Nanomechanical properties of Mg–Al intermetallic compounds produced by packed powder diffusion coating (PPDC) on the surface of AZ91E

Abstract

A packed powder diffusion coating (PPDC) treatment produced two intermetallic layers on the surface of the commercial magnesium alloy AZ91E. The β-phase (Mg 17 Al 12 ) was immediately on top of the AZ91E, on top of which was the τ-phase (Mg 32 (Al,Zn) 49 ). Nanoindentation showed that the elastic modulus and hardness of each of the intermetallic compounds was significantly greater than that of the AZ91E substrate. Staircase displacement bursts occurred during nanoindentation of the intermetallic compounds, attributed to the combination of incipient plasticity at low loads, and the development of dislocation networks due to dislocation pile ups around the indentation at higher loads. Crystallographic analysis of β phase orientations using EBSD showed that the nanomechanical properties of the intermetallic compound produced through PPDC treatment were isotropic.

Review of Recent Developments in the Field of Magnesium Corrosion

“…For example, the surface of an AZ91 Mg alloy after Al hot‐diffusion has a corrosion barrier of β‐phase and high Al content in the matrix phase next to the surface layer. Such a surface can effectively stop corrosion penetration . Similarly, the AZ91D surface with Zn corrodes slower than its counterpart without surface alloying …”

A study of the deformation and failure mechanisms of protective intermetallic coatings on AZ91 Mg alloys using microcantilever bending

“…Heat treatment was utilized to enhance the adhesion of protective coatings on Mg alloys, and showed that the formation of strong metallurgical bonding at the interface provided by the diffusion process could greatly improve the interfacial strength of the coatings. In this work, the phase formation and layer growth during the PPDC treatment are diffusion-controlled processes [6,13,15,31]. At the process temperature of 413°C, the intermetallic layers grew inwards into the substrate alloy.…”

“…Specimens were first cut from the AZ91 ingots, with dimensions of 15 mm × 10 mm × 10 mm and then treated at 413°C for 18 h using PPDC technique. The details of the PPDC process can be found in [6,15]. After the treatment, specimens were cross-sectioned, ground and polished.…”

“…This process has proven to be effective at providing protection from both wear and corrosion for Mg alloys [6,13]. The intermetallic layers exhibited high hardness (approximately a fourfold increase over bulk Mg [14]) and the corrosion resistance of both β-Mg 17 Al 12 and τ-Mg 32 (Al, Zn) 49 phases in NaCl solution was found to be one to two orders of magnitude higher than that of AZ91 Mg alloys [6,9,15]. The β-Mg 17 Al 12 and τ-Mg 32 (Al, Zn) 49 protective layers were also found to exhibit impressively high fracture strengths of 1.76 GPa and 1.05 GPa respectively [16].…”

Influence of Heat Treatment Temperature on Corrosion Characteristics of Biodegradable EW10X04 Mg Alloy Coated with Nd

“…The temperature allows diffusion of the coating into the base metal following changes in the composition and properties of the surface . The growing interest in the development of this process is due to its distinct advantages: (i) it has high adhesion strength and significantly increases the surface hardness, since there is a strong metallurgical bond between the coating and substrate; (ii) the good properties of the Mg alloys such as electrical conductivity, electromagnetic shielding, and high thermal conductivity can be maintained; and (iii) it has the potential to improve corrosion resistance by integrating intermetallic compounds …”