The influence of {332}<113> deformation twinning on the fatigue behavior of Ti-Mn system alloys were investigated focusing on fatigue strength, fatigue crack initiation and propagation. Ti-7Mn (TM7) and Ti-5Mn-3Mo (TMM53) (mass%) alloys which are primarily deformed by dislocation slips and {332}<113> deformation twins, respectively, were subjected to fatigue tests. TMM53 which is deformed primarily by {332}<113> deformation twinning shows higher fatigue strength than TM7 in whole fatigue life region, while these alloys have similar ultimate tensile strength. It was found that the {332}<113> deformation twins are partly responsible for the plastic strain accumulation during cyclic deformation instead of dislocations. Therefore, {332}<113> deformation twinning leads to low dislocation density, and consequently, fatigue crack initiation is suppressed. In addition, {332}<113> deformation twins formed around a crack tip decreases the stress concentration at the tip, resulting in lowering of crack propagation rate.
Table 1 shows tensile properties of studied alloys. These alloys show completely different tensile properties, while they have same phase stability. TMM53 shows comparable ultimate tensile strength as TM7 while yield stress of the alloy is lower than that of TM7. It is also noted that TMM53 exhibits larger EL than TM7.
Figure 1 shows deformation structure of TMM53. As shown in Fig. 1, band like deformation structures which have a misorientation angle of approximately 50.5 degree can be seen in the tensile deformed alloy. This result suggests that the {332}<113> deformation twinning takes place in the alloy during plastic deformation.
Figure 2 shows the maximum cyclic stress-number of cycles to failure (S-Nf) curves for studied alloys, evaluated by cyclic deformation at a stress ratio (R) of 0.1 and a frequency (f) of 10 Hz. The fatigue strength of TMM53 is higher than that of the TM7 in whole fatigue life region, though these alloys have similar UTS. In addition, it was found that there is no significant difference in the fracture surfaces of these alloys. Therefore, it is supposed that the variations in the fatigue strength of these alloys are due to the differences in their fatigue behavior such as crack initiation and propagation during cyclic deformation.
Figure 3 shows transmission electron microscope (TEM) images of TM7 and TMM53 cyclically deformed to 1000 cycles at a maximum applied stress (σmax) of 450 MPa. Numerous dislocations tangled each other can be seen in TM7. On the other hand, as shown in Fig. 4, {332}<113> deformation twins can be seen in TMM53 cyclically deformed to 34086 cycles at σmax = 600 MPa. These results indicate that not only dislocations but also {332}<113> deformation twins are responsible for the plastic strain accumulated during cyclic deformation. As well known, the fatigue crack initiation is caused by high density of dislocations. Thus, it is supposed that the crack initiation life of TMM53 is longer than that of TM7.
Moreover, the average widths of striation for TMM53 is smaller than that for TM7. This indicates that the crack propagation rate of TMM53 is lower than that of TM7. Figure 5 shows an optical microscope (OM) image of crack growth in TMM53 cyclically deformed to 34086 cycles at σmax = 600 MPa. The crack and the twins intersect each other, and consequently, the crack is deflected at {332}<113> deformation twins. This means that the twin boundary acts as an effective barrier to the crack propagation.
