Study On The Notch Sensitivity Of K418 And TiAl Alloys

In this paper, tensile macroscopic mechanical properties and fracture mechanism of TiAl and K418 alloys were studied for different notch root radius flat and round bar specimens, based on the measurements of macro-mechanical properties and notch sensitivity, observation of the fracture surface and microstructure, and the detailed FEM calculations of the specimen with different notch root radius. The main results are as follows:(1) the mechanical properties analysis of the double-notched flat specimen, notched rod specimen, and new specimen with multi-notches indicated that:there are a great different in notch sensitivity for TiAl alloy and K418 alloy:K418 alloys have no notch sensitivity. However, a greater notched sensitivity appears in the TiAl alloys. Therefore, K418 cast defect tolerances standard cannot be directly used on TiAl alloy. (2) SEM fracture surface analysis showed that the fracture mode of K418 is composed of ductile and brittle fracture at the room temperature, ductile fracture is induced by dimple formation, expansion, integration, and its controlling factors is the plastic deformation. The specimen fracture finally due to propagation and connection of internal defects, such as shrinkage; the fracture mode of TiAl alloy is a typical cleavage fracture at room temperature, the controlled factor is the normal stress, and the crack initiation origins of the notch specimens are located at the root, no casting defects are found. So, the fracture mechanisms of TiAl and K418 alloys are different, which results in their different notch sensitivity. (3)FEM analysis results show that:the normal stress syy/sy, the stress triaxiality the equivalent plastic strain e increase with the increasing of the load P and decreasing of the root radius of notches. Specimen deformation is induced from the notch root, but the maximum normal stress is located at a distance from notch root. (4) The results of tensile test for new multi-notch rod specimens show that:when the notch depth is 10% of the diameter of the specimen, fracture strength decreased, while notch depth reaches 20% of the diameter of the specimen, the fracture strength is decreased sharply.