Dynamic phase transformation driven microstructure and texture development during deformation of Zr-2.5wt.%Nb in α+β phase field

This work demonstrates and explains how, during the hot deformation of a Zr alloy in the two-phase field, the texture developed in the α phase is influenced by that developed in the β phase. For this Zr-2.5wt.%Nb alloy was hot compressed in the α+β phase field (700, 750 and 800°C) at 0.1 s ⁻¹ to a true strain of 1.1. The microstructure and texture were characterized using scanning electron microscopy and electron backscatter diffraction. The flow curves showed a sharp drop in flow stress at ε≈0.01 due to plastic deformation induced α→β phase transformation. At 700°C, α-Zr developed 〈 2 1 ¯ 1 ¯ 3 〉 parallel to compression axis (||CA) (as is commonly observed in α-Zr system), whereas at 750 and 800°C 〈 2 1 ¯ 1 ¯ 0 〉 ||CA formed. At all studied temperatures, β-Zr developed { 001 } 〈 110 〉 and { 111 } 〈 110 〉 texture components that were shown to be dependent on the elongated morphology of the α-Zr grains and not on the α-Zr texture. From experimental observations and thermodynamic calculations of the free energy changes in absence and presence of stress, it was shown that the formation of 〈 2 1 ¯ 1 ¯ 0 〉 α − Z r ||CA occurred by migration of β-Zr into 〈 2 1 ¯ 1 ¯ 3 〉 α − Z r followed by the reverse β→α transformation (through Burgers orientation relationship) driven by low flow stress in 〈 2 1 ¯ 1 ¯ 0 〉 α − Z r and low Nb content in the transforming β-Zr. It was concluded that the presence of β-Zr with 〈 001 〉 ||CA and 〈 111 〉 ||CA is a necessary condition for the formation of 〈 2 1 ¯ 1 ¯ 0 〉 ||CA in α-Zr. In addition, temperature promotes the development of 〈 2 1 ¯ 1 ¯ 0 〉 α − Z r through enhanced boundary mobility of β-Zr into harder α-Zr.