Water droplet impact erosion damage initiation in forged Ti–6Al–4V

• Local subsurface plasticity causes the surface protrusions at early incubation.
• Transgranular cracks formation below the protrusion at about 45° suggests LCF.
• Intergranular damage occurs at the surface after appearance of the protrusions.
• Transgranular cracks control the erosion both at initial and more advanced stages.
• Erosion mechanism proceeds due to fatigue as in LCF.

Identifying the mechanisms of water droplet erosion of Ti–6Al–4V parts is a critical issue encountered in many situations from aircraft body exposed to rain during flight to steam turbine blade. Understanding the erosion mechanism and particularly initiation mode of the damage is an essential need for the studies concerning improving erosion resistance of the components. The present work therefore focuses on the early damage stages of forged Ti–6Al–4V parts exposed to high-speed water impact erosion. Qualitative observations and quantitative measurements were done both on and below the surfaces that are undergoing deformation due to water droplet impingements. Progressive cross-sectional polishing revealed surface and sub-surface microplasticity and micro-cracking. Microcracks have intergranular features at surface and present transgranular characteristics below the surface. From these observations a damage mechanism was proposed to explain the early stages of water erosion. These observations, together with information gathered from more advanced erosion stages tend to prove that a mechanism typical of low cycle fatigue may control the nucleation and early growth of cracks below the surface of the parts subjected to high-speed water droplet impingement.