Microstructural evolution and corrosion behavior of friction stir processed fine‐grained AZ80 Mg alloy

Abstract

Friction stir processing (FSP) and subsequent aging heat treatment were used to modify the microstructure of AZ80 magnesium alloy. The influence of FSP and aging heat treatment on the corrosion behavior was systematically studied by using potentiodynamic polarization, immersion, and slow strain rate tensile tests. The results revealed that FSP led to grain refinement, rapid dissolution of β‐phase, and the deflection of c‐axis from transverse direction (TD) and processing direction (PD) by approximately 55° and 25°, respectively, improving the static corrosion and stress corrosion cracking (SCC) resistance. The aging heat treatment rendered a little influence on the grain size and slightly affected the grain orientation. The content of β‐phase in FSP‐5 and FSP‐24 samples was 8.1 and 21.8 wt.%, respectively. Static corrosion and SCC resistance of FSP‐5 and FSP‐24 samples were lower than those of the FSP samples. Compared with FSP‐5 sample, the amount of β‐phase and the proportion of Al2O3 increased in FSP‐24 sample, leading to enhanced static corrosion and SCC resistance. SCC behavior was controlled by anodic dissolution, whereas the presence of hydrogen accelerated the SCC.

Mg/ZrO2 Metal Matrix Nanocomposites Fabricated by Friction Stir Processing: Microstructure, Mechanical Properties, and Corrosion Behavior

“…According to the previous experimental results ( Figures 3 , 4 ), adding ZrO 2 particles into AZ31 Mg with FSP can promote grain refinement, and then improve the corrosion performance. Similar results have been reported ( Esmaily et al, 2016 ; Wang et al, 2018 ; Huang et al, 2019 ). Moreover, Vignesh et al (2019) believed that the dispersion distribution of ZrO 2 particles in composites increases the cumulative corrosion potential of the material, which increases the corrosion resistance of the composite.…”

“…The detailed statistical results about the texture of three samples on the { 0001 } planes are shown in Table 1 . It has been reported that the deflection of c-axis on the {0001} plane of FSP sample is mainly due to the shear stress induced by shoulder and stir pin ( Huang et al, 2019 ). It is worth noting that the deflection angle of c-axis of FSP-ZrO 2 sample is larger than that of FSP sample, which may be due to the fact that ZrO 2 particles increase the friction coefficient of the material during the plastic flow process, causing more grains on the {0001} plane to deflect.…”

“…In the electrochemical corrosion test, the corrosion resistance of FSP sample is better than that of FSP-ZrO 2 sample, while the opposite result appears in the SVET measurement. For Mg alloys, grain size and grain orientation are the main factors affecting their corrosion resistance ( Huang et al, 2019 ). For Mg matrix composites, in addition to the above two factors, reinforced particles also affect the corrosion resistance significantly ( Vignesh et al, 2019 ).…”

“…Generally, grain refinement is considered to effectively improve the corrosion resistance of Mg alloys ( Saikrishna et al, 2017 ; Wang et al, 2018 ; Huang et al, 2019 ). The corrosion resistance depends on the grain size, via the grain boundary defects.…”

“…On the other hand, grain orientation is also a critical factor for influencing the corrosion behavior of FSP-ZrO 2 composites. Previous studies have shown that the (0001) plane of Mg alloy is more resistant to corrosion than (10-10) and (11-20) plane (Liu et al, 2008;Campo et al, 2014;Huang et al, 2019). The (0001) plane of Mg alloy exhibits the highest atomic density (1.13 × 10 19 , atom/m 2 ) and the lowest surface energy, which leads to a low atomic dissolution rate during the corrosion.…”