Tial Intermetallic Compound Fracture Mechanism

TiAl-based alloys with high specific strength are the most potential high temperature structural materials for aerospace application. However, the application of these materials is hindered by low resistance of damage, low room-temperature ductility and fracture toughness and high crack growth rate. There are reverse relationships between room-temperature fracture toughness and tensile ductility with the change of grain sizes in two-phase TiAl-based alloys. But the essence of the reverse relationships is not clear. In order to solve these problems, it is vital to investigate room-temperature fracture mechanisms of TiAl-based alloys.This paper sets about studying room-temperature fracture mechanisms of TiAl-based alloys of fully lamellar and duplex microstructures through the measurements of mechanical properties, in-situ observations of fracture surfaces and finite element method (FEM) calculations, the measurement of macro-parameters, the observations of fracture surface and FEM calculations on notch specimens (4PB) and pre-crack specimens (3PB), detailed observations of configuration changes at pre-crack tips in metallographic cross sections of specimens unloaded at various applied loads and FEM calculation. Based on the results of experiments, simulation and FEM calculation, the following are found: The values of mechanical properties measured in tensile tests are lower than those measured in compression tests. The inferior properties presented in tensile tests are caused by the damages of material, which are produced at much lower applied loads before yielding and resulted in more microcracks. Many micro-cracks initiate in the elastic condition, which shows the driving force initiating a cleavage crack is the tensile stress rather than the shear stress or the plastic strain. The interlamellar strength is lower than the translamellar strength, and even lower than the yield strength. The tensile properties of duplex microstructures are higher than that of fully lamellar, on the contrary, the fracture toughness of duplex microstructure are lower than that of fully lamellar, which is related to the grain sizes in a way. The intergranular fracture and finer interlamellar fracture are the dominant fracture mechanisms in the duplex microstructure materials, the fully lamellar microstructure materials resulted in more translamellar fracture. The fatigue fracture surface and mono-bending fracture surface presents almost the same mode.The cracks extend directly from a notch or a precrack controlled by the tensile stress. The superblunting, burification and deviating from direction of tensile stress of the crack-tip fronting an obstacle are the main toughening mechanism.