Over-aged Conditions Research Articles

Microstructure Evolution and the Resulted Influence on Localized Corrosion in Al-Zn-Mg-Cu Alloy during Non-Isothermal Ageing.

A non-isothermal ageing process was proposed for an Al-Zn-Mg-Cu alloy aiming to accommodate the slow heating/cooling procedure during the ageing of large components. The evolution of microstructure and microchemistry was analyzed by using transmission electron microscopy, high-angle annular dark field imaging, and energy dispersive spectrometry. The age-hardening of the alloy was examined to evaluate the strengthening behavior during the non-isothermal process. The corrosion behavior was investigated via observing the specimens immersed in EXCO solution (solution for Exfoliation Corrosion Susceptibility test in 2xxx and 7xxx series aluminum alloys, referring ASTM G34-01). Secondary precipitation was observed during the cooling stage, leading to increased precipitate number density. The distribution of grain boundary precipitates transits from discontinuous to continuous at the cooling stage, due to the secondary precipitation’s linking-up effect. The solutes’ enrichment on grain boundary precipitates and the depletion in precipitate-free zones develops during the heating procedure, but remains invariable during the cooling procedure. The corrosion in NIA (Non-isothermal Ageing) treated specimens initiates from pitting and then transits to intergranular corrosion and exfoliation corrosion. The transition from pitting to intergranular corrosion is very slow for specimens heated to 190 °C, but accelerates slightly as the cooling procedure proceeds. The transition to exfoliation corrosion is observed to be quite slow in all specimens in non-isothermal aged to over-aged condition, suggesting a corrosion resistance comparable to that of RRA condition.

Impact of Annealing Prior to Solution Treatment on Aging Precipitates and Intergranular Corrosion Behavior of Al-Cu-Li Alloy 2050

The influences of annealing prior to solution treatment on the grain structure, subsequent aging precipitates, and intergranular corrosion (IGC) of Al-Cu-Li alloy (AA2050) sheet with T6 aging at 448 K (175 °C) were investigated. Annealing impedes the full recrystallization during solution treatment, increasing the population density of T1 (Al2CuLi) precipitates, but decreasing that of θ′ (Al2Cu) precipitates, of the aged alloy. Meanwhile, annealing leads to the heterogeneous distribution of T1 precipitates, increasing the alloy hardness, and decreasing the open-circuit potential of the aged alloy. With prolonged aging time, the corrosion mode of the aged AA2050 samples with and without annealing evolved in a similar manner. The corrosion mode as a function of aging may be summarized as local IGC with pitting and general IGC with pitting (following initial aging and under the underaged condition), pitting corrosion (later in the under-aging stage), pitting with slight IGC (near the peak-aged condition), and pitting with local IGC (under the overaging condition). The annealing treatment hinders IGC propagation on the rolling surface while accelerating the IGC on transverse surfaces.

Effect of Retrogression Heat Treatment Time on Microstructure and Mechanical Properties of AA7010

The effect of retrogression time during retrogression and re-aging (RRA) treatment of AA7010 is evaluated by performing tensile tests and characterizing the microchemistry of the grain boundary precipitates (GBPs) using transmission electron microscope coupled with the energy-dispersive spectroscopy. Retrogression time is evaluated so that the ultimate tensile strength of the RRA-treated sample is equal to that of the T6-treated sample and the grain boundary microstructure similar to that of the over-aged (T7451) condition. The investigation reveals that the sample retrogressed at 200 °C for 20 min has UTS of 586 MPa which is equivalent to that of the T6 sample and 11.5% higher than that of the T7451 condition. The fracture toughness of the RRA-treated sample was 41 MPa√m. Microstructure of the RRA-treated sample is similar to T7451, along the grain boundaries and in the grain interior similar to that of the T6-treated sample. Energy-dispersive spectroscopy confirmed the increment of Cu content on the GBP’s with increase in the retrogression time, which is expected to improve the stress corrosion cracking resistance of the alloy.

Effect of Ni and Mn Additions on the Ambient and High-Temperature Performance of Zr-Containing Al–Si–Cu–Mg-Based Alloys: Role of Precipitation Hardening

Alloys such as the 354.1 (Al–9 wt% Si–1.8 wt% Cu–0.5 wt% Mg) alloys show a greater response to heat treatment as a result of the presence of both Cu and Mg. These alloys display excellent strength values at both low and high temperatures. Additions of Zr, Ni and Mn are expected to maintain the alloy performance at still higher temperatures. The present study was carried out to investigate the effects of these additions on the tensile properties of 354.1 alloy castings at ambient and at high temperature (250 °C), using different holding times at testing temperature. Alloys were prepared using 0.2 wt% Ti grain-refined 354.1 alloy, comprising alloy R (354.1 + 0.25 wt% Zr) considered as the base or reference alloy, alloy S (354.1 + 0.25 wt% Zr + 2 wt% Ni) and alloy U (354.1 + 0.25 wt% Zr + 0.75 wt% Mn), containing Ni and Mn. The latter two alloys perform better under high-temperature conditions following 1-h stabilization at 250 °C, displaying strength values of 226.64/230 MPa UTS and 225/226 MPa YS, respectively. For a prolonged stabilization time (200 h at 250 °C), however, the strength values are reduced to 111/96 MPa UTS and 90/78 MPa YS. The decrease in strength and increase in ductility observed after prolonged stabilization are related to alloy softening, which occurs because of the overaging conditions during which the equilibrium precipitates form, leading to loss of coherency strain between the precipitates and the matrix.

Microstructural evolution and mechanical properties of cast high-Li-content TiB2/Al-Li-Cu composite during heat treatment

The microstructural evolution and mechanical properties of cast high-Li-content TiB2/Al-Li-Cu composite during heat treatment were systematic investigated by a combination of various techniques, including transmission electron microscopy (TEM) with HAADF-STEM mode, electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The coarse secondary phases, mixed with clusters of relatively larger TiB2 particles, distributed along grain boundaries were observed in as-cast state. And most of them dissolved into the α-Al matrix after a two-stage solution treatment. The grain size of the studied composite exhibited excelled thermal stability owing to effective pinning grain boundaries by the stable particles at the elevated temperature, resulting in no significant increase after solution treatment. The studied composite showed a strong age-hardening response with the peak-hardness was obtained in about 160 h at 175 °C. TEM results showed much fine δ′ precipitates were uniformly distributed within grains after quenching and it coarsened with the increasing age duration. Fine plate-like θ′ and T1 precipitates were observed in the under-aged (UA, aged for 8 h) condition. The rate of precipitation and growth in its diameter was much greater than its coarsening rate, resulted in the large aspect ratio and high number density of T1 and θ′ precipitates were visible in the peak-aged (PA) condition. Finally, a slight decline in hardness was observed in the over-aged (OA) condition due to the apparent coarsening of θ′ and T1 precipitates. This precipitation behavior was further verified by DSC. After appropriate heat treatment, a good comprehensive mechanical properties could be obtained in the composite.

Dislocation dynamics modeling of precipitation strengthening in Fe–Ni–Al–Cr ferritic superalloys

Abstract

Effects of overaging on microstructure and tensile properties of the 2055 Al-Cu-Li-Ag alloy

The lightweight, unconventional 2055 Al-Cu-Li-Ag alloy exhibits an excellent specific strength in the T83 state, but no literature reports the effects of overaging on this alloy. In the present work, the suitability of the alloy for lightweight components operating at high temperature is evaluated. Thermal exposure in the range 215–305°C was investigated, highlighting its consequences on both microstructure and mechanical properties. In the most severe overaging state (24h at 305°C), the typical T1 precipitates (Al2CuLi) are dissolved, leading to the formation and coarsening of ϑ’ and Ω phases. In all overaging conditions, the alloy performance was superior or at least comparable to that of another third generation Al-Li alloy, AA2099, which is characterised by a slightly lower density and encouraging mechanical properties for high temperature applications. Compared to AA2099, the AA2055 alloy provides a higher specific strength (the basic requirement for mass savings) both in the T83 and in the most severe overaging state (24h at 305°C). This work highlights that AA2055 is a promising candidate for lightweight components operating up to 305°C, and it lays the basis for high temperature tests of the alloy.

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.

Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation

We investigated hydrogen embrittlement in Ni-based superalloy 718 by tensile testing at slow strain rate (10−4 s−1) under continuous electrochemical hydrogen charging. Hydrogen-assisted cracking mechanisms were studied via electron backscatter diffraction (EBSD) analysis and electron channeling contrast imaging (ECCI). In order to elucidate the effects of stress or strain in the cracking mechanisms, material conditions with different strength levels were investigated, including samples in solution annealed (as water quenched) and 780 °C age-hardened states. The microstructure observations in the vicinity of the cracks enabled us to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. Fracture in the hydrogen-charged samples was dominated by localized plastic deformation. Strain-controlled transgranular cracking was caused by shear localization due to hydrogen-enhanced localized plasticity (HELP) and void nucleation and coalescence along {111} slip planes in both, the solution annealed and age-hardened materials. Stress-assisted intergranular cracking in the presence of hydrogen was only observed in the high strength age-hardened material, due to slip localization at grain boundaries, grain boundary triple junction cracking, and δ/γ-matrix interface cracking. To investigate the effect of δ-phase in crack propagation along grain boundaries, the over-aged state (aged at 870 °C) with different precipitation conditions for the δ-phase was also investigated. Observations confirmed that presence of δ-phase promotes hydrogen-induced intergranular failure by initializing micro-cracks from δ/γ interfaces.

The effect of heat treatment on microstructure evolution in artificially aged carbon nanotube/Al2024 composites synthesized by mechanical alloying

Although carbon nanotubes/aluminum (CNT/Al) composites are promising materials in the production of structural components, their mechanical behavior under overaging conditions has not been considered. In this paper the effect of CNTs on the microstructural and mechanical behavior of a 2024 aluminum alloy (Al2024) synthesized by mechanical alloying (MA) and powder metallurgy routes is discussed, as well as the effect of aging heat treatments at different temperatures and aging times. The mechanical behavior of composites was screened by hardness measurements as function of aging time. After 96h of aging time, composites showed mechanical stability in their hardness performance. Images from transmission electron microscopy showed that the mechanical stability of composites was due to a homogeneous dispersion of CNTs in the aluminum matrix and a subsequent alteration in the kinetics of precipitation is due to their presence in the aluminum matrix. Even though strengthening precipitation took place during aging, this was not the main strengthening mechanism observed in composites.

Effect of vanadium micro-alloying on the microstructural evolution and creep behavior of Al-Er-Sc-Zr-Si alloys

Al-Er-Sc-Zr-Si alloys, strengthened by L12-ordered, coherent Al3(Er,Sc,Zr) nanoscale precipitates, can be used for automotive and aerospace applications up to 400 °C. Vanadium, due to its small diffusivity in aluminum and its ability to form L12-ordered tri-aluminide precipitates, is a possible micro-alloying addition for further improving the service temperature of these alloys. Moreover, vanadium-containing Al3(Er,Sc,Zr,V) precipitates are anticipated to have a smaller lattice parameter mismatch with the matrix, thereby improving the alloy's coarsening resistance. In this study, the temporal evolution of microstructural and mechanical properties of an Al-0.005Er-0.02Sc-0.07Zr-0.06Si alloy micro-alloyed with V are investigated utilizing isochronal, isothermal and double-aging treatments and compared to the results obtained from an alloy that does not contain V, but otherwise has the same composition. Both isochronal and isothermal aging treatments reveal slower precipitation and coarsening kinetics for the V-containing alloy. A peak microhardness value of ∼600 MPa is obtained after a double-aging treatment at 350 °C/16 h, followed by aging at 400 °C for 12 h. Transmission electron microscopy reveals a duplex-size precipitate microstructure, with the smaller precipitates having a mean radius <3 nm. Despite the expectation of a reduced creep resistance due to a lower precipitate/matrix lattice mismatch, both alloys have similar creep behavior at 400 °C, characterized by a threshold stress of 7.5 and 8 MPa under peak-aged and over-aged conditions, respectively. Thus, micro-additions of V to an Al-Er-Sc-Zr-Si alloy lead to enrichment of V in the Al3(Er,Sc,Zr,V) nano-precipitates, improving their coarsening resistance without deteriorating their ability to block dislocations under creep at 400 °C.

Grain size versus microstructural stability in the high strain rate superplastic response of a severely friction stir processed Al-Zn-Mg-Cu alloy

An Al-Zn-Mg-Cu, Al 7075, alloy in an overaged state was subjected to friction stir processing (FSP) using a wide range of processing conditions. The goal was to diminish the heat index, increasing therefore the processing severity. Additionally, two backing anvils were selected that strongly influence the heat extraction rate. Grain sizes obtained by transmission electron microscopy were situated in the range 210–820nm. High temperature tensile tests and the scanning electron micrographs of the topography of the tensile samples after testing revealed that grain boundary sliding (GBS) operated at an initial strain rate of ε̇=10−2s−1 in a wide range of temperatures for the processed alloy. That temperature window was wider and situated at lower temperatures for the finer grain sizes. The high processing severity imposed to create such fine microstructures introduces high values of stored energy in the material, which is the driving force for the increasing grain coarsening at high temperatures. This fact limited the operation of GBS at the highest tested temperatures in some conditions. An optimum balance between grain refinement and microstructural stability was obtained using a low heat index, corresponding to the processing conditions of ω=1000rpm, V =500mm/min, and the use of refrigerated backing anvil.

Microstructural characteristics and mechanical properties of cast Al-3Li-xCu-0.2Zr alloy

The microstructures and mechanical properties of cast Al-3Li-xCu-0.2Zr alloy were investigated. The results showed that Cu addition could slightly refine the grain size of as-cast alloy. Cu element was significantly enriched at the grain boundaries, and Cu-rich intermetallic compounds were formed in the Cu-containing alloys during casting. The peak hardness increased and the time to peak ageing decreased with the increase of Cu content. Cu addition also had a pronounced influence on the precipitation behavior of Al-3Li-xCu-0.2Zr alloys, i.e. the number density, size and distribution of T1-Al2CuLi and θ′-Al2Cu phases. The tensile strength was effectively improved after 2wt% Cu addition compared with Al-3Li-0.2Zr alloy. A good combination of strength to ductility could be obtained in Al-3Li-2Cu-0.2Zr alloy after solution and ageing 32h at 175°C. The precipitation-free zones (PFZs) were developed with the coarsening of δ′-Al3Li particles in the peak-aged (PA) and over-aged (OA) state, promoting the poor ductility and premature failure.

Predicting microstructure and strength of maraging steels: Elemental optimisation

A physics–based modelling framework to describe microstructure and mechanical properties in maraging steels is presented. It is based on prescribing the hierarchical structure of the martensitic matrix, including dislocation density, and lath and high–angle grain boundary spacing. The evolution of lath–shaped reverted austenite is described using grain–boundary diffusion laws within a lath unit. The dislocation density provides the preferential nucleation sites for precipitation, whereas descriptions for particle nucleation, growth and coarsening evolution are identified for Ni3Ti, NiAl and its variants, and BCC–Cu clusters. These results are combined to describe the hardness at different ageing temperatures in several FeNi–, FeMn– and FeNiMn–based steels. A critical assessment on individual contributions of typical alloying elements is performed. Ni and Mn control the kinetics of austenite formation, where the latter shows stronger influence on the growth kinetics. Ti additions induce higher hardness by precipitating stronger Ni3Ti, whereas Cu clusters induce low strength. A relationship between the reverted austenite and the total elongation in overaging conditions is also found. This result allows to identify optimal process and alloy design scenarios to improve the ductility whilst preserving high hardness in commercial maraging steels.

Microstructural evolution and mechanical properties of cast Al-3Li-1.5Cu-0.2Zr alloy during heat treatment

The microstructures and mechanical properties of cast Al-3Li-1.5Cu-0.2Zr alloy during heat treatment were investigated. The results showed that large amounts of secondary phases distributed along grain boundaries in as-cast state, and most of them dissolved into the α-Al matrix after solution treatment. The alloy had a very strong age-hardening response with the peak-hardness that was more than double for the as-quenched. TEM showed much fine δ′ (Al3Li) particles were homogeneously distributed after quenching, and long δ′ phases were observed during ageing. Fine T1 (Al2CuLi) precipitate was visible after ageing for 8h at 175°C, and both its number density and size increased with the increase of ageing time. Apart from δ′ and T1, θ′ (Al2Cu) phases of large aspect ratio were precipitated in the over-aged condition. As the ageing proceeded, the δ′ particles coarsened and the Precipitate-Free Zones (PFZs) developed, resulting in poor ductility and premature failure. The studied alloy was largely strengthened by the δ′ phase during ageing. After ageing at 175°C for 32h, a good mechanical property with ultimate tensile strength (UTS)=382MPa, yield strength (YS)=307MPa and elongation (EL)=2.9% was obtained.

Hydrogen Embrittlement Susceptibility and Hydrogen-Induced Additive Stress of 7050 Aluminum Alloy Under Various Aging States

Hydrogen embrittlement susceptibility of 7050 aluminum alloy under various aging states has been investigated by means of cathodic hydrogen permeation, slow strain rate test, hydrogen determinator, x-ray diffraction, and scanning electron microscope, and effect of hydrogen on atomic binding force of charged alloy has been calculated by free electron theory in this paper. Simultaneously, hydrogen-induced additive stress (σad) of 7050 aluminum alloy hydrogen charged with different current densities under various aging states have been investigated by flowing stress differential method. The results showed that hydrogen concentration of examined alloy increased with increasing charging time or current density under the same aging state. Hydrogen segregation occurred at grain boundaries which enlarged the crystal lattice constant, meanwhile, it reduced the average bonding energy and interatomic bonding force of the grain boundary atoms, thus resulting in hydrogen embrittlement; moreover, σad of 7050 aluminum alloy increased linearly with increasing hydrogen concentration under the same aging state, i.e., under aged: σad = −1.61 + 9.93 × 105 C H, peak aged: σad = −1.55 + 9.67 × 105 C H, over aged: σad = −0.16 + 9.35 × 105 C H, correspondingly, σad increased the susceptibility to hydrogen embrittlement (I HE) further. Under the same charging condition, aging states had a great influence on σad and I HE, the under-aged state alloy was of the highest, the over-aged state alloy was of the lowest, and peak-aged was in the middle.

Addressing the issue of precipitates in maraging steels – Unambiguous answer

Despite several decades' long study, the identity of the precipitating phases responsible for the strengthening of maraging steels is still not clear. In the current work, this issue was extensively investigated using experimental and theoretical approaches. First, in-depth characterization of the precipitates in C250 steel and the precipitation order were performed through a combination of various Transmission Electron Microscopy (TEM) methods. In parallel, thermodynamic calculations were used for the prediction of the phase content at equilibrium. Then, in order to isolate the effects of the different precipitates, model alloys were cast and aged. It was shown that the phases responsible for the strengthening during the initial stages of aging are Ni3Mo and Ni3Ti. In the over-aged (close to equilibrium) condition, the steel consists of martensite, reverted austenite, Ni3Ti, and Fe–Mo phases. This conclusion was found to be in perfect agreement with thermodynamic calculations. The formation of Ni3Mo at early stages of aging, despite its calculated lowest driving force for formation, was attributed to a low barrier for formation.

Precipitation Kinetics of M2C Carbide in Severely Ausformed 13Co-8Ni Secondary Hardening Steels

With continuous heating calorimetric data as a basis, the kinetics of M2C formation during isothermal aging was modeled in severely ausformed 13Co-8Ni steels using the Johnson–Mehl–Avrami theory coupled with a variation of effective activation energy with respect to the degree of transformation. These results were compared with small-angle neutron scattering measurements and discussed in terms of variations in the thermodynamic and kinetic behavior of M2C precipitation. In particular, the M2C carbides in the deformed samples contained more Fe content compared with the non-deformed samples. As this can be ascribed to the ausforming effect increasing the driving force for M2C nucleation, it consequently leads to the decrease of the growth/coarsening rate for M2C carbides at over-aged conditions.

The natural aging and precipitation hardening behaviour of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions

The natural aging and artificial aging behaviours of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were investigated using Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterisation. Excess Si and Cu additions enhanced the alloy hardening ability during natural (NA) and artificial aging (AA). Alloys with low Cu and high Si contents exhibited higher precipitation hardening than alloys rich in Mg during artificial aging. In contrast, the alloys with high amounts of Cu were less dependent on the Mg/Si ratio during precipitation hardening due to their similar aging kinetics. The main precipitate phases that contributed to the peak-aging hardness were the L, Q′ and β″ phases. In the over-aging conditions, the alloys rich in Mg and Cu had finer and more numerous precipitates than their Si-rich equivalents due to the preferential precipitation of the L phase. The combination of excess Mg and high Cu resulted in an alloy with a relatively low hardness in T4 temper and a relatively higher hardness after the paint baking cycle. Thus, this alloy has good potential for use in auto body panel applications.

Anomalous hardening and microstructural evolution accompanied by reordering and restoring of plastically deformed Co3Ti

The anomalous hardening and microstructural evolution accompanied by the reordering and restoring of cold-rolled Co 3 Ti alloy annealed at 573–773 K were investigated by hardness testing, SEM and TEM observations, XRD analysis and DSC measurements. The hardening observed in the heavily cold-rolled samples was similar to that observed in precipitation reactions, with the hardness increasing with increasing annealing time to a point and then decreasing. The hardness peak occurred more rapidly at higher annealing temperatures, and the annealing time leading to this peak was insensitive to the degree of cold-rolling reduction. The stored lattice strain evaluated from the half-width of the X-ray reflection peak steadily decreased with increasing annealing time but remained relatively high regardless of the degree of cold-rolling reduction or annealing temperature. The TEM deformation microstructures showed that a high density of dislocations was attained, even in an over-aged state. DSC measurements showed two exothermic peaks at low and high temperatures, which were attributed to the annihilation of point defects and/or reordering and to the rearrangement of dislocations, respectively. The observed anomalous hardening was discussed in association with the reordering of the mechanically disordered structure and the restoring of the plastically deformed microstructure.