The Effect of Hydrogen on the Work-Hardening and Fracture Behaviour of Titanium–Low-Aluminium–Low-Manganese Alloys

Abstract

True stress/true strain curves have been determined for two fine-grained (α + β) titanium alloys containing various amounts of hydrogen, which was present either in solid solution or in solid solution and as a film-like hydride precipitate. In hydride-free specimens three stages, with high, low, and intermediate work-hardening exponents, are identified in the work-hardening of the alloys. The three stages, in order of occurrence, are attributed to partial plastic deformation, easy glide, and parabolic hardening. The presence of hydride lowers the initial flow stress, extends the first stage, eliminates the second stage, and lowers the work-hardening exponent of the third stage of work-hardening. These changes are believed to be caused by the hydride particles acting as stress concentrators, thus enabling localized plastic deformation to occur at low macroscopic stress levels and inducing complex slip at an early stage in work-hardening. Hydride precipitate also causes loss of ductility by markedly reducing the localized deformation but not the uniform deformation. Fracture is initiated by the cracking of the hydride films in numerous places, leading to the formation in the ductile matrix of rounded pores that join up to produce fracture by the ductile-fracture mechanism. Thus, embrittlement is not accompanied by a fundamental change in the fracture mechanism.