The Study on the Crack Initiates and Propagates Mechanism of TC11 Alloy in the Forging Processing
|School||Harbin Institute of Technology|
|Course||Materials Processing Engineering|
|Keywords||TC11 alloy Micro fracture behavior Hot compressive deformation SEM in situ tension TEM in situ tension|
The fracture is one of the most common defects in metal forming. Through the macro-micro experimentals to quantitative observational research and theoretical analysis, it can provides a certain basis of experimental and theoretical for a quantitative theory which use the internal structure of materials, defects and stress-strain distribution to forecast and control the damage law of metal forming. In the present work, the hot compressive tests were used to study the surface cracking process of fine grained structure and widmannstatten structure of TC11 alloy in high temperture. And the techniques of in-situ tension under scanning electron microscope(SEM) and transmission electron microscope(TEM) are used to study the processes of plastic deformation, cracking initiation and propagation in them. The analysis techniques at the same position by HRTEM are also used in the nanoscopic zone of crack-tip.Detailed studies on surface cracking process and fracture feature in the hot compressive tests of TC11 alloy have been carried out. Different structures of TC11 alloy have have different micro-mechanism of elevated temperature fracture behaviror. The fine grained structure give priority to microvoid accumulation fracture behavior. The widmannstatten structure give priority to fracture during lamellar structures and translamellar fracture. At high temperature the plastic of fine grained structure is better than the plastic of widmannstatten structure. Through the SEM anlysis of fracture and structure, the result gave that the macro cracks have relationship with stress-strain distribution and the micro crack initiation and propagation relate to the phase structure distribution.The detailed studies of in-situ tension under scanning electron microscope show that because of the microstructure at room temperature is similar to the microstructure at high temperature, they have same fracture mechanism. The major crack propagation of fine grained structure is connect micro cavities in front of crack tip. The cavities and micro-crack are initiated in the phase boundary and the dimple feature is fine texture. The crack of widmannstatten structure propagate during the phase lamellas. It also connect the micro cavities initiated in translamellar fracture. Its fracture have much shear glide plane and large dimples.Studies about the relationship between structure, dislocation and crack initiation and propagation use the technique of in-situ tension under TEM to observe and anlysis. The results show that the crack first choose to initiate and propagate inβphase or phase boundary. When the micro-cracks were blocked by largeαphase of fine grained structure orαphase lamella of widmannstatten structure, the tip of translamellar crack emittes ring dislocation group inαphase lamella. The piling up of dislocation groups change the crack propagation. The observation under HRTEM show that because of damage the atom arrangement of crack-tip change from long range order to short range order. This studies provide a certain experiment basis for froming mechanism of dislocation-free zones in front of crack-tip.