Microstructural evolution and ultrafine-grain formation during dynamic shear in pure tantalum

In this paper, the mechanical properties of pure tantalum (Ta) hat-shaped samples during dynamic impact were studied, and microstructures at different reductions were characterized. Results showed the ultrafine grains formed in adiabatic shear band (ASB). When the reduction is 40%, that is to say that the shear strain is 4, the shear yield strength is about 703.65 MPa, and then the shear stress decrease with increasing strain. ASB is formed and could be divided into transition and central regions. Most of the grains in the central region are ellipsoidal dynamically recrystallized (DRX) grains with a size of about 760 nm, while numerous grains in transition region are split. The geometric necessary dislocations (GNDs) density (20.13 × 1014 m−2) in the transition region is greater than the density (18.28 × 1014 m−2) in the central region. The average hardness of the matrix region is about 88.80 HV, and the value in the ASB reaches 239.08 HV. The maximum temperature within the ASB is calculated to be approximately 1100 K before ASB formation. According to the mechanically driven subgrains rotation model, subgrains inside ASB slip and rotate under the action of shear forces or normal stresses, and finally a texture 〈110〉 // shear plane normal (SPN) near the top of ASB and 〈110〉 // shear direction (SD) near the bottom is formed.