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At least 1,747 records · Page 97
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DOE OSTI2018

Effect of Localized Corrosion on Fatigue–Crack Growth in 2524-T3 and 2198-T851 Aluminum Alloys Used as Aircraft Materials

Corrosion and fatigue of aluminum alloys are major issues for the in-service life assessment of aircraft structures and for the management of aging air fleets. The aim of this work was to evaluate the effect of localized corrosion on fatigue crack growth (FCG) resistance of the AA2198-T851 Al-Li alloy (Solution Heat Treated, Cold Worked, and Artificially Aged), comparing it with the FCG resistance of AA2524-T3 (Solution Heat Treated and Cold Worked), considering the effect of seawater fog environment. Before fatigue tests, the corrosion behavior of 2198-T851 and 2524-T3 aluminum alloys was verified using open circuit potential and potentiodynamic polarization techniques. Fatigue in air and corrosion fatigue tests were performed applying a stress ratio (R) of 0.1, 15 Hz (air) and 0.1 Hz (seawater fog) frequencies, using a sinusoidal waveform in all cases. The results showed that the localized characteristics of the 2198-T851 and 2524-T3 aluminum alloys are essentially related to the existence of intermetallic compounds, which, due to their different nature, may be cathodic or anodic in relation to the aluminum matrix. The corrosive medium has affected the FCG rate of both aluminum alloys, in a quite similar way. Graphical Abstract: .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Sodium Silicate/Sodium Alginate Additives on Discharge Performance of Mg–Air Battery Based on AZ61 Alloy

The application of Mg–air batteries is limited due to passivation and self-corrosion of anode alloys in electrolyte. In effort of solving this problem, the present work studied the influence of sodium silicate (SS)/sodium alginate (SA) on electrochemical behaviors of AZ61 alloy in NaCl solution by circle potentiodynamic polarization and galvanostatic discharge. The corrosion morphology and discharge product were examined by scanning electron microscopy (SEM) and x-ray diffraction (XRD). Results have shown that sodium silicate/sodium alginate inhibitors have an apparent effect on the self-corrosion of AZ61 alloy without affecting its discharge performance. The discharge capacity and the anodic utilization for Mg–air battery in a 0.6 M NaCl + 0.01 M SS +0.04 M SA solution are measured to be 1397 mAhg{sup −1} and 48.2%, respectively. Electrochemical impedance spectroscopy (EIS) and SEM investigation have confirmed that the sodium silicate/sodium alginate inhibitor can obviously decrease the self-corrosion of AZ61 alloy. SEM and XRD diffraction examinations suggest that the inhibiting mechanism is due to the formation of a compact and “cracked mud” layer. AZ61 alloy can be used as the anode for Mg–air battery in a solution of 0.6 M NaCl + 0.01 M SS +0.04 M SA.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Multiple Reflow Cycles and Al{sub 2}O{sub 3} Nanoparticles Reinforcement on Performance of SAC305 Lead-Free Solder Alloy

The effect of Al{sub 2}O{sub 3} nanoparticles reinforcement on melting behavior, microstructure evolution at the interface and joint shear strength of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder alloy subjected to multiple reflow cycles was investigated. The reinforced SAC305 solder alloy compositions were prepared by adding Al{sub 2}O{sub 3} nanoparticles in different weight fractions (0.05, 0.1, 0.3 and 0.5 wt.%) through mechanical dispersion. Cu/solder/Cu micro-lap-shear solder joint specimens were used to assess the shear strength of the solder joint. Differential scanning calorimetry was used to investigate the melting behavior of SAC305 solder nanocomposites. The solder joint interfacial microstructure was studied using scanning electron microscopy. The results showed that the increase in melting temperature (T{sub L}) and melting temperature range of the SAC305 solder alloy by addition of Al{sub 2}O{sub 3} nanoparticles were not significant. In comparison with unreinforced SAC305 solder alloy, the reinforcement of 0.05-0.5 wt.% of Al{sub 2}O{sub 3} nanoparticles improved the solder wettability. The addition of nanoparticles in minor quantity effectively suppressed the Cu{sub 6}Sn{sub 5} IMC growth, improved the solder joint shear strength and ductility under multiple reflow cycles. However, the improvement in solder properties was less pronounced on increasing the nanoparticle content above 0.1 wt.% of the solder alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fabrication of 7075-0.25Sc-0.15Zr Alloy with Excellent Damping and Mechanical Properties by FSP and T6 Treatment

This study investigated the effects of Sc and Zr additions, friction stir processing (FSP), and subsequent solid solution and artificial aging (T6) treatment on the microstructure, damping capacity, and mechanical properties of 7075 Al alloy. The minor Sc and Zr additions promoted the grain refinement of this alloy during FSP and inhibited the growth of fine equiaxed grains in the FSP sample during subsequent T6 treatment. Thus, the fine equiaxed grain structure and high density of the η′ precipitated phase were simultaneously obtained in the 7075 Al alloy containing Sc and Zr after FSP and T6 treatment, and this sample exhibited superior mechanical properties and damping capacity to those of the 7075 Al alloy after T6 treatment. This work provides an effective strategy for preparing Al alloys with excellent damping capacity and mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Optimization of Hot Deformation Parameters and Constitutive Analysis for As-Cast Mg-5Li-3Zn-0.3Y Alloy Using Processing Maps

Hot deformation behavior of as-cast Mg-5Li-3Zn-0.3Y (wt.%) alloy has been studied by establishing processing maps covering the range of 200-450 °C in temperature and 0.001-10 s{sup −1} in strain rate. The map corresponding to the strain of 0.7 reveals that a domain of dynamic recrystallization occurred at 340-400 °C/0.01-0.158 s{sup −1} with its peak efficiency of approximately 31%. Analysis of the flow behaviors and microstructural observations indicated that dynamic recrystallization occurred in the domain, in which fine microstructure can be achieved. The above conditions represent the optimum processing parameters, and the flow instability regimes are also recognized from the maps. The average deformation in the deformed range was 136.414 kJ/mol which is close to that for lattice self-diffusion in magnesium and far smaller than that for as-cast Mg-5.75Zn-0.73Y-0.36Zr (wt.%) alloy. It is believed that a lower activation energy for Mg-5Li-3Zn-0.3Y alloy can partly result from both the weak particle pinning effects and adding Li into the Mg-Zn-Y-Zr alloy. Results of extrusion experiments conducted at 350 °C show that the alloy can be easily extruded with good mechanical properties of the yield strength of 132 MPa, the fracture strength of 215 MPa and the elongation of 20.9%, and the grains can be well refined.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Welding Parameters on the Corrosion Behavior of Dissimilar Alloy Welds of T6 AA6061 Al-Galvanized Mild Steel

Partial replacement of steel by Aluminium (Al) alloys is a promising approach adopted by automotive sector to improve fuel efficiency without compromising on the strength. However, this results in the generation of dissimilar welds of Al alloys and steel. Understanding the corrosion behavior of Al alloys–steel welds as a function of welding parameters is critical in the successful application of such alloys. The present study reports the intergranular corrosion behavior of T6 heat-treated AA6061 Al alloy-galvanized mild steel lap joints, welded by metal inert gas welding–brazing technique as a function of different welding parameters, viz. weld speed (4-6 m/min) and wire feed rate (0.8-0.9 m/min), following ASTM G 67-04. Weld characterization was performed by field emission scanning electron microscopy (FESEM), x-ray diffraction and nano-hardness measurement. Results indicate heavy dissolution and metal loss at the interface. A high volume fraction of Al-Fe intermetallics was precipitated at the weld interfaces, resulting in high hardness and higher localized corrosion. The thickness of Al-Fe intermetallic layer increased with increasing wire feed rate and lower weld speed which enhanced the severity of galvanic and intergranular corrosion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Minor Er on the Microstructure and Properties of Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr Alloys

As-cast Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr alloys containing different additions of Er were prepared. Then, the specimens were carried out homogenization heat treatment, hot rolling, cold rolling, and annealing. The effects of minor Er on the microstructure and properties of Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr alloy were investigated. Results show that addition of Er can refine the microstructure and improve the mechanical properties of the alloys. As for the annealed alloys, the addition of 0.33%Er can increase the tensile strength by 29 MPa. Strength increment is mainly attributed to refinement and the precipitation of Al{sub 3}Er and Al{sub 3}(Er, Zr). Additionally, the minor Er can improve the corrosion resistance of the alloys, which can be attributed to the compact composite oxide film of Er-O and Al-O, purification, and modification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermoelectric Properties of TiNiSn Half Heusler Alloy Obtained by Rapid Solidification and Sintering

Thermoelectric TiNiSn-based half Heusler compound reveals high Seebeck coefficient and electrical conductivity, leading to high power factor. As a consequence of the solidification path, TiNiSn single phase cannot be obtained directly from the liquid phase. Thus, a long annealing step is needed to homogenize the alloy. In this work, we present a new processing route for half Heusler compound formation, combining arc melting of pure elements, rapid solidification of the molten alloy and sintering. Rapid solidification of the molten alloy allows to obtain almost single TiNiSn phase, limiting the formation of the primary TiNi{sub 2}Sn phase as a consequence of the deep undercooling of the liquid. The rapidly solidified alloy was ground to powder and sintered by open die pressing. As-sintered samples show a density around 95% of the theoretical value. Thermal cycling of the sintered samples shows evolution of the phases, suggesting that after sintering the sample is not in equilibrium yet. After the second thermal cycle thermoelectric properties become reproducible, indicating the attainment of the equilibrium. In conclusion, the proposed processing route allows to obtain dense TiNiSn in bulk form avoiding the time-consuming annealing step, typically used to homogenize this alloy after solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Experimental and Numerical Investigations of Cyclic Plastic Deformation of Al-Mg Alloy

In this work, low-cycle fatigue (LCF) tests are performed on as-received and solution-treated Al-Mg alloy (AA 5754). Solution heat treatment of the as-received alloy is performed at 530 °C for 2 h and then quenched in water at room temperature. The low-cycle fatigue response of the as-received and solution-treated alloy is evaluated at five different strain amplitudes from 0.2 to 0.6%. The strain rate of 0.5 × 10{sup −3} s{sup −1} is kept constant in all the LCF tests. The LCF behavior of AA 5754 is modeled by finite element method (FEM) using Chaboche kinematic hardening cyclic plasticity model. Hardening behavior or fatigue life of as-received and solution-treated alloy is successfully simulated using this model. The results obtained by FEM simulations are compared with those obtained by LCF experiments. Microstructural studies are also performed for as-received and solution-treated alloy. The fracture surfaces obtained from LCF tests are analyzed by scanning electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Glass-Forming Ability, Thermal Stability, and Hardness of Rapidly Solidified Al-Ni-Y-Sc Alloy Ribbons and Rods

The crystallization behaviors and thermal stability of (Al{sub 87}Y{sub 8}Ni{sub 5}){sub 100−x}Sc{sub x} (x = 0, 1.0, 1.5) amorphous ribbons and alloy rods, produced by roller melt-spinning method and inductance casting, were investigated mainly by x-ray diffractometry (XRD) and differential scanning calorimetry (DSC). The glass transition temperatures of the (Al{sub 87}Y{sub 8}Ni{sub 5}){sub 99.0}Sc{sub 1.0} and (Al{sub 87}Y{sub 8}Ni{sub 5}){sub 98.5}Sc{sub 1.5} alloy ribbons were found to be 201 and 197 °C, respectively. Furthermore, the melting temperatures of the alloy ribbons decreased with the Sc content. This revealed that the glass-forming ability (GFA) of the metallic glass increased with increasing Sc content. Crystallization of (Al{sub 87}Y{sub 8}Ni{sub 5}){sub 100−x}Sc{sub x} (x = 0, 1.0, 1.5) amorphous ribbons occurred in the temperature range of 200 to 400 °C in three exothermic reaction stages. The peak temperatures of these three crystallization reactions shifted to higher temperatures at higher heating rates. The kinetic parameters for the first crystallization reaction, determined by Kissinger method, revealed that adding 1.5 at.% Sc increased the activation energy for crystallization from 158 to 167 kJ/mol. On the other hand, the cross-section morphology of the three different alloys revealed that a fully amorphous outer region and a partially crystallized core region formed in each alloy rod. It was also found that the thickness of the amorphous region increased with the Sc content, revealing a positive effect of Sc on the GFA.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Precipitation of Carbides and Dissolution of Widmanstätten Structure for Enhanced Hardness in Ti{sub 2}AlNb-Based Alloys

Carbide-reinforced Ti{sub 2}AlNb matrix composites were spark plasma sintered at 1100 °C by incorporating CNTs into single-B2-phase Ti-22Al-25Nb powders. The as-SPSed alloys were solution treated and then aged in different phase regions. By kinetics analysis and energy-dispersive spectroscopy, it is found carbon will stabilize α{sub 2} phase during solution treatment, and the formation of carbide results in the Nb-depleted regions, which further induces the formation of curly B2 + O Widmanstätten structure during quenching. Different from the general Widmanstätten structure in the aged Ti{sub 2}AlNb alloy, the remained α{sub 2} will consume the B2 within the Widmanstätten structure, when the carbon-added alloy is aged in B2 + O phase region. Compared to the alloy without CNTs, the Vickers hardness of the aged carbon-added Ti{sub 2}AlNb alloy is increased significantly, which is related to the uniform and abundant columnar carbides within the O-phase matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of As-Cast γ-TiAl Alloys with Different Cooling Rates

γ-TiAl alloys composed of Ti-45Al-5Nb-1W-1B (at.%) solidified at three different cooling rates by vacuum electromagnetic induction melting under a controlled degree of vacuum of under 2 Pa. The influence of the cooling rate on the microstructure and mechanical properties of the γ-TiAl alloy samples was investigated. A relatively large temperature gradient at the beginning of solidification led to a thin fine-grained layer around the graphite mold, and undercooling had two main effects: (a) promoting nucleation in the grain-refining surface layer and (b) promoting the growth of columnar crystals beneath the surface. Moreover, all prepared ingots of this alloy could be clearly divided into Regions S, M and C from the surface to the core. The colony size of Region M was larger than those of the other two regions due to composition segregation and the growth of columnar grains. The existence of tungsten led to micro-inhomogeneity, and the mechanical properties of the alloy changed according to concentration fluctuations. The degrees of homogeneity were expressed as H{sub 0.1} and H{sub 0.2}. The alloy with the highest values, which possesses the best mechanical properties, solidified at a relatively low cooling rate. The deformation mechanism is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Mechanical Properties and Corrosion Behavior of Spray-Formed 7075 Alloy with One-Stage Aging

The mechanical properties, corrosion resistance and microstructure of the spray-formed 7075 alloy after one-stage aging treatments were studied by using a tensile tester, hardness tester, slow strain rate test, electrical test, intergranular corrosion test, electrochemical corrosion test, scanning electron microscope and transmission electron microscope. The results show that the strength and hardness of the alloy have double peaks with the one-stage aging treatments. With aging at 120 °C for 24 h (first peak), abundant tiny η′ phases and η phases precipitate in the matrix. The tensile strength of the alloy increases to 760 MPa, and the elongation is 4.8%. After aging at 120 °C for 132 h (second peak), blocky η phases appear. The tensile strength and elongation of the alloy are 757 MPa and 5.9%, respectively. The grain boundary precipitates dissociate, and the precipitate-free zones are widened. The corrosion resistance of the alloy increases.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Multidirectional Forging on Microstructural, Mechanical, and Corrosion Behavior of Mg-Zn Alloy

Multidirectional forging (MDF) was applied to Mg-6%Zn alloy up to 5 passes successfully at 280 °C. MDF-processed materials were characterized using optical microscope, scanning electron microscope, electron backscatter diffraction, transmission electron microscope, and x-ray diffraction. Obtained results showed a significant reduction in grain size (up to 3.8 μm) having a large fraction of high-angle grain boundaries after 5 passes of MDF process. Maximum tensile strength of 230 MPa was achieved for 5-pass MDF-processed Mg-6%Zn alloy which is about ~ 2.0 times higher in comparison with that of homogenized alloy (117 MPa) and was attributed to higher dislocations density and grain refinement. Corrosion behavior of the alloy was investigated in 0.1 M NaCl solution using potentiodynamic polarization test, electrochemical impedance spectra analysis, and immersion tests. It was found that the corrosion rate of 5-pass MDF sample improved (0.34 mm/year) ~2.5 times in comparison with that of homogenized Mg-6%Zn alloy (0.86 mm/year) due to fine grain structure, which creates more grain boundaries that act as a corrosion barrier.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Application of Canned Deformation to a Hard-Deformed Spray-Deposited Al-Zn-Mg-Cu Alloy

A wide variety of industrial applications require Al-Zn-Mg-Cu alloys with high strength and ductility. However, Zn-rich Al-Zn-Mg-Cu alloys prepared by rapid solidification methods usually show excellent strength but poor formability, which restricts their wide application in complex forming. In this study, we developed a new deformation method for the spray-deposited Zn-rich Al-Zn-Mg-Cu alloy to avoid crack formation and achieve isotropic properties. The microstructure evolution and crystallographic textures were analyzed and compared with the results of a normal deformation process. Crack-free forgings with the ultimate tensile strength (UTS), yield strength (YS) and elongation of 727 ± 5 MPa, 690 ± 3 MPa, 5.0 ± 0.7% along the radial direction and 735 ± 2 MPa, 693 ± 3 MPa, 6.3 ± 0.2% along the tangential direction were finally obtained. The improvement in the mechanical properties can be attributed to the larger amount of subgrains and substructures generated in the canned deformed alloy. The results of this study show excellent advantages and potential application prospects of the canned deformation to hard-deformed Al-Zn-Mg-Cu alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Rolling Deformation on the Microstructure and Mechanical Properties of an Extruded Mg-Zn-Ca Alloy

The microstructure and mechanical properties of an as-extruded Mg-4.5Zn-1.1Ca (wt.%) alloy subjected to unidirectional rolling and cross-rolling were compared in the present study. The results indicated that rolling deformation refined the microstructure, dispersed second phases and modified the texture, which resulted in an improvement in the mechanical properties of the as-rolled alloys compared with those of the as-extruded alloy. The yield strength of the Mg-4.5Zn-1.1Ca (wt.%) alloy increased from 173 to 297 MPa after unidirectional rolling. The initial fragmented Ca{sub 2}Mg{sub 6}Zn{sub 3} phases were further refined after rolling deformation, which clearly restricted the grain growth and gave rise to a fine microstructure. It was also found that rolling deformation intensified the basal texture in comparison with the as-extruded alloy, which was related to particle pinning.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On the Effect of Quenching on Postweld Heat Treatment of Friction-Stir-Welded Aluminum 7075 Alloy

This work focuses on the effect of postweld heat treatment (PWHT) on the mechanical properties and microstructural evolution of aluminum 7075 alloy processed via friction stir welding (FSW). FSW is known to be capable of grain refinement in the nugget zone (NZ). Two different quench media (water and air) were employed for PWHT. Regardless of the quench media, the PWHT led to the occurrence of grain growth in the NZ of the FSWed aluminum 7075 alloy. Abnormal grain growth occurred in the water quenched specimen. It is shown that ductility and strength of FSWed aluminum 7075 alloy are strongly dependent on the quenching rate. Changes in the mechanical properties and microstructure reveal that only at lower cooling rate this alloy is prone to the formation of precipitate-free zones (PFZs) in the vicinity of grain boundaries. Eventually, the PFZs deteriorate mechanical properties of this alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of the Composition on the Solidification Path, Microstructure Evolution and Mechanical Properties of Al-Cu-Mg Alloys

The influence of the initial composition on the solidification path, microstructure evolution, thermophysical properties and mechanical properties of Al-Cu-Mg alloys was investigated. The solidification paths of the investigated alloys were determined by analyzing the solidification structure, and then the experimentally determined paths were compared with those from the calculated results. Three of the investigated alloys experienced typical ternary eutectic solidification, and the other four alloys experienced quasiperitectic reactions. Due to differences in the initial compositions, different eutectic morphologies were formed during the solidification process. The binary eutectic phases, (α-Al + Al{sub 2}Cu) and (α-Al + Al{sub 6}CuMg{sub 4}), and the (α-Al + Al{sub 6}CuMg{sub 4} + Al{sub 8}Mg{sub 5}) ternary eutectic phase tended to exhibit divorced growth, and the (α-Al + Al{sub 2}CuMg), (α-Al + Al{sub 6}CuMg{sub 4}) and (α-Al + Al{sub 2}Cu + Al{sub 2}CuMg) eutectic phases tended to exhibit coupled growth. The results indicated that the initial composition, solidification path, microstructure and morphology could greatly affect the melting enthalpy, microhardness and compressive strength of the investigated Al-Cu-Mg alloys.

36 MATERIALS SCIENCE↗