A Review on the Role of Microstructural Heterogeneities in the Failure of Aerospace Components: A Metallographer’s Perspective

Abstract

Wrought components show improved mechanical properties under both static and dynamic conditions against their cast counterparts. This is basically attributed to the homogeneous and fine microstructure introduced by the thermomechanical processing. However, thermomechanical processing also leads to mechanical property anisotropy especially when the microstructural heterogeneities (second phase, inclusions) are not randomly distributed and get oriented. This usually occurs due to the nature of the thermomechanical processing adopted to fabricate the products. The microstructural heterogeneities can be both metallic (such as delta ferrite stringers in steels) and non-metallic (such as MnS inclusions in steel) in nature. Physical orientation of grains and the crystallographic orientation due to thermomechanical processing also add to the woes of heterogeneities in these materials. When materials with non-metallic inclusions are subjected to large plastic deformation, such as wire drawing and are used for applications such as springs (which are subjected to dynamic loading), failures can occur in these components due to the propagation of cracks from the incoherent interface between inclusion and matrix. Similarly, when these materials are subjected to secondary fabrication processes such as cold swaging or high strain rate deformation, failures can occur due to the propagation of cracks along the interface of the second phase with the matrix. Therefore, the purpose of the present paper is to highlight the role of microstructural heterogeneities in the failure of components with specific emphasis to aerospace structural materials. Finally, it is concluded that materials subjected to dynamic loading conditions during fabrication or service should be microstructurally clean and second phase, if present should be fragmented and uniformly distributed in the matrix to avoid cracking/failure.