Impact Properties Of Ti-15Mo Alloy With TWIP Effect

?-type titanium alloys have been widely used in the aerospace,biomedicine and energy industries due to their structural and functional properties,such as high temperature oxidation resistance,excellent corrosion resistance,low elastic modulus,twinning induced plasticity/transformation induced plasticity(TWIP/TRIP)effects.Conventional titanium alloys have been severely limited the application in industrial production due to trade-off between strength and toughness.In addition,high plastic deformation ability and high resistance on crack initiation and propagation are key factors in determining high toughness.However,the new metastable?-type Ti-Mo alloys obtained good combination of strength and ductility via the deformation mode control.Due to{332}<113>twinning induced dynamic Hall-Petch effect,making that alloys have significant work hardening behavior,therefore,resulting in the higher uniform elongation.It was generally believed that the toughness of the materials was related to the product of strength-ductility.However,the response characteristics of{332}<113>twinning under dynamic impact loading and its effect on crack initiation and propagation behaviors have not been studied.In this paper,the Charpy impact properties of{332}<113>twinning type Ti-15Mo alloy and dislocation slip type Ti-15Mo-1Fe alloy were studied by optical microscope,scanning electron microscope and electron backscattering diffractometer.The response characteristic of twinning and its influence on crack initiation and propagation were discussed by characterizing the fracture morphology and deformation microstructure of alloys and instrumented impact testing analysis at room temperature.Simultaneously,the effect of temperature on the Charpy impact performance of alloys was discussed,meanwhile,the fracture mechanism and deformation microstructure evolution of alloys at different temperatures were analyzed.The results showed that Ti-15Mo alloy exhibited an high impact toughness of average250 J/cm~2,which was almost equivalent to four times of that Ti-15Mo-1Fe alloy.The microscopic fracture morphology was composed of elongated dimples,shallow dimples with different sizes and serpentine glide features.Large amounts of{332}<113>twins formed under impact loading,which induced adequate plastic deformation to absorb the impact energy.The formation of dense and fragmental twins effectively released stress concentration due to the dynamic grain refinement effect,and the abundant micro-cracks along{332}<113>twin boundaries remarkably increased the crack propagation path.Thus,the delayed crack initiation and enhanced crack propagation resistance further consumed the impact energy.The energy required for the crack propagation stage of Ti-15Mo alloy was higher than that of the initiation stage,and the influence of crack propagation process on impact toughness was more pronounced.The corresponding energy required for the crack initiation stage of Ti-15Mo-1Fe alloy was greater than the propagation stage,and the influence of crack initiation process on impact toughness was dominant.As the temperature decreased,impact toughness of Ti-15Mo and Ti-15Mo-1Fe alloys decreased continuously,and ductile-brittle transition temperature(DBTT)was-90°C and-10°C respectively.The fracture modes of the two alloys were all changed from dimple fracture to cleavage fracture,the number of dimples and the area fraction of shear lip decreased continuously.The number of twins in the grains and the propagation distance of the main crack were both decreased,that is,the plastic deformation ability and the resistance to crack propagation were gradually weakened.At the same test temperature,the twin area and the main crack propagation distance of Ti-15Mo alloy were higher than those of Ti-15Mo-1Fe alloy,that is,the plastic deformation degree of the former was larger than that of the latter,and the crack propagation resistance was higher than the latter,thereby absorbing more impact energy.Hence,based on the response characteristic of{332}<113>twinning,the Ti-15Mo alloy with TWIP effect possessed excellent impact properties.