Static and fatigue behaviour of structural light alloys in air and aggressive environment – investigation and innovation with thin hard coatings

Today, the constant need for increased energetic efficiency and resources economy leads the research in the field of structural materials, towards the investigation of new applications of structural light alloys. To increase design efficiency, high strength-to-mass ratio alloys are adopted in weight critical structures. Ti-6Al-4V (grade 5) Titanium alloy is certainly the most widespread metallic material, being developed for highly stressed structural components of jet fighters and of new generation jet liners, although it presents high Stress Corrosion Cracking (SCC) sensitivity in solutions with very high concentrations of methanol. Recent experiments produced some very interesting results on the effects of methanol aggressive environments in conjunction with heat treatments. This concern raises a critical warning to the adoption of Ti-6Al-4V alloy in environments with particular chemical conditions, such as the oil & gas sector, chemical treatment plants, and fuel cells. In addition to this experimental work, numerical studies have been concluded, by means of Finite Element Method (FEM) analysis of fatigue specimens of Ti-6Al-4V in air, inert environment (paraffin oil) and NaCl environment. In seeking high performance structural materials, also aluminium high strength light alloys must be properly investigated. The most interesting material from the high-strength 7000 series is certainly the 7075-T6. Developed initially in the aeronautic field, the material usage is now limited, due to its reduced corrosion resistance in aggressive environments. Low temperature Physical Vapour Deposition (PVD) techniques have been adopted on 7075-T6 fatigue specimens, to assess the effect of thin hard coatings of tungsten Carbide/Carbon (WC/C) and low temperature Diamond Like Carbon (DLC) on fatigue behaviour in air and aggressive methanol environment. Scanning Electron Microscopy (SEM) has been used to identify the mechanisms involved in the fatigue behavior of coated and uncoated specimens, in rotating bending, R = -1, step-loading fatigue tests.