Effects of Quenching Cooling Rate on Residual Stress and Mechanical Properties of a Rare-Earth Wrought Magnesium Alloy

To investigate the effect of quenching rate on microstructure, residual stress (RS) and mechanical properties of a rare-earth wrought magnesium alloy Mg-Gd-Y-Zr-Ag-Er, RS in 20 °C water quenching (WQ (20 °C)), 100 °C water quenching (WQ (100 °C)) or air cooling (AC) conditions were measured and compared with the simulation results, corresponding mechanical properties and microstructure in quenching and aging state were studied. The decrease of quenching rate has little effect on the grain size but makes the twinning disappear, precipitates increase and the texture weakened, leading to easier brittle fracture after aging. WQ (100 °C) is the best quenching condition in this study, with a significant decline in RS and only 4.9% and 3.7% decrease in yield stress (YS) and hardness compared with WQ (20 °C). The results make it feasible to invent an appropriate quenching method of greatly reducing RS while maintaining mechanical properties. The research conclusions would be beneficial to the application of the alloy.

Effects of the Quenching Rate on the Microstructure, Mechanical Properties and Paint Bake-Hardening Response of Al-Mg-Si Automotive Sheets

The quenching rate of Al-Mg-Si alloys during solution treatment is an important parameter for the automotive industry. In this work, the effect of the different quenching rates on the microstructures, mechanical properties, and paint bake-hardening response of Al-Mg-Si sheets was studied. Large dimples form on the fracture surface of a sample at a quenching rate of 0.01 °C/s. When the quenching rate increased to 58.9 °C/s, the dimples became smaller. The recrystallized grains and textures were slightly affected by quenching rates beyond 1.9 °C/s. Thus, higher r values of the samples were achieved with slower quenching rates. Furthermore, only the Al(FeMn)SiCr insoluble phases were observed in samples with a rapid quenching rate. Sufficient solute atoms and vacancies resulted in the improvement of the precipitation kinetics and paint bake-hardening capacity for Al-Mg-Si sheets at rapid rates. With a decrease in the quenching rate, the formation of the rod-like coarse β' phases consumed many solute atoms and vacancies, leading to the deterioration of the paint bake-hardening capacity. This study provides a critical reference on quenching rates for industrial practices, so that good mechanical properties can be achieved using precision control of the quenching process.

Development of a Singlet Oxygen Absorption Capacity (SOAC) Assay Method. Measurements of the SOAC Values for Carotenoids and α-Tocopherol in an Aqueous Triton X-100 Micellar Solution

Recently, a new assay method for the quantification of the singlet oxygen absorption capacity (SOAC) of antioxidants (AOs) and food extracts in homogeneous organic solvents was proposed. In this study, second-order rate constants (k_Q) for the reaction of singlet oxygen (¹O₂) with eight different carotenoids (Cars) and α-tocopherol (α-Toc) were measured in an aqueous Triton X-100 (5.0 wt %) micellar solution (pH 7.4, 35 °C), which was used as a simple model of biomembranes. The k_Q and relative SOAC values were measured using ultraviolet-visible (UV-vis) spectroscopy. The UV-vis absorption spectra of Cars and α-Toc were measured in both a micellar solution and chloroform, to investigate the effect of solvent on the k_Q and SOAC values. Furthermore, decay rates (k_d) of ¹O₂ were measured in 0.0, 1.0, 3.0, and 5.0 wt % micellar solutions (pH 7.4), using time-resolved near-infrared fluorescence spectroscopy, to determine the absolute k_Q values of the AOs. The results obtained demonstrate that the k_Q values of AOs in homogeneous and heterogeneous solutions vary notably depending on (i) the polarity [dielectric constant (ε)] of the reaction field between AOs and ¹O₂, (ii) the local concentration of AOs, and (iii) the mobility of AOs in solution. In addition, the k_Q and relative SOAC values obtained for the Cars in a heterogeneous micellar solution differ remarkably from those in homogeneous organic solvents. Measurements of k_Q and SOAC values in a micellar solution may be useful for evaluating the ¹O₂ quenching activity of AOs in biological systems.

Kinetic study of the quenching reaction of singlet oxygen by α-, β-, γ-, δ-tocotrienols, and palm oil and soybean extracts in solution

Measurements of the singlet oxygen ((1)O2) quenching rates (kQ (S)) and the relative singlet oxygen absorption capacity (SOAC) values were performed for 11 antioxidants (AOs) (eight vitamin E homologues (α-, β-, γ-, and δ-tocopherols and -tocotrienols (-Tocs and -Toc-3s)), two vitamin E metabolites (α- and γ-carboxyethyl-6-hydroxychroman), and trolox) in ethanol/chloroform/D2O (50:50:1, v/v/v) and ethanol solutions at 35 °C. Similar measurements were performed for five palm oil extracts 1-5 and one soybean extract 6, which included different concentrations of Tocs, Toc-3s, and carotenoids. Furthermore, the concentrations (wt%) of Tocs, Toc-3s, and carotenoids included in extracts 1-6 were determined. From the results, it has been clarified that the (1)O2-quenching rates (kQ (S)) (that is, the relative SOAC value) obtained for extracts 1-6 may be explained as the sum of the product {Σ kQ(AO-i) (S) [AO-i]/100} of the rate constant (kQ(AO-i) (S)) and the concentration ([AO-i]/100) of AO-i (Tocs, Toc-3s, and carotenoid) included.