Research On Evolutions Of Mechanical Properties And Spall Fracture Behavior Of Titanium Alloys At The Dynamic High-Pressure Loading

Tita nium a lloys are wide ly use d in nationa l defe nse, aviation,aerospace a nd other fie lds because of the ir be tter machina bility, high spec ific stre ngth, low density and corrosion resista nce. However,whe n titanium a lloys are used in these fie lds, they must be subjected to dyna mic high-pressure loading. Given this, the shock response of two titanium a lloys, Ti-6Al-4V and Ti-47 Nb, are syste mica lly investigated in this thesis. The effects of a lloying c omponent, phase and morphology on the mecha nica l properties and spa ll fracture be havior of tita nium alloys loa ded at high-pressure are explored through plate impact experime nts, quasistatic a nd dynamic re loa d compression tests at room te mperature, as we ll as careful microscopic a na lyses using optica l microscopy(OM), scanning e lectronic microscopy(SEM) and X-ra y diffraction(XRD).The major conclusions are as follows.The measured values agree well with the estimated impact Hugoniot parameters(ρ0, C0 and λ0) and impact pressure(PH). Under the range of estimated impact pressure(PH<15 GPa), two titanium alloys follow the Dugdale-Mac Donald formula, and the relationship between shock velocity and particle velocity(D-u) of them is general linear. The relative location of shock Hugoniot in forms of impact pressure versus particle velocity(PH-u) from bottom to top for two alloys is Ti-6Al-4V and Ti-47 Nb in turn, suggesting that the higher molybdenum equivalent([Mo]eq) lead to the steeper Hugoniot curves. Therefore, when the shock pressure is same, the flyer velocity of Ti-47 Nb al oy is less than Ti-6Al-4V al oy.Ti-47 Nb and Ti-6Al-4V alloys are shocked with different pressure, and the plasticity of former is better than the latter under quasistatic reloading and dynamic reloading conditions at room temperature. The flow stress and yield strength of Ti-6Al-4V alloy are obviously higher than Ti-47 Nb alloy, showing shock- induced strengthening effect. After dynamic high-pressure loading, the increase of internal defects density is the main reason of shock- induced strengthening effect. Moreover, β-to-α″ phase transition in Ti-47 Nb alloy is also the important factor causing shock-induced strengthening effect.The complete spall fractures of Ti-6Al-4V and Ti-47 Nb are observed at 10.24 GPa and 6.73 GPa respectively, suggesting the resistance to spall fracture of Ti-6Al-4V is stronger than the Ti-47 Nb. The fracture surfaces of two titanium alloys are covered by ductile dimples. al oys are completely ductile rupture.The micro- voids in Ti-6Al-4V alloys almost nucleate at the α/β phase boundaries and propagate along the phase boundaries to micro-cracks. Subsequently, voids and cracks propagate along the transformed adiabatic shear band(ASB) and complete spall fracture is generated. The micro- voids in Ti-47 Nb alloys mainly nucleate and grow at the grain boundaries or triple junctions and the micro-cracks are direct connected through micro-voids. Subsequently, voids and cracks propagate along the deformed ASB and complete spall fracture is generated.