Grain Boundary Wetting Transition in the Mg-Based ZEK 100 Alloy

Abstract

Modern magnesium-based alloys are broadly used in various industries as well as for biodegradable medical implants due to their exceptional combination of light weight, strength, and plasticity. The studied ZEK100 alloy had a nominal composition of 1 wt.% zinc, 0.1 wt.% zirconium, and 0.1 wt.% rare earth metals (REMs) such as Y, Ce, Nd, and La, with the remainder being Mg. It has been observed that between the solidus (Ts = 529.5 ± 0.5 °C) and liquidus temperature (Tl = 645 ± 5 °C), the Mg/Mg grain boundaries can contain either the droplets of a melt (incomplete or partial wetting) or the continuous liquid layers separating the abutting Mg grains (complete wetting). With the temperature increasing from Ts to Tl, the transformation proceeds from incomplete to complete grain boundary wetting. Below 565 °C, all grain boundaries are partially wetted by the melt. Above 565 °C, the completely wetted Mg/Mg grain boundaries appear. Their portion grows quickly with an increasing temperature until reaching 100% at 622 °C. Above 622 °C, all the solid Mg grains are completely surrounded by the melt. After rapid solidification, the REM-rich melt forms brittle intermetallic compounds. The compression strength as well as the compression yield strength parameter σ02 strongly depend on the morphology of the grain boundary layers. If the hard and brittle intermetallic phase has the shape of separated particles (partial wetting), the overall compression strength is about 341 MPa and σ02 = 101 MPa. If the polycrystal contains the continous intergarnular layers of the brittle intermetallic phase (complete wetting), the overall compression strength drops to 247 Mpa and σ02 to 40 Mpa. We for the first time observed, therefore, that the grain boundary wetting phenomena can strongly influence the mechanical properties of a polycrystal. Therefore, grain boundary wetting can be used for tailoring the behavior of materials.

Coexistence of Intermetallic Complexions and Bulk Particles in Grain Boundaries in the ZEK100 Alloy

“…The focus of this study is the investigation of GB phase transitions in the ZEK100 alloy, which is another significant magnesium-based alloy . The ZEK100 alloy shares similar alloying elements with EZ33A, albeit at lower concentrations [3]. Consequently, it is reasonable to anticipate a substantial difference in the structure and morphology of ZEK100, primarily due to the reduced volume of the secondary phase present in its GBs.…”

“…The investigation of magnesium-based alloys has gained significant popularity in modern materials science [1][2][3][4]. The interest in these alloys is evident due to their widespread use in technologically demanding industries.…”

“…Our previous investigations have primarily focused on the EZ33A alloy, revealing the presence of GB wetting phase transitions involving second liquid and solid phases at various temperatures [3,4,96]. The EZ33A is a widely used magnesium casting alloy in the industry, particularly for manufacturing bulk parts like differentials' housings.…”

“…Studies [3,4,96] have revealed that just above the solidus temperature, at 530 • C, more than 10% of the GBs in EZ33A are already completely wetted by the bulk liquid phase. As the temperature rises, the fraction of wetted GBs gradually increases, reaching 85% at 580 • C. However, at this temperature, the fraction of wetted GBs seems to saturate, which could be attributed to experimental errors during quenching from elevated temperatures or the presence of GBs with low misorientation angles and/or coincidence site lattice GBs in the material.…”

“…These GBs have lower σ GB energies and might remain unwetted in the alloy until the melting temperature is reached. Leveraging their understanding of the GB wetting phase transition in the EZ33A alloy, the authors proposed a heat treatment method to reduce the number of wetted GBs and thereby enhance the ductility of the alloy [3,4].…”