Composition formulas for Mg-Al industrial alloy specifications

Abstract

Mg-Al alloys are the most widely used Mg-based industrial alloys, but their composition rules behind the apparent industrial specifications are largely unknown, which hinders the development of new alloys. As is well known, industrial alloys often undergo the process of a high-temperature solution treatment, and the final structures originate from the single-phase solid solution parent state. Since solid solutions are characterized by short-range chemical orders, necessarily the optimum alloy composition should be related to the presence of a certain short-range chemical structure unit. In the present paper, by introducing our cluster-resonance model for short-range-order structure description of solid solutions, a chemical structure unit of Mg-Al binary solid solution is established,[Al-Mg12]Mg1, which represents the characteristic short-range-order structure, with the bracketed part being the nearest-neighbor cluster centered by Al and shelled by 12Mg and with one glue atom Mg located between the clusters. Because of the existence of other alloying elements besides Al, a general formula[(Al, A)1-Mg12]-(Mg, B) is then proposed, where A represents the elements showing a negative mixing enthalpy with Mg, while B showing a positive one. This formula is used to explain the multi-component Mg-Al industrial alloys. Based on this chemical formula, typical Mg-Al industrial alloy specifications in ASTM handbook are well explained. For instance, cast AZ63A alloy is formulated as[Al0.78Zn0.16-Mg12]Mg1.04Mn0.02, cast AZ81A as[Al0.97Zn0.03-Mg12]Mg0.98Mn0.02, and wrought AZ80A as[Al1.02n0.03-Mg12]Mg0.94Mn0.01. The deviations from the ideal chemical structure unit in different Mg-Al alloys are well correlated to their corresponding alloy performances. Those alloys, where the numbers of center atoms are close to ones in their cluster formulas, exhibit excellent comprehensive mechanical performances in both strength and plasticity. While the alloy with less than one center atom only shows good plastic performance with a relatively poor strength, and the one with more than one center atom shows just the reverse tendency. Among cast Mg-Al alloys, AZ81A, whose cluster formula completely matches the stable chemical structure unit, exhibits the optimized combination of strength (275 MPa) and plasticity (elongation 15%). Among wrought Mg-Al alloys, AZ61A and AZ80A, whose cluster formulas show minor deviations of -0.11 and 0.05 in the center site from the ideal chemical structure unit, also have good comprehensive mechanical properties, respectively with the strengths of 310 MPa and 380 MPa, and the elongations of 16% and 7%. Based on the results in the present paper, the simple composition rule behind the complex industrial alloy specifications as unveiled here, can be a powerful approach to the development of Mg-Al alloys.