Papers by Author: Carlos de Moura Neto

Abstract: The present study aimed to explore possible thermal processing conditions of a steel AISI 43100, seeking the best combination of properties, especially strength and ductility. Different routes of heat treatments for continuous cooling were applied to the material, and the mechanical properties were evaluated by tensile and hardness tests. The microstructure and fracture micromechanisms were characterized by scanning electron microscopy (SEM). It was observed a strong influence of cooling rate on the formation of microstructure, reflecting directly in mechanical properties.

Abstract: This work studies the effect of double aging heat treatment on superalloy Inconel 718. The heat treatment followed two steps: Solid solution at 1095oC for 1h, and two steps of aging 955oC/1h - 720oC/6,5h - 720oC/1,5h + 620oC/8h. Parameters such as grain size, secondary phases distribution and qualitative analysis via SEM/EDS and Vickers hardness are used in order to evaluate the microstructural evolution. The results show increase of grain size and Vickers hardness, as well as a better distribution of secondary phases along the matrix.

Abstract: The evaluation of stability of secondary phases in superalloys is of great importance because of its application under high homologue temperatures. The precipitation hardening Inconel 718 superalloy has high stress rupture and creep resistance, good LCF behavior and high corrosion resistance. Those properties can be obtained due to the high stable matrix based on Ni, Fe and Cr, as well as secondary phases promoters elements such as Ti, Al and Nb. This study aims to analyze the phase stability among a range of temperature from 500°C to approximately 1400°C. The thermodynamic modeling has a high importance to predict the behavior of alloys among equilibrium transition. The microstructure of the system was evaluated using Thermo-Calc software and the selected databases: SSOL4 and TTNI6. This analysis made possible the prediction of the phases presented, compositions and amounts. Results from SEM analysis were used to compare the results obtained, showing good coherence between them.

Abstract: Superalloys are used primarily in aerospace applications. These applications require a material with high mechanical strength, good resistance to fatigue and creep, good corrosion resistance and ability to operate continuously at elevated temperatures. These alloys were developed for elevated temperature service, where relatively severe mechanical stressing is encountered, and where high surface stability is frequently required. Inconel 718 has being investigated because it is one of the most widely used superalloys. Constant load creep tests were conducted on a standard creep machine at 600 and 700°C and stress levels of 300 MPa. Sets of curves and experimental parameters for the primary, secondary and tertiary regions, as a function of stress and temperature applied were obtained. The ductility, the creep rate and lifetime was evaluated.

Abstract: Ti-6Al-4V is the most used of titanium alloy and presents some important properties as metallurgical stability, high specific strength, corrosion and creep resistance [. The aim of this study is to evaluate the creep behavior of Ti-6Al-4V alloy with equiaxed and bimodal microstructures and determine the creep parameters of Ti-6Al-4V in these conditions. It was used a Ti-6Al-4V alloy forged and annealed at 190°C for 6 hours and cooled in air. The material in this condition shows an equiaxed microstructure. For bimodal microstructure, the material was heat-treated at 950°C for 60 minutes and cooled in water until room temperature. After this the material was heat-treated at 600°C for 24 hours and cooled in air until room temperature. Creep tests were performed at 600°C in stress conditions of 125, 250 and 319 MPa at constant load. The alloy with Bimodal microstructure shows higher creep resistance with a longer life time in creep.

Abstract: Titanium and its alloys have been used a lot for its special and unique properties, and characteristics under different conditions [. When we want to use one of its alloys in aerospace industry, we have to assure that in these conditions the material will resist and keep its integrity [1,. We wanted to analyse a titanium based superalloy by modeling a routine and extracting information from it about the dependence between Gibbs free energy and the amount of weight from two components, titanium and aluminium, which is the second major element according to X-ray diffraction analysis. It was concluded that increasing Ti amount, or making the alloy richer on titanium, the system leads to a higher level of energy. The opposite behaviour happens with aluminium; increasing it, the energy of the system decreases which is great, once the equilibrium is obtained with lower levels of energy. Although the analysis had been done with a general database available in the software, its possible to predict how the material will be influenced under several situations in a quick and reliable method.

Abstract: A superalloy is an alloy developed for elevated temperature service, where relatively severe mechanical stressing is encountered, and where high surface stability is frequently required. High temperature deformation of Ni-base superalloys is very important since the blades and discs of aero engine turbine, because need to work at elevated temperature for an expected long period. The nickel-base alloy Inconel 718 has being investigated because it is one of the most widely used superalloys. The objective of this work was to evaluate the creep behavior of the Inconel 718 focusing on the determination of the experimental parameters related to the primary and secondary creep states. Constant load creep tests were conducted with at 650, 675 and 700°C and the range of stress was from 625 to 814 MPa to according to ASTM E139 standard. The relation between primary creep time and steady-state creep rate, obeyed the equation for both atmospherics conditions at 650, 675 and 700°C. The microstructural characterization employing the technique of scanning electron microscopy has been a valuable tool for understanding the mechanisms of creep.

Abstract: Aluminum alloys have low specific weight, relatively high strength and high corrosion resistance and are used in many applications. Aluminum Alloy 2024 is widely used for aircraft fuselage structures, owing to its mechanical properties. In this investigation, Aluminum Alloy 2024 was given solid solution treatments at 495, 505, and 515°C followed by quenching in water. It was then artificially aged at 190 and 208°C. Subsequently, hardness measurements, tensile tests as well as impact and fatigue tests were carried out on the heat treated alloys to determine the mechanical properties. The tensile and hardness tests revealed similar mechanical properties for specimens of this alloy that were given the three solid solution treatments. Aluminum Alloy 2024 specimens that were solid solution treated at 515°C and artificially aged at 208°C for 2h exhibited the highest yield and tensile strength. In general, the increase in strength was accompanied by a decrease in ductility. Cyclic fatigue studies were conducted with symmetric tension-compression stresses at room temperature, using a bending-rotation test machine. The alloy solution heat treated at 515°C and aged at 208°C/2h was fatigue tested at constant frequency. The relation between stress amplitude and cycles to failure was established, enabling the fatigue strength to be predicted at more than 7.8x10⁶ cycles, with maximum stress of 110.23 MPa. The fracture surfaces of specimens that failed after fewer cycles showed mainly precipitates and micro voids, whereas specimens that fractured after a higher number of cycles indicated that cracks initiated at the surface. The high cycle fatigue fracture surfaces revealed pores that could be due to precipitates from the matrix.

Abstract: The present study is about the effect of oxide layers in creep of Ti-6Al-4V alloy, in different atmospheres (air, nitrogen and argon). Ti-6Al-4V alloy was treated during 24 hours in a thermal treatment furnace at 600°C in different atmospheres (argon, nitrogen and air). The samples were analyzed by High Resolution X-Ray Diffraction, Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM) and microhardness test. The polished samples of Ti-6Al-4V allloy were treated during 24 hours at 600°C and the oxidation behavior in each case using argon, nitrogen and air atmospheres was observed. The oxidation was more aggressive in air atmosphere, forming TiO2 film in the surface. The oxidation produced a weight gain through the oxide layer growth and hardening by oxygen dissolution. Ti-6Al-4V alloy specimens also were produced in order to test them in creep, at 250 MPa and 600°C, with argon, nitrogen and air atmospheres. When the Ti-6Al-4V alloy was tested under argon and nitrogen atmospheres oxidation effects are smaller and the behavior of the creep curves shows that the creep life time was better in atmospheres not so oxidant. It is observed a decreasing of steady state creep in function of the oxidation process reduction. It is shown that, for the Ti-6Al-4V alloy, their useful life is strongly affected by the atmosphere that is submitted, on account of the oxidation suffered by the material.

Abstract: This study aimed to evaluate the resistance of a Ti-6Al-4V alloy in creep after heat treatments. It was used a Ti-6Al-4V alloy in cylindrical bars forms, forged condition and annealing at 190oC for 6 hours and cooled in air. The microstructure of Ti-6Al-4V alloy was evaluated after heat treatment and was submitted to creep tests at 600oC and stress conditions from 125 to 319 MPa at constant load. The Widmanstätten structure was obtained by heat treatment. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco Type 9 MB. The alloy with Widmanstätten structure and ceramic coating shows greater resistance to creep and oxidation with a longer life time in creep. At higher stress condition, 600°C and 319 MPa, the Ti-6Al-4V alloy with ceramic coating didn’t show higher creep resistance. This condition presented higher tp value and the value. It occurred because at high stress condition the coating is very fragile, decreasing your creep resistance.