Study On The Relationship Between Dynamic Mechanical Property And Ballistic Property Of Titanium Alloys

The main objective of this thesis is to establish relationship between dynamic mechanical property and ballistic property of titanium alloys for purpose of their application as armor material. Quasi-static compression test, split Hopkinson pressure bar (SHPB) test and terminal ballistic test were carried out respectively on the selected a type, a+β type and near β type titanium alloys.The macro-morphology, microstructure and adiabatic shear bands (ASB) of recovery specimen were observed by optical microscope and scanning electron microscope. The main research results are as follows:Firstly,the dynamic deformation behaviors of TA7 and TA11 alloys as a type, TC4 as a+(3 type, Ti-5Al-5Mo-5V-3Cr, Ti-5Al-5Mo-5V-3Cr-1Fe, Ti-5Al-5Mo-5V-3Cr-1Zr and TC18 alloys as near P type were obtained using SHPB by means of uniaxial compression while strain rates ranged from 1.5×103s-1 to 5.0×103s-1.The alloys fractured along the maximum shearing strength orientation,45-degree angle to the compression axis when subjected at failure loading conditions due to adiabatic shearing according to the macro morphology of recycle specimen after SHPB tests. The failure surface composed of shear zones characteristic with smooth surface and tension zones characteristic with parabolic dimples.The type of true stress-strain (σ-ε) curves of TA7, TA11 and TC4 alloys was strain hardening while that of near β type alloys was thermal softening. The maximum of impact absorbing energy in descending order was Ti-55531Zr alloy when subjected at strain rate of 4900s-1, Ti-55531Fe alloy when subjected at strain rate of 4800s-1, TA7, Ti-5553, TC18, TA11 and TC4 alloys when subjected at strain rate of 4000s-1.No adiabatic shear band (ASB) was founded by optical microscope within TA7 alloy, whereas several ASBs existed without regular direction in TA11 alloy. The direction of ASBs (1μm-30μm wide) of alloys except TA11 was 45-degree angle against the compression axis, consistent with the failure crack direction.Results of Johnson-Cook constitutive equation of TA7, TC4 and TC18 alloys showed that the dynamic flow stress was independent on the strain rate hardening coefficient C which was 10-2 magnitude and the value of the three alloys was similar, while the strain hardening type of true σ-ε curve of TA7 alloy due to its maximum value of strain hardening coefficient B and strain hardening index n.Then, resistance to penetration against 7.62mm armor-piercing incendiary at 0-degree angle impact and damage behaviors of 8mm targets of TA7, TC4, Ti-55531Zr (high-strength and high-toughness) and TC18 alloys which selected based on the SHPB test results while considering impact absorbing energy and type of σ-ε curves were evaluated by terminal ballistic tests.The V50 in descending order of TA7, high-toughness Ti-55531Zr, TC18, high-strength Ti-55531Zr, and TC4 were 406m/s,390m/s,370m/s,330m/s and 308m/s respectively. The damage mode contained petal shaped damage of TA7 and high-toughness Ti-55531Zr, plugging damage of high-strength Ti-55531Zr and TC4, and spallation damage or hole expansion damage of TC18.There were cracks instead of ASB observed by optical microscope around the bullet holes of TA7 target which were in accordance with the SHPB results. As a contrast, there were large amount of ASBs and cracks around the bullet holes of high-toughness Ti-55531Zr, TC4 and TC18 targets and most of the crack tips located within the ASB while cracks appearred alternately with ASB within one ASB which indicated that ASB was the preferred location of crack to nucleate and propagate.Finally, relationship between dynamic mechanical property and ballistic property was as follows:The ballistic property of a type TA7 target was the best, followed by nearly β type high-strength and high-toughness of Ti-55531Zr and TC18 targets, last for α+β type of TC4 target.The maximum impact absorbed energy of TA7 and TC4 were 560MJ/m3 and 315 MJ/m3 respectively while the type of true σ-ε curve were both strain hardening, corresponding to the value of V50 was 406m/s and 308m/s. The maximum impact absorbed energy of high-toughness Ti-55531Zr, TC18 and high-strength Ti-55531Zr were 700 MJ/m3,550 MJ/m3 and 340 MJ/m3 respectively while the type of true σ-ε curve were all thermal softening, corresponding to the value of V50 were 390m/s,370m/s and 330m/s. It indicated that alloy with higher impact absorbed energy could be expected better ballistic property when the type of true stress-strain curve was the same.