Over-aged Samples Research Articles

Micro-mechanisms of failure in nano-structured maraging steels characterised through in situ mechanical tests

High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time using in situ characterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements. It is seen that the phenomenon of planar slip leads to strain softening under uniaxial deformation and to crack branching under a triaxial stress state in hot rolled maraging steels. On the other hand, nano-structuring after HPT processing creates a large number of high angle grain boundaries as dislocation barriers, leading to strain hardening under uniaxial tension and nearly straight crack path with catastrophic fracture under triaxial stress state. Upon overaging, the hot-rolled sample shows signature of transformation induced plasticity under uniaxial tension, which is absent in the HPT processed overaged samples, owing to the finer reverted austenite grains containing higher Ni concentration in the latter. In the overaged fracture test samples of both the hot-rolled and HPT conditions, crack tips show a signature of strain induced transformation of the reverted austenite to martensite, due to the accompanying severe strain gradients. This leads to a higher fracture toughness even while achieving high strengths in the overaged conditions of the nanocrystalline HPT overaged samples. The results presented here will aid in design of suitable heat treatment or microstructure engineering of interface dominated nano-scale maraging steels with improved damage tolerance.

Microstructure and work hardening behavior of Al–Cu–Li alloy (AA2198) in different temper conditions

In this paper, Al–Cu–Li alloy (AA2198) sheets have been imparted under different temper conditions to obtain different precipitate morphologies. The tensile testing of the samples in the solution treated, peak-aged, and over-aged conditions at ambient and cryogenic temperatures has been carried out to compare the hardening behavior. The strain hardening in the alloy is a strong function of the nature of obstacles present in individual temper conditions and extrinsic factors such as temperature. The decreased strain hardening capacity of peak-aged and over-aged samples is manifested in terms of a considerable decrease in uniform elongation. The effect of temperature is mainly manifested in terms of a decrease in dynamic recovery parameter and increase in dislocation storage capacity, irrespective of temper condition.

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.

Effect of Grain Boundary Characters on Precipitation Behavior and Local Deformation Behavior in Al-Mg-Si Alloy

Nanoindentation tests were conducted near the grain boundary (GB) of the Al-Mg-Si alloy, and the influence of GB character on the aging precipitation behavior and the mechanical properties was confirmed. After obtaining the GB characters by electron back scattered diffraction (EBSD) analysis and nanoindentation tests were carried out on under-aged, peak-aged, and over-aged samples. And then, the indentation areas were observed by back scattered electrons imaging (BSE) in order to identify indentation positions to the GB. In this study, for the GB character, focusing on the rotation angle, the high-angle GB (HAGB) and the low-angle GB (LAGB) were selected. In addition, coincidence site lattice GBs (CSL) were selected as the special GB. In the 180℃ under-aged sample, the nano-hardness near GB is higher than that far from GB, while 180℃ peak-aged and 250℃ aged samples, the nano-hardness is lower than that far from GB. Then the amount of change in hardness of HAGB was larger than that of the LAGB. This suggests that the GB character affects the aging precipitation behavior and mechanical properties.

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.

Microstructural evolution and tensile property of 1Cr15Ni36W3Ti superalloy during thermal exposure

1Cr15Ni36W3Ti was thermally exposed at 580 ℃ and 680 ℃, respectively, up to 3000 h. The γ’ phase and intergranular TiC carbides continuously coarsened during exposure. None of η, laves or σ phase was discovered in the exposed samples, indicating good microstructure stability under the present exposure conditions. The ripening process of the γ’ phase could be well modelled utilizing the LSW theory. The evolutions of the yield and tensile strengths were monotonous during exposure at 580 ℃. However, a transition point in strengths was detected in the tensile samples exposed at 680 ℃ for 300 h. Accordingly, the critical γ’ diameter was measured to be 13−14 nm. The γ’/dislocations interaction mechanism transformed from shearing to looping with the γ’ diameter exceeding the critical point. The combination of the weakly coupled dislocations model and the Orowan looping model yielded a critical diameter of 13.1 nm which coincided well with the measured one, indicating the applicability of these two strengthening models for 1Cr15Ni36W3Ti. The present exposure conditions did not exert a profound effect on the fracture mode. All the tensile samples underwent a typically ductile fracture with a dimple pattern dominating the fracture surface. The dispersed deformation induced by the prevalence of dislocation looping in the over-aged tensile samples retarded the propagation of intergranular cracks. The declined precipitation hardening increment and the enhanced deformation homogeneity partially recovered the tensile ductility in the over-aged samples exposed at 680 ℃.

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.

Relationship between mechanical properties and age-hardening behavior around grain boundaries of Al-Mg-Si alloy

Nanoindentation tests were conducted near the grain boundary (GB) of the Al-Mg-Si alloy, and the influence of GB character on the aging precipitation behavior and the mechanical properties was confirmed. After obtaining the GB characters by electron back scattered diffraction (EBSD) analysis, nanoindentation tests were carried out on under-aged, peak-aged, and over-aged samples. And then, the indentation areas were observed by back scattered electrons imaging (BSE) in order to identify indentation positions with respect to the GB. In this study, for the GB character, focusing on the rotation angle, the high-angle GB (HAGB) and the low-angle GB (LAGB) were selected. In addition, coincident site lattice GBs (CSL) were selected as the special GB. In the 180°C under-aged samples, the nano-hardness near GB is higher than that far from GB, while 180°C peak-aged samples, the nano-hardness is lower than that far from GB. Then the range near the GB where the hardness changes was larger at HAGB than at LAGB and CSL3. This suggests that the GB character affects the aging precipitation behavior and mechanical properties.

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.

Effect of precipitation state on recrystallization texture of continuous cast AA 2037 aluminum alloy

Various states of precipitation were produced by variation of the aging treatments with and without the solution treatment and pre-deformation. The effect of the precipitation state on the recrystallization texture of continuous cast AA 2037 aluminum alloy was investigated in detail. The results show that as the precipitation state changes from the solution-treated state through the peak-aged state to the over-aged state, the strength of the cube texture decreases, whereas the strength of the R texture increases. In the peak-aged and over-aged samples, the pre-deformation prior to aging decreases the strength of the cube texture and increases the strength of the R texture, and the strength of the R component decreases significantly with increasing annealing temperature. The effect of precipitation on recrystallization in directly cold-rolled hot band is different from that in the cold-rolled solution-treated alloy. The coarser precipitates formed during long-time aging of the hot band are responsible for the stronger cube texture and coarser grains.

Fracture toughness of cast and extruded Al6061/15%Al2O3p metal matrix composites

This paper reports fracture toughness data on aluminium alloy 6061 reinforced with 15 vol% of Al2O3 particulate of average size 13 μm. Composites were cast/extruded and heat-treated to give under-aged, peak-aged and over-aged samples, with both longitudinal (L-S) and transverse (T-S) properties being measured to determine the effects on toughness of the banded structure produced via extrusion. This anisotropy was reflected in the toughness results, primarily in the tearing modulus. Comparisons were made to the behaviour of layered/laminated composites where the effects are more dramatic and designed into the structure. This suggests that greater improvements to the fracture critical properties (at least in one orientation) could be obtained by purposely segregating the reinforcement during casting, then extruding/rolling to create a more segregated composite than that tested presently. The current results are compared with those from other similar composites reported in the literature.

Microstructural evolution and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment

The microstructure, electrical conductivity and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment were investigated. The results indicated that a population of secondary phases enriched along grain boundaries in as-cast studied alloy, most of which dissolved into the α-Al matrix after solution treatment. Profound enhancement in mechanical properties of the studied alloy was obtained after T6 heat treatment. TEM observations revealed competitive precipitation of T1 (Al2CuLi) and S′ (Al2CuMg) phases during ageing, while no θ′ (Al2Cu) phase was observed. The anomalous decrease in conductivity observed during ageing was attributed to the precipitation of T1 phases. Fine δ′ (Al3Li) particles were homogeneously distributed after quenching, gradual coarsening of which was observed with prolonged ageing time. As ageing proceeded, the δ′ coarsened and δ′-Precipitate-Free Zones (PFZs) developed, resulting in diminishing ductility observed from as-quenched to peak-aged state. Precipitation and continuous coarsening of T1 phases kept on at the expense of gradual δ′ dissolution. The improvements of ductility observed in over-aged samples arose mainly from the presence of T1, S′ phases, Al3(Li,Zr) composite precipitates and progressive δ′ dissolution. Moreover, continuous coarsening of T1 and S′ phases contributed primarily to the strength loss in over-aged studied alloy.

Influence of Carbide Modifications on the Mechanical Properties of Ultra-High-Strength Stainless Steels

The mechanical properties of ultra-high-strength secondary hardened stainless steels with varying Co, V, and C contents have been studied. A reduced-Co alloy based on the chemical composition of Ferrium S53 was made by increasing the V and C content. This changed the M2C-strengthened microstructure to a MC plus M2C-strengthened microstructure, and no deteriorative effects were observed for peak-aged and over-aged samples despite the large reduction in Co content from 14 to 7 wt pct. The mechanical properties according to alloying modification were associated with carbide precipitation kinetics, which was clearly outlined by combining analytical tools including small-angle neutron scattering (SANS) as well as an analytical TEM with computational simulation.

Low cycle fatigue behavior of casting A319 alloy under two different aging conditions

To determine the influence of peak- and over-aging on low-cycle fatigue (LCF) behavior of casting A319 alloy, the strain controlled low-cycle fatigue tests was studied at room temperature under different total strain amplitudes. At lower total strain amplitudes (0.2–0.3%), the A319 alloy presented initial cyclic hardening and then, steady stage and cyclic softening; while at higher total strain amplitudes (0.35–0.4%), it presented continuous hardening in the peak-aged specimens and constant softening in the over-aged specimens. At a given total strain amplitude, the hysteresis loops in the peak-aged specimens were relatively narrower than those in the over-aged specimens. The fatigue life of the peak-aged samples was higher than that of the over-aged samples at any given total strain amplitude. Fractographic morphology indicated that the size of the fatigue crack propagation zone at peak-aged condition was larger than that at over-aged treatment. The crack characteristic in the propagation region and the fast fracture zone also showed some differences between two aging conditions.

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.

Hydrogen embrittlement of ultra high strength 300 grade maraging steel

Slow strain rate tests were conducted in air and aqueous 0.6M NaCl at −1.2 VSCE environments to investigate hydrogen embrittlement (HE) of ultra high strength 300 grade maraging steel. The results showed that the samples were susceptible to HE in different levels after submitted to heat treatments. Moreover, the over-aged samples were more resistant to HE than the peak-aged ones. Reverted austenite contained on the microstructure increased the fracture toughness of the material. EBSD revealed that the paths for intergranular crack propagation were mainly along grain boundaries with {001}//ND fiber orientation, while {111} and {110}//ND fibers were resistant to propagation.

Effect of Solution Treatment Temperature on Tensile Strength of Al-Mg-Si Alloy

This work examines the effect of solution heat treatment temperature on the tensile strength of Al-Mg-Si aluminum alloy. All samples were machined according to the ASTM B557. The samples were solution treated at 450°C and 530°C and followed by ageing at 160°C for 0, 5, 10, 15 and 20 hours. Tensile test was carried out on the samples after heat treatment. The results showed that the highest tensile strengths (201.69 MPa) was given by the sample solution heat treated at 530 °C for 5 hours followed by 20 hours aging at 160 °C. Whereas, the lowest tensile strength (98.52MPa) was given by the sample solution treated at 450°C for 20 hours. Tensile strength was increased with ageing process and decreased for over-aged samples.

The Age-Precipitations Structure Of Al-Mg-Ge Alloy Aged At 473K

Abstract The Al-Mg-Ge alloy is one of the age-hardening aluminum alloy after solution heat treatment. It has been proposed that the age-precipitation behavior of Al-Mg-Ge alloy is different from that of Al-Mg-Si alloy according to our previous works about the microstructure on Al-Mg-Ge alloy over-aged at 523K. For example, The hardness of peak aged Al-1.0mass%Mg2Ge alloy is higher than that of Al-1.0mass%Mg2Si alloy. The precipitates in the over-aged samples have been classified as some metastable phases, such as the β’-phase and Type-A precipitates and equilibrium phase of β-Mg2Ge by TEM observation. There a few reports about microstructure on Al-Mg-Ge alloys observed by TEM for different aging times. The age-precipitations structure of Al-Mg-Ge alloy has not been became clear. In this work, TEM observation was investigated the microstructure on Al-1.0mass%Mg2Ge alloy for difference aging times aged at 473K.

Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

The corrosion behavior and microstructure of AA7055 alloy with a high Cu content, and a Cu-free Al-Zn-Mg alloy in various aging conditions were studied. Polarization measurements and ASTM B117 salt-spray exposure were used to assess the corrosion characteristics. Transmission electron microscope and atom probe tomography were used to evaluate the microstructure and the composition of precipitates. The matrix composition of AA7055 changed with aging because of precipitation. Thus, the galvanic relationship between Al7Cu2Fe particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. The susceptibility to pitting corrosion damage of AA7055 was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. However, aging had only a small influence on pitting corrosion damage for the Cu-free Al-Zn-Mg alloy. Furthermore, despite the more active breakdown potential of the Cu-free Al-Zn-Mg alloy, this alloy exhibited less severe pitting corrosion damage than AA7055, in particular for the over-aged samples. This was attributed to the weaker galvanic coupling between Al3Fe particles and the surrounding matrix. Although Cu in the intermetallic particles is detrimental to pitting resistance, the galvanic effect can be reduced by increasing the matrix Cu content.

Hot Deformation Behavior and Microstructural Evolution of Al-Zn-Mg-Cu Alloy with Different Starting Structures

Hot Deformation behaviors and microstructural evolution of Al-Zn-Mg-Cu alloy with as-homogenized, as-forged and as-over-aged starting structures were studied at temperatures in the range of 300-420°C, strain rates in the range of 0.01-1s-1, and reductions in the range of 20%-80% by high-temperature compression tests. The flow stresses increase with increasing strain rate or decreasing temperature, which can be described by a hyperbolic-sine equation with the deformation activation energies of 246.35KJ/mol, 188.87KJ/mol and 178.25KJ/mol for the homogenized, the forged and the over-aged samples respectively. At the same deformation condition, the flow stress of the homogenized samples is greater than that of the forged and over-aged samples. For the homogenized samples, dendritic coarse grains elongated along the deformation direction, and interdendritic boundaries within coarse grains disappeared gradually due to diffusion at higher temperatures. When deformation is heavy, microstructures became into homogenous and geometric recrystallization occurs and new small grains appear within the severe elongated grains. For the forged samples, higher temperatures promote dynamic recrystallization. Recrystallized new small grains were developed along prior grain boundaries at large strains. For the over-aged samples, prior grains elongated along the deformation direction, and there is not much more dynamic recrystallized grains observed.