Influence of Aging on the Microstructure, Mechanical Properties and Stress Corrosion Cracking of High Strength Al-Zn-Mg-Sc-Zr Alloy
Date Issued
2008
Date
2008
Author(s)
Wu, Ling-Mei
Abstract
In this study, the effect of homogenization temperature on the recrystallization behavior and dispersoid distribution; while the influence of aging on the mechanical properties, microstructures and resistance to stress corrosion cracking (SCC) of an Al-Zn-Mg-Sc-Zr alloy have been investigated. The mechanical and SCC properties were investigated through microhardness, tensile and SCC tests. The microstructural characterizations were analyzed by using differential scanning calorimetry (DSC), Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy and high resolution transmission electron microscopy (TEM and HRTEM). In addition, the size and morphology of precipitates evolution after various isothermal aging have been studied together with small angle X-ray scattering (SAXS) and TEM. The experimental results show that an optimum homogenization is obtained by precipitating a large number and uniformly distribution of small particles that is optimized with regard to inhibit recrystallization. This is observed to have a lower recrystallization fraction in hot rolled and solution treated plate. Hence, a higher recrystallization fraction led to an increase in incoherent interfaces which provided very effectively heterogeneous nucleation site, and then the equilibrium η (MgZn2) phase easily nucleated on the dispersoids. Due to a decrease of solute supersaturation of the matrix, it would reduce the capabilities of precipitation hardening and lower the strength of the alloy. For the mechanical properties, hot extruded specimens show the highest strength than that in as-rolled condition. The higher values of mechanical properties for all tempers of hot extruded condition are a result of grain refinement. On the other hand, the peakaged condition (T6) exhibited the highest ultimate strength and the smallest elongation. It can be clearly observed that the UTS value of RRA temper was similar to the T6 temper, while the Two-Step temper had the lowest value. The wide precipitatedfree zone (PFZ) and coarse grain boundary precipitates of the Two-Step temper lead to the loss of strength comparing with the T6 and RRA tempers. However, the T7 temper indicated an overaged condition and had the widest PFZ and largest grain boundary precipitates lead to severely reduce the strength comparing with the T6, RRA and Two-Step tempers. The SCC susceptibility is the most severe for the T6 temper, intermediate for the RRA and Two-Step tempers, and minimal for the T7 temper. In T7 temper, the sizes of matrix precipitates and grain boundary precipitates are grater than those in the T6, RRA and Two-Step tempers, which make the planar slip more difficult. This results in decreasing of stress concentration at the grain boundary and greatly improving the SCC resistance, but loss of the strength. Thus, increasing the size of matrix precipitates and grain boundary precipitates can increase the SCC resistance of the alloys, because it can result in the change dislocation slip type from the planar slip to homogenous slip mode. In addition, the addition of Sc and Zr resulted in Al3(Sc,Zr) dispersoids that greatly refined grains and restrained recrystallization process, which enhanced the strength and SCC resistance. The concentrations of stress at the grain boundary may be reduced significantly due to the Al3(Sc,Zr) dispersoids suppress the planar slip of dislocation, which decreased the susceptibility to corrosion. This alloy artificially aged at 120°C for various time. It can be found that the hardness value increased with increasing aging time, and then reached the maximum hardness after 48h aging (peakaged T6 temper), finally decreased gradually with longer aging times. The initially increase is attributed to the nucleation and growth of the large number of coherent GP zones and semicoherent metastable η’ phase in the matrix. Hence, the decrease of hardness value for the overaged T7 temper due to the precipitates transformed gradually from the metastable η’ phase to the stable η phase. The structural features of this alloy are based on TEM and HRTEM observations, smallclusters formed rapidly after water quenching. These smaller clusters grow to spherical GPI zones at the earliest stages of aging. However, GPII zones have not presented after quenching from a solution temperature of 480°C and aging at 120°C. Besides, the presence of zirconium suppresses the formation of GPII zones. With increasing aging times, the GPI zones dissolve gradually and η’ precipitates form simultaneously. HRTEM result shown that, small edge-on η’ precipitates lying on {111}Al matrix planes and are close to the spherical GPI zone. After 48h aging, it is proposed that metastable η’ particles are plate-like precipitates on {111}Al with a size of about 7 nm. After 96h aging, the microstructure contains mainly η1 and η2 precipitates, with smaller amount of the η4 type and minor quantities of η’ particles. The η1-plates lying on {100}Al , η2-plates on {111}Al and η4-rods along <110>Al. Furthermore, the isothermal aging was performed at 100°C, 120°C and 140°C, respectively, for different periods of time. Aging hardness curves display the normal shapes: at 100°C aging, the hardness value increased with increasing aging time; at 120°C aging, a maximum hardness was reached before overaging; at 140°C aging, diffusion rates were faster and peak hardness was thereby achieved after shorter aging time. The peakhardness value is lower than that of 120°C aging. Both TEM observation and SAXS measurement obtain quantitative information about the size and morphology of precipitates at various aging conditions. SAXS results shown that, at 100°C aging, the size of precipitates have not show significant evolution from the nucleation radius during aging treatment. The size of precipitats increased rapidly at higher temperatures. The precipitates were only a few nanometers even at higher temperature (140°C). This is consistent with the TEM observations.
Subjects
Al-Zn-Mg-Sc-Zr alloy
Homogenization treatment
Aging treatment
Mechanical properties
Stress corrosion cracking
Microstructure
Al3(Sc,Zr) dispersoids
Type
thesis
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