Under-aged Alloy Research Articles

Effects of Sc and Zr microalloying additions on the microstructure and mechanical properties of new Al–Zn–Mg alloys

Effects of Sc and Zr microalloying additions on the microstructure and mechanical properties of new Al–Zn–Mg alloys alloyed with a small amount of copper were investigated comparatively by tensile tests and microscopy methods. Compared with the strength of peak-aged Al–Zn–Mg alloy, the yield strength increased by 66MPa after adding 0.10wt.% Sc and 0.10wt.% Zr, and improved by 96MPa after adding 0.25wt.% Sc and 0.10wt.% Zr, respectively. Introduction of 0.10wt.% Zr to Al–Zn–Mg alloy, the grain refinement effect of scandium did not occur by adding only 0.10wt.% Sc, but appeared with an increase in scandium content to 0.25wt.%. In the presence of 0.10wt.% Zr, antirecrystallized effect of scandium appeared at 0.10wt.% scandium concentration, and enhanced with an increase in the content of scandium. Main aging precipitates were two kinds of GP zones in under-aged alloys, and η′ phases in peak-aged alloys, respectively. Small additions of Sc and Zr did not retard or suppress the formation of aging precipitates in Al–Zn–Mg–Sc–Zr alloys. The strengthening effect from aging precipitates was much stronger than from Sc and Zr microalloying additions in Al–Zn–Mg–Sc–Zr alloys. In addition, the most important strengthening mechanisms of Sc and Zr microalloying in aged Al–Zn–Mg alloys was Orowan strengthening of secondary Al3(Sc, Zr) particles.

Al-Cu-Mg-Ag耐热铝合金高温蠕变行为

The creep behaviors of heat-resistant alloy Al-5.3Cu-0.8Mg-0.5Ag-0.3Mn-0.15Zr (Al-Cu-Mg-Ag) were studied in a temperature range of 100—210℃and a load range of 150—300 MPa, and the effect of aging on the properties of Al-Cu-Mg-Ag alloy at elevated temperatures was also investigated. The results show that the steady creep rate of the under-aged alloy is much lower than that of the peak-aged alloy at the same creep conditions. The creep fracture time is 75 h for the under-aged Al-Cu-Mg-Ag alloy at 210℃/200 MPa, while that for the peak-aged alloy is only 21 h. The precipitates grow gradually during the creep process and the growth rate in the peak-aged alloy is higher than that in the under-aged alloy. Dynamic precipitation happened in the under-aged alloy. The fine precipitates inhibite the motion of the dislocations during the creep, which leads to lower creep deformation rate compared to the peak-aged alloy. The steady creep rates of the under-aged Al-Cu-Mg -Ag alloy at 100—150℃keep a relatively lower level, but increases to a high level when the creep temperature increased to 180℃. The steady creep rate increased with increasing temperature or stress, which can be described by a constitutive equation with an apparent activation energy of 102 kJ/mol.

Microstructure and Mechanical Properties of Al-Cu-Mg-Ag Alloy during Thermal Exposed at Elevated Temperature

The thermal stability of Al-Cu-Mg-Ag alloys under-aged and peak-aged was analyzed by tensile testing and transmission electron microscopy (TEM). And the microstructural evolution and mechanical properties of Al-Cu-Mg-Ag alloy during thermal exposed at elevated temperature were also studied. The results show that the under-aged Al-Cu-Mg-Ag alloy possessed better thermal stability compared with the peak-aged alloy when exposed at 150 . The tensile strength of the peak-aged alloy decreased with prolonging the holding time. While that of the under-aged alloy increased and then decreased with a peak of 524 MPa after exposed for 20 h, which was 19 MPa higher than that of the peak-aged alloy. Meanwhile the precipitations were distributed dispersively and amount of fine phases were observed. The tensile strength of the under-aged alloy was 434 MPa after exposed for 1000 h at 150 . When exposed at 200 , the tensile strength of the under-aged sample decreased and the prolongation increased with prolonging the thermal exposure time. The tensile strength of the sample decreased dramatically when exposed at 250 and 300 and the strengthening phases coarsened dramatically and evolved to the equilibrium phases . The width of the precipitation free zone (PFZ) increased with increasing the thermal exposure temperature and the distribution of the precipitations on the boundaries became discontinuous.

Microstructure and properties of Al-Cu-Mg-Ag alloy exposed at 200 °C with and without stress

The effect of stress on the microstructure and properties of an Al-Cu-Mg-Ag alloy under-aged at 165 °C for 2 h during thermal exposure at 200 °C was investigated. The tensile experimental results show that the remained tensile strength of both specimens at room temperature after being exposed at 200 °C with and without applying stress rises firstly, and then drops with the increasing of exposure time. The peak value of the remained strength reaches 439 MPa for non-stress-exposure for 10 h, and 454 MPa after being exposed with stress loaded for 20 h at 220 MPa. The elongation change is similar to that of strength. After being exposed for 100 h, specimen exposed at 220 MPa still remains a tensile strength of 401 MPa, larger than that exposed without applying stress. TEM shows that the microstructure of under-aged alloy is dominated by Ω phase mainly and a little ϑ′ phase. The ϑ′ and Ω phases are believed competitive with increasing exposure time. The width of precipitation free zone(PFZ) increases and the granular second phase precipitates at grain-boundary correspondingly. It is shown that the mechanical properties of alloy decrease slightly and present good thermal stability after thermal exposure at 200 °C and 220 MPa for 100 h.

Temperature-Dependent Shear Strain Localization of Aluminium-Lithium Alloy in Uniaxial Compression Using Zerilli-Armstrong and Gradient Plasticity Models

Gradient-dependent plasticity where a characteristic length is involved to consider the microstructural effect (interactions and interplaying among microstructures due to the heterogeneous texture) is introduced into Zerilli-Armstrong model based on the framework of thermally activated dislocation motion. Effect of initial temperature on the distributions of plastic shear strain and deformation in adiabatic shear band (ASB), the axial compressive stress-axial compressive strain curve, the shear stress-average plastic shear strain in ASB curve and the plastic shear strain corresponding to the occurrence of shear strain localization is investigated. The axial deformation within aluminum-lithium alloy specimen in uniaxial compression in strain-hardening stage is considered to be uniform. Beyond the peak compressive stress, a single ASB with a certain thickness determined by internal length is formed and intersects the specimen. The axial plastic deformation is decomposed into uniform deformation and localized deformation due to the shear slip along ASB. Lower temperature leads to earlier occurrence of shear strain localization, i.e., lower critical plastic compressive strain, steeper post-peak shear stress-average plastic shear strain in ASB curve, higher peak shear stress and more apparent shear strain localization. The calculated distributions of plastic shear strain and deformation in ASB are highly nonuniform due to the microstructural effect, as cannot be predicted by classical elastoplastic theory applicable to completely homogenous material. The predicted average plastic shear strains in ASB for different widths of ASB agree with the measured values for under-aged Al-Li alloy at 298K and at strain rate of approximately 103s-1.

Precipitate evolution in underaged Al–Mg–Si alloy during thermal cycling between 25 °C and 65 °C

The evolution of metastable precipitates and the aging response in underaged Al–Mg–Si alloy during environmental temperature cycling was investigated using transmission electron microscopy (TEM) and hardness tests. After the alloy underwent thermal cycling between 25 °C and 65 °C, the hardness tests revealed that hardness decreased slightly, rather than following a concave downward curve, with the cycle times. Needle-shaped G.P. zones transformed during the environmental thermal cycling. The fraction of the zones declined sharply from almost 100% to only approximately 10% after 90 cycles, accompanied by an increase in the fraction of lath-shaped precipitates and the formation of β″ precipitates in the matrix. The precipitate developed with the 25–65 °C cycling time as follows: needle-shaped G.P. zones ⇒ lath-shaped ppt + β″ ppt + needle-shaped G.P. zones ⇒ lath-shaped ppt + β″ ppt + rod-shaped ppt + needle-shaped G.P. zones. Therefore, three or four precipitates coexisted in the underaged alloy following prolonged cycling. The formation of a limited number of β″ precipitates and the presence of a rod-shaped phase in the alloy during environmental temperature cycling reduced the hardness as the cycle time increases.

Deformation and fracture behavior of an Al–Li alloy 8090

Deformation and fracture behavior of an Al–Li alloy 8090

Determination of Hardness of Pre-Aged AA 6063 Aluminum Alloy by Means of Artificial Neural Networks Method

A lot of experiments must be conducted in order to find an appropriate technology for the calculation of strength of the materials, which wastes both man power and money. For this reason artificial neural networks (ANNs) have been used to search the optimum technology proper for pre-aged AA 6063 aluminum alloy. In this study, ANNs were used to compare experimental results and test data were used for teaching of the ANNs. This paper examines the changes in the hardness of AA 6063 alloys when heat treated at different pre-aging treatments. The alloy was solution treated for 1 hour at 525±3 °C and quenched in water. After quenching, samples were subjected to five different pre aging times, 2 hours, I day, 3 days, 7 days. On the other hand, some specimens were not pre-aged. Artificial age temperatures were selected as 160 °C and 180 °C. The hardness values of these under-aged alloys were measured. When the pre-aging time was 7 days, the hardness values of the specimens increased. An excellent correlation was found between experimental hardness results and ANNs hardness results.

Al-5.6mass%Cu-4.0mass%Si合金時効材の被削性に及ぼす切削油剤の効果(生産加工)

The effect of the cutting fluids on the machinability of Al-Cu-Si alloys which is candidate of the free cutting aluminum alloy without lead have been studied. The turning test was carried out using the cutting tool with the chip breaker. It was found that surface roughness after machining of the alloys became roughened, while the generation of the adhesion on the cutting tool was increased either in the dry or the wet cutting conditions. In the dry cutting, the chip breaking was better in the over-aged alloys where the adhesion was easily generated. On the other hand, the chip breaking in the wet cutting was better in the under-aged alloys where the adhesion was hard to generate. These results can be explained in terms of generation of the adhesion on the cutting tools; in the dry cutting, the chip was greatly bent by the existence of the adhesion on the cutting edge of the cutting tool though, in the wet cutting, the chip breaker acted effectively by the adhesion was not easily generated.

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R=0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in Δ K th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.

Influence of Aging Treatment on Chip Breaking and Surface Roughness during Turning in Al-5.6 mass%Cu-4.0 mass%Si Alloys.

The turning test was carried out under the dry cutting condition to evaluate the machinability of aluminum alloys (Al-5.6mass%Cu-4.0mass%Si) which aged up to several stages, as a candidate of the free-cutting aluminum alloy without lead. The chip breaking during turning were low in the under-aged alloys, but that were increased in the over-aged alloys. The chip breaking of the most over-aged alloys was equal to that of the free-cutting aluminum alloy with lead. The changing of the chip breaking by aging treatment for the alloys was closely related to generation of the adhesion on the cutting tool. Although the increase of the adhesion caused increasing the chip breaking, that also caused roughing of machined surface.

Fatigue behavior of A356/357 aluminum cast alloys. Part II – Effect of microstructural constituents

A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (SDAS), eutectic structure, matrix strength, and fatigue behavior of two Al–7% Si–Mg casting alloys with magnesium contents of 0.4% and 0.7%, respectively, has been conducted. In the absence of casting defects, the influence of microstructural features on the fatigue performance becomes more pronounced. The degree and rate of microdamage (microcracking) is strongly affected by the strength of the matrix, and especially by the eutectic particle size, morphology, and distribution (clustering). A soft matrix (under-aged alloys) will generate more local microdamage compared to a peak-aged one. Large and elongated eutectic particles present in unmodified alloys result in lower fatigue lives. A decrease of fatigue life with increasing Mg and Fe content is observed, mainly due to the increased sizes of Fe-rich intermetallic particles. Microstructures with similar eutectic particle size and morphology as the Sr-modified ones, show a minimum fatigue life at intermediate SDAS values (∼60 μm), which is related to the continuity of particles on the dendrite cell boundaries. For coarser microstructures ( SDAS>60 μ m), the increase in fatigue life is attributed to the reduced damage rate along the cell boundaries.

The microstructure and environment influence on fatigue crack growth in 7049 aluminum alloy at different load ratios

The role of microstructure and environmental influence on fatigue crack has been investigated on a high strength 7049 aluminium alloy, artificially aged to underaged (UA) and overaged (OA) microstructures with approximately the same yield strength properties, but differing in the mode of slip deformation. The UA alloy deforms by planar slip while the OA alloy by wavy slip. Crack growth measurements were performed at constant load ratios, between 0.8 and −1, in ambient air and vacuum. The influence of load ratio is discussed in terms of slip deformation mechanisms, microstructure, and the influence of environment using the two intrinsic parameters, Δ K and K max.

Influence of aging parameters on the mechanical properties of 6063 aluminium alloy

The 6063 aluminium alloy were given various heat treatments at under-aged, peak-aged and over-aged temperatures. The effect of precipitation on the tensile strength, yield strength, hardness, ductility and number of cycles required to fail the alloy at constant stress was investigated. The variation in time and temperature have improved the mechanical properties of the Al-alloy, whereas the ductility has decreased. The experimental work has revealed that time and temperature play a very important role in the precipitation hardening process of the Al-alloy. The initial increase in the tensile strength, yield strength, hardness and fatigue is due to vacancies assisted diffusion mechanism and formation of high volume fraction of guinier preston (GP) zones, which disturbs the regularity in the lattices. In over-aging of the alloy, the size of the individual particle increases, but the number of particles decreases. This causes few obstacles to the movement of dislocations, therefore, the mechanical properties decreases. The scanning electron microscope (SEM) study of the under-aged alloy have exhibited facet fatigue fracture surface, whereas the peak-aged and over-aged alloy show a mixed mode of fracture, i.e. facet fracture with striation and also intergranular fracture.

Influence of microstructure and load ratio on fatigue threshold behavior in 7075 aluminum alloy

The influence of microstructure and load ratio on fatigue crack propagation in the near-threshold regime has been investigated on the 7075-aluminum alloy. The alloy heat treatment was varied to give underaged and overaged microstructures with approximately the same yield strength properties. Crack growth measurements were performed at constant load ratios between R=−1 and +0.8 in ambient air. The influence of load ratio is discussed in terms of deformation slip, crack branching and fracture mode. Vacuum data from the literature are compared with the current air results. For both environmental conditions, it was observed that the crack growth and threshold properties of the underaged alloy were better than the overaged alloy. The overall fatigue behavior is discussed in terms of the two parameter concept namely Δ K and K max.

STRESS CORROSION CRACKING OF THE ALZN5MG1 ALLOY

The results of stress corrosion and corrosion tests of the AlZn5Mg1 alloy of different tempers are shown to be related to test solution composition. The most susceptible to stress corrosion cracking was the underaged alloy tested under constant load in HC1- containing solutions. The comparison of stress corrosion and corrosion tests shows that the cracking may be considered in terms of hydrogen-related crack propagation. The remarkable degradation of the underaged alloy is assumed to relate to the grain boundary microstructure.

The effect of aging on the hydrogen-assisted fatigue cracking of a precipitation-hardened Al-Li-Zr alloy

The corrosion fatigue behavior of an Al-2.5 pct Li-0.12 pct Zr alloy has been studied as a function of heat treatment by performing smooth specimen fatigue life experiments on differently aged alloys in dry air and humid nitrogen. Results indicated that aging decreased the fatigue life of the Al-Li-Zr alloy in dry air. However, exposure to water vapor reduced the fatigue resistance of the underaged (UA) alloys but increased the fatigue life of the overaged alloys (OA) alloys. Hydrogen precharging experiments (either exposure to moist air or cathodic charging in HC1 solutions) followed by fatigue testing in dry air confirmed that the UA alloys were susceptible to hydrogen embrittlement and that the OA alloys were insensitive to a hydrogen effect. The experimental results suggest that the susceptibility of the Al-Li-Zr alloy to hydrogen-assisted fracture is essentially related to the effectiveness of hydrogen transport to the region ahead of the crack tip, which is controlled by the microstructure of the alloy. Environmental and aging effects which influence the fatigue characteristics of the studied alloy are discussed.

Corrosion fatigue of a precipitation-hardened Al-Li-Zr alloy in a 0.5 M sodium chloride solution

Corrosion fatigue (CF) experiments have been performed on a high-purity Al-2.5Li-0.12Zr alloy in a deaerated 0.5 M sodium chloride solution as a function of aging time. The results of these tests were compared to the results of fatigue tests performed in dry air to investigate the effect of aging on the CF susceptibility of the alloy. It was found that the high cycle fatigue strength of the alloy was dramatically reduced by the aqueous environment. Examinations of the relative fatigue strength (ΔσNaCl/Δσair) indicated that the underaged (UA) alloys were more susceptible to CF than the overaged (OA) alloys over the stress ranges studied, but the difference of the susceptibility between the UA and the OA alloys was reduced by decreasing the applied cyclic stress. The evidence suggests that, for the UA Al-Li-Zr alloys, the CF resistance is determined by both slip-enhanced dissolution and hydrogen embrittlement at high stress ranges, while at low stress ranges, the CF life is predominantly controlled by pitting-induced crack initiation regardless of the aging condition of the alloy.

Ａｌ‐Ｌｉ系合金の腐食挙動と応力腐食割れ性

Three type Al-Li alloys (8090, 8091 and 2090) sheets were aged at 443K in various temper conditions, and evaluated for corrosion behavior and resistance to stress corrosion cracking (SCC) initiation in sodium chloride solution. The underaged alloys were susceptible to intersubgranular corrosion, apt to lead localized corrosion pits. The peak-aged alloys suffered from dimple type pitting, on alloy 8090 and alloy 8091 preferentially along subgrain and grain boundaries, and on alloy 2090 within grains also. The resistance to SCC initiation was in agreement with that to localized corrosion owing to intersubgranular corrosion, improved with increasing aging time, and ranked in the order of alloy 8090>alloy 2090>alloy 8091.

Effects of microstructure of the behavior of an aluminum alloy and an aluminum matrix composite tested under low levels of superimposed hydrostatic pressure

Experiments have been conducted on an aluminum alloy and an aluminum matrix composite tested in tension under the influence of superimposed hydrostatic pressure. Monolithic alloys heat-treated to underaged (UA) and overaged (OA) conditions exhibited significant differences in their responses to the superimposition of hydrostatic pressure during tension testing. Significant increases in ductility were obtained with moderate increases in confining pressure for the OA alloy, while the UA alloy exhibited little effect of pressure. In contrast, significant increases in ductility were obtained for the composites, regardless of the matrix aging condition. The effects of pressure on frature are determined in light of the micromechanisms of fracture in these materials.