The fracture morphology of the cast ti-48al-2cr-2nb alloy with total strain range δ ε (T) = ± 0.7% and service life 2NF = 1448 is shown in the figure. The macro morphology of the fracture shows obvious fatigue source region, crack growth region and instantaneous fracture region, as shown in Figure a.
Figure B, figure C, figure 8D and figure e show the morphology of fatigue source region and instantaneous fracture region respectively. It can be seen from Fig. 8b that the fatigue crack of the sample originates from the spot welding thermocouple. Due to the difference between the Ti Al alloy sample and the common metal sample, the thermocouple can not be welded by the common spot welding process. After the test, a more suitable welding process is found to ensure that the thermocouple will not fall off during the test. However, the fracture observation shows that the fatigue crack originated from the welding area and the structure of the source area changed (Fig. b).
This shows that the welding process has a certain impact on the fatigue performance of the sample, and the welding process should be further improved or the temperature measurement method should be changed to minimize the impact. After spot welding, the affected area is accompanied by interlaminar cracking and interlaminar cleavage fracture. In the further enlarged figure C, the flat interlaminar cracking in area a and the interlaminar cleavage fracture in area B can be seen, which shows a typical river pattern.
Due to the unique full lamellar structure of TiAl alloy, the fracture surface of its thermomechanical fatigue specimen is quite different from that of other superalloys. There is almost no fatigue band in the crack growth area, and there is almost no dimple in the instantaneous fracture area. The main cracking modes of Ti Al alloy with full lamellar structure are intralamellar fracture, interlaminar fracture and through lamellar tear [14-18]. Fig. B and Fig. C show that there are interlaminar and intralaminar cleavage fractures in the crack source area, and Fig. D instantaneous fracture area is almost all tear type fractures through the lamina. Further enlarged, we can see the secondary cracks of interlaminar cracks, as shown in Fig. e.