The Effect Of Multi-level Microstructure On Mechanical Properties Of TC21 Alloy

With the development of aerospace devices toward large-scale,high-speed and complex,the selected criteria for aircraft are changed from the past "safety-life" to "damage tolerance design".The lamellar structure is more conducive to the damage tolerance performance of the alloy.Although a large number of studies have been carried out on the relationship between lamellar microstructure and strength and toughness,there is no report on the structural control unit of strength and toughness of the alloy,especially the coupling relationship between multi-level lamellar microstructure and strength and toughness in titanium alloys is also rarely studied.In the present paper,TC21 alloy was used as the research object.Firstly,the DIL805A/D phase transformation instrument combined with OM,SEM,EBSD,and TEM was performed to study the effect of heat treatment process on multi-level lamellar microstructure parameter.Then the relationship between the mechanical properties such as tensile properties,impact toughness and fracture toughness and the multi-level lamellar microstructure of TC21 alloy was analyzed by Hall-Petch formula to find the effective microstructure unit for controlling strength and toughness of TC21 alloy.The main conclusions of the study are as follows:When the annealing temperature is constant,the ? grain size and the ? colony size increase with the increase of the solution temperature while the average size of the ? plate decreases slightly with the solution temperature increase.The tensile properties(tensile strength,yield strength,elongation and reduction of area)of TC21 alloy decrease with increasing solution temperature,but the toughness(impact toughness and fracture toughness)increases.The ? grain size remains basically constant with the increase of the annealing temperature when the solution temperature is constant,but ? colony and ? plate grow gradually.The tensile strength and yield strength of TC21 alloy decrease with increasing annealing temperature,while plasticity(elongation,reduction of area)and toughness(impact toughness,fracturetoughness)increase gradually.The tensile strength and yield strength of TC21 alloy follow the Hall-Petch relationship with the reciprocal square root of ?-grain size and ? colony size,and ?colony is regarded as an effective control unit for controlling strength of TC21 titanium alloy.Under the different solution treatment conditions,the fracture toughness and impact toughness obey the Hall-Petch relationship with the reciprocal square root of the thickness of the ? plate.The thickness of the ? plate is an effective control unit for controlling fracture toughness and impact toughness.Under different annealing mechanism,both the reciprocal square root of the ?colony size and the ? plate cannot obey the Hall-Petch relationship with fracture toughness.The volume fraction of low angle boundaries increase with the increasing of annealing temperature.Meanwhile,the increase of the fracture toughness is mainly determined by the bending,rotation and final shear of ? plate during the plastic deformation process by EBSD analysis.Therefore,? plate is regarded as microstructure unit for controlling the fracture toughness of the TC21 alloy.