Thermodynamics-Based Computational Design of Al-Mg-Sc-Zr Alloys

Alloying additions of Sc and Zr raise the yield strength of Al-Mg alloys significantly. We have studied the effects of Sc and Zr on the grain refinement and recrystallization resistance of Al-Mg alloys with the aid of computational alloy thermodynamics. The grain refinement potential has been assessed by Scheil-Gulliver simulations of solidification paths, while the recrystallization resistance (Zener drag) has been assessed by calculation of the precipitation driving forces of the Al(3)Sc and Al(3)Zr intermetallics. Microstructural performance indices have been derived, used to rank several alloy composition variants, and finally select the variant with the best combination of grain refinement and recrystallization resistance. The method can be used, with certain limitations, for a thermodynamics-based design of Al-Mg and other alloy compositions

Environmentally assisted cracking behavior of peak-aged 7010 aluminum alloy containing scandium

The 7010 Al alloy with and without addition of 0.25 wt pct Sc in peak-aged condition was examined for its environmentally assisted cracking (EAC) behavior. Slow strain rate testing (SSRT) per ASTM standard G129-00 was employed to investigate EAC. The base 7010 Al alloy showed 10 pct elongation, 9.9 pct reduction in area, and 561 MPa ultimate tensile strength (UTS), when tested in air. The ductility of the base alloy dropped to 3 and 3.3 pct in terms of elongation and reduction in area, respectively, when tested in 3.5 pct NaCl solution, showing its high susceptibility to EAC. On the other hand, the 0.25 wt pct Sc containing alloy showed a significant improvement in ductility not only in air but also in 3.5 pct NaCl solution, without any loss in the UTS. Thus, the 0.25 wt pct Sc containing alloy exhibited 13.4 pct elongation, 15.8 pct reduction in area, and 560 MPa UTS in air and 12.5 pct elongation, 16.4 pct reduction in area and 560 MPa UTS in 3.5 pct NaCl solution. The study for the first time shows that the high resistance to EAC of 7010 alloy can be imparted even in peak-aged condition by the addition of 0.25 wt pct Sc

Influence of heat treatment and scandium addition on the\ud electrochemical polarization behavior of Al-Zn-Mg-Cu-Zr alloy

In this study, the electrochemical polarization behaviors of Al-Zn-Mg-Cu-Zr (7010) alloy in three different heat treatments, namely, underaged, peak-aged, and overaged, were examined in 3.5 wt pct NaCl solution. Experimental results show that the cathodic current increases marginally\ud in the order of underaged < peak-aged < overaged alloys, which has been attributed to an increase in copper content of the precipitates in general and the grain boundary precipitates(GBPs) in particular. The change in the precipitate chemical composition has been found to\ud affect the anodic polarization behavior even in a more significant way. Thus, both the anodic polarization curves of underaged and peak-aged alloys exhibit two distinct breakdown potentials and current reversal immediately below the second breakdown potential, whereas such a\ud phenomenon is found to be absent in the overaged alloy. The overaged alloy exhibits only one breakdown potential without any current reversal. Detailed study of the polarization data and corroded surfaces of the alloy shows that the anodic current reversal is due to H2 evolution on\ud the alloy surface just after the occurrence of passive film breakdown along the grain boundary. Notably, it is only those heat treatments that are prone to intergranular corrosion (IGC) seems\ud to exhibit the tendency to reduce H+ ions, when they are anodically polarized. The chemical\ud composition of the precipitates that can be altered by heat treatments is responsible for this\ud behavior. The addition of 0.25 wt pct scandium to type 7010 Al alloy did not show any\ud improvement in the corrosion resistance of the alloy. The Ecorr of scandium containing alloy\ud shifted toward the active direction as compared to the base alloy. Noticeably, the peak-aged\ud scandium containing alloy also exhibited two distinct breakdown potentials in the anodic\ud polarization curve similar to the peak-aged base alloy, thus revealing its susceptibility to IGC\ud and pitting corrosion

The diffusion coefficient of scandium in dilute aluminum-scandium alloys

The diffusion coefficient of Sc in dilute Al-Sc alloys has been determined at 748 K, 823 K, and 898 K (475 °C, 550 °C, and 625 °C, respectively) using semi-infinite diffusion couples. Good agreement was found between the results of the present study and both the higher temperature, direct measurements and lower temperature, indirect measurements of these coefficients reported previously in the literature. The temperature-dependent diffusion coefficient equation derived from the data obtained in the present investigation was found to be D(m2/s)=(2.34±2.16)*10-4(m2/s)exp((-(167±6)(kJ/mol))/RT). Combining these results with data from the literature and fitting all data simultaneously to an Arrhenius relationship yielded the expression D(m2/s)=(2.34±0.84)*10-4(m2/s)exp((-(167±2)(kJ/mol))/RT). In each equation given above, R is 0.0083144 kJ/mol K, T is in Kelvin, and the uncertainties are ±1 standard error