PSI - Issue 13

Yoshimasa Takahashi et al. / Procedia Structural Integrity 13 (2018) 1010–1013 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1011

2

Aluminum (Al) and its alloys, typical non-ferrous metals, on the other hand, do not possess such a characteristic. This necessitates engineers to use a cycle- dependent strength that needs to be evaluated through time-consuming high-cycle fatigue tests. This motivated the authors to develop an Al alloy having a distinct fatigue limit. Since the existence of fatigue limit in ferrous steels had been connected to strain-aging (e.g. Sinclair (1952)), the applicability of such a concept to an Al alloy was examined by Shikama et al. (2012). They added small amount of extra magnesium (Mg) to a standard 6061-T6 alloy and successfully observed a distinct fatigue limit. The new Al alloy also showed eminent strain aging capability represented e.g. by serrated flow (Portevin-Le Chatelier effect) and negative strain-rate-sensitivity, see Takahashi et al. (2015). Although the drastic effect of Mg on fatigue property was interesting, the results were obtained by experiments conducted only in ambient air. In order to probe the effect of Mg on small crack behavior that controls the fatigue limit, a comparative experiments conducted under different environments may provide valuable information as the crack tip slip in Al alloys is known to be influenced by the environment, see, e.g., Lynch (1988), Ro et al. (2012). The aim of this study is to clarify the effect of environment on the fatigue limit property of the new alloy. 2. Materials and methods The material used in this study was fabricated by adding excess Mg (ca. 0.5wt%) to the base alloy (standard 6061 alloy) having a stoichiometric Mg 2 Si composition. Table 1 shows the chemical composition of the new Al alloy along with the base alloy. The ingot of the new alloy was die-extruded and shaped into a f 23 mm rod. Since the excess Mg is known to reduce the solubility of Mg 2 Si, the alloy was solutionized at relatively high temperature to promote the solution of Mg 2 Si (813 K, 1 hour) followed by water quenching. Then the alloy was aged (463 K, 4 hour) to obtain the peak hardness. Fig. 1 shows the shape and dimension of a fatigue specimen. The longitudinal direction of the specimen is parallel to the extrusion direction. A small blind hole ( f 300  m) was introduced at the centre of the gauge portion. The fatigue test was conducted by using a cantilever-type rotary bending machine operated at 50 Hz. The test environments were ambient air, dry air and dry nitrogen. The environment around the specimen, except for the ambient air, was controlled by supplying the gas into an attached gas chamber.

Table 1. Chemical composition of Al alloys used in this study (wt%). Mg Si Fe Cu Mn Ti

Cr

Zr

Al

New alloy Base alloy

1.43 0.95

0.54 0.52

0.19 0.20

0.20 0.20

0.09 0.09

0.02 0.02

0.25 0.23

0.16

Bal. Bal.

-

A

f 0.3

A

20 f 5 R50

f 10

0.3

25

5

5

45

Fig. 1. Shape and dimension of fatigue specimen.

3. Results and discussion Fig. 2 shows the S - N curves of the new alloys. In each environment, the number of cycles to failure ( N f ) abruptly increases from the 10 6 -10 7 order to the run-out cycles (10 8 ) as the stress amplitude (  a ) decreases. The fatigue limit indicated in the figures are: 92.5 MPa for ambient air, 87.5 MPa for dry nitrogen and 82.5 MPa for dry air. Note that such a behaviour was not observed for the base alloy. Figure 3 shows the relation between fatigue cycles and crack length for run-out specimens. The crack length shown here is defined as the projected total length including the hole diameter. The crack growth is shown to be retarded soon after the initiation and almost arrested regardless of the environment. The fatigue limit shown in Fig. 2 is therefore regarded as the threshold  a against small crack propagation. Figure 4 shows the results of  a step-up tests for pre-cycled specimens. After the pre-cycles (10 8 ), the specimens were loaded at a slightly higher  a (+ 2.5 MPa) until another cycle set (10 7 ) was achieved. The step-up/fatigue was repeated until the final failure. The results