Under-aged Alloy Research Articles

Precipitate-mediated fatigue crack propagation behavior of Al–Zn–Mg–Cu alloy tailored by non-isothermal aging

The fatigue crack propagation behavior of non-isothermally aged Al–Zn–Mg–Cu alloy was examined at different aging stages. It has been revealed that non-isothermal aging (NIA) improved the fatigue crack resistance by adjusting the thermodynamics, and thus modified the precipitation configuration. Enhanced crack propagation threshold and decreased crack propagation rate were achieved when NIA was applied and carried out thoroughly. Fine and dispersed GP zones existing in underaged alloy significantly intensified the slip localization, which effectively mitigated the stress concentration near the crack tip and showed an approximately 8.2 MPa·m1/2 crack propagation threshold. Smaller width of the precipitate-free zone enabled the slip bands to propagate through the grain boundaries. However, relatively low strength and intense slip localization led the fatigue crack to propagate along the crystallographic slip bands, responsible for the higher crack propagation rate. For minorly over-aged alloys, coarsening and elevated number density of intragranular precipitate effectively hindered the movement of dislocation around, where strain concentration was frequently and preferentially induced, causing the degradation of resistance to fatigue crack propagation.

On the unusual R-ratio dependence of the high cycle fatigue performance of precipitate strengthened Al alloys

The high cycle fatigue (HCF) performance of precipitate strengthened Al alloys is important for their use in moving structures applications such as transportation. It was first shown in the 1950′s, and has been confirmed multiple times since, that sometimes the weaker underaged alloys can show longer high cycle fatigue lives under R = -1 loading than the stronger peak aged materials. However, under alternative R-ratio loadings (e.g. R = 0.1), the underaged alloys tend to show the same, or even worse, HCF performance as the peak aged alloys. This unusual R-ratio dependence is studied in this work using a combination of fatigue testing on smooth samples, optical profilometry of the evolution of surface roughness during cycling, and transmission electron microscopy.We demonstrate that the unusual R-ratio effect is really an effect of the maximum applied stress, and the critical role that this has in determining the strain localization that occurs during the first cycle of loading. This initial strain localization sets the path for subsequent plasticity accumulation which eventually leads to fatigue failure. The maximum applied stress plays a stronger role in determining the initial strain localization than either the applied stress amplitude or R-ratio and this is the critical factor determining the fatigue life of the precipitate strengthened Al alloys considered in this study.

On the low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing

The low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing (NIA) was examined at different strain amplitudes. We showed that NIA improved the low cycle fatigue life (more than 7000 cycles) by optimising the precipitate configuration within 5.5 h while maintaining comparable mechanical properties (570 MPa for tensile strength) and conductivity (nearly 39% IACS) to conventional isothermal ageing, simultaneously. Experimental observation combined with molecular dynamic simulation revealed that precipitation configuration manipulated by NIA had a crucial effect on fatigue resistance. A great number of repeatedly sheared and locally destructed GP zones enhanced co-planar slip and slip localisation in the under-aged alloy during the early stage of NIA, responsible for the dramatic displacement steps on the surface and resultant poor fatigue performance. As the NIA further proceeded, moderately coarsened precipitates with an average dimension of 6.0 nm and elevated number density effectively impeded the dislocation movement and weaken the slip localisation to a great extent, improving the fatigue performance within a few hours.

Effect of small Sn addition on the initial strengthening and microstructural evolution of an Al-Cu-Mg-Ag alloy

The influence of small Sn addition on the mechanical properties and microstructure of an Al-Cu-Mg-Ag alloy is systematically investigated in this work. The age-hardening level of the material at 180 ℃, as well as the strength properties during exposing at 200 ℃, is found to decrease with small Sn addition. With Sn, the morphology of the tensile fracture surface gradually evolves from a transgranular mode to an intergranular one during the prolonged exposing at 200 ℃. Quantitative transmission electron microscopy (TEM) analysis proposes the negative role of Sn on the dense precipitation of Ω phase by accelerating the θ' formation, thereby leading to a loss of strength properties and the lower thermal stability of Sn-added alloy. This detrimental effect by Sn, as revealed by atom probe tomography (APT), is primarily ascribed to the suppressed Mg-Ag co-clustering, which reduces the nucleation rate of Ω phase. Our calculation also indicates that a reduction of the cluster hardening by Sn is mainly responsible for the observed loss in the strength of underaged alloy. The contributions of modulus hardening and order hardening to the cluster hardening are also quantified for each cluster. A similar recrystallized grain size in Sn-free and Sn-added alloys eliminates the grain refinement effect by Sn.

Dependence of Creep Properties on Aging Treatment in Al–Cu–Mg Alloy

The mechanical properties are strongly associated with the heat treatment applied in the Al–Cu–Mg alloys. The peak aging treatment is normally employed following solution treatment to achieve a peak aging hardening effect. However, herein, it is shown that the creep resistance is improved at under‐aged (UA) conditions, where the material is transiently weaker than that at peak condition. Under the creep condition of 185 °C and 150 MPa, the creep fracture time of the UA (4 h) alloy is the longest at 146.5 h, followed by 128 h of the peak‐aged alloy and 72 h of the over‐aged alloy. The stress exponents are about 4, which indicates that the creep deformation mechanism of the alloy is mainly due to fracture caused by dislocation slip and grain boundary (GB) migration. Combined with the first‐principles calculation, the reason for the longer creep fracture time of the UA alloy is that the solutes Cu and Mg retained in the UA alloy interact with GBs and dislocations during creep.

Low cycle fatigue response of differently aged AA6063 alloy: Statistical analysis and microstructural evolution

Understanding the dynamic structural changes during cyclic deformation of Al-alloys is a discipline of great interest to material technologists. The primary objective of this report is to assess the low cycle fatigue (LCF) behaviour of AA6063 alloy in under-aged (UA), peak-aged (PA) and over-aged (OA) states and to understand the energetic aspects of cyclic plastic behaviour in the background of the nature and density of the dislocations. Two parameters Weibull distribution analyses, for the first time, establish that despite inferior monotonic strength, LCF life of UA alloy is twice compared to PA alloy at higher (≥0.5%) strain amplitudes. This corroborates well with the fact that the cumulative dissipated plastic strain energy density (CPSED) values exhibit reverse variation with applied strain amplitude in the case of UA alloy as compared to PA and OA alloys. TEM analyses confirm that strain-induced vacancy-driven dynamic precipitation causes dramatic cyclic hardening in UA alloy resulting in improved fatigue life. One can also evidence this from the fivefold increase in the plastic strain energy dissipation and 2-times increased of post-fatigue hardness. In contrast, fracture surface analyses and XRD-based dislocation density measurements coupled with TEM examinations reveal that the shearing or by-passing of precipitates by the dislocations in the PA or OA alloys, respectively, lead to strain localization and cyclic softening, and consequently, reduce the plastic strain energy dissipation and lower fatigue life. The study provides important guidelines for the development of high strength Al-alloys with improved fatigue performance.

Effects of Aging Treatment on Corrosion Behavior of a Tensile Deformed Al-Cu-Mn-Fe-Zr Alloy in 3.5% NaCl Solution.

In this paper, the effects of an aging treatment on the corrosion resistance/mechanism of a tensile deformed Al-Cu-Mn-Fe-Zr alloy are investigated. The impedance magnitude and polarization resistance increase, while the corrosion current decreases with the increased aging time and temperature. The discontinuously-distributed precipitates and precipitation-free zone, which can cut the corrosion channels, appear at grain boundaries when the temperature is relatively high and the aging time is relatively long. They can improve the corrosion resistance. Additionally, the intergranular and pitting corrosion are the main mechanisms. The intergranular corrosion is likely to occur in an under-aged alloy. This is because the potential difference between the grain boundaries and grains is high, due to the segregation of Cu atoms. When the aging degree is increased, the grain boundary precipitates reduce the potential difference, and the intragranular precipitates make the surrounding matrix prone to dissolution. As such, the pitting corrosion is likely to occur in the over-aged alloys.

Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy.

The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.

Commencement of Ordering in Al–0.69Mg–0.31Si Alloy

Early stages of ordering in dilute Al–0.69(at.%)Mg–0.31(at.%)Si alloy, aged at 373 K for 120 h and 7200 h and at 453 K for 20 h, have been studied using scanning transmission electron microscopy techniques. Starting from the solute enrichment in (110)-type parallel planes, merely after 120 h of aging, isothermal growth at 373 K has been observed to lead to the formation of the small-ordered region after 7200 h of aging. Scanning transmission electron microscopy images from the aforesaid three varieties of samples and the corresponding Fourier-transformed images have been the basis to suggest the possible path of evolution of β″ ordering in this dilute alloy. Based on this, the present study reveals the mechanism of formation of silicon pillar, which is the early stage of β″ that transforms to a fully developed β″. This corroborates with the concept of getting the final β″-strengthened product from an under-aged alloy by a suitable heat treatment.

Mechanical property and microstructure evolution of aged 6063 aluminum alloy under high strain rate deformation

The dynamic impact tests were performed on 6063 alloy with different initial aging conditions of under-aged (UA), peak-aged (PA) and over-aged (OA) at different strain rates ranging from 1 × 103 to 5 × 103 s−1 by split Hopkinson pressure bar. The results show that dynamic deformation behavior is affected by initial aging condition, as well as strain rate. The PA alloy exhibits the highest flow stress and strain rate sensitivity (SRS), while the UA alloy displays the lowest flow stress and SRS. The dislocation and precipitate evolution were analyzed by TEM observations. During high strain rate deformation, the density of the dislocation increases with strain rate, the GP zones in the UA alloy are almost dissolved into the matrix, the number density of the β′′ precipitates in the PA alloy decreases with increasing strain rate due to the dynamic dissolution. The impact deformation results in a great energy differences between precipitates and matrix, precipitates dissolution occurs in order to balance the disrupted thermodynamic equilibrium. The dynamic evolution of the precipitates has a great effect on the mechanical property of the impacted 6063 alloy, it is necessary to consider the microstructure evolution in material selection and structure design.

Dislocation dynamics simulations of precipitation-strengthened Ni- and Co-based superalloys

Two-dimensional dislocation dynamics (DD) simulations are carried out to simulate the increase in yield stress of nickel- and cobalt-based superalloys by order-strengthened γ’(L12)-precipitates, under conditions where precipitates are sheared and/or looped by single and super-dislocations. Parametric studies are performed for precipitate volume fractions between 5 and 70% and radii between 1 and 100 nm, resulting in yield stress increments due to a mixture of precipitate bypassing and shearing by single and super-dislocations of edge or screw character. DD yield stresses are compared to a variety of analytical models for strengthening by ordered precipitates showing that the closed-form solutions do not agree with DD predictions over all regimes of precipitate sizes and volume fractions. The DD yield stresses, converted to hardness values, are also compared to experimental hardnesses from archival literature on binary Ni-13Al (at. %) as well as Ni-10Al-8Cr(-2 W) (at.%) alloys aged at various temperatures and times, for which γ’(L12)-precipitate radii and volume fractions had been measured via atom-probe tomography and scanning electron microscopy. DD predictions are within 5–10% of experimental hardnesses in underaged alloys, but they typically overestimate hardness for peak and overaged samples. Finally, DD is used to predict the hardnesses for a γ’(L12)-strengthened Co-8.8Al-7.3 W (at. %) superalloy for 50–130 nm average precipitate radii within 10% of experimental values.

The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloy

The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloys were systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. In under-aged alloys, most of precipitates have a disordered structure, with a substructure of β″ (LDC) and Cu sub-unit cluster or C unit cell. After aging to peak strength, disordered precipitates including β″, QP1 and QP2 phases are formed. The disordered QP1 and QP2 phases, which contain the unit cells of Q′ and C phases, respectively, are the precursor phases of Q′ phase in these alloys. The β″ phase can transform into the disordered QP1 phase by incorporating Cu atoms, forming Cu sub-unit clusters and QP lattice. When the alloy is over-aged, the ordering and transformation of QP1 to Q′ occurs by the formation of Cu sub-unit clusters, the ordering of QP lattice, and the ordering of QC lattice. In contrast, the transformation of the disordered QP2 phase are rather sluggish. After sufficient aging, Q′, C and disordered QP2 transform into the Q phase. During the evolution of the precipitates in these alloys, a continuous incorporation of Mg, Si and Cu atoms and release of Al atoms occur. These findings provide new insights in understanding precipitation in Al-Mg-Si-Cu alloys.

Initiation of Intergranular Corrosion on Extruded AA6005 Aluminium Alloy with Low Copper Content

6000-series aluminum alloys are normally resistant to intergranular corrosion (IGC). However, underaged alloys, containing Cu as little as 0.1%, may be susceptible due to grain boundary sensitization by precipitation of Cu-rich nanofilm and depletion of Si in certain tempers [1]. The purpose of this work is to understand the initiation mechanism of IGC on as received and modified surface of extruded alloy AA6005. Methods for surface modification consisted of metallographic polishing, argon sputtering and alkaline etching. IGC initiation and testing was performed according to the standard BS-ISO 11846, consisting of immersion in acidified NaCl solution (pH 1) for different periods. Initiation of IGC was slower on as-received surface compared to the modified surface. Glow Discharge Optical Emission Spectroscopy (GD-OES) analysis indicated this was due to the presence of an approximately 50 nm thick mixed metal oxide layer. IGC initiated at the primary α-Al(Fe,Mn)Si precipitates for all types of surfaces. However, these phases corroded rapidly in the test solution causing formation of dealloyed Cu on the exposed particle surface, which act as increasingly effective external cathode for IGC. The AlMgSiCu (Q-phase) precipitates, present as primary and secondary particles, were relatively stable in the test solution both against anodic oxidation and as cathodic sites. In fact, the Q-phase particles appeared to act as physical barriers against IGC propagation. These results were in accordance with earlier preliminary results for the Ar-sputtered surface [2].

Influence of ageing on the low cycle fatigue behaviour of an Al–Mg–Si alloy

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–Si alloy at under-aged (UA), peak-aged (PA) and over-aged (OA) conditions has been examined to understand the micromechanism of fatigue and the associated dynamic structural changes in this alloy. The LCF behaviour of the differently aged AA6063 alloys has been studied at strain amplitudes ranging between 0.2 and 1.0% under strain control mode. The UA state exhibits pronounced cyclic hardening unlike the PA and the OA states at strain amplitudes greater than 0.4%. The PA and the OA states show hardening only for a few cycles followed by prolonged softening. Characterisations of the micro- and the sub-structural alterations due to LCF establish that the phenomenon of dynamic precipitation results in cyclic hardening the UA alloy. The softening of PA alloy occurs due to shearing of precipitates and that in the OA alloy takes place owing to reversibility of slip by the formation and annihilation of the Orowan loops around the β (Mg2Si) precipitates. Analyses of the hysteresis loops reveal Masing, nearly-Masing and non-Masing behaviour in the UA, OA and PA states, respectively. Analyses of the asymmetry factor of the hysteresis loops assist to infer that the Masing behaviour in the UA alloy is due to dislocation–dislocation interactions, whereas the nearly-Masing behaviour in the OA alloy and the non-Masing behaviour in the PA alloy are the consequence of varying degrees of dislocation–precipitate interactions associated with inhomogeneous deformation.

Effects of pre-treatments on precipitate microstructures and creep-rupture behavior of an Al–Zn–Mg–Cu alloy

Abstract

Influence of Dynamic Precipitation During Low Cycle Fatigue of Under-Aged AA6063 Alloy

The present study is aimed to understand the influence of dynamic precipitation on the low cycle fatigue (LCF) behavior of an under-aged (UA) AA6063 Al–Mg–Si alloy. This was accomplished by the estimation of plastic strain energy density (PSED) at varied isolated cycles during LCF of the UA alloy with subsequent comparison of these results with those of peak-aged (PA) and over-aged (OA) ones. The LCF tests of the UA alloy were carried out in the range of strain amplitudes of 0.2–1.0 % together with the evolution of hardness and tensile properties. The UA alloy shows Masing behavior, evaluated in terms of the variation of Bauschinger strain with plastic strain amplitude, and exhibits continuous hardening till failure unlike the PA and OA alloys. Higher average PSED value for the UA alloy in comparison to that for the PA and the OA alloys indicates dynamic precipitation during cycling; the magnitudes of average PSED were calculated using a proposed method. In addition, pronounced increase in the post LCF hardness values substantiate the dynamic precipitation.

Elevated-temperature mechanical properties and thermal stability of Al-Cu-Mg-Ag heat-resistant alloy

The elevated-temperature mechanical properties and thermal stability of Al-Cu-Mg-Ag heat-resistant alloy were studied by tensile test, transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The results show that with the increase of Ag content, the tensile strength and yield strength increase, which is attributed to the increase of the precipitations number and the decrease of the size. The same conclusions are drawn in the study of increasing Mg content. The alloy possesses excellent thermal stability. At 100–150 °C, the strength of the under-aged alloy increases at the initial stage, and after reaching the peak strength, it remains the same. The secondary precipitation of the under-aged alloy occurs in the process of exposure at 150°C, and it distributes diffusely after thermal exposed for 20 h. Then, the tensile strength decreases gradually with increasing the thermal exposure time at 200–250 °C. The strength of the peak-aged alloy decreases gradually, and the precipitation grows up, but the number decreases gradually with prolonging the exposure time at 100–250 °C. The strength of two kinds of alloys decreases with elevating of exposure temperature.

PRECIPITATION KINETICS AND YIELD STRENGTH MODEL FOR NZ30K-Mg ALLOY

Age-hardening effect is considerably strong in magnesium alloys containing Nd, making it possible to develop magnesium alloys with low cost and high strength. Although there have been massive researches about the precipitation product sequence and strengthening models in magnesium, aluminum and other light alloys during their ageing processes, those of NZ30K-Mg alloy, a newly-developed magnesium alloy, has not been carefully investigated. The present work mainly focuses on the model of precipitation kinetics and strengthening of NZ30K-Mg alloy. The precipitation kinetics has been investigated using electrical resistivity testing during continuous heating with different heating rates and formulated based on the isoconversional method. Two related model parameters, modified pre-exponential factor A'αrand activation energy Eαrwere respectively determined. The precipitation behavior of NZ30K-Mg alloy during ageing processes can also be intrinsically explained from the variations of Eαrand ln A'αr with the precipitation fraction. This kinetics model with two above-mentioned parameters can accurately describe the precipitation of strengthening phase during different ageing processes. The yield strength of under-aged and peak-aged NZ30K-Mg alloy have been tested and the results show that the testing samples isothermally aged at different temperature from 180 to 250 ℃ have almost the same peak yield strength of about 150 MPa, indicating that the strengthening effect of under-aged and peak-aged NZ30K-Mg alloy is only determined by the precipitation fraction within a certain range of temperatures. The precipitation strengthening model of NZ30KMg alloy has been carefully derived, and the parameter C in the model has then been determined by least squares method based on the tested yield strength data. The value of C is about 93 MPa. The prediction of yield strength of under-aged and peak-aged NZ30K-Mg alloy has been performed and fit well with the tested ones, demonstrating the effectiveness of precipitation strengthening model and its engineering application prospects.

Influence of Microstructure, Environment and Temperature on Fatigue Crack Propagation in 2XXX Aluminium Alloys

Aluminum alloys, widely used for constitutive parts of aircrafts are confronted to a wide range of temperature depending on altitude and climate, from room temperature on the ground down to some 223K at high altitude. The fatigue crack growth behavior of two new generation aluminum alloys, 2024A in T351 temper and 2022 in T351 and T851 tempers, have been investigated at both temperatures. It is shown that temperature and air humidity do not affect the crack growth resistance of the peak aged 2022 while these two parameters widely influence the crack growth in the under-aged alloys which exhibit in cold air a crystallographic retarded propagation similar to that in vacuum. The respective role of microstructure, temperature, atmosphere residual humidity and crack closure is discussed on the basis of a preexisting modeling framework for environmentally assisted fatigue.

Modeling of Strain Hardening in the Aluminum Alloy AA6061

In this paper, the evolution of work-hardening and dynamic recovery rates vs the flow stress increase (σ − σy) in Al-Mg-Si alloys is presented. The experimental data have been extracted from stress–strain curves. All curves show an initial very rapid decrease in slope of the σ–e curve, which is associated with the elastic–plastic transition. After the elastic–plastic transition, there are typically two distinctive behaviors. For underaged alloys, there is an approximately linear decrease of work-hardening rate as (σ − σy) increases. However, for overaged alloys after elastic–plastic transition, there is a plateau in the work-hardening rate followed by an almost linear decrease. The maximum work-hardening and dynamic recovery rates are found to be dependent on the aging state. In order to investigate these phenomena, a model has been employed to simulate the work-hardening behavior of Al-Mg-Si alloys. The model is based on a modified version of Kocks–Mecking–Estrin (KME) model, in which there are three main components: (1) hardening due to forest dislocations, grain boundaries, and sub-grains; (2) hardening due to the precipitates; and (3) dynamic recovery. The modeling results are discussed and compared with the experimental data.