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Junhua You - One of the best experts on this subject based on the ideXlab platform.

  • Retrogression on corrosion behavior of spray formed al 7075
    Journal of Materials Research, 2017
    Co-Authors: Junhua You
    Abstract:

    The effects of Retrogression and re-aging (RRA) treatment on intergranular corrosion (IGC), exfoliation corrosion (EXCO), stress corrosion cracking (SCC) behavior and microstructure of spray formed Al-7075 were investigated by a scanning electron microscope, a transmission electron microscope, slow strain rate test, and EXCO and IGC test. The results show that the precipitates are redissolved in the matrix of the alloy after Retrogression at 200 °C for a suitable time (8 min), and the grain boundary precipitates are discrete and the obvious precipitate free zones are left at the grain boundaries. After RRA with suitable retrogressed time, thin homogeneous dispersive precipitates are separated out again in the matrix. After Retrogression at 200 °C for 8 min and re-aging, the ultimate tensile strength, elongation, IGC depth, EXCO rating, and SCC index of spray formed Al-7075 are 791 MPa, 8.5%, 29.8 μm, EA, and 0.155, respectively.

  • study on a new Retrogression and re aging treatment of spray formed al zn mg cu alloy
    Journal of Materials Research, 2016
    Co-Authors: Junhua You
    Abstract:

    Conventional Retrogression and re-aging (RRA) treatment could not be put to good use for combination property of Al-Zn-Mg-Cu alloys. The new RRA treatment fitted for spray formed Al-Zn-Mg-Cu alloy was investigated by transmission electron microscope, tensile, and conductivity tests. The results show that the pre-aging treatment with under aging of 120 °C for 16 h is beneficial for the redissolution of matrix precipitates during Retrogression treatment. With the Retrogression of 200 °C for 8 min, grain boundary precipitates are discrete and the corrosion resistance of the alloy is drastically increased. After re-aging (120 °C for 24 h) the strength of the alloy is increased again. According to the above-mentioned new RRA treatment, the ultimate tensile strength, yield strength, elongation, and conductivity of the alloy are 791 MPa, 736 MPa, 8.5%, and 39.5% IACS respectively, which is higher than that after conventional RRA treatment.

Eric M Taleff - One of the best experts on this subject based on the ideXlab platform.

  • Retrogression and Reaging of AA7075 and AA6013 Aluminum Alloys
    Metallurgical and Materials Transactions A, 2021
    Co-Authors: Katherine E Rader, Jon T Carter, Louis G Hector, Eric M Taleff
    Abstract:

    Retrogression and reaging (RRA) is of interest to the automotive industry for manufacturing components of high-strength aluminum alloys. RRA heat treatments are investigated for AA7075-T6 and AA6013-T6 materials. Retrogression is demonstrated to be a thermally activated process reasonably characterized with a single activation energy. Activation energies for Retrogression are measured as 97 ± 7 and 160 ± 30 kJ/mol for AA7075-T6 and AA6013-T6, respectively. Critical Retrogression times, t _R* and t _R ^max , are defined and measured across a range of Retrogression temperatures. These data are used with the concept of reduced time to predict combinations of temperature and time that produce successful Retrogression heat treatments. Recommended Retrogression heat treatments are 200 °C for 3 to 12 minutes for AA7075-T6 and 240 °C for 7 minutes for AA6013-T6. Data from reaging heat treatments confirm a significant RRA response in AA6013. Recommended reaging heat treatments are 120 °C for 24 hours for retrogressed AA7075 and 190 °C for 1 hour for retrogressed AA6013. A reaging heat treatment that simulates the automotive paint-bake cycle, 185 °C for 25 minutes, is almost as effective as the recommended reaging heat treatment for AA6013 but is significantly less effective than the recommended reaging heat treatment for AA7075.

  • review of Retrogression forming and reaging for aa7075 t6 sheet
    2021
    Co-Authors: Katherine E Rader, Jon T Carter, Louis G Hector, Eric M Taleff
    Abstract:

    Retrogression forming and reaging (RFRA) is a new warm-forming process designed to produce automotive structural components from high-strength aluminum alloys. A scientific approach is described to determine appropriate RFRA conditions for AA7075-T6 and is applied to laboratory-scale forming experiments. The concept of reduced time is used with the activation energy of Retrogression measured for AA7075-T6 to predict appropriate times and temperatures for Retrogression forming. Conditions recommended for AA7075-T6 are Retrogression at 200 °C for 3 to 12 min while forming at strain rates of up to 10–1 s−1. The recommended reaging heat treatment to fully restore strength to the T6 condition after Retrogression forming is 120 °C for 24 h. These RFRA conditions were successfully applied in laboratory-scale experiments to form AA7075-T6 Alclad sheet and produce a final strength equivalent to the T6 condition. Data from tensile tests provide flow stresses and tensile ductilities across the range of conditions appropriate for RFRA.

  • Retrogression forming and reaging of aa7075 t6 alclad to produce stampings with peak strength
    2020
    Co-Authors: Katherine E Rader, Jon T Carter, Louis G Hectorjr, Eric M Taleff
    Abstract:

    A Retrogression heat treatment was combined with simultaneous warm forming to produce cross-shaped stampings from AA7075-T6 Alclad sheet. This process is termed Retrogression forming. A maximum-allowed-Retrogression-forming-time, which includes sheet heat up, transfer, and stamping, was predicted by calculation to achieve peak-aged strength through a single reaging heat treatment after forming. Sheets of 1.6-mm-thick AA7075-T6 Alclad were stamped at 200 °C to a depth of 45 mm within 2 s without splitting. The formed geometry exhibits a complexity appropriate to automotive structural components. These stampings were then subjected to one of two reaging heat treatments. A full reaging heat treatment of 120 °C for 24 h produced strength levels in excess of the original, peak-aged T6 alloy sheet. A simulated paint bake heat treatment at 185 °C for 25 min recovered 95% of the strength lost during warm forming. Successful Retrogression forming and reaging of AA7075-T6 provides new possibilities for stamping high-strength aluminum alloys into complex geometries without sacrificing strength.

  • Retrogression and reaging applied to warm forming of high strength aluminum alloy aa7075 t6 sheet
    Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2019
    Co-Authors: Thomas A Ivanoff, Jon T Carter, Louis G Hector, Eric M Taleff
    Abstract:

    This study introduces Retrogression forming, a new warm-forming methodology for high-strength aluminum alloys such as AA7075-T6. Retrogression forming combines a Retrogression heat treatment with simultaneous warm forming at approximately 200 °C to improve upon the room-temperature ductility of AA7075-T6, approximately 10 pct in tension, and can then use the automotive paint-bake cycle to restore nearly peak-aged strength. Experimental data indicate that Retrogression forming at 200 °C provides a tensile ductility of 20 pct, approximately double that of room temperature, and that subsequent reaging by a simulated paint bake restores hardness to within 10 pct of the original peak-aged (T6) hardness. To explore the Retrogression-forming concept, the Retrogression behaviors of two AA7075-T6 sheet materials from different suppliers are characterized between 200 °C and 350 °C. The times and temperatures of Retrogression treatments suitable for simultaneous warm forming are determined, and tensile ductilities under these conditions are measured. Tensile ductilities during high-temperature deformation (up to 520 °C) are measured and compared to those possible under Retrogression-forming conditions. Plastic flow at temperatures of 460 °C to 520 °C is governed by dislocation-climb-controlled creep, a mechanism different from the behaviors observed under Retrogression-forming conditions.

  • Retrogression reaging behavior in aluminum aa6013 t6 sheet
    2019
    Co-Authors: Katherine E Rader, Jon T Carter, Louis G Hector, Eric M Taleff
    Abstract:

    The kinetics of Retrogression in a high-strength aluminum AA6013-T6 (Al-Mg-Cu-Si) sheet are investigated to determine Retrogression heat treatments that allow subsequent reaging to approximately peak-aged hardness and strength. The goal is to combine Retrogression with simultaneous warm forming of AA6013-T6 sheet. This new approach is termed Retrogression forming. Differential scanning calorimetry is used to measure the activation energy of Retrogression in AA6013-T6, which was determined to be 160 ± 30 kJ/mol. Heat treating experiments determine the change in hardness during Retrogression as a function of time for temperatures ranging from 230 to 250 °C. The concept of temperature-compensated time is used to predict Retrogression times and temperatures that allow for the recovery of nearly peak-aged strength through a subsequent reaging treatment. Different reaging heat treatments are investigated for recovering the hardness lost during Retrogression.

Yann Rolland - One of the best experts on this subject based on the ideXlab platform.

  • petrochronology of the terre adelie craton east antarctica evidences a long lasting proterozoic 1 7 1 5 ga tectono metamorphic evolution insights for the connections with the gawler craton and laurentia
    Gondwana Research, 2020
    Co-Authors: M O Naumenkodezes, Yann Rolland, G Lamarque, Guillaume Duclaux, S Gallet, Jerome Bascou, Renepierre Menot
    Abstract:

    Abstract The Terre Adelie Craton displays superimposed strain fields related to the Neoarchean (2.6–2.4 Ga, M1) and Paleo-Mesoproterozoic (1.7–1.5 Ga, M2) metamorphic events. M1 is a regional granulite facies event, constrained by P-T modelling at ~0.8–1.0 GPa – 800–850 °C, followed by a decompressional Retrogression in the upper amphibolite facies at ~0.6 GPa – 750 °C. M2 Stage 1 P-T peak is constrained at 0.6–0.7 GPa – 670–700 °C, followed by a steep P-T path down to 0.3 GPa – 550 °C. Retrogression after M2 PT peak occurred in a context of dextral shearing along the Mertz Shear Zone along with thrust motions within the eastern Terre Adelie Craton. In this paper, we present a series of 63 new 40Ar/39Ar ages of biotite and amphibole pairs in mafic rocks from a complete traverse of the Terre Adelie Craton. 40Ar/39Ar dating constrains M2 amphibolite facies metamorphism at a regional scale between 1700 and 1650 Ma, during stage 1 peak metamorphism. During Retrogression, lower amphibolite facies recrystallization mainly occurred along vertical shear zones and mafic dykes between 1650 and 1600 Ma (Stage 2), followed by amphibolite to greenschist facies metamorphism until after 1500 Ma (Stage 3). At the scale of the Mawson continent, this event is related to the growth of an active margin above an oblique subduction zone. The supra-subduction model best explains opening of Dumont D'Urville and Hunter basins at 1.71 Ga followed by their rapid closure and metamorphism at 1.70 Ga. In this context, episodic shear zone reactivation and magmatic dyke emplacement led to a partial reequilibration of the 40Ar/39Ar system until

  • blueschists of the amassia stepanavan suture zone armenia linking tethys subduction history from e turkey to w iran
    International Journal of Earth Sciences, 2009
    Co-Authors: Yann Rolland, Michel Corsini, Marc Sosson, Ghazar Galoyan, Sandra Billo
    Abstract:

    The Amassia–Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic melange within an accretionary prism. This blueschist melange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most blueschists show evidence of greenschist Retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT Retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. 40Ar/39Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite Retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.

David A Wardle - One of the best experts on this subject based on the ideXlab platform.

  • micro arthropod community responses to ecosystem Retrogression in boreal forest
    Soil Biology & Biochemistry, 2017
    Co-Authors: David A Wardle, Stef Bokhorst, Matty P Berg
    Abstract:

    Explaining the variation in communities of soil organisms across plant communities or ecosystems remains a major challenge for ecologists. Several studies have explored how soil communities are affected along ecosystem successional gradients but most of these are based on relatively short term chronosequences. To address the impact of ecosystem age on micro-arthropod communities, we utilized a 5000 year old post-fire chronosequence, which consists of thirty lake islands differing greatly in time since fire in the boreal forested zone of northern Sweden. The Acari community did not change along this chronosequence, indicating that Acari rapidly (<60 yr) reach equilibrium after forest fire and that they are relatively unresponsive to subsequent long term changes in plant community composition and soil quality. The Collembola community composition, however, showed greater responsiveness to the chronosequence and this was best explained through their functional traits. Notably, the youngest (most recently burned) islands, which had the highest ecosystem productivity and fungal mass turnover, were dominated by soil-dwelling (eu-edaphic) Collembola species that are best positioned to take advantage of resource input to the soil. Although plant community characteristics did not emerge as powerful drivers of the Collembola community, we found that Collembola community composition was related to the quality (N and P) of the soil substrate, which reflects a long term legacy of the plant community. Collembola life history characteristics proved to be important for understanding how abundances of different taxa varied relative to one another across the gradients of plant diversity and substrate quality gradients that occur across long-term chronosequences. The causal connection between vertical stratification of Collembola and substrate quality is at present unclear but is likely to be related to their feeding preferences and microhabitat conditions. Because the soil-dwelling Collembola showed a strong decline in abundance with ecosystem Retrogression while surface-dwelling Collembola did not these two life history groups may operate as functionally distinct groups within the soil food web across these long-term chronosequences.

  • coordination of aboveground and belowground responses to local scale soil fertility differences between two contrasting jamaican rain forest types
    Oikos, 2015
    Co-Authors: David A Wardle, Peter J Bellingham, Paul Kardol, Reiner Giesler, Edmund V J Tanner
    Abstract:

    There is growing interest in understanding how declining soil fertility in the prolonged absence of major disturbance drives ecological processes, or ecosystem Retrogression'. However, there are fe ...

  • decoupled responses of tree and shrub leaf and litter trait values to ecosystem Retrogression across an island area gradient
    Plant and Soil, 2013
    Co-Authors: Anna Lagerstrom, Mariecharlotte Nilsson, David A Wardle
    Abstract:

    In the long term absence of catastrophic disturbance ecosystem Retrogression occurs, and this is characterized by reduced soil fertility, and impairment of plant biomass and productivity. The response of plant traits to Retrogression remains little explored. We investigated how changes plant traits and litter decomposability shift during Retrogression for dominant trees and understory shrubs. We characterized changes in intraspecific, interspecific and community-averaged values of plant traits and litter decomposability, for six abundant species across thirty lake islands in boreal forest that undergo Retrogression with increasing time since fire. For understory shrubs, trait values and litter decomposability often changed as soil fertility declined in a manner reflective of greater conservation (versus acquisition) of nutrients, particularly at the interspecific and whole community levels. Such responses were seldom observed for trees, meaning that trees and shrubs show a decoupled response to declining soil fertility during Retrogression. Our results only partially agree with previous studies on temperate and subtropical retrogressive chronosequences. Because traits of only shrubs were responsive, they also highlight that impairment of belowground ecosystem processes during Retrogression is primarily driven by changes in the trait spectra of understory vegetation rather than that of the trees.

  • lichen specific thallus mass and secondary compounds change across a retrogressive fire driven chronosequence
    PLOS ONE, 2012
    Co-Authors: Aron Sandling, Johan Asplund, David A Wardle
    Abstract:

    In the long-term absence of major disturbances ecosystems enter a state of Retrogression, which involves declining soil fertility and consequently a reduction in decomposition rates. Recent studies have looked at how plant traits such as specific leaf mass and amounts of secondary compounds respond to declining soil fertility during Retrogression, but there are no comparable studies for lichen traits despite increasing recognition of the role that lichens can play in ecosystem processes. We studied a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. We used this system to explore how specific thallus mass (STM) and carbon based secondary compounds (CBSCs) change in three common epiphytic lichen species (Hypogymnia phsyodes, Melanohalea olivacea and Parmelia sulcata) as soil fertility declines during this Retrogression. We found that STMs of lichens increased sharply during Retrogression, and for all species soil N to P ratio (which increased during Retrogression) was a strong predictor of STM. When expressed per unit area, medullary CBSCs in all species and cortical CBSCs in P. sulcata increased during Retrogression. Meanwhile, when expressed per unit mass, only cortical CBSCs in H. physodes responded to Retrogression, and in the opposite direction. Given that lichen functional traits are likely to be important in driving ecological processes that drive nutrient and carbon cycling in the way that plant functional traits are, the changes that they undergo during Retrogression could potentially be significant for the functioning of the ecosystem.

  • response of photosynthetic carbon gain to ecosystem Retrogression of vascular plants and mosses in the boreal forest
    Oecologia, 2012
    Co-Authors: Sheel Bansal, Mariecharlotte Nilsson, David A Wardle
    Abstract:

    In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characterized by reduced nutrient availability [notably nitrogen (N) and phosphorus (P)] and net primary productivity. Few studies have considered how Retrogression induces changes in ecophysiological responses associated with photosynthetic carbon (C) gain, and only for trees. We tested the hypothesis that Retrogression would negatively impact photosynthetic C gain of four contrasting species, and that this impact would be greater for vascular plants (i.e., trees and shrubs) than for non-vascular plants (i.e., mosses). We used a 5,000-year-old chronosequence of forested islands in Sweden, where Retrogression occurs in the long-term absence of lightning-ignited wildfires. Despite fundamental differences in plant form and ecological niche among species, vascular plants and mosses showed similar ecophysiological responses to Retrogression. The most common effects of Retrogression were reductions in photosynthesis and respiration per unit foliar N, increases in foliar N, δ(13)C and δ(15)N, and decreases in specific leaf areas. In contrast, photosynthesis per unit mass or area generally did not change along the chronosequence, but did vary many-fold between vascular plants and mosses. The consistent increases in foliar N without corresponding increases in mass- or area-based photosynthesis suggest that other factor(s), such as P co-limitation, light conditions or water availability, may co-regulate C gain in retrogressive boreal forests. Against our predictions, traits of mosses associated with C and N were generally highly responsive to Retrogression, which has implications for how mosses influence ecosystem processes in boreal forests.

Xiaoyan Liu - One of the best experts on this subject based on the ideXlab platform.

  • microstructure and mechanical properties of al zn cu mg sc zr alloy after Retrogression and re aging treatments
    Journal of Central South University of Technology, 2011
    Co-Authors: Qinglin Pan, Yanping Xiao, Xiaoyan Liu
    Abstract:

    The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with Retrogression and re-aging treatment (RRA), the conductivity increases with the Retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn2) at grain boundary aggregate apparently with Retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn2) and η(MgZn2) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.

  • influence of Retrogression and re aging treatment on corrosion behaviour of an al zn mg cu alloy
    Materials & Design, 2011
    Co-Authors: Yanping Xiao, Qinglin Pan, Xiaoyan Liu
    Abstract:

    Abstract Influence of Retrogression and re-aging treatment on the microstructure, strength, exfoliation corrosion, inter-granular corrosion and stress corrosion cracking of an Al–Zn–Mg–Cu alloy has been investigated by means of optical microscope (OM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The results show that Retrogression and re-aging treatment can increase the size and the distribution discontinuity of the grain boundary precipitates, and lead to the increase of the corrosion resistance without the loss of strength and ductility. In addition, the analysis of electrochemical impedance spectroscopy shows that Retrogression and re-aging treatment can enhance the resistance to exfoliation corrosion.