Overview: Age-Hardenable Microalloying in Magnesium

Abstract

As light weighting of vehicles becomes an urgent way to achieve increasingly stringent environmental standards, the introduction of low-density and high-abundance materials is becoming more important. Among these, magnesium (Mg) alloys are prime candidates for structural applications. However, critical issues with strength and ductility remain a roadblock for structural Mg alloys relative to Al alloys and steels. New designs of Mg alloys are needed to achieve in full the potential benefits lightweight magnesium can provide. These designs must necessarily use novel mechanisms of strength and ductility control to improve fracture resistance and formability over current alloys if complex system-critical Mg components are to be manufactured at scale and at cost. A primary method for improving strength in cast and wrought Mg alloys is age hardening. Upon quenching a supersaturated solid solution of alloyed Mg below the single-phase hexagonal close-packed (hcp) solvus, precipitation of secondary phase(s) occurs during aging. Typically, a series of phase transformations takes place and produces a procession of metastable secondary phases that vary in composition, crystal structure, and morphology. Because alloy hardness depends on microstructure, it also evolves with aging temperature and time; it achieves maxima due to a particular precipitate structure. During the past 10 years, significant advances have been made in our mechanistic understanding of structure–property relationships in Mg alloys. They have been fueled primarily by novel microscopy techniques and theoretical computations. Precipitation sequences of many Mg alloys have been evaluated in great experimental detail, revealing specific strengthening mechanisms for specific alloys. For instance, during aging of Mg alloys containing light rare-earth elements, b¢¢ plate-type particles tend to precipitate on prismatic planes of the hcplattice, preventing basal slip and thus improving strength. Theoretical ab initio calculations have discovered that the cause of this particular prismatic precipitate morphology is a specific balance of anisotropies in the chemical interfacial energy and the elastic coherency strain energy. Currently, efforts are underway to search for less costly elements that can produce similar precipitates using this mechanistic model. An understanding of strengthening mechanisms in existing Mg alloys is necessary for the development of novel alloys. Alloy design fundamentally depends on such processing-structure–property relationships (e.g., aging fi precipitate morphology fi strength). Thus, the development of a competitive Mg alloy for significant light weighting in vehicles requires exploration of fundamental strength and ductility mechanisms. To that end, the following articles being published under the topic of Age-Hardenable Microalloying in Mg provide excellent details and research on the subject. To download any of the papers, follow the URL http:// link.springer.com/journal/11837/67/10/page/1 to the table of contents page for the October 2015 issue (vol. 67, no. 10). The first article by Mendis, Kainer, and Hort is titled, ‘‘High-Strength Magnesium Alloys Through Precipitation Hardening and Microalloying: Considerations for Alloy Design.’’It reviews recent advances in Mg strengthening through microalloying and summarizes design strategies that should be explored to achieve the properties necessary for the broad commercial utilization of Mg. Three primary microstructural features are identified as degrees of freedom upon which alloying can affect the hardness of base Mg. These are grain boundaries, matrix composition and structure, and precipitates. A critical assessment of the current state of research into these pathways for alloying culminates in a primary criterion for enhancing the agehardening response by alloying: The formation of a large number of heterogeneous nucleation sites to James E. Saal and Dmitry Orlov are the guest editors for the Magnesium Committee of the TMS Light Metals Division (LMD); and coordinators of the topic Age-Hardenable Microalloying in Magnesium JOM, Vol. 67, No. 10, 2015