Over-aged Conditions Research Articles

The effect of cold rolling, double aging and overaging processes on the tensile property and precipitation of AA2024 alloy

In this investigation, the effect of the amount of cold rolling on the single stage and double stage aging responses of an AA2024 aluminum alloy was assessed. Two cold deformation amounts were applied on the solution heat treated AA2024 sheets. The effect of these deformation on a two-step short time aging performed at low, and subsequently at high temperatures, were studied in terms of precipitation and mechanical properties variations. As well, the effect of rolling percentage on the singe stage of overaging was assessed. Microstructural evolution was studied by optical microscope (OM) and Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM). Mechanical properties were assessed through tensile testing and microhardness variation. Electrical conductivity (EC) measurements were also made to track the changes of precipitation condition. Results showed that applying cold rolling substantially increased the strength and slightly reduced ductility. In addition, performing the cold rolling created better condition, through dislocation generation, for the strengthening precipitates. Under such a condition, a short time aging through double aging process provided higher strength level compared with the long aging process of the overaging treatment. TEM studies showed the development of sub-structure through dislocation cells formation for the cold deformed cases. It also showed that very small size S′ hardening precipitates are present for the highest level of strength/hardness achieved in the double aging process. Also TEM studies suggested that no morphological changes occurred in the highest strength level achieved for the double aged conditions. Microhardness and electrical conductivity variation followed the same trend as the strength variation, for both the double aging process and the overaging condition.

Degradation of impact toughness in cast stainless steels during long-term thermal aging

Cast austenitic stainless steels (CASSs) have been extensively used for the large components of nuclear reactor primary coolant systems. Since the cast steels inevitably contain degradable metastable phases and replacement of the large coolant system components is impractical, the thermal embrittlement of CASS components has been a serious concern in the extended-term operation of nuclear power plants. This study aimed to systematically measure and analyze the effect of long-term thermal aging on the Charpy impact toughness to provide a comprehensive understanding of thermal degradation behavior and a practical aging model to predict the degree of thermal degradation in the cast stainless steels. The materials tested in the research include eight CASS alloys (two CF3s, one CF3M, three CF8s, and two CF8Ms) and two reference wrought materials (304L and 316L), in which the nominal δ-ferrite content ranges from ~2% to 33%. These stainless steels have been thermally aged at two light water reactor (LWR) temperatures (290 and 330 °C) and at two accelerated-aging temperatures (360 and 400 °C) for up to 30,000 h; these include both under-aged and over-aged conditions relative to the extended service lifetime (80 years). Charpy impact testing was performed for aged and non-aged specimens, and the impact (absorbed) energy parameters were correlated with a new aging parameter (A). Both the reduction of impact fracture toughness and the shift of ductile-brittle transition temperature were strongly dependent on the δ-ferrite content and degree of thermal aging. A linear relationship was found between the increasing rate of the index transition temperature T41J and aging parameter A; base on which an empirical model was proposed for prediction of the transition temperature as a simple function of the aging parameter (A) and δ-ferrite content (F). Finally, the critical aging parameter for embrittlement (AC) was evaluated and compared with the existing δ-ferrite content criteria.

Structural Evolution in the Quenched Al–Zn–Mg–Fe–Ni Alloy during Severe Plastic Deformation and Annealing

The evolution of the structure and phase composition of the Al–Zn–Mg–Fe–Ni alloy (nikalin) during severe plastic deformation by high pressure torsion and annealing has been investigated using high-resolution TEM techniques. The deformation of the quenched alloy has been found to result in a submicron composite comprising an Al matrix and various fine aluminides. The severe structural refinement takes place along with phase transformations, which dissolve the metastable Al3Zr-phase particles and cause the nanosized strengthening phases Т (Al2Mg3Zn3) and η' (MgZn2) to precipitate from the Al solid solution which has been supersaturated in result of quenching and deformation. Nikalin has been determined to retain its submicrocrystalline state during low-temperature annealing (200°C, 4 h) due to the barrier effect of fine hardening phase particles, fixing grain boundaries. High-temperature heating to 400°C for four hours transfers nikalin to the overaged state, in which it is characterized to consist of a recrystallized matrix with grains 2–3 µm in size, eutectic micron-sized aluminides, and stable Al2Mg3Zn3 and MgZn2-phase particles with sizes of no more or equal to 500 nm.

Effect of Overaging Conditions on Microstructure and Mechanical Properties of Maraging Steel

The microstructure and mechanical properties of maraging steel 350 is studied after aging at 500 – 650°C. The yield strength, the ultimate strength, the impact energy, and the hardness of the steel are determined. The volume fraction of reverted austenite after the aging is assessed. The impact energy, the elongation, and the product of the ultimate strength by the elongation are shown to depend on the fraction of reverted austenite in steel 350.

Effect of Precipitate Characteristics on the Corrosion Behavior of a AZ80 Magnesium Alloy

In this study, a systematic investigation on the effect of both aging temperature (250 °C and 330 °C) and time (chosen based on aging curves) on the distribution and volume fraction of the Mg17Al12 precipitates and subsequently on the corrosion behavior of the AZ80 magnesium alloy was carried out by conducting both immersion and dynamic polarization test in a freely aerated 3.5% NaCl solution. The analysis of corrosion products using Raman spectroscopy and X-ray diffraction and investigation of corrosion morphologies using scanning electron microscopy suggested galvanic corrosion of the anodic α-Mg matrix in contact with the cathodic Mg17Al12 precipitates. The higher corrosion resistance of the alloy aged at lower temperature (250 °C) was attributed to the higher number density of the Mg17Al12 precipitates than that at higher temperature (330 °C). In the peak aged condition, the average number densities of precipitates (1/µm2) were measured to be ~ 0.74 and ~ 0.32 and average corrosion rates (mm/y) were measured to be ~ 3.0 and ~ 5.0 at the aging temperatures of 250 °C and 330 °C, respectively. At a particular aging temperature, the corrosion resistance increased to a maximum at peak-aged condition followed by a decline in the over-aged condition and further increase with prolong aging, which was correlated with the number density, size and distribution of the Mg17Al12 precipitates.

Suppression of Discontinuous Precipitation in Cu-Ti Alloys by Aging in a Hydrogen Atmosphere

Age-hardened Cu-Ti alloys are widely used as conductive materials in small electrical devices, so it is important for them to have suitable mechanical and electrical properties. One of the principal guidelines for improving the mechanical properties of Cu-Ti alloys is to suppress the formation of coarse discontinuous precipitates within them. Herein, we investigated the microstructure and properties of a Cu-4 at. pct Ti alloy aged under hydrogen pressures of 0 to 0.6 MPa, focusing on discontinuous precipitation at the grain boundaries. During the early stages of aging, fine needle-shaped metastable β′-Cu4Ti precipitates formed continuously in the matrix. With further aging, these β′-Cu4Ti precipitates continued to nucleate and grow, and simultaneously cellular components composing of the lamellae of Cu solid solution and the stable β-Cu4Ti discontinuous precipitates were formed at grain boundaries. Nevertheless, the formation of these cellular components was significantly suppressed when aging under hydrogen pressures of above 0.03 MPa. Upon aging under higher hydrogen pressures (> 0.10 MPa), some granular TiH2 precipitates formed, and the preformed β′-Cu4Ti precipitates decomposed. Upon aging under 0.03 MPa hydrogen pressure, fine metastable β′-Cu4Ti precipitates were formed dominantly, because the formation of stable β-Cu4Ti laminates was suppressed and the TiH2 precipitates was still not formed. Hence, the hardness was maintained under over-aging conditions.

Mechanical properties and corrosion behavior of artificially aged Al-Mg-Si alloy

This article reports the influence of the state of artificial ageing on the hardness, tensile properties and corrosion behavior of an Al-Mg-Si alloy. Pitting corrosion in 0.1M NaOH solution and susceptibility to intergranular corrosion (IGC) have been systematically investigated, covering highly under-aged to highly over-aged states. In alkaline solution, corrosion rate increases with the progress of ageing; while for a given state of ageing, it initially increases with immersion time followed by rapid reduction since corrosion is controlled by the competition of formation and dissolution of Al(OH)3 plus Al2O3 film as determined by FESEM, EDS, and XRD characterizations. The IGC susceptibility of the selected alloy is found to be governed by the microstructure as determined by the state of ageing since it is controlled by the anodic dissolution of the precipitate free zones while closely neighboured grain boundary precipitates act as cathode. It is identified as slight to moderate pitting in highly under-aged, moderate to heavy pitting in under-aged, pitting with localized ICG in peak-aged, localized to uniform IGC in over-aged, and uniform IGC to etching in highly over-aged state. It is adjudged that apart from the inclusions like AlCrMnFeSi, the changes in the precipitation such as β” (Mg5Si6), β' (Mg9Si6) and β (Mg2Si) within the grains and at the grain boundaries with progress of artificial ageing play a pivotal role on mechanical properties and corrosion characteristics of the Al-Mg-Si alloy.

3D Time-Resolved Observations of Fatigue Crack Initiation and Growth from Corrosion Pits in Al 7XXX Alloys Using In Situ Synchrotron X-ray Tomography

In situ three-dimensional X-ray synchrotron tomography was used to visualize and quantify the fatigue crack initiation and growth from corrosion pits in high strength aluminum alloys. Corrosion pitted Al 7075 and Al 7475 samples of peak-aged, over-aged, and highly over-aged conditions were prepared by soaking in 3.5 wt pct NaCl solution for fifteen days. These samples were fatigue tested in situ in 3.5 wt pct NaCl solution using synchrotron X-ray tomography to analyze the fatigue crack initiation and growth characteristics (4D). Increases in maximum pit depth and fatigue crack growth rate variability were found with increased aging. A detailed analysis of crack morphology and deflection, as a function of aging, was carried out and it discussed.

The effect of isothermal heat treatment on hydrogen environment-assisted cracking susceptibility in Monel K-500

The effect of isothermal heat treatment on hydrogen environment-assisted cracking (HEAC) susceptibility in a single heat of Monel K-500 was evaluated via slow-rising stress intensity (K) testing while immersed in 0.6 M NaCl solution and exposed to applied potentials ranging from −1000 to −1200 mVSCE (vs. saturated calomel electrode). Four heat treatments, corresponding to the non-aged, under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature of 923 K, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the non-aged and over-aged conditions, respectively. Microstructural variables that are expected to vary under this isothermal heat treatment protocol are grain size, grain boundary character, grain boundary impurity concentration, yield strength, strain hardening, and precipitate morphology. A detailed assessment of grain size, grain boundary character, and grain boundary impurity concentration evolution with aging time suggests these variables do not explain the observed variation in HEAC susceptibility of Monel K-500. Differences in the crack tip hydrostatic stress field induced by differences in yield strength and strain hardening were directionally consistent with the observed HEAC susceptibility, but do not capture the enhanced susceptibility of the under-aged heat treatment relative to the peak-aged condition. However, transmission electron microscopy of each heat treatment revealed the presence of planar slip in the under-aged and peak-aged specimens, as compared to wavy slip in the non-aged and over-aged specimens, suggesting that the propensity for strain localization correlates well with HEAC susceptibility.

Aging condition and trapped hydrogen effects on the mechanical behavior of a precipitation hardened martensitic stainless steel

PH13-8Mo is a precipitation hardened martensitic stainless steel, known for its high strength but also for its high sensitivity to hydrogen embrittlement. The aging condition of the alloy is known to play a key role in the mechanical behavior of the as-hydrogen-charged state. Yet the exact effects of residual trapped hydrogen on this behavior were not explored. In this study a novel simple methodology utilizing the small punch test (SPT) and thermal desorption analysis (TDA) is developed. It is used to investigate the effects of hydrogen trapping at different aging conditions on the mechanical properties and fracture behavior of PH13-8Mo steel. The effects of hydrogen were tested at the fully-aged (550 °C for 4 h) and over-aged (600 °C for 4 h) conditions. Samples were cathodically hydrogen charged and were tested in both the as-charged condition and also after holding at ambient condition for specific delay times. SPT load-displacement behavior and fractographic scanning electron microscopy (SEM) examinations were performed. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses were used as complementary characterization tools. It is shown that the over-aged condition (related specifically to the reverted austenite content), in contrast to the general notion, exhibits an embrittling effect on the mechanical behavior due to residual trapped hydrogen. The significance of these findings and their relation to the microstructure are discussed.

Friction-stir-welded overaged 7020-T6 alloy joint: an investigation on the effect of rotational speed on the microstructure and mechanical properties

Commercial A7020-T6 plates in the overaged state were subjected to friction stir welding with four different tool rotational speeds of 500, 710, 1000, and 1400 r/min and a single traverse feed rate of 40 mm/min. The resultant changes in the welding heat input, microstructure, and the mechanical properties of the joints were investigated. The changes were related to the processes of growth, dissolution, and re-formation of precipitates. The precipitate evolution was examined by differential scanning calorimetry, and the microstructural analysis was conducted using optical, scanning, and transmission electron microscopes. The results showed that the grain size in the stirred zone (SZ) decreased substantially compared with the base metal, but increased with tool rotational speed because of the rise in temperature. We found that the width of the heat-affected zone increased with tool rotational speed. The hardness and the tensile strength in the SZ increased with increasing heat input compared with the base metal in the overaged condition. This recovery in mechanical properties of the joints can be attributed to the dissolution and re-formation of precipitates in the SZ and the thermomechanically affected zone. This process is referred to as an “auto-aging treatment.”

Development of high strength sand cast Mg–Gd–Zn alloy by co-precipitation of the prismatic β′ and β1 phases

In order to design a high strength sand cast Mg–Gd–Zn alloy, the microstructures and mechanical properties of cast Mg–12Gd–xZn–0.4Zr (x = 0, 0.4, 0.8) (wt%, signed as GZ1200K, GZ1204K and GZ1208K) alloys were studied. Zn addition changes the constitution of the strengthening phases in Mg12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of γ″ and γ′, accelerated-precipitation of the β1 phases and lower number density of the prismatic β′ phases, and modifies the morphology of the prismatic β′ precipitates. Compared with the Zn-free alloy, the Zn-containing alloys (GZ1204K & GZ1208K) having higher yield strength in peak-aged and over-aged conditions are mainly due to the extra prismatic β1 precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic β′ and β1 phase indicates a good combination of strength and ductility at room temperature: yield strength of 270 MPa, ultimate tensile strength of 348 MPa and elongation of 2.6%.

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.

Precipitation processes during the peak-aged and over-aged stages in an Mg-Gd-Y-Zr alloy

The precipitation processes during the peak-aged and over-aged stages of the as-extruded Mg-11.9Gd-1Y-0.44Zr (wt.%) alloy were investigated in detail by transmission electron microscopy. We revealed that, in addition to the dominant precipitate β′, both βT and βF' precipitates were also found from the peak-aged stage. The transformation from βT to βF' gives rise to the formation of neck morphology that connects different β-phases. The lengthening rate of precipitate in the <101¯0>α is larger than the thickening rate in the <12¯10>α. In the over-aged condition, self-stress orienting phenomenon leads to the reduction of the variants of precipitates. The age-hardening of this alloy strongly depends on the formation of the size of the near-continuous network composed of the {112¯0}α prismatic precipitates. The near-continuous precipitate network was formed in the peak-aged condition while the breakdown of the near-continuous precipitate network results in the significant decrease of hardness.

Effect Of The Application Of Constant And Variable Temperatures at the Overaging Stage in the Mechanical Properties of Trip780 Steel

The effect of the application of constant temperatures (isothermal plateau) and variables (cooling ramp) during the overaging (OA) stage of the continuous annealing cycle in the microstructural characteristics and mechanical properties of a TRIP steel of the class of 780 MPa was investigated. It was simulated six overaging conditions in the equipment Gleeble. The microstructures presented variable fractions of ferrite, bainite, retained austenite and a small martensite portion. All the conditions achieved the minimum strength of 780 MPa it is possible to highlight the conditions for initial OA temperature of 440 °C where presented the best results of mechanical properties and typical TRIP steels behaviors.

Precipitation behavior of Cu-3.0Ni-0.72Si alloy

CuNiSi alloys have been widely applied in electronic and electrical industries. The precipitation behavior of some of the CuNiSi alloys is still not well understood. In this study, the precipitation behavior of the Cu-3.0Ni-0.72Si alloy aged at 600 °C for different times was investigated by transmission electron microscopy, atom probe tomography and phenomenological theory of precipitation crystallography. A new orientation relationship (OR) between the precipitates and the Cu matrix was found in the over-aged condition and a coarsening mechanism of the metastable precipitates was put forward. The two- and three-dimension invariant line theories were successfully applied in interpreting the evolution of the ORs and the morphologies in the Cu/δ-Ni2Si (Cu/δ) system. At the early stage of aging, the fine metastable δ’-(Cu, Ni)2Si precipitates are coherent with the Cu matrix, with a quasi-Bain OR of (110)Cu||(100)δ′ and [001]Cu||[001]δ′, and four pairs of parallel conjugate planes: (1¯1¯1)Cu||(3¯01)δ′, (111)Cu||(301)δ′, (1¯11)Cu||(021)δ′, and (11¯1)Cu||(02¯1)δ’. The precipitates have a δ-Ni2Si structure, with some Ni atoms substituted by Cu atoms. During growth, the core region of the metastable δ’-(Cu,Ni)2Si precipitate transforms into stable δ-Ni2Si, with a quasi-NW OR of (1¯11)Cu||(021)δ and [110]Cu||[100]δ, while a layer of metastable δ’- (Cu, Ni)2Si still exists around the core. With prolonging aging time, the δ-Ni2Si precipitates with the OR of (1¯11)Cu||(021)δ and [110]Cu||[100]δ grow two-dimensionally to form a plate-like shape, while those with the OR of (111)Cu||(301)δ and [1¯10]Cu||[010]δ grow one-dimensionally to form a fiber-like shape.

Strain-controlled fatigue characteristics of a cast Mg–Nd–Zn under peak-aged and over-aged conditions

Strain-controlled fatigue characteristics of peak-aged and over-aged Mg96.47Nd2.9Zn0.21 magnesium alloys containing 0.42Zr, including stress response, strain resistance, hysteresis loops, strain–life and corresponding low-cycle fatigue life prediction model, were studied. In the peak-aged state (T61: 540 °C × 8 h + 200 °C × 14 h), the alloy shows higher cyclic stress response, but lower ductility than the alloy in the over-aged state (T62: 540 °C × 8 h + 200 °C × 400 h). The yield strength and ultimate tensile strength of the alloy under T61- and T62-treated conditions are close. Compared with T61-treated alloy, the steady stress amplitude occurred in T62-treated alloy is due to higher ductility and more homogenous deformation. In T61 state, the fatigue cracks in the alloy first initiate along the cracked persistent slip bands and then propagate in the trans-granular mode, while in the T62 state, the fatigue cracks initiate along grain boundaries and then propagate in the inter-granular mode.

Enhancement of Mechanical Properties of Cast Beta-Type Titanium Alloy by Aging Treatment

Beta type Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) is one of the titanium alloys which have gained much attention in dental applications. Dental precision casting is predominant for fabricating dental prostheses. However, there is a possibility for the mechanical properties of its casting to be degraded because of a α case, shrinkages and pores and a dendrite structure. One of the ways to enhance their mechanical properties is heat treatment process. Therefore, the aim of this study is to investigate the effect of aging treatment on mechanical properties and microstructure of TNTZ cast into magnesia based mold in order to improve its mechanical properties. As results, the Vickers hardness of the cast TNTZ after solution treatment is larger than that of the wrought TNTZ. The aging curve of the cast and the wrought TNTZ at an aging temperature of 673 K and 723 K exhibit almost similar pattern. For each aging time, the higher the aging temperature, the smaller the Vickers hardness for both alloys. Microstructures of cast TNTZ at various aging conditions consist of a dendritic structure and the average diameters of their grain size are around 40 μm. The diffraction peaks of precipitation of α and β phase s are detected in under aging (UA), peak aging (PA) and over aging (OA) conditions for both aging temperatures. However, the diffraction peak of ω phase is observed only in OA condition for cast TNTZ at aging temperature of 673 K. The highest tensile strength of the cast TNTZ and the wrought TNTZ at both aging temperatures are in PA condition and the elongation decrease continuously by increasing aging time. The tensile strengths of cast TNTZ in UA, PA and OA conditions at an aging temperature of 723 K are lower and their elongations are higher in comparison with those of 673 K. The high oxygen content seems to contribute to the poor elongation. SEM fractographs of the cast TNTZ at aging temperatures of 673 and 723 K in UA, PA and OA conditions show the brittle morphology with intergranular fracture that increases with increasing of aging time.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Enhancement in age hardenability of sintered Ti–5Fe alloy by Zr addition processed by pulsed electric current sintering

ABSTRACTThe age-hardenable Ti–5Fe–5Zr (wt. %, 5Zr) alloy has been consolidated by pulsed electric current sintering, following a β solution treatment, and the results are compared with a Ti–5Fe (0Zr) alloy. The precipitation sequence measured at 640°C ageing is β + athermal ω → β + isothermal ω → β + α. At the peak hardness isothermal ω phase forms at 20 s of ageing. The Zr addition retards the precipitation kinetics of the α phase; as a result, the α phase nucleates at latest at 300 s ageing in the overaged state. Fe is partitioned into β, while it is depleted from the α phases. There is Zr enrichment near the α/β interface when the α phase precipitates due to a solute drag effect; the growth rate of the α phase in the 5Zr alloy is significantly reduced compared with that in the 0Zr alloy.