13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- stress level or the decrease in frequency to 6 Hz alters the shear mode to the tensile mode. In the present study, microstructural analyses of the extruded 7075 T6 Al specimens subjected to fatigue tests were carried out by EBSD and XRD, in order to obtain an insight into the influence of extrusion texture on the fatigue properties in high humidity environment. Since EBSD analyses are restricted in local regions and insensitive to internal stresses, XRD analyses were also conducted to obtain the information on the macroscopic texture structure as well as the internal stresses remaining due to extrusion. The microstructural analyses were carried out on another specimen into which different microstructures were introduced by using different heat treatment. In order to deepen the understanding of the relation of fatigue properties influenced by humid environment with microstructures, some of the previous results on fatigue tests were reassessed and compared with the results of microstructural analyses. 2. Experimental procedure Commercial rods of 7075 T6 Al alloy with the diameters of 18 mm were used as a starting material. The alloy has the chemical composition listed in Table 1, but the degree of extrusion was not known. Some of the as-received rods were solution treated at 733 K for 10.8 ks, and some rods were solution treated at 773 K for 118.8 ks. These solution treatments were carried out to increase grain sizes. The solution treated specimens were aged at 393 K for 86.4 ks, i.e., under ordinary T6 temper condition. In order to distinguish the heat conditions, the as-received specimens and the specimens solution treated at 733 K and 773 K will be hereafter called A, B and C specimens, respectively. Table 2 shows the tensile properties and grain size of these rods. It is clear from this table that the solution treatment increased grain size considerably, and lowered the yield strength markedly compared to that of as-received ones. The fatigue tests were conducted for A and B specimens, while C specimens were used only for the comparison of microstructure with these specimens. A specimens were subjected to rotating bend (RB) tests at 50 Hz and ultrasonic loading (US) tests at 20 kHz. On the other hand, B specimens were RB-tested at 50 Hz and 6 Hz. The shape and size of the fatigue test specimens are shown in Fig. 1. The humidity was varied from 0% RH in N2 gas to 100% RH in distilled water. The details of the fatigue tests and conditions had been mentioned in the previous studies [3-7]. EBSD and XRD analyses were conducted by using disk specimens with a height of 10 mm cut from A, B and C rods (Fig. 2). The surfaces of these specimens to be used for EBSD analyses were mechanically polished to the final polishing with 20 nm silica particles, followed by low-energy Ar ion etching. XRD profiles were obtained by using Cu Kα beam from the same transverse cross-section as was subjected to EBSD analyses. Subsequently the disc specimens were cut longitudinally at the Table 1. Chemical composition of 7075 T6 Al. Element Zn Mg Cu Cr Si Fe Mn Ti + Zr Al Mass fraction (%) 5.46 2.56 1.47 0.19 0.09 0.25 0.03 0.03 bal. Atomic fraction (%) 2.35 2.96 0.65 0.10 0.09 0.13 0.02 0.01 bal. Table 2. Tensile properties and grain size of 7075 T6 Al. Specimen Heat treatment Yield strength (MPa) Tensile strength (MPa) Reduction in area (%) Nominal grain size (μm) A as-received ordinary T6 630 691 9.5 8 B 733 K, 10.8 ks 393 K, 86.4 ks 527 673 11.3 13 C 773 K, 118.8 ks 393 K, 86.4 ks 553 685 11.6 40
RkJQdWJsaXNoZXIy MjM0NDE=