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 〈21¯1¯3〉 parallel to compression axis (||CA) (as is commonly observed in α-Zr system), whereas at 750 and 800°C 〈21¯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 〈21¯1¯0〉α−Zr||CA occurred by migration of β-Zr into 〈21¯1¯3〉α−Zr followed by the reverse β→α transformation (through Burgers orientation relationship) driven by low flow stress in 〈21¯1¯0〉α−Zr 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 〈21¯1¯0〉||CA in α-Zr. In addition, temperature promotes the development of 〈21¯1¯0〉α−Zr through enhanced boundary mobility of β-Zr into harder α-Zr.