Effect of Mn dispersoid on the fatigue crack propagation of Al-Zn-Mg alloys
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Table I.
Chemical Composition of Alloys (in Weight Percent)
Alloy
Zn
Mg
Mn
Zr
Si
Fe
A1
1 2
4.20 4.26
1.70 1.54
0.10 0.79
0.15 0.15
0.01 0.01
0.01 --
bal bal
Effect of Mn Dispersoid on the Fatigue Crack Propagation of AI-Zn-Mg Alloys BONG LAE JO, DONG SEOK PARK, and SOO WOO NAM For commerical aluminum alloys, a number of studies have been conducted on the role of various dispersoids, such as Zr, Mn, and Cr, in affecting fracture toughness,t ~41 grain refinement,ts,6~ and quench sensitivity.I7.Sl In particular, much attention has been paid to the effect of Zr and Cr dispersoids on the mechanical properties of aluminum alloys. However, there have been few studies concerning the effect of Mn dispersoids on both monotonic properties and cyclic behavior. A previous studyI91has been carried out to investigate the role of Mn dispersoid in low cycle fatigue life of the A1Zn-Mg alloys. The main purpose of the present study is to understand the role of Mn dispersoid in the fatigue crack propagation behavior of A1-Zn-Mg alloys. The aluminum alloy whose composition is listed in Table I was prepared by melting 99.99 pct pure aluminum with the appropriate master alloys and casting in an inert atmosphere. The ingots were homogenized by heating for 24 hours at 733 K and were hot-extruded at 683 K with an extrusion ratio of 8.3. Subsequent aging treatments and tensile tests followed those reported in a previous study. I9] Thin foils for transmission electron microscopy (TEM) were prepared by electrolytic twin-jet polishing techniques.[lOl Fatigue crack propagation tests were performed with compact tension test specimens, as specified by the ASTM E647-88 standard. Fatigue test specimens were machined to a width of 40 mm from the full thickness of the plate in the L-T (crack growth perpendicular to fiber) direction. All testing was carried out in controlled room-temperature air using a servohydraulic testing machine (Instron model 1350) operating at 2 Hz sinusoidal frequency with a load ratio (R = gmin/Kmax) of 0.25. Crack lengths were measured using a
BONG LAE JO, Engineer, is with Ki-A Motor Company, Kyung-Ki Do, Korea. DONG SEOK PARK, Senior Engineer, is with the Korea Defense Quality Assurance Agency, Seoul, Korea. SOO WOO NAM, Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Korea. Manuscript submitted September 26, 1994. 490~VOLUME 27A, FEBRUARY 1996
Fig. 1--TEM micrographs showing the size and distribution of Mn dispersoids: (a) alloy 1 and (b) alloy 2.
Table II.
Tensile Properties
Alloy
YS (MPa)
UTS (MPa)
El. (Pct)
n
1 2
258 360
344 428
20 16.5
0.05 0.06
Note: YS = yield strength; UTS = ultimate tensile strength; and El. = elongation.
traveling microscope, and other testing was performed in accordance with ASTM E647-898. The TEM micrographs in Figure 1 show that the added manganese forms the rodlike Mn dispersoids. This result is consistent with the results in other articles,t",12] in which the added manganese d
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