Underaged Research Articles

The effect of heat treatment on the shock induced mechanical properties of the aluminium alloy, 7017

Abstract The yield and tensile strengths of under-aged (UA) and peak-aged (PA) aluminium 7017 have been measured at quasi-static and shock induced strain-rates, showing that PA has higher yield strengths whilst tensile strengths are similar in both states. Microstructural evidence shows failure is controlled by crack initiation at grain boundary inclusions.

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.

Hydrogen-assisted deformation and fracture behaviors of Al 8090

In the present study, the hydrogen embrittlement (HE) and the hydrogen-assisted fracture (HAF) behaviors of electrochemically hydrogen-charged Al 8090 with different specimen orientations and aging practices were examined using smooth bar and single-edge notch (SEN) specimens. It was found that the cathodic hydrogen charging substantially reduced the tensile ductility and the resistance to fracture of Al 8090. The susceptibility to HE and HAF of Al 8090 was strongly dependent on specimen orientations and aging practices. Hydrogen attack was the most significant along the grain boundaries, and, consequently, T-S oriented SEN and T-oriented smooth-bar specimens showed the highest susceptibility among the orientations studied. The susceptibility to HE and HAF decreased from underaging (UA) temper to overaging (OA) temper. It is speculated that the formation and development of a precipitate-free zone (PFZ) along the grain boundaries, rather than the change in slip planarity with prolonged aging is responsible for the reduced susceptibility to HE and HAF. Further studies are, however, required to confirm this notion.

The effects of test temperature, temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated.

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 µm compared to 10 µm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 µm. These grain refinements contributed to strengthening of the control alloy and the composite.

Fracture behaviour of an Al–Mg–Si industrial alloy

The tensile fracture behaviour of an Al–0.89 wt.% Mg 2Si industrial alloy is studied. It is found that the alloy deforms in a highly localised way only in the underaged (UA) state. In the peak aged (PA) condition the hardening precipitates are often by-passed by an Orowan mechanism, which predominates in the overaged (OA) state. In the UA condition, the stress concentration is shown to be large enough to nucleate cracks at the particle interfaces provided the shear strain has reached the critical value for cutting the precipitates into pieces. In the PA and OA conditions, failure results from coalescence of voids nucleated at large AlFeSi and smaller (Mg+Si) particles. It is shown that none of these particles are responsible alone for fracture. In contrast, ductility and rupture surfaces are consistent with a damage mechanism whose nucleation step is the coalescence of voids nucleated at large AlFeSi and neighbouring (Mg+Si) particles.

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.

Some observations on cyclic deformation structures in the high-strength commercial aluminum alloy AA 7150

Load-controlled fatigue testing of the aluminum alloy AA 7150 has been conducted using four-point bending with an R ratio of +0.1 over a range of maximum stress levels from 60 to 120 pct of the 0.2 pct proof stress. The alloy, in the form of 12.5-mm rolled plate, was investigated in underaged (UA), peak-aged (PA), and overaged (OA) conditions, corresponding to a change in average precipitate sizes from 5 nm in the UA condition to 21 nm in the OA condition. Three orientations of the plate were investigated. Orientation and aging condition influenced the degree of surface topographical development but not fatigue life. Detailed transmission electron microscopy (TEM) of the fatigued surface indicated that deformation in all aging conditions occurred by planar slip. Slip was generally restricted to a single slip system within each grain, and subgrain boundaries offered little resistance to dislocation movement facilitating long slip line lengths (measured up to 310 µm) between adjacent high-angle grain boundaries. Planar slip observed in the OA condition is attributed to shearing of large strengthening precipitates, which is promoted by long slip line lengths. No evidence of surface specific changes in slip character was observed.

Optimization of the strength-fracture toughness relation in particulate-reinforced aluminum composites via control of the matrix microstructure

The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulate-reinforced aluminum alloy 6092-matrix composite has been studied as a function of postfabrication processing and heat treatment. It is demonstrated that, by the control of particulate distribution, matrix grain, and substructure and of the matrix precipitate state, the strength-toughness combination in the composite can be optimized over a wide range of properties, without resorting to unstable, underaged (UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness than that displayed in the peak-aged condition. Further, it is demonstrated that, following an appropriate combination of thermomechanical processing and unconventional heat treatment, the composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced alloy. In the high- and low-strength matrix microstructural conditions, the matrix grain and substructure were found to play a substantial role in determining fracture properties. However, in the intermediate-strength regime, properties appeared to be optimizable by the utilization of heat treatments only. These observations are rationalized on the basis of current understanding of the grain size dependence of fracture toughness and the detailed microstructural features resulting from thermomechanical treatments.

Hydrogen Effects on the Mechanical Properties of Al-Li 2090 Alloy in an Acid Environment

Although aluminum-lithium alloys are attractive materials for aeronautic and aerospace applications because of their lower density and higher elastic modulus compared to conventional aluminum alloys, their bane is their poor ductility and susceptibility to environment induced cracking (EIC). EIC of Al-Li alloys has been fairly well researched and the role of hydrogen in causing this phenomenon has been proposed. Of the various mechanisms proposed to explain the phenomenon of hydrogen embrittlement (HE) in Al-Li alloys, the formation of brittle hydrides, LiH and LiAlH{sub 4} has been indicated as the cause of embrittlement. This mechanism has been accepted widely, both for precharged specimens and in stress-corrosion cracking. Literature is not consistent with regard to the phenomenon of HE in Al-Li alloys, believed to be due to the widely varying experimental conditions used by each group of investigators. There is also no general agreement on the effect of aging temper on HE in these alloys. Some investigators note that the HE resistance is poor for the overaged (OA) condition while others claim the underaged (UA) temper as having poor resistance to HE. Some others have also reported that the peak aged (PA) temper is most resistant to HE. Most studies of HEmore » for the Al-Li alloy system has been carried out in a NaOH environment. The main objective of this study is to obtain an understanding of the effect of hydrogen on the mechanical properties of a 2090 commercial alloy using electrolytic charging conditions in an acid environment for different tempers.« less

Effects of Heat Treatment and Reinforcement Size on Reinforcement Fracture during Tension Testing of a SiCp Discontinuously Reinforced Aluminum Alloy

The effects of heat-treatment, matrix microstructure, and reinforcement size on the evolution of damage, in the form of SiCp cracking, during uniaxial tension testing of an aluminum-alloy based composite have been determined. A powder metallurgy Al-Zn-Mg-Cu alloy reinforced with 15 vol pct of either 5 or 13 μm average size SiCp was heat treated to solution annealed (SA), underaged (UA), and overaged (OA) conditions. The SA treatment exhibited lower yield strength and higher ductility for both particulate sizes compared to the UA and OA conditions. The evolution of damage, in the form of SiCp fracture, was monitored quantitatively using metallography and changes in modulus on sequentially strained specimens. It is shown that the evolution of SiCp fracture is very dependent on particulate size, matrix aging condition, and the details of the matrix-reinforcement interfacial regions. SiCp fracture was exhibited by the UA and OA treatment over a range of strains, while a preference for failure near the SiCp/matrix interfaces and in the matrix was exhibited in the OA material. While thepercentage of cracked SiCp at each global strain typically was equal or somewhat lower in the material reinforced with 5 μm average size SiCp, theabsolute number of cracked SiCp was always higher at each global stress and strain in the material containing 5 μm average size SiCp, for each heat treatment. Damage(e.g., voids) in the matrix and near the SiCp/matrix interfaces was additionally observed, although its extent was highly matrix and particle-size dependent. It was always observed that increases in stress (and strain) produced a larger amount of fractured SiCp. However, neither a global stress-based nor a global strain-based model was sufficient in converging the amount of SiCp fractured for all heat treatments and particle sizes tested.

The effects of superimposed hydrostatic pressure on deformation and fracture: Part II. Particulate-reinforced 6061 composites

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 + 15 pct Al203 composites heat-treated to underaged (UA) and overaged (OA) conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected, while the ductility significantly increased with each increment in pressure. At 300 MPa pressure, the monolithic 6061 and 6061 composite exhibited nearly identical ductility. It is shown that the levels of pressure chosen significantly inhibit void growth and coalescence in the composite. Void nucleation in the composites occurredvia the fracture of the reinforcement, followed by void growth and coalescence in the matrix. Tests conducted with 500 MPa pressure additionally provided evidence for suppression of void nucleation. Neither the ductility nor the pressure response was significantly affected by the heat treatments chosen.

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.

Influence of precipitate microstructure on flow and forming properties of an aluminum alloy sheet

A 7075 aluminium alloy sheet was cold rolled, solution heat treated and artificially aged to underaged (UA) and overaged (OA) conditions. A careful fabrication and heat treatment procedure was followed in order to obtain UA and OA sheets having the same isotropic crystallographic texture and uniaxial yield strength. Hydraulic bulge tests, plane strain, and uniaxial tension tests were performed to characterize the mechanical behavior of these materials. The forming limit diagram (FLD) was also obtained for these two conditions. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the precipitate microstructure, slip line patterns and fracture surfaces. Experimental observations showed that both macroscopic and microscopic responses of the materials to plastic deformation was strongly affected by the precipitate microstructure. FLDs were theoretically computed with the Marciniak and Kuczynski (MK) analysis, using the yield locus calculated with the Taylor/Bishop and Hill (TBH) theory for polycrystalline plasticity and the stress-strain curve corresponding to both UA and OA conditions. The predicted results were in good agreement with experiments for sheet in the OA condition, and they were not for the material in the UA condition. This was explained in terms of yield surface shape, consistent with the slip line characteristics, though differences in fracture behavior may contribute to these observations. The yield surface shape effect was emphasized since it provides, besides fracture, an alternate interpretation of failure mechanism near balanced biaxial tension, and since it can be described fairly well with a simple phenomenological description of plastic behavior.

Crack initiation and growth toughness of an aluminum metal-matrix composite

The effects of systematic changes in matrix microstructure on crack initiation and growth toughnesses were determined on an AlZnMgCu alloy containing 0, 15, 20% by volume of SiC particulates. Materials were heat treated to underaged (UA) and overaged (OA) conditions of equivalent matrix microhardness and flow stress. Although both the fracture initiation and growth toughnesses, as measured by J Ic and tearing modulus, were similar for the unreinforced materials in the UA and OA conditions, significant effects of microstructure on both J Ic and tearing modulus were observed in the composites. SEM and TEM observations of fracture paths in the two conditions are utilized to rationalize these observations in light of existing theories of ductile fracture propagation.

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.

Effects of aging condition on the fracture toughness of 2XXX and 7XXX series aluminum alloy composites

Results are presented on the effects of matrix aging condition (i.e., matrix temper) on the fracture toughness of 2XXX and 7XXX Al matrix alloys reinforced with SiC particulates, and the results are compared with the mechanical behavior. Fracture toughness testing was conducted on fatigue precracked bend specimens, and fracture surfaces were examined using SEM. Results revealed dramatic differences in the effect of matrix microstructure on the fracture properties of the two composite series. In the 7XXX material, the toughness values decreased from the underaged (UA) condition to the overaged (OA) condition by approximately 40 percent, while in the 2XXX series composite, the effect of matrix microstructure was marginal. In the 7XXX series composites, a transition in fracture mode from particle cracking (in UA) to matrix and linear-interface failure (in OA) was observed, while the 2XXX series composite failed predominantly by particle cracking.