Intragranular Void Formation In Shock-spalled Tantalum:Mechanisms And Governing Factors

Because of their high shock resistance and fracture toughness and other characteristics,tantalum metal holds promise for light weight structural applications in many advanced technology and automotive industries,such as aircraft engine combustion chamber structure material.With the employment of tantalum metal in these applications,the structural design must face the high strain rate loading conditions.The awareness of tantalum metal in high stress and high strain rate under the condition of dynamic response,for predicting damage of engineering materials and evolution as well as the ground and extraterrestrial impact events is critical.During service,tantalum metal may be subjected to impulsive loading such as impact,and their dynamic responses under high strain rate loading have been investigated with split Hopkinson compression or tension bars(102-104 s-1).The investigation on high strain rate deformation behaviors of tantalum metal is still very rare,with gas gun plate impact at higher strain rates(?105 s-1 or higher).Therefore,the purpose of thesis is to study the tantalum mechanical response and deformation mechanism under high strain rates.Understanding how tantalum deforms at high strain rates is not only a requirement of light weight structural application but also can broaden our understanding of the mechanism of plastic deformation of tantalum.In this thesis,high purity polycrystalline tantalum(99.99%)was used as the research object in a series of one dimensional plate impact experiments.Their crystal microstructures under different loading conditions were analyzed in polycrystalline tantalum holes nucleation mechanism.The conclusions are listed as follows:(1)We conduct four spallation experiments on one thick sample and three thin samples.The results showed that HELs of three thin samples are?3.9 GPa and weakly dependent on grain size and impact velocity.HELs of three thin samples are?3.9 GPa and weakly dependent on grain size and impact velocity.Spall strengths increases by 6%with grain size increasing from 20?m to 40?m at similar impact velocities(-330 m/s),and about by 20%with shock velocity increasing from 327 m/s to 443 m/s at the same grain size(20?m).(2)The experiments of polycrystalline tantalum with different grain size were compared.The results showed that smaller grain size is favorable for intra-and intergranular voids than larger grain size,since denser grain boundaries can enhance multipleslip for intragranular voids and supply more nucleation sites for intergranular voids.Intragranular voids tend to nucleate near grain boundaries,and are larger in number but smaller in size than intergranular voids.On the other hand,twins/intragranular voids tend to nucleate at high-angle/medium-angle grain boundaries,and are strongly/weakly dependent on grain orientation,respectively.Grain boundary mediated stress and shear component along a twinning or slip system of applied stress depend on grain boundary misorienation and grain orientation,respectively.Both of them can supply shear stress for single twins,while only the former can offer complex stress for multiple-slip at low shock pressures.(3)The molecular dynamic simulation of multi-crystal tantalum impact simulation was carried out.The results showed that intragranular voids induced by multiple-slip show strong dependence on grain boundary misorientation,grain size and shock pressure but weak dependence on grain orientation.grain boundary-induced multiple-slip and slip-slip intersections near grain boundaries act as a prerequisite to intragranular void formation.