Study On The Microstructure Parameters And Mechanical Properties Of Multi-layered TC21 Titanium Allo

With the high-speed development of aerospace industry,marine engineering,and high-end equipment,traditional metallic materials have been difficult to meet the service requirements of key mechanical components.Due to its excellent mechanical properties,TC21 titanium alloy is universally acclaimed.The multi-level lamellar microstructure has excellent damage tolerance property and is one of the typical service microstructure of high-strength titanium alloys,so an in-depth study of its process-microstructure-property relationship is very important to ensure the safety and stability of high-strength titanium alloy components.This paper explored the relationship between heat treatment process parameters and multi-level lamellar microstructures of TC21 alloy by using optical microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,laser scanning confocal microscope（LSCM）and electron backscatter diffraction（EBSD）;investigated in depth how multi-level lamellar microstructure of the alloy affected mechanical properties under static and impact loads;identified strong plasticity control unit of the alloy through the fitting of the Hall–Petch formula;systematically studied crack initiation and crack propagation,and established the crack propagation mechanism model.The main conclusions are as follows.After the triple heat treatment,the element distribution and microstructures are greatly changed.Ti,Al elements are less at theα/βinterface,while Mo elements increase at theα/βinterface.The solid solution temperature mainly affects theβgrain size;different annealing and aging temperatures cause segregation of alloy elements,leading to the different microstructure.As the annealing temperature increases,the size ofα_c increases,the width ofα_p andβ_t increases gradually,and the number ofα_sprecipitated onβ_t increases sharply,while its size decreases;the aging temperature basically does not change the microstructure characteristics ofβgrains,G_αandα_c.The higher the aging temperature corresponds to the larger size ofα_s,the less number,and the firstly increase and then decrease in V_α.As observed,the increasingα_c size and decreasingα_p andα_s size are beneficial to the increase of strength and the decrease of plasticity by Hall-petch formula fitting.α_c,α_p andα_s are the control units of strength,andα_s is the minimum unit.The tissue parameters,α-phase volume fraction and alloy element segregation jointly determine the tensile properties of TC21 titanium alloy.The fineα_s increases the total phase boundary area,reduces the dislocation accumulation and element segregation,and thus effectively increase the alloy toughness.In addition,in the mixed fracture mechanism,the larger the area ratio of the microvoid area to the fracture,the greater the toughness.Although a large number of cleavage fractures and intergranular fractures increase the crack propagation path,both of these fractures belong to low-energy interfaces which contribute little to the toughness.In the study of crack initiation and crack propagation,it was found that crack initiation occurs easily when the stress is concentrated at theα/βinterface and G_α,and the crack initiation process follows the strength principle and stress principle.The wideα/βphase interface and grain boundaries make it easy for dislocation accumulation,therefore,many cleavage fractures and intergranular fractures are found in the fractures,both of which contribute to the increase of crack propagation paths.Meanwhile,after the analysis of various fractures and fracture profiles,it was found that theα_c size has a great influence on the crack propagation path,and larger or smaller size ofα_c is not conducive to the change of crack propagation direction.