Carlos de Moura Neto

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (hi), secondary dendrite arm spacing (λ2), corrosion potential (ECorr), corrosion rate (iCorr), polarization resistance (R1), capacitances values (ZCPE), ultimate tensile strength (UTS, σu), yield strength (YS, σy), and elongation. It is shown that σU decreases with increasing λ2 while the CR increases with increasing λ2, for both alloys experimentally examined. A combined plot of CR and σU as a function of λ2 is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.

Study of the non-linear stress-strain behavior in Ti-Nb-Zr alloys

The aim of this work is to study the elastic behavior of some Ti-Nb-Zr alloys (Ti-8Nb-13Zr, Ti-13Nb-13Zr, Ti-18Nb-13Zr and Ti-41.1Nb-7.1Zr) developed to biomedical applications. These alloys were produced by arc melting under argon atmosphere. Uniaxial tensile tests, carried out in a MST servo-hydraulic machine, were employed in their mechanical characterization. The occurrence of non-linear stress-strain behavior in the conventional elastic region (total strain values up to 0.2%) lead to the Ramberg-Osgood relationship, modified by Hill, in order to analyze that portion of the obtained curves. The present study involves the following properties: initial elastic modulus, tangent modulus, secant modulus, proof stress and the Ramberg-Osgood parameter (n). The results demonstrate that these alloys are not similar with respect to the mechanical behavior. Furthermore, it is shown that the degree of non-linearity in the stress-strain behavior is quantified by the parameter (n).

Low Carbon Content NiTi Shape Memory Alloy Produced by Electron Beam Melting

Earlier works showed that the use of electron beam melting is a viable process to produce NiTi shape memory alloy. In those works a static and a semi-dynamic processes were used producing small shell-shaped and cylindrical ingots respectively. The main characteristics of those samples were low carbon concentration and good composition homogeneity throughout the samples. This paper presents the results of scaling up the ingot size and processing procedure using continuous charge feeding and continuous casting. The composition homogeneity was very good demonstrated by small variation in martensitic transformation temperatures with carbon content around 0.013wt% compared to 0.04 to 0.06wt% of commercial products.

Influence of the microstructure on the degree of sensitization of a duplex stainless steel UNS S31803 aged at 650ºC

This work evaluates the phase transformations during aging of an UNS S31803 DSS at 650oC and its influence on the DOS. The material was solution treated at 1175oC and then aged at 650oC for times up to 360 h. SEM-BSC images indicate the formation of Cr2N, σ and χ phases in the samples aged at 650oC. The analysis of DL-EPR curves, obtained in a 1 M H2SO4 + 0.25 M NaCl + 0.01 M KSCN solution, shows an increase in DOS values for samples aged at 650oC. Probably, this increase observed in DOS values is mainly related to the presence of Cr- and/or Mo-depleted α, as a result of σ phase, χ phase and/or Cr2N formation at 650oC. Moreover, a possible healing up of Cr- and/ or Mo-depleted areas is observed between 1 h and 4 h of aging at 650oC.

Creep Behavior of the Inconel 718 Superalloy

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.

The Effect of Widmanstätten and Equiaxed Microstructures of Ti-6Al-4V on the Oxidation Rate and Creep Behavior

Ti-6Al-4V is currently used in aeronautic and aerospace industry mainly for applications that require resistance at high temperature such as, blades for aircraft turbines and steam turbine blades. The titanium affinity by oxygen is one of main factors that limit the application of their alloys as structural materials at high temperatures. Notable advances have been observed in the development of titanium alloys with the objective of improving the creep properties. Increased oxygen levels are associated with increased microhardness and decreased ductility in titanium. In spite of this, Ti-6Al-4V containing an (+) structure continues to be the workhorse of the titanium industry due to their high specific strength, corrosion resistance, excellent high temperature properties and metallurgical stability. The objective of this work was to study the influence of equiaxed and Widmanstätten microstructures on oxidation rates and creep behavior of the Ti-6Al-4V alloy. The samples were exposed to different conditions of time and temperature to evaluate the oxidation rates. This influence on the oxidation rates was evaluated in terms of weight gain, -case depth and microhardness profile at 500 and 600 °C. Preliminary results indicated that the equiaxed microstructure with average grain size of 10 m exhibits faster oxygen diffusion. Short-term creep tests were performed under constant load in a stress range from 291 to 472 MPa at 500 °C and in a stress range from 97 to 291 MPa at 600 °C. The stress exponents obtained lie in the range from 4.0 to 11.3. The apparent activation energies for steady-state creep determined in the present work were estimated to be 316 and 415 kJ/mol at 291 MPa for the equiaxed and Widmanstätten microstructures, respectively. On the basis, the creep of Ti-6Al-4V is consistent with the lattice diffusion-controlled dislocation climb process in -Ti, for both microstructures. The creep rates of Widmanstätten microstructure were two orders of magnitude lower than of equiaxed microstructure in both temperatures. Apparently, the higher creep resistance with a Widmanstätten microstructure can be attributed to / interfaces acting as obstacles to dislocation motion and to the average grain size of 395 m, which reduces the grain boundary sliding, dislocations sources and the rate of oxygen diffusion along grain boundaries.

Effect of Artificial Aging on the Mechanical Properties of an Aerospace Aluminum Alloy 2024

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.

Determination of Creep Parameters of Ti-6Al-4V with Bimodal and Equiaxed Microstructure

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.

Mechanical Characterization of a Steel AISI 43100 Submitted to Different Routes of Heat Treatments

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.

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

B composition. The present study concerns the preparation of titanium alloys that contain such phase mixed with α-titanium and other intermetallic phases. High-purity powders were initially processed in a planetary ball-mill under argon atmosphere with Ti-18Si-6B and Ti-7.5Si-22.5B at. (%) initial compositions. Variation of parameters such as rotary speed, time, and ball diameters were adopted. The as-milled powders were pressureless sintered and hot pressed. Both the as-milled and sintered materials were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometry. Sintered samples have presented equilibrium structures formed mainly by the α-Ti+Ti