Multi-scale Study On The Microstructure Uniformity Of High-purity Ta Sheet And Corresponding Influence Mechanism Of Rolling Parameters

Tantalum(Ta)is an important metallic material in industry and possesses high melting point and density,excellent ductility,and high corrosion resistance.Due to its superior comprehensive performance,Ta has been widely used in electronics,chemical,military,aerospace and national defense industries,etc.In the production and processing of Ta plates,it is easy to generate undesirable microstructure and texture characteristics such as texture gradients,texture bands and coarse grain size.The above undesirable aspects are in essence closely related to the deformation and recrystallization behavior of Ta with strong orientation dependence.However,the understanding of orientation-dependent deformation and recrystallization behavior in Ta has not been studied systematically and in-depth.Thus,more systematic work based on the above scientific problem has been done.In this paper,Ta plates processed by four different rolling techniques,namely,unidirectional rolling,135^o cross rolling,135^o warm cross rolling and asymmetric cross rolling are taken as the research object,focusing mainly on the strain path,deformation strain,pass number,rolling temperature,and shear strain on the orientation-dependent deformation behavior and microstructure homogeneity of Ta sheets,and the corresponding deformation and annealing mechanism are analyzed in depth.The main conclusions are as follows:(1)Compared with unidirectional rolling(UR),more complete?and?fiber textures are formed in 135^o cross rolling(CR)Ta sheets along the thickness,and the difference in maximum texture intensity is smaller.The probability of primary slip system being activated within the deformed{111}grains in UR sample is higher and many microshear bands(MSBs)and microbands(MBs)are formed in deformed{111}matrix,which leads to significant increase in its stored energy,while in deformed{100}grains multiple silp systems can be operated easily and only a few dislocation cells can be observed.For 135^oCR sample,the orienation-dependent deformation behavior existing in UR sample can be significantly weakened,and grain splitting and stored energy distribution for{111}and{100}orientations along the thickness is relatively uniform.Upon annealing,more uniform and fine grain size combined with random crystal orientation is formed in CR sample along the thickness due to relatively uniform microstructure fragmentation and stored energy distribution.(2)The increase of strain in 135^oCR can weaken the orientation-dependent heterogeneous grain splitting.The calculation of grain average misorientation(GAM)and geometrically necessary dislocations(GNDs)density displays that there is a significant difference in grain splitting for{111}and{100}orientations in 70%sample,while the{111}and{100}grains in 87%sample deform evenly.Taylor model analysis demonstrates that there are fewer slip systems being activated within the deformed{111}grains in 70%sample,resulting in a higher dislocation pile-up and local-region strain concentration,and thus leading to the formation of MBs and MSBs.Due to more preferred nucleation sites and higher stored energy in the deformed{111}grains in 70%sample,this sample recrystallized faster than 87%sample during annealing.(3)More uniform texture and stored energy distribution are formed in Ta sheets along its thickness by increasing the pass number in CR.The average grain orientation spread(GOS_average)indicates that deformation is relatively uniform in 16-pass sample along the thickness,and the degree of microstructure subdivision within the deformed grains for different orientations is relatively low.Relative Schmid factor(?)analysis shows that multiple slip systems are activated in deformed{111}grains in 16-pass sample,and the activation of multiple slip systems easily leads to dislocation rearrangement and annihilation,which results in the formation of cell blocks(CBs).The CBs in deformed{111}grains can weaken the orientation-dependent deformation behavior in Ta sheets.The recrystallized grains with relatively random orientation and uniform size are formed in 16-pass sample along the thickness in annealing because of relatively uniform deformation microstructure and lower stored energy gradient.(4)Due to the increase of rolling temperature,the Peierls stress needed to overcome by dislocation movement during 135^owarm cross rolling(WCR)is reduced,and the dislocation movement rate is increased.Meanwhile,the strain path change in WCR introduces the reverse strain and increases the probability of dislocations interaction(such as dislocation rearrangement and annihilation),thus reducing the proportion of low-angle grain boundaries(1.5-5.5^o).Dynamic recovery is promoted by WCR,leading to the formation of numerous subgrains with thermodynamical instability and the increase of grain boundary energy in deformed matrix.After the completion of recrystallization,more randomly oriented grains are formed and the size difference for different orientations is relatively small in WCR sample.(5)Asymmetric cross rolling(ACR)can effectively weaken the texture gradient of Ta plate along the thickness.Meanwhile,great shear strain introduced by ACR significantly enhances the microstructure splitting of deformed{100}grains,thus weakening the orientation-dependent stored energy difference.Kernel average misorientation(KAM)and its standard deviation combined with microhardness suggest that the increase of strain in ACR can improve the uniformity of grain splitting along the thickness.Taylor model analysis and contour strain distribution demonstrate that the increase of strain in ACR can improve the splitting ability of deformed grains,especially{100}grains,causing relatively high average shear strain.Besides,the increase of strain can reduce strain concentration in local region,thereby improving the region-dependent microstructure uniformity.Uniform and fine grain size combined with random crystal orientations can be obtained in 80%ACR sample after the completion of recrystallization because of relatively random deformation texture and uniform microstructure subdivision.