Initiation of stress corrosion cracks in aluminum alloys
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graphite mold, enclosed in a stainless steel bomb and mounted in a vertical resistance heated furnace. The bomb was agitated vigorously to stir the melt. Unidirectional cooling and solidification were obtained by placing the base of the bomb on a large steel block, while retaining the upper part in the furnace. This technique minimized pipe formation and yielded an ingot of 25.5 mm diameter and 152 mm length. The ingots were homogenized, machined and sectioned into billets having a 24.4 mm diameter and 76 mm length. These were extruded at temperatures of 380 ~ (A1-Zn), 390 ~ (A1-Mg) and 415 ~ (A1-Mg-Zn) into rods having a diameter (D) of 3.86 mm.
2.2 Heat Treatment and Specimen Preparation The extruded rods were sectioned into lengths of 50 mm, solution treated (ST) and water quenched (WQ). Following ST, the A1-Mg and A1-Mg-Zn alloys were machined to shape and then aged to conditions of either peak hardness or slight overaging. The A1-Zn alloy was machined after peak aging, due to the low aging temperature (20 ~ of this alloy. All ST and aging treatments were conducted in air, with water quenching at 20 ~ The A1-Mg alloy was ST at 450 ~ for 0.5 h, WQ, aged at 200 ~ for 7 h (peak hardness) and WQ. The A1-Mg-Zn was ST at 460 ~ for 4 h, WQ, aged at 160 ~ for 24 h (slightly overaged) and WQ. The A1-Zn was ST at 500 ~ for 1 h, step quenched to 300 ~ for 0.5 h and aged for 1.5 h at 20 ~ 14. This corresponded to peak hardness, which remained unchanged for at least 11 days. The heat treated AI-Zn specimens were kept refrigerated until required. Machining of the specimens involved threading the ends of the sectioned rods to mate with tensile grips, and introducing a circumferential notch at the center. The angle of the notch was 60 deg, notch tip radius was 0.05 mm and diameter at the base of the notch (d) was 1.93 mm (i.e. d = D / 2 ) . It was not possible to machine the specimens before ST, because quenching of notched specimens from solutionizing temperatures introduced either distortion or cracking. All machining was carefully conducted on a jeweller's lathe.
ISSN 0360-2133/81/0413-0607500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, APRIL 1981--607
Final surface preparation of the notched and aged specimens involved etectropolishing in a solution of 45 vol pct H N O 3 + 45 vol pct CH3OH + 5 vol pct conc. HCI at 15 ~ Polishing was conducted at 4 to 6 V and 0.02 A / m m 2. Specimens were subsequently washed in water, dried and ultrasonically degreased in chlorethane for 0.25 h. Microscopical examination of specimens following electropolishing showed that the final notch tip radius was 0.05 mm. Conventional tensile tests were conducted on heat treated cylindrical specimens obtained from the extruded alloy rods. A gage length of 20 mm and gage diameter of 1.93 mm were employed. The 0.2 pct offset yield strengths for the aging treatments employed for scc are shown in Table I, together with the grain sizes.
Table I. Yield Strength and Grain Size of Heat Treated Alloys
Alloy
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