Initial Stage Of Aging Research Articles

Effect of Natural Aging and Cold Working on Microstructures and Mechanical Properties of Al-4.6Cu-0.5Mg-0.5Ag alloy

This research investigates the effects of natural aging and cold working prior to artificial aging on microstructures and mechanical properties of Al-4.6Cu-0.5Mg-0.5Ag alloy. Mechanical properties relative to microstructure variations were elucidated by the observations of the optical microscope (OM), differential scanning calorimeter (DSC), electrical conductivity meter (pct IACS), and transmission electron microscopy (TEM). The results showed that natural aging treatment has little noticeable benefit on the quantity of precipitation strengthening phases and mechanical properties, but it increases the precipitation strengthening rate at the initial stage of artificial aging. Cold working brings more lattice defects which suppress Al-Cu (GP zone) and Mg-Ag clustering, and therefore the precipitation of Ω phase decreases. Furthermore, more dislocations are formed, leading to precipitate the more heterogeneous nucleation of θ′ phase. The above-mentioned precipitation phenomena and strain hardening effect are more obvious with higher degrees of cold working.

Crack Growth by the Isothermal Martensitic Phase Transformation in Tetragonal Zirconia Polycrystals

In this study, we investigated the crack nucleation and growth and propagation on the surface of Y-TZP during isothermal phase transformation by low temperature ageing. Crack initiation and growth on the surface of Y-TZP specimen was dependent on the sintered microstructure, i.e, sintered density, grain size, pore structure, residual stress etc.. In the case of Y-TZP with 2mol % yttria content, phase transformation of tetragonal to monoclinic began on the surface and induced a crack nucleation of specimen at the initial stage of low temperature ageing. Most of cracks in 2Y-TZP by low temperature ageing were firstly formed on the surface of specimens (free surface, weak bonding grains, etc.) where the change of strain free energy for a tetragonal to monoclinic transformation was small, and surface cracks grew into the bulk interior through the grain boundaries.

On the Cu precipitation behavior in thermo-mechanically embrittlement processed low copper reactor pressure vessel model steel

Cu precipitation behavior in 30% pre-deformed reactor pressure vessel model steel has been investigated during thermal ageing. In this work, a thermo-mechanical treatment based embrittlement process, including high temperature quenching, pre-deformation and thermal ageing, has been applied to a low-Cu (0.17wt.%) RPV model steel to produce Cu-rich precipitates. Ultrafine Cu-rich precipitates were observed by transmission electron microscope (TEM) and energy dispersive X-ray microanalysis following this embrittlement process. Moreover, differential scanning calorimetry was used to study the structure transformation of Cu precipitates during long-term thermal ageing. It is noted that for dilute Cu containing steel, high temperature quenching and pre-deformation was found to promote the production of high density of vacancies in calculation, which may facilitate the formation of the fine Cu-rich precipitation. As a result, it leads to a significant hardness peak in the initial stage of thermal ageing. The results were discussed to link with the thermo-mechanical embrittlement process, which provides a method for studying the Cu-rich precipitates formation mechanism in steels containing less concentration of impurity Cu.

Stress–thermooxidative aging behavior of polyamide 6

AbstractThe long‐term stress–thermooxidative aging behavior of polyamide 6 (PA6) was studied in terms of the creep behavior, mechanical properties, chemical structure, crystallization, and orientation behavior. During aging, a thermooxidation reaction occurred, which included molecular chain degradation and crosslinking, in PA6. Meanwhile, when the samples were subjected to stress, crystallization, orientation, and chain scission were induced. In the initial stages of aging, the stress‐induced crystallization and orientation dominated; this resulted in an increase in the creep deformation, mechanical strength, crystallinity, and orientation factor. Molecular degradation and chain scission dominated in the subsequent aging process and resulted in a decrease of the mechanical strength, reduced viscosity, crystallinity, and orientation factor and an increase in the formation of oxide and peroxide products. The stress may have promoted the chain scission of PA6 during thermal aging and resulted in a decrease in the reduced viscosity and an increase in the carboxylic acid concentration. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (Tg ) in network glasses is developed for the archetypal chalcogenide glass, As20Se80, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below Tg . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.

Effects of Cu and Ag Addition on Nanocluster Formation Behavior in Al-Mg-Si Alloys

Two types of nanoclusters, termed Cluster (1) and Cluster (2) here, both play an important role in the age-hardening behavior in Al-Mg-Si alloys. Small amounts of additions of Cu and Ag affect the formation of nanoclusters. Two exothermic peaks were clearly detected in differential scanning calorimetry(DSC) curves by means of peak separation by the Gaussian method in the base, Cu-added, Ag-added and Cu-Ag-added Al-Mg-Si alloys. The formation of nanoclusters in the initial stage of natural aging was suppressed in the Ag-added and Cu-Ag-added alloys, while the formation of nanoclusters was enhanced at an aging time longer than 259.2 ks(3 days) of natural aging with the addition Cu and Ag. The formation of nanoclusters while aging at <TEX>$100^{\circ}C$</TEX> was accelerated in the Cu-added, Ag-added and Cu-Ag-added alloys due to the attractive interaction between the Cu and Ag atoms and the Mg atoms. The influence of additions of Cu and Ag on the clustering behavior during low-temperature aging was well characterized based on the interaction energies among solute atoms and on vacancies derived from the first-principle calculation of the full-potential Korrinaga-Kohn-Rostoker(FPKKR)-Green function method. The effects of low Cu and Ag additions on the formation of nanoclusters were also discussed based on the age-hardening phenomena.

Ageing Precipitation and Strengthening Behavior of the Spray Formed Al-Zn-Mg-Cu Alloy

In this study, the Al-Zn-Mg-Cu alloy was prepared by spray forming process. The ageing precipitation and strengthening behavior of the spray formed Al-Zn-Mg-Cu alloy were studied by microhardness tests, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) .The results show that the main strengthening precipitates of the spray formed Al-Zn-Mg-Cu alloy are GPI (solute-rich clusters), GPII (vacancy-rich clusters) under peak aging condition. The effect of the microstructure on ageing hardness is controlled by the size and amount of the existing precipitates, and therefore depends critically on the ageing temperature and ageing time. Strength of this alloy is influenced significantly during natural ageing at room temperature, which is related to the dominant formation of GPI. During the initial stage of artificial ageing at 120°C, strong age-strengthening response is due to the formation of GP zones (both GPI and GPII ) .During later stages of artificial ageing , GPII precipitation is accelerated significantly but GPI is not affected clearly.

Precipitation Behavior of Al-Cu-Mg Alloy During Electric Field-Assisted Aging

In this article, we report the effect of electric field (EF) on the precipitation behavior of Al-Cu-Mg alloy. It is found that the applied field increases the hardness of Al-Cu-Mg alloy at initial aging stage. The peak of the S phase formation determined by differential scanning calorimetry also shifts to lower temperature with the applied EF. These are attributed to a lower decreasing rate of density of vacancies under field which is verified by positron annihilation measurements. The built-in field caused by the rearrangement of vacancy-solute atom complexes might lower the barrier for vacancy to escape from a complex. It leads to a longer survival time of vacancies, resulting in more possible nucleation sites for clusters.

25Cr-7Ni-4Mo 수퍼 2상 스테인리스강의 충격인성에 미치는 χ의 영향

This study has been carried out to investigate the precipitation behavior of phase and effect of phase precipitation on the impact toughness of 25%Cr-7%Ni-4%Mo super duplex stainless steel. It was proved that the phase was a intermetallic compound, which represented the higher chromium and molybdenum concentration than the matrix phases, and also showed the higher molybdenum concentration than the phase. The phase was precipitated at the interface between ferrite and austenite or inside the ferrite matrix in the early stage of aging. The number of phase precipitates increased with increasing aging time, however, after showing the maximum value, the number was decreased due to the gradual transformation of phase into -phase. Aging ferrite phase was decomposed by the phase and -phase. Impact toughness rapidly decreased with time in the initial stage of aging at phase start to precipitate. Thus, the impact toughness was greatly influence for the precipitation of phase.

Growth behavior study of second phase particles in a Zr–Sn–Nb–Fe–Cr–Cu alloy

The growth behavior of the second phase particles (SPPs) in a Zr–Sn–Nb–Fe–Cr–Cu alloy during aging (600–700°C, 30–1800min) has been investigated using scanning electron microscopy (SEM) and an image analysis technique. Microstructural observations show that the precipitated SPPs distribute linearly along the boundaries of α plates at the initial aging stage. As the aging time prolongs and/or temperature increases, the linear distribution feature of SPPs weakens gradually and finally a random distribution is formed. A numerical analysis confirms that the growth of SPPs follows well the second order kinetics of Kahlweit theory and the activation energy is experimentally determined to be 194kJ/mol. The reason for the larger growth activation energy of the SPPs in the experimental alloy, compared to that in Zircaloy-4, is discussed and ascribed to the addition of Nb element.

大功率LED的电流老化特性分析

Insulation layer of aluminum substrate for LED encapsulation was made by hard anodizing technology.White and blue GaN-based light-emitting diodes were encapsulated on the aluminum plate directly.White LED was fabricated by blue chip coated with YAG∶Ce phosphor,while blue LED didn't coated with phosphor.Accelerated aging tests at 500 mA and 700 mA were carried out for about 1 000 h on the new type of integrated high-power.Then optical parameters of LEDs were tested per 24 h.Compared with the blue and white LED,we could get the following conclusions: White LED luminous flux attenuates seriously than the blue LED,while optical power is opposite.There are two stages in LED attenuation: The chip and phosphor decay at the same time in initial stage of aging,well blue chip attenuation is dominant after a certain period of time.

Theoretical Analysis of the Influence of the Electron Pulse Modification on Al-Cu-Mn Alloy Initial Aging Stage with Valence Electron Structure

Statistical values of valence electron structure parameter for phase structures of super saturation solid solutions in the initial aging stage of Al-Cu-Mn alloy were calculated by using the Empirical Electronic Theory in solid and molecules (EET), then the influence of various kinds of unit cells on the aging initial stage of the Al-Cu-Mn alloy subjected to the electron pulse modification was analyzed. The results show that the a-Al-Cu-Mn phase structure unit (cell) with larger atomic distortion provides the driving force for the saturated Al-Cu-Mn alloy to form G.P quickly and results in the increase in the primary nucleus of Al-Cu-Mn alloy; thus the a-Al-Cu-Mn phase structure unit (cell) playes an important role in the improvement of alloy aging. After the application of pulsed electric field into Al-Cu-Mn alloy melt, the movement of atoms is accelerated, and the number of segregation units of Al-Cu-Mn is increased, as a result the number of the second phase is increased; generating the remarkable aging strengthening.

RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)2Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [ $\bar 1$ 2 $\bar 1$ 0]p//[ $\bar 1$ 10]B2, where the suffixes p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)2Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.

Effect of χ Phase Formation on the Mechanical Properties of 25Cr-7Ni-4Mo-0.2N Super Duplex Stainless Steel

Abstract In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging andits effects on the mechanical properties of 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel. The R-phase was mainlyprecipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging. It wastransformed into the σ-phase with an increase of the aging time. The ferrite phase was decomposed into a newaustenite(γ 2 )phase and the σ-phase by an aging treatment. The R phase was an intermetallic compound showing highermolybdenum and tungsten concentrations than the matrix and also showed higher molybdenum and tungsten concentrations thanthe σ phase. In the initial stage of aging, precipitation of the R-phase did not change the hardness, the strength and theelongation. The hardness and the strength increased upon a longer aging time, but the elongation rapidly decreased. These resultsshow that the R-phase did not significantly affect the hardness and the strength, though it did influence the elongation.Key wordssuper duplex stainless steel, hardness, tensile properties, R-phase, σ phase.

Nano-structural evolution of Ti(O, N, C) precipitates in V–4Cr–4Ti alloys studied by 3-dimensional atom probe

To investigate the effect of atmospheric impurities on the behavior of Ti(O, N, C) precipitates in V–4Cr–4Ti alloy at elevated temperatures, chemical and structural analysis were carried out using the laser-assisted 3-dimensional atom probe. After aging at 973K for 40h Ti(O, N, C) precipitates were observed around the depth of 1μm from the specimen surface. The coarse precipitates with a diameter of about 50nm consisted of mostly Ti, Cr, O and N. The O/N ratio of 142 was much larger than the value of 2 obtained from chemical composition of NIFS-HEAT1 alloy. The fine precipitates, which shows high O/N ratio of 4 seemed to be formed at the initial stage of aging. Thus, it is thought that the ingressed O affects the chemical composition and precipitation behavior of Ti(O, N, C) precipitates within the surface region.

Effects of Cu Addition on Nanocluster Formation and Two-Step Aging Behaviors of Al–Mg–Si Alloys

Two types of nanoclusters, i.e., Cluster (1) and Cluster (2), are detected in both Cu-free and Cu-added alloys. The Cu addition to the Al-Mg-Si alloys highly suppresses the formation of Cluster (1) at the initial stage of natural aging and causes higher peak hardness. The over-aging is suppressed in the Cu-added alloy, showing higher thermal stability.

Effect of Aging Treatment on Ultra-Fine Grains and Si-Phase in Al-0.5%Si Alloy Fabricated by ARB Process

Using TEM observation and Vickers micro-hardness measurements, this study investigated microstructural changes resulting from aging treatment of ultrafine-grained (UFG) Al-0.5%Si alloy fabricated with a six-cycle accumulative roll-bonding (ARB) process, which includes severe plastic deformation. Results show that the mean grain size of the UFGed Al-0.5%Si alloy produced by the ARB was 253 nm. The hardness of specimens aged at 373 K and 423 K decreased monotonously with increasing aging time in the initial stage of aging. The hardness value of the specimen aged at 373 K then became higher than the value of the specimen aged at 423 K after 10 ks. TEM observation revealed that the mean grain size of the UFG specimen aged at 373 K was less than that of the specimen aged at 423 K. Moreover, results confirmed that Si phase precipitates on the {111} planes on the matrix in the UFG specimen aged at 373 K and 423 K. The mean size of Si-phase in the UFG increased with aging time. The Si phase formation in the UFG specimen aged at 373 K is less than that of 423 K in long-term aging. However, the formation of Si phase on the grain boundaries is more conspicuous than in UFG specimens aged at 373 K and 423 K. These results suggest that restraint of the growth of UFG with aging resulted from Si phase precipitation on the grain boundary and lowering of the aging temperature. Moreover the formation of Si-phase in the UFG suggests precipitation hardening in the ARB-processed Al-Si alloy. [doi:10.2320/matertrans.L-M2011819]

Pre-treatment to Improve the Bake-hardening Response in the Naturally Aged Al-Mg-Si Alloy

This paper describes pre-treatment methods to improve the bake-hardening response of naturally aged AA6022 (Al-0.6Mg-1.0Si), which is an alloy used in automotive body panels. These methods are used to accelerate the artificial aging process, which proceeds 30-day period of natural aging. The precipitation behavior of the sheets of this aluminum alloy was analyzed by differential scanning calorimetry (DSC) and electrical conductivity measurements, and subsequently confirmed by microstructure observations by transmission electron microscopy (TEM). Tensile tests and microhardness measurements were performed to determine the mechanical properties of the samples. Under simulated paint-baking conditions (30 min at 170°C), the microhardness and the yield strength (YS) of the pre-strained + pre-aged samples were found to be 113 HV and 225 MPa, respectively. These values are considerably higher than those of pre-aged samples, and they did not decrease significantly during the initial stage of artificial aging.

In situdetermination of aging precipitation in deformed Fe-Cu and Fe-Cu-B-N alloys by time-resolved small-angle neutron scattering

We performed in situ time-resolved small-angle neutron scattering (SANS) measurements on high-purity Fe-Cu and Fe-Cu-B-N alloys during isothermal aging at $550\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ in order to study the potential self-healing of deformation-induced defects by nanosized Cu precipitation. Three different samples with 0%, 8%, and 24% prestrain were used to study the influence of variable defect levels on the Cu precipitation kinetics. The time-resolved SANS measurements show the presence of two contributions corresponding to spherical precipitates and precipitation at dislocations and/or interfaces, as confirmed by complementary transmission electron microscopy experiments. For the Fe-Cu alloy, prestrain was found to accelerate the formation of spherical precipitates in initial aging stage and lead to a significant enhancement of copper precipitation at dislocations and/or interfaces. For the Fe-Cu-B-N alloy, the addition of boron and nitrogen accelerates the formation of spherical precipitates but suppresses the precipitation along dislocations in the prestrained samples.

Deleterious phases precipitation on superduplex stainless steel UNS S32750: characterization by light optical and scanning electron microscopy

Deleterious phases precipitation in superduplex stainless steels is the main concern in fabrication by welding and hot forming of this class of material. Sigma, chi and secondary austenite phases are considered deleterious phases because they produce negative effects on corrosion resistance. Besides, sigma and chi phases also promote strong decrease of toughness. In the present work, the precipitations of sigma, chi and secondary austenite under aging in the 800-950 °C interval were studied in two UNS S32750 steels with different grain sizes. The deleterious phases could be quantified by light optical microscopy, with no distinction between them. Scanning electron microscopy was used to distinguish the individual phases in various aging conditions. The results elucidate the influence of the aging temperature and grain size on the kinetics precipitation and morphology of deleterious phases. The kinetics of deleterious phases is higher in the fine grained material in the initial stage of aging, but the maximum amount of deleterious phases is higher in the coarse grained steel.