New aspects of globularization crystallography and dynamic phase evolution during thermomechanical processing of Ti–6Al–4V alloy

Abstract

Microstructural and crystallographic aspects arising from thermomechanical processing of a Ti–6Al–4V alloy in β and α+β phase fields were studied by electron backscatter diffraction and both conventional and high energy synchrotron X-ray diffraction. The impact of deformation in the single β field on the final microstructure has been acknowledged by an increase of the retained β phase content and decrease of martensitic c/a ratio with the reduction of strain rate, although no significant influence of strain rate on the well-defined fiber texture of α' has been observed. During deformation in the α+β field, texture weakening has been observed, and misorientation angle/axis distribution analysis has evidenced randomization and loss of the Burgers orientation relationship between α and β upon globularization. The behavior of deformed α lamellae in the α+β field was also evaluated in terms of internal misorientation spread and lattice rotation around the deformation axis, corroborating that globularization takes place through dynamic recrystallization, boundary splitting and shearing. Globularized grain size was shown to be exponentially proportional to strain rate decrease. Lower strain rates in the α+β field also increase the β phase fraction and decrease the c/a ratio of the α phase, leading to values closer to the ideal one for titanium. However, an excessive amount of β causes its instability during quenching, resulting in its decomposition into secondary α during cooling.