| Titanium alloys are excellent light–weight armor materials for their attractiveproperties, such as low density, high specific strength, good plasticity and ductility,and corrosion resistance, and so on. The armor materials would be subjected tohigh–velocity impacts by the projectiles or fragments in the process of the armorservice environments. Due to the high strength and low heat conductivity of theTitanium alloys, adiabatic shear bands (ASB) are prone to forming underhigh–velocity or high strain rate loading conditions. Microcracks are easily tonucleate, propagate and coalesce in ASB, which would cause the adiabatic shearfracture of the materials. Therefore, the adiabatic shear behavior and ballisticimpact properties of Ti–6Al–4V alloys with equiaxed, quenched and aged, andlamellar microstructures were systemically investigated. The ballistic impact tests,the MTS810, the Gleebe1500machine, and the split Hopkinson press bar (SHPB)were systematically employed to study the mechanical and ballistic impact properties ofthe Ti–6Al–4V alloys. Optical Microscopy (OM), Scanning Electron Microscopy (SEM)and Transmission Electron Microscopy (TEM) were used to perform careful microscopicanalyses. The formation mechanism, distribution features of the ASB under ballistic impactwere investigated, as well as the ASB interactions such as bifurcation and intersection, andthe effect of ASB and interactions among them on the ballistic performance of Ti–6Al–4Valloys with different microstructures. The major results and conclusions are as follows.Craters with the diameter of approximately10mm and the depth of approximately1mm were formed in the Ti–6Al–4V targets under ballistic impact using flat–headedprojectiles. The diameter and depth of the craters increased with the increasing impactvelocity in the targets with the same thickness. The projectiles would be upset, partlyfragmented or even totally fragmented. Multiple parallel ASBs were formed in the targetstogether with ASB interactions, such as bifurcation, intersection, truncation, and so on. Inaddition, microvoids and microcracks were nucleated and propagated in and along ASBs.The formation of multiple ASBs and the distribution feature were caused by the distributionof the maximum shear stress. The length of the longest ASB in the targets increased withthe increasing impact velocity firstly and decreased when the velocity reached a certainvalue. When impacted at the velocity, the length of the longest ASB in the targets withlamellar microstructure was the largest among the three microstructures, the targets with quenched and aged microstructure took the second place, and the length in the targets withequiaxed microstructure was the smallest.ASB consisted of equiaxed grains and strip subgrains, of which the formation was dueto the elongation, fragmentation, rotation and re–crystallization of original grains under theeffect of shear stress. The equiaxed grains formed from grains which experienced the wholedynamic recrystallization process, while the strip subgrains formed from grains which wereshear deformed or incompletely and dynamically recrystallized.ASB bifurcation was caused by the deformation incongruity becoming seriouslyenough between the strip subgrains and surrounding equiaxed grains. The number of ASBbifurcation was less in the Ti–6Al–4V alloys with equiaxed microstructure than withquenched and aged or lamellar microstructures. Most of the bifurcations in the alloy withquenched and aged microstructure distributed in different ASBs while in the alloy withlamellar microstructure, one single ASB was likely to bifurcate more than once. In addition,the grains in different colonies generally arrange along different directions, causing moreserious deformation incongruity at the boundaries of the colonies. Hence ASBs were morelikely to bifurcate at these boundaries of the colonies.During the process of ASB intersection, the elongated subgrains in the ASB whichpropagated firstly to the intersection area would be re–sheared and distributed into the ASBwhich later propagated to the area. ASB intersection would hinder the propagation of theASB and when the hindering effect became larger than the propagation ability, ASBtruncation would happen. The hindering effects of the ASB in alloys with the quenched andaged and lamellar microstructures were larger than with equiaxed microstructure.The effects of ASB on the anti–multiple–hit ability of the Ti–6Al–4V targets could bedivided into two types. One type was that ASB would cause the reduction of the loadingcapacity of the materials for the nucleation and propagation of cracks in ASBs. The othertype was that ASB would enhance the anti–multiple–hit ability of the materials because ofthe hindering effect of the prior–formed ASBs on the propagation of ASBs caused by thesecond hit.In the crater–partly–overlapped double–hit tests, the number of ASBs in theoverlapped area increased and the ASBs near the crater edges were bended. Some of theseASBs expanded further due to the effect of the second hit and became apparently longerthan ASBs caused by single–hit. ASB buckling and ASB interactions would absorb moreenergy, which would be benefit of the anti–multiple–hit ability of the Ti–6Al–4V targets. In the crater–nearly–overlapped double–hit tests, macro cracks would form in ASBs and plugswould form at the rear side of the targets. It could be confirmed that ASBs were bad for theanti–multiple–hit ability of the Ti–6Al–4V targets at this kind of loading condition.The quasi-static and dynamic mechanical properties of the affected materials of threekinds of Ti–6Al–4V alloys presented different variation laws with respect to the originalmaterials. This was due to the density and morphology of the dislocation in the materialsand the different microstructures of the three alloys. Because of the variation, the depth ofthe craters and the bulges in the rear side of the targets caused by the first hit increasedslightly under the effects of the second hit but no macro crack and plug occurred.According to the damage evolutionary analysis in the ASB center, the damage variableD(0) of the affected materials apparently increased compared to the original materials in theTi–6Al–4V alloy with lamellar microstructure, while the D(0) basically unchanged in theTi–6Al–4V alloys with equiaxed, quenched and aged microstructures. Microvoids ormicrocracks and adiabatic shear fracture were more easily to form in the ASB of theaffected materials than the original materials in Ti–6Al–4V alloy with lamellarmicrostructure. |
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