Mechanical behaviour and low cycle fatigue performance of plasma sprayed ZrO2 coated Al-7075-T6 alloy under subzero temperature condition

This study investigates the mechanical and low-cycle fatigue (LCF) behaviour of ZrO₂-coated Al-7075-T6 alloy under subzero temperature conditions (-60 °C). Tensile and LCF tests were conducted in accordance with ASTM E08 and E606 standards at both room and subzero temperatures, with strain-controlled fatigue tests performed at various strain amplitudes ($\Delta {\varepsilon }_t/2$= 0.65 %, 0.75 %, 0.85 %, and 0.95 %). The LCF performance of both coated and uncoated specimens were evaluated through hysteresis loop analysis, plastic strain life curves, plastic strain energy density, cyclic stress-strain responses, and Basquin-Coffin-Manson relationship curves. The microstructural changes in the fatigue-fractured surfaces were examined using Optical Emission Microscopy (OEM), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). The results showed that the ZrO₂ coating significantly enhanced the fatigue life of Al-7075-T6 under subzero conditions. This was attributed to the thermal barrier effect of the coating, which mitigated strain localization, reduced dislocation activity, and suppressed microcrack initiation. SEM analysis revealed the fracture morphologies at both room and subzero temperatures, including ductile-to-brittle transitions at low temperatures. XRD analysis showed variations in crystalline size and microstrain in the fractured surfaces, further supporting the enhanced fatigue performance of the ZrO₂-coated specimens. These findings emphasize the role of ZrO₂ coatings in improving the mechanical and fatigue resistance of Al-7075-T6 alloy, offering a promising solution for enhancing the durability of lightweight alloys in aerospace and other extreme environmental applications.