Early Stage Of Aging Process Research Articles

Phase field study of effect of Al on Cu-rich precipitates in Fe-Cu-Mn-Al alloys

Low carbon steel plays an important role in many applications due to its high strength. Its high strength comes from the strengthening effect of nano-Cu-rich phase precipitates. In order to effectively adjust the microstructure of Cu-rich phase precipitates and obtain Fe-Cu-based steel with the best properties by adding different alloying elements (Mn, Al), it is necessary to understand the precipitation process of Cu particles. In this paper, based on the Ginzburg-Landau theory, the previous phase field model is modified, and the continuous phase field method is used to simulate the precipitation mechanism of nanometer Cu-rich precipitates and the inhibiting of the effect of Al content on Cu-rich precipitates of Fe-15%Cu-3%Mn-<i>x</i>Al (<i>x</i> = 1%, 3%, 5% mass fraction) alloy at 873 K isothermal aging. Combining with the free energy derived from thermodynamics database, the microstructure evolution corresponds to the real alloy system. By calculating the composition field variables and structural order parameters, the evolution of phase separation and precipitated phase morphology in aging process are simulated. Moreover, the influence law of morphology, quantity density, average particle radius, growth and coarsening of Cu-rich precipitated phase are discussed. The results show that in the early stage of aging process, the nano-Cu-rich phase precipitates through the spinodal decomposition mechanism, and is randomly distributed in the iron matrix. Furthermore, due to the difference in atomic diffusion rate, the core-shell structure with Cu-rich phase as a core is formed. With the aging time extending, the structure of Cu-rich phase precipitates changes from bcc to fcc. Because of the synergistic effect between Al and Cu, the diffusion of Cu is slowed down. Besides, with the Al and Mn atoms precipitating, Al/Mn clusters are segregated around the Cu-rich precipitates, forming the Al/Mn intermetallic core-shell structure, and gradually wrapping the Cu-rich phase uniformly. During the evolution of the precipitation stage, the Al/Mn clusters are isolated around the Cu-rich precipitation phase, forming a gradually uniform Al/Mn intermetallic phase core shell structure covering the Cu-rich phase, which is to hinder the buffer layer from forming in the precipitation stage of the reservoir. In addition, with the Al content increasing, the Al/Mn intermetallic phase promotes the growth of the buffer layer and hinders the Cu-rich precipitate phase from growing and coarsening.

Atomic-scale investigation into precipitated phase thickening in Al-Si-Mg-Cu alloy

The microstructure variation in Al-Si-Mg alloys added with high content of Cu during aging process is investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). At the early stage of aging process, there are needle-like GP zone and β″ containing Cu precipitating in succession. A kind of bright contrast step type structure unit (step-unit), consisting of Cu atoms, is observed in the edge of these precipitates. The amount of step-unit increases with precipitates thickening. Step-unit is resulted by the translation distribution of a kind of atomic coordination geometry and the specific occupation of Cu atoms in the precipitates. This kind of atomic coordination is called Unit T in β'' and Unit T' in GP zone, respectively. The thickening process of these needle-like precipitates is realized by the formation of Unit T(T′) and local diffusion of solute atoms.

Age-hardening mechanism by precipitation in a zinc- and indium-added silver-palladium alloy

The age-hardening mechanism of a copper-free silver-palladium alloy is unclear yet because the age-hardening mechanism is variable even with slight compositional changes. The aim of present study is to elucidate the age-hardening mechanism and relation of age-hardenability with the phase transformation and microstructural changes of a Zn- and In-added Ag-Pd alloy by means of the hardness test, X-ray diffraction study, field emission-scanning electron microscopic observation and energy dispersive spectrometer analysis. The matrix was composed of the Ag-rich α1 phase containing small amounts of solute elements, and the particle-like structures were composed of the PdZn-based β1 phase containing relatively large amounts of In. The lamellar-forming grain boundary reaction which is usually observed in Ag-Pd alloys containing Cu was not observed in the Zn- and In-added Ag-Pd alloy. The grain interior precipitate composed of the Pd-Zn-rich phase containing In resulted in apparent hardening from the early stage of aging process, and its coarsening by prolonged aging resulted in softening. In the specimen alloy, Zn and In formed extra phase preferentially with Pd, and the amount of Zn and In which took part in the precipitation hardening was very small.

Controllable Synthesis and Ostwald Ripening of Silver Nanoparticles

Silver nanocrystals with different particle sizes were prepared by chemical reduction of silver nitrate with sodium borohydride in the presence of citrate. The formation and ripening mechanisms of silver nanocrystals were also proposed. TEM images show that the mean diameter of silver nanocrystals is about 2 nm by addition of NaBH4 all at once and 10 nm by addition of NaBH4 drop by drop. UVVis spectra show that the mean diameter of the nanocrystals prepared at 0°C is smallest and then increases with the temperature. The aging time has obvious influence on the small nanocrystals and little influence on the big nanoparticles. Only the crystal planes of small nanocrystals have a high possibility of selective adsorption of citrate at the early stage of aging process. However, the smaller nanocrystals in the colloidal solution have aggregated into larger nanoparticles after 8 h of aging. Keywords: Adsorption, citrate, controllable synthesis, mechanism, Ostwald ripening, silver nanocrystals.

관교의치용 Au-Ag-Cu-Pt-Zn 합금의 시효경화성과 관련된 상변태와 입계석출

Purpose: The age-hardening mechanism of an Au-Ag-Cu-Pt-Zn alloy for crown and bridge fabrication was investigated by means of hardness test, X-ray diffraction study and field emission scanning electron microscopic observation. Methods: Before hardness testing, the specimens were solution treated and then were rapidly quenched into ice brine, and were subsequently aged isothermally at <TEX>$400-450^{\circ}C$</TEX> for various periods of time in a molten salt bath and then quenched into ice brain. Hardness measurements were made using a Vickers microhardness tester. The specimens were examined at 15 kV using a field emission scanning electron microscope. Results: By the isothermal aging of the solution-treated specimen at <TEX>$450^{\circ}C$</TEX>, the hardness increased rapidly in the early stage of aging process and reached a maximum hardness value. After that, the hardness decreased slowly with prolonged aging. However, the relatively high hardness value was obtained even with 20,000 min aging. By aging the solution-treated specimen, the f.c.c. Au-Ag-rich <TEX>${\alpha}_0$</TEX> phase was transformed into the Au-Ag-rich <TEX>${\alpha}_1$</TEX> phase and the AuCu I ordered phase. Conclusion: The hardness increase in the early stage of aging process was attributed to the formation of lattice strains by the precipitation of the Cu-rich phase and then subsequent ordering into the AuCu I-type phase. The decrease in hardness in the later stage of aging process was due to the release of coherency strains by the coarsening of tweed structure in the grain interior and by the growth and coarsening of the lamellar structure in the grain boundary. The increase of inter-lamellar space contributed slightly to the softening compared to the growth of lamellar structure toward the grain interior.

Age structural transition in South Africa

This chapter deals with the current age structure and the age structural transition (AST) underway in South Africa. We have used the national Census data for 1996 and 2001 and the Community survey data for 2007. In 2007, 31 percent of the population was enumerated below 15 years of age, 64 percent between 15- 64 and 5.4 percent 65+ implying the early stages of aging process. The median age in 2007 is 24.3 years, increased from 23 years in 1996. The proportion of young dependents is declining whereas that of the economically active and aged populations are increasing. Generally, the Black population appears younger and the Whites older. The other two ethnic groups are placed in the middle. This is, perhaps, a reflection of the differential fertility across the ethnic groups. The proportion of labour force population in 2007 is the highest for Asians & Indians followed by Whites; and the lowest for Blacks. So far as the proportion of the aged is concerned, Whites lead relatively high (12.4 %) as compared to coloured and Black population groups. During 1996-2007, the age structural transition for the total population is only marginal in South Africa. The median age of the total population grew from 23 years to 24.3 years, an increase of 5.6 percent. The female median age grew from 24.5 to 25.2 years, slightly better than that of males. The old age population has increased by 12.5 percent during the same period along with the global trend of female advantage. Again, the labour force population has increased by 5.5 percent. AST is highest for the white population, followed closely by Asians & Indians. This calls for continued higher investments in primary and secondary education, health care service delivery including reproductive health and social security. Again, for a country like South Africa where unemployment is quite high, the enormity of labour force (64%) has to be viewed seriously. Rapid employment generation, especially for the weaker groups, is the key. Further, one fifth (20.4 %) of the population is in the 15-24 years age group which is the target group for tertiary education. The slow process of AST among Black population warrants for preferential resource allocations especially in the areas of reproductive health and employment.

Grain interior precipitation and related lamellar forming grain boundary reaction in Ag–Pd–Cu–Au–Zn alloy

Grain interior precipitation and related lamellar forming grain boundary reaction in an Ag–Pd–Cu–Au–Zn alloy were examined by characterising the hardening behaviour, phase transformations, changes in microstructure and changes in element distribution during aging. The solution treated specimen was composed of the Ag–Au rich α1 matrix and the CuPd β1 particle-like structures of various sizes containing Zn. The α1 phase decomposition into α2 and α3 in the grain interior accompanied the microstructural changes into fine tweed-like structure containing lattice strains in the interphase boundaries, resulting in hardening in the early stage of aging process. The increased lattice strains in the grain interior induced the grain boundary reaction, stimulating the development of coarse lamellar structure composed of the α2 and α3 phases. The increase in volume fraction of the lamellar structure accompanied the decrease in hardness by consuming the dense interphase boundaries of the fine tweed-like structure.

Hardening and overaging Mechanisms in an Au-Ag-Cu-Pd alloy with In additions

Hardening and overaging mechanisms were examined in a semi-precious Au−Ag−Cu−Pd dental alloy with small amounts of In, Zn and Ir. The alloy showed maximum age-hardenability at the aging temperature of 400°C. The hardness value increased to reach the maximum value, and then decreased continuously with aging time. In the early stage of aging process, the matrix of the single α0 phase separated into the α1 and AuCu I phases, and the fine InPd-based precipitates containing Zn and Cu formed at the grain boundaries. During further aging, the grain boundary precipitates grew toward the grain interior. In overaged specimens, the original matrix was replaced by the coarse lamellar structure composed of the AuCu I phase containing Pd and Zn and the Ag−Au-based α1 phase of Cu-, Pdand Zn-depleted. The hardness increase in the early stage of aging process was caused by the nucleation of the InPd-based phase and the AuCu I phase in the ga0 matrix; this introduced significant lattice strains into the interface with the matrix. The hardness decrease in the latter stage of aging process was caused by the formation and coarsening of the lamellar structure composed of the α1 phase and the AuCu I phase. The minor constituent, In formed InPd-based grain boundary precipitates prior to the lamellar structure formation of α1 and AuCu I.

Phase transformation and microstructural changes during ageing process of an Ag–Pd–Cu–Au alloy

Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α 1 phase as a matrix, the Cu–Pd α 2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α 1 and Cu–Pd α 2 phases were transformed into the Ag-rich α ′ 1 and Cu 3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α 1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α 1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α 2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

Phase Transformation and Microstructural Changes during Ageing Process of an Ag-Pd-Cu-Au Alloy

Abstract Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α1 phase as a matrix, the Cu–Pd α2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α1 and Cu–Pd α2 phases were transformed into the Ag-rich α ′ 1 and Cu3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

New Hydroxamic Acid Derivative Prevents Sq-OOH Induced Wrinkle

Lipid peroxidation has been suggested to play a key role in damaging biological processes resulting from excessive exposure to UV light. Aging of skin has been associated with increased peroxidation of skin lipid. In particular, squalene, which is a main component of skin surface polyunsaturated lipids, is easily peroxidized. Squalene-monohydroperoxide (Sq-OOH), the initial product of peroxidized squalene, is produced at the human skin by natural exposure to sunlight during day time activities. Recently, a few researchers reported on the damages of hairless mice skin by the topical application of Sq-OOH. Repeated application of Sq-OOH to the hairless mice induced its the skin wrinkles with clear and fine appearance. Histologically were seen, more keratinocyte hypertrophy with Sq-OOH treated skin than with chronic UVB irradiated group in epidermis. In dermis, characteristics, found in UVB irradiated skin, such as the loss of polarity, fractured collagen bundle and elastosis, were not observed severely. The alteration of epidermal layer might be the main factor for the formation of wrinkle caused by Sq-OOH. From the previous works, Sq-OOH is considered as an important source of damage in the early stage of aging process to result in epidermal wrinkle. Thus, it is very important to minimize such damage that would be induced mainly by Sq-OOH against healthy skin condition. Therefore, we tried to develop new anti-aging materials for protecting skin damage by Sq-OOH. Hydroxamic acids, the naturally occurring products exhibit low toxicities in general, and are of interest for many therapeutic applications. Recently, hydroxamic acids and derivatives were reported largely as effective antibacterial and antifungal agents. Their activities originate from their ability to coordinate on metal ions. That is, hydroxamic acid group is regarded as an one electron donor. Hydroxamic acid moiety has also drawn much attention as a reactive oxygen species (ROS) scavenger. In this study, a new skin protecting compound, (4-(Nhydroxycarbamoyl)phenyl)-N-adamantanylcarboxamide (1) is synthesized to show inhibitory activities of lipid peroxidation both in vitro and in vivo. The targeted compound 1 is a hydroxamic acid derivative which possesses an amide linker between hydroxamic acid and adamantane moiety. Adamantane group was adopted as a hydrophobic moiety to optimize antioxidant activity of hydroxamic acid in animal test. Compound 1 is believed to be adequate in skin permeation because of its balance in hydrophilic and hydrophobic character.

冷間加工可能な永久磁石，クロムを含むバイカロイについて

Air-cooled and cold-rolled specimens of Vicalloy containing chromium were aged at 420∼600°C for various lengths of time. The saturation induction, the residual induction, the coercive force, and the Vickers hardness were measured during the aging process. The coercive force increases with a decrease of the saturation induction, that is, with an increase of the precipitated volume of the non-magnetic gamma phase. The experimental results obtained agree with Néels theory which claims that the coercive force is closely connected with some kind of inhomogeneity in the material. A remarkable increase of the Vickers hardness was observed in the early stage of aging process. This hardening is due to the change from the disordered alpha phase to the ordered phase.