Over-aged Sample Research Articles

Hydrogen-induced failure in a partially-recrystallized Al-Zn-Mg-Cu alloy with different aging conditions: Influence of deformation behavior dominated by microstructures

Hydrogen-induced failure in a partially-recrystallized Al-Zn-Mg-Cu alloy with different aging conditions was studied by in-situ electrochemical hydrogen charging method, and the hydrogen-induced cracking mechanism was elucidated by investigating the microstructure-dominated deformation behavior. It was found that in naturally-aged and peak-aged samples, cracks initiate at sample surfaces, and deformation dominated by shearing facilitates hydrogen diffusion, resulting in a high hydrogen embrittlement susceptibility. However, in the over-aged sample, deformation dominated by bypassing cannot transport hydrogen efficiently, which exerts no effect on crack initiation at the sample center. In addition, the same cracking pattern exists on all samples in silicone oil and in-situ hydrogen charging environments. The grain boundary of recrystallized grains is prone to strain localization, leading to intergranular cracking. In unrecrystallized grains, transgranular cracks propagate along intersections of slip lines and subgrain boundaries. This study improves the understanding of hydrogen embrittlement in new generation Al-Zn-Mg-Cu alloy by linking the existing hydrogen embrittlement mechanism to the deformation behavior dominated by microstructures.

Effect of aging route on the precipitation behavior and thermal stability of Al–Cu–Mg–Ag alloy

In this study, the evolution of Ω plates during artificial aging and the effect of peak-aging treatment and over-aging treatment on the thermal stability of Al–Cu–Mg–Ag alloy were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was shown that the diameter distribution of Ω plates in the over-aged sample was narrower compared to the peak-aged one, which suppressed the Ostwald ripening in the over-aged sample. In addition, the Ω plates in the over-aged sample has a larger aspect ratio than that in the peak-aged sample, which makes it more likely for the soft impinge to inhibit the growth of the Ω plates. Quantitative analysis showed that after heat exposure, Ω plates in the over-aged sample contributed more to the yield strength compared to peak-aged sample. This study shows that better thermal stability can be obtained in exchange for a little loss of room temperature mechanical properties, improving the application potential of Al–Cu–Mg–Ag alloys.

Role of grain boundaries in hydrogen embrittlement of alloy 725: single and bi-crystal microcantilever bending study

In situ electrochemical microcantilever bending tests were conducted in this study to investigate the role of grain boundaries (GBs) in hydrogen embrittlement (HE) of Alloy 725. Specimens were prepared under three different heat treatment conditions and denoted as solution-annealed (SA), aged (AG) and over-aged (OA) samples. For single-crystal beams in an H-containing environment, all three heat-treated samples exhibited crack formation and propagation; however, crack propagation was more severe in the OA sample. The anodic extraction of H presented similar results as those under the H-free condition, indicating the reversibility of the H effect under the tested conditions. Bi-crystal micro-cantilevers bent under H-free and H-charged conditions revealed the significant role of the GB in the HE of the beams. The results indicated that the GB in the SA sample facilitated dislocation dissipation, whereas for the OA sample, it caused the retardation of crack propagation. For the AG sample, testing in an H-containing environment led to the formation of a sharp, severe crack along the GB path. • Single crystal (SC) and bi crystal (BC) in situ cantilever bending test is used. • SC of over-aged sample experienced severe cracking than the aged sample. • Load drop for SC over aged cantilevers occurs at lower distance than aged sample. • Bi crystal cantilever of over-aged was more resistant than aged sample. • Grain boundary in solution annealed cantilever mitigated the risk of embrittlement.

Study of pre-strain before artificial aging on mechanical property and stress corrosion behavior of 2297 Al–Cu–Li alloy

The influence of pre-strain (2%, 5%, 8% and 12.5%) and aging state (under, peak and over aging) on mechanical properties and stress corrosion behavior of 2297 Al–Cu–Li alloy was studied. The peak microhardness of the alloy reaches 156.3 HV at 15 h with 12.5% pre-strain, and that of the alloy is only 115.6 HV at 31 h without pre-strain. The slow strain rate testing results indicate the low stress corrosion cracking factor for the samples without pre-strain and high stress corrosion cracking factor for the samples with pre-strain, and the stress corrosion cracking factor of the peak-aged sample with 12.5% pre-strain reaches to 33.3%. From the transmission electron microscopy characterization, it can be inferred that the quantity and distribution of T1 (Al2CuLi) precipitates significantly affect the stress corrosion behavior of 2297 Al–Cu–Li alloy. The over-aged sample exhibits poor stress corrosion resistance, resulting from the large numbers of coarse T1 precipitates and precipitation free zone at the grain boundary.

The Effect of Hydrogen on the Nanoindentation Behavior of Heat Treated 718 Alloy

In this study, the effect of precipitates on the surface mechanical properties in the presence of hydrogen (H) is investigated by in situ electrochemical nanoindentation. The nickel superalloy 718 is subjected to three different heat treatments, leading to different sizes of the precipitates: (i) solution annealing (SA) to eliminate all precipitates, (ii) the as-received (AR) sample with fine, dispersed precipitates, and (iii) the over-aged (OA) specimen with coarser precipitates. The nanoindentation is performed using a conical tip, and a new method of reverse imaging is employed to calculate the nano-hardness. The results show that the hardness of the SA sample is significantly affected by H diffusion. However, it could be recovered by removing the H from its matrix by applying an anodic potential. Since the precipitates in the OA and AR samples are different, they are influenced by H differently. The hardness increase for the OA sample is more significant in −1200mV, while for the AR specimen, the H is more effective in −1500mV. In addition, the pop-in load is reduced when the samples are exposed to cathodic charging, and it cannot be fully recovered by switching to an anodic potential.

Effects of artificial aging on microstructure, mechanical properties and stress corrosion cracking of a novel high strength 7A99 Al alloy

A recently developed 7A99 Al alloy was subjected to hot extrusion, solid solution and artificial aging. The microstructure evolution and precipitation behavior were examined, and their effects on mechanical properties and stress corrosion cracking were analyzed. The results showed that the coarse Mg(Zn, Cu)2 phases were broken and refined by hot extrusion. Most of the phases dissolved into Al matrix during solution, and only insoluble Al7Cu2Fe phase was remained. GPI zones and η′ appeared in the under-aged and peak-aged samples. With the extension of aging time, stable η phase with large size formed and became the dominant precipitate in the over-aged sample. It was concluded that the precipitation of 7A99 Al during aging at 150 °C followed the sequence of solid solution → GPI zones → metastable η′ → stable η. The peak-aged sample owned the high yield and ultimate tensile strength of 588 MPa and 622 MPa, and a low elongation of 10.2%. The under-aged and over-aged samples exhibited relatively lower strength and higher elongations. 2% pre-stretching prior to aging could further improve the tensile properties. Both the under-aged and peak-aged samples were susceptible to the stress corrosion cracking, and the intergranular cracking was observed. In contrary, the over-aged sample showed much better corrosion resistance with a typical ductile fracture, due to the coarsening and separation of grain boundary precipitations.

Dynamic Compression Behavior of a Mg–Gd-Based Alloy at Elevated Temperature

The dynamic compression behavior and microstructure evolution at 400 °C of an extruded Mg–8Gd–4Y–Nd–Zr alloy with different tempers were investigated. The peak-aged samples exhibit the highest compressive strength, followed by as-extruded samples and over-aged samples. The highest dynamic compressive strength of 582 MPa was achieved by peak-aged sample compressed at 1224 s−1. The high strength was attributed to the formation of abundant thermally stable βʹ precipitates and some dynamic precipitates. The dynamic compressive strength of peak-aged sample and over-aged sample is not sensitive to strain rates, while that of the as-extruded sample is sensitive to strain rates. The dynamic compressive strength of the as-extruded alloy can reach 535 MPa when compressed at 2024 s−1. The high strength was mainly ascribed to the formation of numerous dynamic precipitates and the work hardening effect caused by dislocations. The cracks are composed of crack that is 45° to loading direction on the cylindrical surface and crack on the compressed surface. Microstructure observation indicates that the crack was easily propagated along the interface between the adiabatic shear band and matrix, grain boundaries. The equilibrium phase β in over-aged sample was unable to hinder the crack propagation.

Precipitation in Mg-Sm binary alloy during isothermal ageing: atomic-scale insights from scanning transmission electron microscopy

The paper reports on an atomic-scale investigation into the precipitation hardening process in Mg-2.5Sm(wt%) alloy, using advanced Cs-corrected high angle annular dark field – scanning transmission electron microscopy (HAADF-STEM). Various novel precipitate structures existing in different ageing conditions are identified and discussed in depth. In the under-aged sample, three nucleation mechanisms are observed: βL, βH and β′, which are uniformly-scattered atomic-size rare earth clusters, displaying a significant strengthening effect in a very short ageing time of 30min. In the peak-aged sample, the main strengthening structures are needle-like precipitates composed of zigzag line repetitive units and hexagonal repetitive units. In the over-aged sample, the needle-like precipitates grow coarse and a subset of the precipitates transform into β1 precipitates, both of which are detrimental to the mechanical properties. From an engineering perspective, it is of great interest to design advanced Mg-RE materials with the uniformly-scattered atomic-size rare earth clusters, because they are effective strengthening structures requiring short ageing time. In the second place, the paper reveals three types of transformation processes occurring during the precipitation: the transformation between βL precipitates and needle-like precipitates, the transformation within needle-like precipitates along [0001]Mg and the transformation between needle-like precipitates and β1 precipitates. The RE atoms remain coherent with the α-Mg matrix before they transform into β1 precipitates.

Physically Based Model of the Yield Strength for an Al-Mg-Si-Cu-Zn Alloy

The aim of this work is to implement recently developed modeling approaches to predict the mechanical behavior of a precipitation-hardened Al-Mg-Si-Cu-Zn alloy. Assuming that precipitates act as weak or strong obstacles to dislocation motion, a yield strength model, originally introduced for AA6111 alloy, is used to derive the precipitate strengthening formulations. The application of the model provides accurate predictions for the evolution of yield strength of the alloy during artificial aging. The transition point, at which shearable precipitates become non-shearable ones, has been identified directly from the result of tensile tests of AA6011(m) samples at different stages of artificial aging using work-hardening rate model. The linear/non-linear behavior of work-hardening rate of AA6011(m) samples at different stages of artificial aging is also studied. It is explained that high dynamic recovery rate in presence of non-shearable precipitates causes the non-linear work-hardening behavior of massively overaged sample. The modeling results for underaged samples show better agreement with measured values of yield strength when the weak obstacle model has been implemented, while strong obstacle model shows relatively good agreement between the experimental and calculated results for the peak-aged and overaged samples.

Dry wear behavior of ultra-high strength Cu–10Ni–3Al–0.8Si alloy

Cu–10Ni–3Al–0.8Si alloy exhibited desirable physical and mechanical properties and wear resistance for bearing applications. An electrical conductivity of 18.1% IACS, hardness of 309.3HV, tensile strength of 1004.2MPa, coefficient of friction of 0.58 and volume wear rate of 4.69×10-6mm3/mm were obtained with over-aged sample. Adhesion and oxidation were observed to be the two main mechanisms under dry wear condition. The wear resistance of the experimental alloy increased with aging time until the over-aging stage due to the increment of micro-hardness. Adhesion was observed to be the dominant wear mechanism for the cold-rolled sample while tribo-oxidation dominated for the over-aged sample. The latter was attributed to the effect of precipitates on the mechanically mixed layer of the wear sample.

Age-Hardening Behavior of Mg-Al-Zn Alloys Produced by Sand Mold Casting

In recent years, Mg-Al-Zn system alloy has been used for the parts in the automobile for weight reductions. The age-hardening behavior of Mg-6mass%Al (-1mass%Zn)-0.3mass%Mn alloys sand mold castings were investigated by Vickers hardness measurement and optical microscopic observation. Both alloys were solution-treated and then isothermal-aged at 473, 498 and 523K. For each aging temperature, both alloys were indicated age-hardening obviously, and decreased the value of maximum hardness and shorten time to maximum hardness with heighten aging temperature. Age-hardening curves for both alloys approximately showed the same change of hardness. However, these optical micrographs after aging treatment are observed different microstructure. In case of AM60 magnesium alloy, discontinuous precipitation preferentially occurred in aged sample. The volume fraction of discontinuous precipitation was larger than that of continuous precipitation. On the other hand, in case of AZ61 magnesium alloy, the volume fraction of continuous precipitation was larger than that of discontinuous precipitation. Furthermore, over-aged sample of both alloys were consisted of discontinuous precipitation, continuous precipitation and pre-precipitation area.

Effect of the Precipitated Second Phase during Aging on the Damping Capacity Degradation Behavior of M2052 Alloy

In this study, the damping capacity degradation in Mn-20Cu-5Ni-2Fe alloy (M2052) aged at 435 °C for 2-24 hours was investigated. The damping capacity was measured by a torsion pendulum. XRD, SEM and EDS analysis methods were used to characterize the microstructure of specimens. The results showed that aging at 435 °C for more than 4 h leaded to the precipitation of α-Mn in the matrix of M2050, which resulted in the decrease of the original damping capacity. Holding the aged samples in 60°C environment, the damping capacity of under-aged and peak-aged M2052 samples decreased rapidly with the time increasing. While no obvious degradation of damping capacity occurred in the over-aged sample even after 40 d, which is probably due to the precipitation of α-Mn phase in the matrix of alloy. M2050 alloy with high and stable damping capacity can be obtained by aging at 435°C for 8 h.

Microstructure effect on hydrogen-induced cracking in TM210 maraging steel

Hydrogen embrittlement (HE) of TM210 maraging steel was studied by slow strain rate tensile and constant load tests. The over-aged sample exhibited the best resistance to HE, since HE susceptibility of the maraging steel does not depend on the strength, but rather on the reverted austenite content. The hydrogen concentration, observed by scanning Kelvin probe force microscopy, was enriched in the reverted austenite at the grain boundaries and martensite lath boundaries, resulting in hydrogen-induced cracks propagating along the grain boundaries and martensite lath boundaries.

Effect of Cu Particles Precipitation on Wear Behaviour of Fe-3mass%Cu under Dry Sliding Condition

Effect of peak-aged and over-aged Cu particles on wear behaviour of ferritic iron was investigated by means of Pin-on-Disc wear test under dry sliding condition. It was found that hardness of the peak-aged sample was higher than the over-aged sample. The specific wear rates of peak-aged samples were in the range of 0.20 × 10-4 to 0.89 × 10-4 mm3/(Nm) while the over-aged samples were in the range of 0.21 × 10-4 to 1.29 × 10-4 mm3/(Nm). Although both samples possessed moderate wear behaviours, the peak-aged samples had better wear resistance. Scanning Electron Microscopy observation found that most wear mechanism were due to plastic ploughing phenomenon. Transferred materials from the counterface tool was also proven by Energy Dispersive Spectroscopy test. However, the roughness test showed that the peak-aged sample surface was finer than the over-aged sample surface. Average roughness of peak-aged samples were in the range of 0.49 to 1.79 μm while the over-aged samples were in the range of 3.28 to 4.02 μm. Hence, it can be concluded that the peak-aged Cu particles can improve the wear resistance of steel.

Identification of β Phase Particles in an Isothermally Aged Al–10 mass%Mg–0.5 mass%Ag Alloy

The structure and morphology of coarse-scale globular and smaller spheroidal particles in an isothermally aged Al-10Mg-0.5Ag (mass%) alloy have been charcterised by means of transmission electron microscopy. Coarse-scale globular and smaller spheroidal particles were commonly observed in an over-aged sample aged for 72 h at 240°C. The coarse-scale globular and smaller spheroidal particles were identified by electron microdiffraction to have the face-centered cubic structure of equilibrium β phase (Al 3 Mg 2 ) (lattice parameter a = 2.824nm). The orientation relationship between the β phase and α-Al matrix was such that: (100) β // (100) α and [001] β // [001] α . This orientation relationship differs from those reported previously for the β phase formed in binary Al-Mg alloys. The coarse-scale globular β particles had invariably internal faulted structures.