A study of the mechanism of hardness change of Al-Zn-Mg alloy during retrogression reaging treatments by small angle X-r
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I.
INTRODUCTION
The Al-Zn-Mg alloys are susceptible to stress corrosion cracking (SCC). The degree of susceptibility depends on the aging condition and can be reduced by overaging, which leads to a sacrifice of the maximum strength. Recently, a new heat-treatment procedure called retrogression and reaging (RRA) was devised for 7000 system alloys by Cina.[1] This method produces a several-fold increase in the threshold stress for SCC over that of 7075T6 without a sacrifice of maximum strength. The RRA treatment consists of a short time retrogression anneal applied to 7075-T6 in a temperature range within the twophase field of the phase diagram, followed by water quenching and a final reaging treatment equivalent to the original T6 tempe.[1] Some authors studied the changes of microstructure during the retrogression and reaging in 7075 alloy by means of transmission electron microscopy (TEM), and some controversial views upon the nature of the benefits of the RRA treatment were proposed. Although some authors believed that the high strength of the 7075 alloy in reaged sample is considered to arise from the high overall concentration of the particle in this structure, no quantitative results have been obtained.[2–6] A Small Angle X-ray Scattering (SAXS) study on the change of microstructure in Al-Zn-Mg alloy during RRA treatments has been performed. On the basis of previous work,[7] this article investigated the mechanism of hardness change during RRA treatment quantitatively by means of SAXS.
CHAOFU MENG, Professor, HOUWEN LONG, Associate Professor, and YONG ZHENG, Student, are with the Materials Science Institute, Jilin University, Changchun 130023, People’s Republic of China. Manuscript submitted April 30, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL
The alloy composition is 1.5 wt pct Cu, 5.8 pct Zn, 2.4 pct Mg, 0.4 pct Mn, 0.5 pct Fe, 0.4 pct Si, and balance Al. The sizes of the sample were 35 3 10 3 0.08 mm3 for SAXS experiments and 20 3 10 3 1.5 mm3 for hardness measurement. The samples were polished mechanically on two surfaces with grits. The RRA treatment consists of three stages: (1) heating at 470 7C for 40 minutes, followed by water quenching and aging at 120 7C for 24 hours; (2) retrogression treatment at 200 7C for various periods, followed by water quenching; and (3) reaging at 120 7C for 24 hours. For the sample treated at each stage of the RRA treatments, SAXS intensity and Rockwell hardness with A scale were measured. The SAXS experiments were carried out on a D/max-rA diffractometer with a long-slits collimation system. The widths of the collimation slits are 0.04, 0.03, and 0.05 mm, respectively, while that of the detector slit is 0.1 mm. The Cu Ka radiation with a Ni filter, a tube current of 180 mA, and a tube voltage of 55 kV was used. A step scan technique was used with a step width of 0.02 deg, accounting time of 20 seconds for each step, and a scanning range from 0.1 to 2 deg. The background intensity was subtracted.
III.
RESULTS
To simplify the description, the
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