisticated tests and simulations to predict creep behavior, meet design needs, and save time and resources. This demands an appropriate mathematical formalism that can represent creep data for different materials and loading conditions. In this context, mathematical models such as θ-Projection and ν Concept can be applied to predict the behavior of materials under creep conditions. The main goal of this study involves the application of such methodologies to predict a creep curve for Ti-6Al-4V. In a preliminary assessment, both equation parameters, determined by computational techniques and an experimental database, enabled the determination of the strain-time curve of Ti-6Al-4V at 465 MPa and 500 ºC. The analysis showed that both methods have acceptable accuracy to predict primary, secondary, and tertiary creep behavior for Ti-6Al-4V. However, results indicated that the ν Concept produced a creep curve with better agreement with the experimental data. The technique used for obtaining parameters is promising and should be improved for future research.

Abstract Over the years, the demand for high engine efficiency has resulted in the development of new generation superalloys with improved elevated temperature properties, especially creep resistance. This study aimed to evaluate creep and mechanical behavior of Inconel 718 superalloy. Creep tests were performed at temperature range of 650 to 700 °C and stress range of 625 to 814 MPa. Hot tensile and oxidation tests were performed and the characterization techniques used in this study were scanning electron microscopy (SEM) for microstructural and fracture surface analysis, transmission electron microscopy (TEM) for precipitates analysis; grazing X-ray diffraction for analysis of oxide formation and Vickers microhardness. The analyze of stress exponent value (n = 36.48) and activation energy (Qc = 512.97 kJ/mol), suggested that creep mechanism at 650 °C was the climb dislocation mechanism. The Inconel 718 presented ductile fracture at 650 and 700 °C and intergranular fracture to 675 °C.

RESUMO A aplicação de um filme de SiC/Cr em liga Ti-6Al-4V para indústria aeroespacial visa a proteção química a ação do oxigênio na liga de titânio, e aumento do tempo de vida em fluência e do tempo de fratura e a diminuição da taxa de fluência da liga a deformação plástica em relação a liga sem proteção. O objetivo deste trabalho foi investigar e avaliar a evolução microestrutural, textura e as correlações cristalinas da liga Ti-6Al-4V recoberta com filme de SiC/Cr via High Power Impulse Magnetron Sputtering (HiPIMS) antes e após os ensaios de fluência. Os ensaios de fluência foram executados em temperaturas de 500, 600 e 650 °C, na modalidade de carga constante, e nas tensões de 125, 250 e 319 MPa. As análises das amostras foram realizadas por meio de microscopia eletrônica de varredura (MEV) utilizando a técnica de difração de elétrons retroespalhados (EBSD) e espectroscopia de raios X por dispersão em energia (EDS). Os resultados na liga Ti-6Al-4V a partir de EBSD identificaram que houve mudanças microestruturais e da textura para um conjunto de direções preferencias à medida em que aumentou a tensão do ensaio de fluência, aumentou a texturização das fases Ti-α e Ti-β.

ABSTRACT The application of a SiC/Cr film in Ti-6Al-4V alloy for the aerospace industry aims to chemically protect the action of oxygen in the titanium alloy, and to increase the creep life and fracture time and reduce the creep rate of plastic deformation alloy compared to unprotected alloy. The objective of this work was to investigate and evaluate the microstructural evolution, texture and crystal correlations of the Ti-6Al-4V alloy coated with SiC/Cr film by High Power Impulse Magnetron Sputtering (HiPIMS) before and after creep tests. The creep tests performed at temperatures of 500, 600 and 650 °C, in constant load mode stresses of 125, 250 and 319 MPa. The analyses of the samples were carried out by means of scanning electron microscopy (SEM) using the technique of backscattered electron diffraction (EBSD) and energy dispersion X-ray spectroscopy (EDS). The EBSD results on Ti-6Al-4V alloy were identified that there were microstructural changes and texture for a set of preferred orientations that with increased the creep test load stresses, the Ti-α and Ti-β phases texturization increases.

Abstract Titanium and its alloys are widely used as implant materials and many studies to accelerate the osseointegration have been performed. This work aims to evaluate the formation of a bioactive surface in commercially pure titanium (cp-Ti) grade 4 after alkali (AT) and heat treatments at 600 °C (AHT600) and 900 °C (AHT900). Characterization techniques were SEM, AFM, Raman, TF-XRD, wettability, nanoindentation and indentation adhesion. Additionally, SBF soaking tests were performed to evaluate apatite growth and showed that alkali and heat treatment accelerates apatite growth. The AT samples formed sodium hydrogen titanate (1 µm thick), and AHT600 and AHT900 formed sodium titanate (1 µm thick), while rutile TiO2 increased with temperature, reaching up to 5 µm thick and the surface changed from slightly hydrophilic to fully hydrophilic. Roughness and surface area increased, especially in AHT900. The hardness of the surface layer was significantly increased by the heat treatment.

The objective of this study is to evaluate the surface physicochemical properties of Ti6Al4V alloy after etching with a piranha’s solution at temperatures of 25 °C, 40 °C, and 60 °C for 30 min. Scanning electron microscopy with energy-dispersive X-ray spectroscopy, optical profilometry, X-ray diffraction, and wettability analyses were used to characterize the surface of the samples. In addition, corrosion tests were performed using potentiodynamic polarization and electrochemical impedance spectroscopy. The proposed surface treatment did not alter the wettability and surface chemistry (crystallinity), but the texture, surface roughness, superficial area, and corrosion resistance of the alloy could be changed by varying the etching temperature. Considering that these properties are indirect indicators of the alloy’s potential success in the osseointegration of a dental implant, the surface treatment of the Ti6Al4V alloy using a piranha solution while controlling the etching temperature has attractive biomedical applications.

Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 µm layer composed of ε-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45º shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.

AbstractIn this paper, chrome (Cr) thin films were deposited and used as interlayer between SiC films and Ti-6Al-4V substrates. Films and interlayers were obtained by using HiPIMS (High Power Impulse Magnetron Sputtering) technique. Interlayers were growth for 5, 30, and 60 minutes. The films were analyzed with respect to morphology, stoichiometry, thickness, roughness, and adhesion. The results showed that the HiPIMS technique was efficient to produce dense thin films and that the adhesion increased with Cr thickness.