Correlation of Microstructure and Corrosion Behavior of squeeze-cast Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca (wt%) Alloys Constituting W and LPSO Secondary Phases

Abstract

The present study aims to explore the effect of microstructural evolution via forging on corrosion behavior of squeeze-cast Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca (wt%) WZ44 alloys comprising W-type (Mg3Y2Zn3), long-period stacking order (LPSO, Mg12YZn) as secondary phases and alpha-Mg matrix. In particular, the as-cast ingot was solutionized at 400 omicron C for 20 h, followed by forging at 450 omicron C by applying a compressive pressure of 280 kg/cm2 for 45 min, which has a pronounced effect on the grains orientation, texture, and distribution of the second phases. The forging inflicted a spreading of (0002) grains along the longitudinal direction and strong basal texture, causing improved tensile yield strength and ductility by strain hardening phenomena. The corrosion behavior of alloy specimens assessed in 0.1 M NaCl solution and benchmarked against pure Mg using pH, open circuit potential (OCP) variation with immersion duration, mass loss, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) reveal inferior corrosion resistance of alloy specimens owing to the presence of highly cathodic LPSO and W-phase causing severe pitting of alpha-Mg with corrosion products comprising Mg, Y-rich complex phases. Nevertheless, forged alloy exhibited excellent corrosion protection ability (similar to 4-5 h) during 48 h long EIS analysis owing to a reduction in Volta potential difference between W-type and LPSO phase (98 +/- 2.3 mV) with alpha-Mg matrix confirmed by scanning Kelvin probe force microscopy (SKPFM) diminishing severe pitting of grains concomitant with a dense protective barrier of oxide/hydroxide layers preventing ingression of Cl--ions. Overall, the work emphasizes that W-phase and LPSO-rich WZ44 alloy, which is prone to corrosion, can exhibit excellent mechanical properties and slightly improved saltwater corrosion resistance provided by texture effect, and second-phase distribution via forging at elevated temperature.