An investigation of fatigue crack nucleation and growth in a Ti―6Al―4V/TiB in situ composite

摘要

This paper presents the results of an experimental study of fatigue crack nucleation and growth in a Ti―6Al―4V/TiB in situ whisker-reinforced composite. The onset of crack nucleation is shown to correspond to ～20% of the total life at a stress range of 480 MPa. This is associated with transverse cracking across TiB whiskers, and interfacial decohesion between the TiB whiskers and the Ti~6Al-4V matrix. Subsequent cracking occurs by the formation of multiple cracks across the elongated α grains. These cracks are retarded initially by the β phase. However, subsequent fatigue damage results in transgranular crack growth across α and β phases prior to the onset of catastrophic failure. The long fatigue crack growth rates in the Paris regime in the Ti―6Al―4V/TiB composite are comparable to those of Ti 6A1-4V processed under nominally identical conditions. However, the fatigue crack growth rates in the composite are faster than those in the matrix alloy at lower ΔK values. Cyclic deformation of the composite is associated with strain softening, presumably as a result of progressive interfacial decohesion around the TiB whiskers early in the fatigue deformation process. The implications of the results are assessed for potential structural applications of the Ti―6Al―4V/TiB composite.