Research On Microstructure Control And Fatigue Behavior Of TiZr-based Alloys

An increasing scarcity of resources and their growing expense demands a reduction in energy consumption for passenger and goods transportation and here the aerospace sector plays a special role with respect to the application of high specific-strength materials.Owing to high specific strength and attractive corrosion-resistant properties TiZr-based alloys can be used as structural materials and will inevitably be subjected to alternation load which leads to fatigue failure.It has been estimated that 90% of mechanical failures of metallic components occur due to fatigue and it is very important to improve the service life of structural components by understanding the fatigue properties and mechanisms of applied materials.In this paper the microstructure can be controlled by different processing technology and the investigations on the fatigue behavior of TiZr-based alloys with different microstructures not only have practical values,but also provide a reliable theoretical basis for the fatigue resistant design and safety usage of structural components.Equiaxed microstructure of T20 Z alloy was obtained through appropriate hot working technology and the globularization mechanism was analyzed.It is found that the fine lamellar microstructure before rolling and low deformation temperature benefit globularization procedure.Both dynamic recrystalization and boundary splitting occur during hot rolling process and fine globularized particles grow into equiaxed grains when annealing at temperature below 850?.By post-deformation annealing following water quench the fine equiaxed T20 Z alloy shows an average yield strength of 1000 MPa,ultimate tensile strength of 1073 MPa and ductility of 20.18%;compared with the coarse lamellar microstructure by direct annealing,the tensile strength and ductility are improved by 13.5% and 124.9%,respectively.The obtainment of equiaxed microstructure have changed the embarrassing situation of T20 Z alloy with only lamellar microstructure and laid the cornerstone for microstructural diversity,therefore the microstructure can be controlled according to the working conditions.High cycle fatigue test of T20 Z alloy with lamellar microstructure was conducted at different temperatures using MTS electro-hydraulic servo test system.The result shows that T20 Z alloy demonstrates a high fatigue endurance limit of 775 MPa at room temperature and fatigue ratio of 0.67 which is higher than other common metallic materials,besides the equation of S-N curve is fitted as ?max=775+52444.13×?2N_f?^-0.519 as a matter of convenience for engineering applications.With increasing the temperature from -60? to 100?,the fatigue lifetime decreased gradually at a same stress level.Combined with TEM observation on dislocation configurations near the fracture surface,it is believed that severe local stress concentration due to extensive dislocation pile-ups at ?/? interf ces is responsible for the cr ck initiation.Effect on microstructure on high cycle fatigue property and crack growth behavior was investigated and found that the fatigue limit of equiaxed T20 Z alloy is 575 MPa,which is lower than that of lamellar microstructure?775MPa?.At stress levels above 825 MPa,the fatigue lifetime of equiaxed microstructure is longer than that of lamellar microstructure and opposite result is observed below 825 MPa.The fatigue crack growth rate of equiaxed microstructure is always higher than that of lamellar microstructure at both low and high stress ratio?0.1 and 0.6?.As for the same microstructure,the fatigue crack growth resistance increases with the decrease of stress ratio.Based on the results,it is believed that due to better compatible deformation capability of the equiaxed T20 Z alloy than the lamellar one,the fatigue crack initiation lifetime is longer at high stress level above 825 MPa.The fatigue behavior of as-cast Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass was systematically investigated under tension-tension and three-point bending mode.The fatigue endurance limit under tension-tension mode is 630 MPa which is 2.8 times higher than that under three-point bending condition?225MPa?.However,the fatigue lifetimes are almost the same at stress level higher than 675 MPa under the two fatigue conditions.The results have been analyzed and indicate that fatigue crack would initiate quickly from shear bands or defects when the stress level is higher than 675 MPa and the total fatigue lifetime is controlled by crack growth lifetime.Otherwise the total fatigue lifetime is determined by crack initiation lifetime because fatigue crack always initiates from weaker defects rather than shear bands,the quantity of defects determines the total fatigue lifetime.The study suggests that defects have a significant influence on the mechanical properties of bulk metallic glasses and if the quantity of defects was decreased by downsizing bulk metallic glasses,the security and reliability of bearing structural components would be improved efficiently.