Effect of phase morphology on fatigue crack growth behavior of α-β titanium alloy -: A crack closure rationale

Effect of phase morphology on fatigue crack growth (FCG) resistance has been investigated in the case of an alpha-beta titanium alloy. Fatigue crack growth tests with on-line crack closure measurements are performed in the microstructures varying in primary alpha (elongated/equiaxed/Widmanstatten) and matrix beta (transformed/metastable) phase morphologies. The microstructures comprising metastable beta matrix are observed to yield higher FCG resistance than those for transformed beta matrix, irrespective of primary alpha phase morphology (equiaxed of elongated). But, the effect of primary alpha phase morphology is dictated by the type of beta phase (transformed or metastable) matrix. It is observed that in the microstructures with metastable beta matrix, the equiaxed primary alpha as second phase possesses higher FCG resistance as compared to that of elongated a morphology. The trend is reversed if the metastable beta matrix is replaced by transformed beta phase. The fatigue crack path profiles are observed to be highly faceted. The detailed fractographic investigations revealed that tortuosity is introduced as a result of cleavage in alpha or beta or in both the phases, depending upon the microstructure. The crack closure concept has been invoked to rationalize the phase morphology effects on fatigue crack growth behavior. The roughness-induced and plasticity-induced crack closure appear to be the main mechanisms governing crack growth behavior in alpha-beta titanium alloy.