Fast grain boundary: A FORTRAN-77 program for calculating the effects of retrograde interdiffusion of stable isotopes

Exchange of stable isotopes between coexisting minerals is recognized widely as an important factor in the interpretation of stable isotope geochemistry of plutonic and high-grade metamorphic rocks. Where retrogression has occurred without major recrystallization events, the rate limiting step for stable isotope exchange will be diffusion. The mathematics of diffusion are well known for many problems, but no analytical solution, including that for closure temperature, adequately describes the complex and highly variable controls of rate and mass balance that will dominate many diffusion processes in rocks. We have implemented a model describing diffusional exchange for rocks in which grain boundary diffusion is sufficiently rapid that a representative volume of rock (typically millimeter to centimeter) is able to have mutual equilibration of all grain boundaries for the time scale of cooling. This Fast Grain Boundary model explicitly links intracrystalline diffusion rates and abundances of all minerals in a rock, and allows study of the impact of rock type on stable isotope thermometry, retrogression, and zonation.The FORTRAN-77 program for the Fast Grain Boundary model presented here can be used with a personal computer to solve typical problems in minutes. Input includes the grain size(s), model abundance(s), diffusion coefficient, and fractionation factor for each constituent mineral, and a cooling rate for the rock. Output includes the diffusion profile and integrated (bulk) composition of every mineral in a rock, as well as the apparent temperatures that would be observed by applying bulk-mineral stable isotope thermometry to such a rock.     

The effect of retrogression and re‐aging treatment on mechanical and tribological features of AA7075 aluminium alloy

In this study, T651‐applied AA7075 alloy was subjected to retrogression and re‐aging (RRA) process. Various retrogression temperatures (180 °C, 280 °C, 370 °C) and times (15 min, 30 min, 90 min) were used to determine the effects of temperature and time on the mechanical and tribological properties of the AA7075 alloy. All re‐aging stages were performed at 120 °C for 24 hours. Retrogression and re‐aging‐applied specimens were characterized by scanning electron microscope, transmission electron microscope, x‐ray diffraction, Charpy V‐notch impact and tensile tests. Brinell hardness measurements and ball‐on‐disc type tribometer measurements by using AISI 316 ball as a counterpart have also been conducted. Grain boundary precipitates in the T651‐applied specimen was transformed from continuous to the discontinuous structure after retrogression and re‐aging process. Continuous MgZn₂ precipitates at grain boundaries were disintegrated and re‐precipitated along the grain boundaries. The sizes of intragranular precipitates have become coarsened by comparison with the T651 condition. Hardness, tensile strength and wear resistance were decreased whereas impact toughness values were increased with increasing retrogression temperature and time. The best wear resistance was obtained in the sample treated at 180 °C for 15 minutes.     

喷射成形7075合金回归再时效处理的组织和抗应力腐蚀性能

喷射成形7075铝合金是典型的时效强化合金,通过T6处理可得高强度,但其抗应力腐蚀开裂(Stress corrosion cracking, SCC)等性能较差;T7处理虽可改善抗SCC性能,但合金的强度却损失严重。为此,采用常温拉伸测试、电导率测试和透射电镜观察等手段,研究4种回归再时效(Retrogression and re-ageing, RRA)处理对喷射成形7075铝合金显微组织及抗SCC性能变化的影响,分析探讨RRA处理与合金性能变化之间的关系。结果表明,合金经200/10 min 回归和再时效处理,抗拉强度和电导率均较好,分别是748 MPa和22.8 MS/m;随着回归处理的进行,基体中的GP区和η′相逐渐回溶,当200 ℃回归处理超过10 min,合金晶粒内再次析出η相,使合金强度小幅上升;回归过程中η相先回溶再析出,因此不同回归状态下RRA处理后的合金晶内组织形貌差异较大。RRA处理后晶内大量再析出的η相与晶界处粗化并断开的η相可以在保持合金高强度的基础上显著改善其抗SCC性能。     

Retrogression, reaging and residual stresses in 7010 forgings

Open die forgings of the aluminium alloy 7010 (DTD5636) have been retrogressed at temperatures between 200 and 240 °C and then reaged (RRA). Tensile, fracture toughness and stress corrosion cracking values are reported. Retrogression at 200 °C offers the most promising combination of properties, and the long retrogression time of approx. 2500 s will enable thick sections to be treated. Residual stresses induced during quenching are also predicted, and initial experimental results suggest that some stress relief occurs during the retrogression heat treatment.     

蠕变时效制度对回归态Al-Zn-Mg-Cu合金析出相的影响

基于回归再时效(RRA)工艺,提出一种新的回归－应力时效制度(RSA)用于Al-Zn-Mg-Cu合金。系统研究了应力时效制度(时效时间和应力)对回归态Al-Zn-Mg-Cu合金析出相的影响。透射电镜(TEM)观察结果表明：在回归处理后,合金内部存在大量的基体析出相(MPts)和轻微不连续的晶界析出相(GBPs)。时效时间和应力对回归态合金析出相的影响十分显著。随着时间和应力的增加,基体析出相的尺寸增加而密度减少;同时,晶界析出相的尺寸、间距和无沉淀的宽度也增加。相比于回归再时效工艺,回归-应力时效工艺使得晶内析出相尺寸增加,无沉淀析出带变窄且晶界析出相更不连续。     

Non-isothermal “retrogression and re-ageing” treatment schedule for AA7055 thick plate

In order to apply the retrogression and re-ageing (RRA) treatment to AA7055 thick plate subject to the non-isothermal retrogression condition, a kinetic model based on the mean size evolution of grain boundary precipitate (GPB) is developed firstly. An optimized RRA treatment schedule derived from AA7055 thin plate is established subsequently. Taking the retrogression temperature and time as the references, the “equivalent retrogression time” in selected non-isothermal retrogression schedule of AA7055 thick plate was predicted by the kinetic model for several thermal paths with different heating rates. The efficiencies in processing and enhancements in strength that may be realistically achieved are also examined. The results reveal that, the “equivalent retrogression time” of the selected non-isothermal retrogression schedules can be predicted successfully. The similar properties to those from the referential RRA treatment can be achieved by more than one non-isothermal RRA schedule. However, the thick plate under non-isothermal retrogression schedule with quick heating rate could achieve the same properties by less energy consumption.     

Microstructures and properties evolution of Al-Zn-Mg-Cu alloy under electrical pulse assisted creep aging

Creep aging (CA) is a promising forming technology for integral panels with complex structures. However, how to balance excellent corrosion resistance and strength is still a challenge for CA parts. By synchronously applying electrical pulse (300 Hz, 15 A/mm²) in the middle of steady-state CA of Al-Zn-Mg-Cu alloy, the electrically assisted CA (ECA) is proposed to induce retrogression rapidly for realizing a three-step aging (retrogression and re-aging, RRA) in CA to tailor the needed properties balance. It is compared with conventional CA with one-step aging (peak aging, T6 or over aging, T7), two-step aging (over aging, T73) and RRA in hardness, intergranular corrosion and microstructures. For the ECA of pre-aging and re-aging for 20 h plus electropulsing for 10 min, the hardness is 10.5%, 20.5% and 18.9% higher than those of CA with T6, T7 and T73 processes, respectively, while the corrosion resistance is higher than T6 process and lower than T7 and T73 processes. Although the hardness and corrosion resistance are comparable to those of the CA with the RRA process, ECA takes one hour less due to the accelerated effect of the electropulsing and is not limited by the thick plates. The improved comprehensive performance of the ECA sample is due to both fine intragranular precipitates η′ and large discontinuous grain boundary precipitates η.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00404-2     

Influence of retrogression temperature and time on the mechanical properties and exfoliation corrosion behavior of aluminium alloy AA7150

The tensile properties, exfoliation corrosion behavior and microstructures of the retrogression and re-aging (RRA) treated aluminum (Al) alloy AA7150 were studied. AA7150 was retrogressed at different temperatures (175 °C, 185 °C and 195 °C) for various times. It is found that as the hardness of the retrogressed AA7150 approaches the near-peak condition, the corresponding RRA treated AA7150 possesses good exfoliation corrosion resistance without strength loss. By retrogressing at 175 °C, the retrogression time can be extended to 3 h, the RRA treated AA7150 possesses a strength as high as that of conventional AA7150-T6, and its exfoliation corrosion resistance is in the vicinity to that of AA7150-T73. This enhanced exfoliation corrosion resistance was associated with the more separated η precipitates at the grain boundary. AA7150-T6 is mainly strengthened by fine GP zones with high number density, while the intra-grain micro-structure of AA7150-RRA retrogressed at 175 °C for 3 h is characterized by relatively coarse η′ precipitates.     

Microstructural and mechanical properties of Al-Zn alloy 7075 during RRA and triple aging

Since aluminum is used in many essential applications, it has become a focus of researchers, mainly aluminum alloy 7075, because of its importance in the aircraft industry. The alloy 7075 incorporates high-strength materials such as Al-Cu-Mg, but with Zn through the primary alloy ingredient, instead of copper. Variations in the properties achieved in heat handling of Al-Zn-Mg ingredient 7075 are caused by solution and hardening process precipitation. The word heat-treatment solution means heat-treatment of a metallic structure to remove precipitated particles in the matrix. This work aims at studying the effect of solution treatment, the aging process, and the retrogression process on the mechanical properties of the Al 7075. Test measurements were taken by heating to 470 °C, intended for 30 min, then water quenching from goods, machines, and solutions. For example, not numerous of these collections were aged at R.T. Across 120 h. Other participants aged 24 h at 120 °C artificially. Then the retrogressed for 35 min at 180 °C; the group of these samplings was typically aged in R.T. across 120 h. Other groups were chemically aged for one day at 120 °C and retrogressed for 8 min at 200 °C, and all these samples were naturally aged at the average temperature for 120 h. Many classes were aged chemically for one day at 120 °C. Materials were evaluated by studying their microstructure, hardness, and tensile strength. It has been concluded that the best heat treatment values are the condition (1), indicating that the triple artificial aging gives the highest values of Hardness 49.4 HB and UTS 690 MPa, which were seen and convinced with the microstructure taken for each specimen.     

Extrusion Processing of High-Strength Al Alloy 7055

The effects of a combination of extrusion processing parameters and aging schedules on the microstructure and mechanical properties of the 7055 Al alloy were investigated. A safe extrusion processing zone is determined through a limit diagram constructed over the experimental initial billet temperature ranging from 380° to 420°C, extrusion ratio from 10:1 to 40:1, and the ram speed ranging from 1 to 15 mm s^-1. Microstructural characterization of as-extruded, solution-treated, and artificially-aged materials was carried out using polarized light microscopy (for grain structure) and transmission electron microscopy (for precipitate morphology). A combination of hardness and tensile tests was used to evaluate mechanical properties. It is shown that in 7055 Al alloy, the optimization of alloy composition, extrusion processing parameters, and peak aging treatment results in reproducible tensile properties of 0.2% P.S. = 725 MPa, UTS = 750 MPa, and % elongation = 12.9. In order to improve the stress-corrosion resistance of peak aged material, retrogression and reaging (RRA) temper was established. A strength-electrical conductivity relationship has been established for the RRA temper between 36% and 37% International Annealed Copper Standard (IACS) electrical conductivity to enable selection of suitable combination of properties.