| Tantalum(Ta)is a typical transition metal with body-centered cubic.Due to its outstanding physic-chemical properties like high melting point,density,corrosion resistance and excellent extending ability,Ta and its alloys have been widely used in electronics industry,military,aerospace and medical fields,etc.High purity Ta can serve as ideal barrier layer between copper and silicon substrate to prevent the integrated circuit from the influence of copper silicon alloy introduced by their diffusion.Grains with small size and homogeneous orientations for sputtering target put forward a request on microstructural regulation of Ta.Thus,it is still a hot topic to control the microstructures of Ta upon its industrial applications.In conventional unidirectional rolling methed,very strong strain concentration always appearing in material and results in inhomogeneous deformation and the subsequent strong recrystallization textures during annealing.The change of strain path may do a favor for the controlling of deformation,leading to homogeneous microstructures to some extent.Thus,to improve the design and process techniques,it is very necessary to investigate the mechanisms responsible for deformation and recrystallization of Ta.In this study,multiple techniques,such as scanning electron microscopy(SEM),electronic backscatter diffraction(EBSD),X-ray diffraction and transmission electron microscopy(TEM),etc.were applied to characterize the deformation,recovery and recrystallization microstructures of Ta samples with different strain paths.The responsible deformation and annealing mechanisms were analyzed in depth.The main conclusions include:(1)?-fiber and?-fiber texture formed in unidirectional rolled(UR)and clock rolled(CR)samples,but grains with different orientations exhibit different microstructures.Many micro-bands and micro-shear bands occurred in{111}(<111>//ND,Normal Direction)grains due to its uniserial slipping in UR sample.while orientation gradient appeared in{100}(<100>//ND)grains in UR sample and{100},{111}grains in CR sample due to the multiple slipping.The deformation mechanism difference was further proved by Schmid Factor(SF)and the derived Schmid Factor Difference Ratio(SFDR).The stored energy was estimated for differently oriented grains,which was found to depend on grain orientations.Clock rolling diminish the gap of stored energy between{111}and{100}grains and leads to a homogeneous deformation.(2)The through-thickness stored energy distribution was homogenized and the energy gradient was reduced by the change of strain path.For one single layer,clock rolling impeded the formation of microbands and microshear bands in{111}grains and narrowed the stored energy gap between{111}and{100}grains,leading to homogeneous energy distribution.The contents of?and?fiber texture changed greatly from the surface to center layer of UR sample,but they changed little in CR sample.Especially,?fiber texture distributed homogeneously through the thickness of CR sample.(3)The?-?GB and?-?GB possess the highest and lowest Vickers hardness values,irrespective of rolling paths.The calculation of geometrical necessary dislocations(GND)revealed that,in UR specimen,the GND density is 14.854×1014 m-2 in{111}grains,which is almost three times of that in{100}grains.In contrast,this density difference in CR specimen is largely narrowed to 1.94 times.Besides,MSBs seem to transmit across?-?GB in UR specimen,due to the activation of slip systems with similar directions in contiguous?grains.Instead of the transgranular MSBs,{100}grains exhibit distinct orientation gradients because of multiple slipping.(4)UR sample recrystallized fast than that of CR sample.In UR sample,new grains with large size formed via high angle boundary migration in deformed{111}matrix,and these grains are mainly with{111}orientation.Nucleation in CR sample mainly occurred in IRs via subgrains nucleation mechanisms.The new grains are with small size and their orientations are close to the surrounding points with higher distortions.(5)The recrystallization microstructures can be optimized by changing the weight of high-angle boundary migration mechanism and subgrain nucleation mechanism via regulating pre-recovery degree.The dislocation morphology can be changed by pre-recovery,and the distribution of stored energy is influenced by pre-recovery time.During recovery,sub-grains rotated and coalesce into larger ones,and these sub-grains tended to be equiaxed.The two-step annealing schedule,long time recovery and the subsequent high-temperature annealing,is an effective annealing method for Ta.Grains with small size and equiaxed shape can be formed,and the macrotexture can be homogenized by this schedule. |
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