Strength of zirconium-titanium martensites and deformation behaviour

Abstract The deformation behaviour of pure zirconium and of zirconium-titanium alloys containing 5, 10, 15 and 20 wt% titanium was studied in two heat treated conditions: furnace cooled and water quanched from the β phase field. By comparing the flow stresses of the furnace cooled α and the water quenched α′ (martensite) structures it was possible to isolate the strengthening contributions of the martensitic structure (comprising the contributions due to the small size of the martensite units and to the distributions of defects like dislocations and internal twins) from those arising from the solid solution. The internally twinned plate martensite structure in the Zr-15%Ti and the Zr-20%Ti alloys was found to be responsible for a significant increase in strength, while the strengthening due to the dislocated lath martensite structure in the more dilute alloys was only marginal. Stress relaxation experiments revealed that the strengthening associated with the martensite structure was mainly due to an increase in the athermal component of the flow stress. The effectiveness of the lath boundaries and the {1011} twin boundaries in offering resistance to an approaching deformation front (either slip or twin) was examined. While the lath boundaries were found to be transparent with respect to the propagation of slip dislocations and deformation twins, a majority of plate as well as twin boundaries were effective barriers against their propagation. TEM observations showed an extensive accumulation of geometrically necessary dislocations in the plastically deformed twinned martensites. The enhanced work hardening, observed in these martensites, was seen to be related to the geometric slip distances in these structures in accordance with Ashby's one parameter work hardening theory for plastically inhomogeneous materials. The effect of the martensite structure on different components of the flow stress (dependent on or independent of grain size and strain) was also discussed.