Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.     

Phenomenological observations on mechanical and corrosion properties of thixoformed 357 alloys: A comparison with permanent mold cast 357 alloys

In the present study, the mechanical and corrosion properties of thixoformed 357 alloys were examined with different reheating temperatures, and the results were compared with those of permanent mold cast (PMC) 357 alloys. It was found that the thixoforming process significantly improved the mechanical properties (i.e., tensile elongation, impact energy, and resistance to fatigue crack propagation) and the corrosion resistance of 357 alloys. A 380 pct increase in tensile elongation and a 120 pct increase in impact energy were, for example, observed with the thixoforming process of 357 alloy in the T1-tempered condition, as compared to the PMC counterparts. The impact energy was extremely sensitive to reheating temperature due to the coarsening of eutectic Si particles. The resistance to fatigue crack propagation was also much higher for the thixoforming process than the PMC process in the T1-tempered condition. The resistance to both general corrosion and stress corrosion cracking was also greatly improved with thixoforming process. The present observations strongly suggest that the enhancement with thixoforming 357 alloy is largely associated with the size and shape of eutectic Si particles.     

Tensile behavior of friction-stri-welded Al 6061-T651

In the present study, tensile behavior of friction-stir-welded Al 6061-T651 with varying welding parameters, including rotating and welding speeds, was examined. The 4-mm-thick Al 6061-T651 alloy plates were FSW with varying tool rotating speeds, 1000, 1400, 1600, 2000, and 2500 rpm, and welding speeds, 0.1, 0.2, 0.3, to 0.4 mpm (m/min). Tensile specimens were prepared with the tensile direction perpendicular to the welding direction, so that the weld zone is located in the middle of the specimen. It was found that the tensile elongation of friction-stir-welded Al 6061-T651 decreased with decreasing welding speed or increasing rotating speed. The yield and ultimate tensile strength were also affected, but to a significantly lesser degree, with varying welding parameters. The micrographic and fractographic observations strongly suggested that the change in tensile behavior of friction-stri-welded Al 6061-T651 was largely related to the clustering of coarse Mg₂Si precipitates, due to the whirling and hurling action by severe plastic flow in the weld zone. Low welding speed or high rotating speed tended to encourage the plastic flow per unit time and consequently the clustering of coarse precipitates.     

Plastic deformation behavior of aluminum casting alloys A356/357

The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multi-loop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

Stress corrosion cracking behavior of friction-stir-welded Al 6061-T651

In the present study, the stress corrosion cracking (SCC) behavior of friction-stir-welded AI 6061-T651 alloy was examined of −650 mV vs Ag/AgCl using a slow strain rate testing technique. The resistance to SCC was correlated to the percent change in tensile elongation with exposure to 3.5 pct NaCl aqueous solution with respect to the reference environment. It was demonstrated the the SCC resistance of friction-stir-welded Al 6061-T651 was considerably higher than that of parent material at an anodically applied potential. In friction-stir-welded Al 6061-T651 alloy, the stress corrosion cracks occur only locally in the boundary region between the dynamically recrystallized zone (DXZ) and the heat affected zone (HAZ) regions. However, the HAZ has much lower strength properties compared with the rest of the material, and thus, fracture occurs there despite the increase in stress intensity due to corrosion at the DXZ and HAZ boundary. Eventually, the tensile fracture in friction-stir-welded A1 6061-T651 was relatively unaffected by the SCCs formed in 3.5 pct NaCl aqueous solution.     

Tensile behavior of friction-stir-welded A356-T6/Al 6061-T651 bi-alloy plate

Abnormally low tensile ductility has often been reported for the friction-stir-welded (FSWed) dissimilar metals. The mechanism(s) for such a low tensile ductility has, however, not been established. In the present study, the tensile behavior of FSWed A356-T6/Al 6061-T651 bi-alloy plate was studied to understand the underlying mechanism for the reduced tensile ductility with the friction stir welding of dissimilar metals based on thorough micrographic and fractographic observations. The present study also demonstrated that the tensile ductility of the friction-stir-welded A356-T6/Al 6061-T651 bi-alloy specimen was substantially lower than that of the weighted mean value of the uni-alloy counterparts, including A356-T6 and Al 6061-T651 alloys. Interestingly, a relatively large number of acicular shaped Si particles were observed locally in the FSWed bi-alloy specimens compared to the dominantly globular shaped particles in the FSWed uni-alloy counterpart. Moreover, these acicular shaped Si particles were found to be mostly aligned parallel to the tool-rotating direction. Such agglomerated areas of the preferentially oriented, acicular Si particles in the present study appeared to serve as initiation sites for the tensile fracture and eventually caused low tensile ductility.     

Deformation behavior of bimodal nanostructured 5083 Al alloys

Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 μm. Compression loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation associated with the occurrence of a Lüders band was observed in the bimodal alloys. As the volume fraction of coarse grains was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.     

Deformation and Fracture Behavior of Alumina Particle-Reinforced Al 6061-T6 Composite during Dynamic Mechanical Loading

Alumina particle-reinforced aluminum 6061-T6 composites are subjected to deformation at high strain rates in torsion and in compression using Hopkinson bars. The volume content of alumina particles in the aluminum alloy are 10 and 20 pct. While occurrence of adiabatic shear bands is evident under compression, the specimens fractured rapidly in torsion at high strain rates. Fracture occurs in both testing modes by ductile shear failure with elongated dimples originating mostly from the particle matrix interface. Most of the reinforcing particles along the crack propagation path fractured in torsion. However, excessive thermal softening inside the shear bands in the specimens under compression loading caused debonding rather than fragmentation of most particles as the crack propagates along the shear bands. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

高压扭转制备SiC_p/Al复合材料的断裂行为

为了研究大塑性变形对颗粒增强复合材料断裂行为的影响规律,在不同高压扭转工艺(high-pressure torsion,HPT)工艺参数下制备SiC_p/Al复合材料,测量试样真应力-应变曲线和观察试样的断口形貌,并分析SiC-Al界面的EDS谱。在分析各参数下材料断口形貌和界面原子扩散的基础上,讨论颗粒增强复合材料的断裂机理。研究发现:SiC_p/Al复合材料包含韧性断裂和脆性断裂2种性质的断裂,断口韧窝的大小和数量与材料的工艺参数有关;HPT变形可以有效改善SiC颗粒与Al基体的界面连接强度,提高该类材料的断裂性质。基体内气孔和颗粒与基体间孔隙的连接是金属基复合材料的主要断裂机制。     

Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Effect of casting defects

The influence of casting defects on the room temperature fatigue performance of a Sr-modified A356-T6 casting alloy has been studied using un-notched polished cylindrical specimens. The numbers of cycles to failure of materials with various secondary arm spacings (SDAS) were investigated as a function of stress amplitude, stress ratio, and casting defect size. To produce pore-free samples, HIP-ed and Densal™ treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental effect on fatigue life by shortening not only the crack propagation period, but also the initiation period. Castings with defects show at least an order of magnitude lower fatigue life compared to defect-free ones. The decrease in fatigue life is directly correlated to the increase of defect size. HIP-ed alloys show much longer fatigue lives compared to non-HIP-ed ones. There seems to exist a critical defect size for fatigue crack initiation, below which fatigue crack initiates from other competing initiators such as eutectic particles and slip bands. A fracture mechanics approach has been used to determine the number of cycles necessary to propagate a fatigue crack from a casting defect to final failure. Fatigue life of castings containing defects can be quantitatively predicted using the size of the defects. Moreover, the fatigue fracture behavior of aluminum castings is well described by Weibull statistics. Crack originating from different defects (such as porosity and oxide films) can be readily identified from the Weibull modulus and the characteristic fatigue life. Compared with oxide films, porosity is more detrimental to fatigue life.     

Investigation of the segregation behavior of boron in Ni3Al alloys by PTA technique

The particle-tracking autoradiograph (PTA) technique has been used, combined with quantitative statistical analysis using the image processing system, to study the effect of such factors as aluminium content of base alloy, the bulk boron concentration and different heat treatments on the grain boundary segregation behaviors of boron in Ni₃Al alloys. Moreover, the mechanical properties of Ni₃Al alloys subjected to different heat treatments and therefore with various quantities of segregated boron have been tested. Their fracture surfaces have also been observed in SEM. The relation between the level of boron segregation and ductility has been inspected.     

Low cycle fatigue behavior of polycrystalline Ni3Al alloys at ambient and elevated temperatures

The low cycle fatigue (LCF) resistance of polycrystalline Ni₃Al has been evaluated at ambient, intermediate (300 °C), and elevated (600 °C) temperatures using strain rates of 10⁻²/s and 10⁻⁴/s. Testing was conducted on a binary and a Cr-containing alloy of similar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and intermediate temperatures, the cyclic constitutive response of both alloys was similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually softened for the remainder of the life. Initial hardening resulted from the accumulation of dislocation debris within the deformed microstructure, whereas softening was related to localized disordering. For these experimental conditions, crack initiation resulted within persistent slip bands (PSBs). At the elevated temperature, diffusion-assisted deformation resulted in a rate-dependent constitutive response and crack-initiation characteristics. At the high strain rate (10⁻²/s), continuous cyclic hardening resulted from the accumulation of dislocation debris. At the low strain rate (10⁻⁴/s), the diffusion of dislocation debris to grain boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constitutive response on the driving force for environmental embrittlement. Chromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven. Formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA     

Fatigue Crack initiation and microcrack growth in 2024-T4 and 2124-T4 aluminum alloys

A metallograph was mounted directly on a closed-loop electrohydraulic testing unit and initiation of fatigue cracks was directly observed on polished notches at magnifications up to 800 times in aluminum alloys 2024 and 2124 in the T-4 condition. The latter is a high purity version of 2024 and contains considerably fewer constituent particles. At high stresses on the notch surface the fatigue cracks initiated on coarse slip lines in both alloys. At low stresses almost all of the cracks in 2024 initiated in the matrix adjacent to constituent particles. In 2124 at low stresses 50 pct of the cracks initiated near constituent particles and 50 pct in the matrix not near constituent particles. The probability that a constituent particle in 2024 initiates a fatigue crack falls off very rapidly as the particle size decreases below 6 μm. Growth of microcracks is impeded by grain boundaries. This research was done at Northwestern University.     

Microstructure of laser treated Al alloys

In this study the microstructures of laser treated ultra pure Al and two AlSi alloys (Al0.4 Si and Al0.75 Si) were investigated. In ultra pure Al a large number of dislocation loops were found especially at higher laser scan velocities. During annealing only at laser scan velocities above 2 cm/s a large quantity of dislocation loops became visible. Both results indicate that at high laser velocities vacancies are frozen in, but at laser velocities around 1 cm/s there is still enough time at high temperature to reduce the vacancy concentration towards lower super-saturation. In AlSi alloys the dislocation density rises with higher laser scan velocities probably caused by the smaller distances between the eutectic cell walls. In these alloys entangled dislocation structures were found in contrast to ultra pure Al. For solidification structures consisting of an eutectic material with a high hardness as for an eutectic structure in AlSi alloys it was found that the hardness can be described by a pile up mechanism, which depends on the difficulty to exert stresses on neighbouring cells due to thick and hard walls. The hardness has been described by a 1/d² dependence, i.e. it is mainly determined by the small size d, of the solidification structure.