Lab1 / main

and correspondingly large elongation. After ECAP, the ultimate tensile strength is increased by 60attributed to considerable grain re nement through severe deformation by ECAP. The tensile elongation was, however, drastically decreased, by 31to the decrease of strain hardening capability after ECAP, which commonly occurs in many metallic alloys after ECAP [2].

3.3. Fatigue properties

Fig. 4 shows the S-N curves for unECAPed and ECAPed Ti with and without notches. Their S-N curves can be described by the

following equations

a = 311 N 0:023

f for plain specimens (unECAPed);

R = 0:92

Ti (=380 MPa) with the same grain size studied by Valiev et al. [4]. The tensile and fatigue test results of the unECAPed and ECAPed Ti are summarized in Table 1, together with the data from other investigators for pure Ti [4,7-9].

The fatigue notch factor, Kf , is de ned as follows:

where e and en are the nominal fatigue limit of the minimum cross-sectional area for plain and notched specimens, respectively. Based on the data in Fig. 4, the values of KF for the ECAPed and unECAPed samples were computed. The values were 2.12 and 1.28, respectively. The theoretical stress concentration factor Kt for the notched specimen

The result of the smooth bar tests will be discussed rst. According to Fig. 4, the fatigue limit e of the pure Ti increased from 210 and 350 MPa after ECAP, which is a factor of 1.67 increase. This result indicates that a signi - cant improvement in high-cycle fatigue life can be achieved by applying ECAP on pure Ti. This is in contrast to the cases for the Al and Mg alloys after ECAP where little enhancement in high-cycle fatigue performance was observed [5,6]. The fatigue limit of 350 MPa is comparable to that of the ECAPed