High temperature ductility loss in titanium alloys -- A review

Abstract

It is well known that two phase titanium alloy systems suffer from an abrupt drop in ductility at elevated temperatures in the range of 1000 to 1150°K. This loss of ductility is manifested by easy decohesion of polycrystalline aggregates along the grain boundaries of the high temperature be__aphase. If the alloy is in a state of tensile stress at the aforementioned temperatures, :racks ini_,_ate at the grain boundaries and propagate readily through the alloy, leading to premature failure. This phenomenon is a cause of major concern in titanium alloy fabrication and welding. Several mechanisms have been proposed to explain high temperature crack nucleation and growth along the boundaries. A critical review of the phenomenon and possible mechanisms responsible for the observed behavior will be discussed. INTRODUCrlON Several near c_ and (x-l] titanium alloys have been found to have poor hot workability under certain processing conditions. This poor workability, and subsolidus weld cracking, result from a ductility loss in the temperature range of the to 0_transformation. This phenomenon has been studied and found to vary with alloy content, specimen thermal history, and a number of microstructural characteristics. This paper will review and summarize the results of several investigations of this high temperature ductility drop. !_i_ !:__:II:_E R I. Background The high temperature ductility loss observed in 0_-_ titanium alloys is illustrated in Figure 1. This data, from Lewis and Caplan (1), plots %RA for tensile tests performed both on heating and on cooling during a simulated GTA weld thermal cycle. The key features of the thermal cycle were a peak temperature above