Transformation induced crack deflection in a metastable titanium alloy and implications on transformation toughening

The deflection of cracks in grain interior was observed in a metastable β-phase Ti alloy (Ti-24Nb-4Zr-7.9Sn) with transmission electron microscopy during high cycle fatigue. This peculiar phenomenon is induced by the β → α″ martensitic transformation in front of crack tips in grain interior when cracks propagate along the {011}_β slip planes with acute angles between α″ lamellae and cracks approximately of 45°. The α″ product phase has lattice dilation and contraction along two main transformation directions, thus introduces compressive and tensile strains perpendicular and parallel to crack propagating planes, respectively. This exerts beneficial effect on the crack deflection during the subsequent cyclic-loading.