Development of creep-resistant Mg–Sn-based alloys utilizing rapid phase transformation during deformation

Abstract

Fuel efficiency has become a critical issue in the transportation industry, driving a growing demand for lightweight structural materials. Among these materials, Mg, with its exceptionally low density, is considered a key material for vehicle weight reduction. In addition, Mg exhibits outstanding damping capacity and EMI shielding effects, offering multifunctional advantages. Consequently, commercial AZ91 alloy has been widely adopted for various automotive components, including seat frames and dashboard panels. With increasing demands for further weight reduction, Mg alloys are increasingly considered for applications in components operating at high temperatures, including transmission gears and engine blocks. However, the use of Mg–Al-based alloys, for example, AZ91 and AZ80, has been restricted at high temperatures due to their limited creep resistance. To overcome the limitations of Mg–Al-based alloys, researchers have developed thermally stable phases by alloying with Ca, Si, Sr, and rare earth (RE) elements to suppress grain boundary sliding (GBS). (Mg,Al)2Ca, Mg2Si, Al4Sr, Al11RE3, and Al2RE phases were found to be effective for pinning grain boundaries. In addition, controlling grain matrix deformation (GMD) is also essential for enhancing the creep resistance of Mg alloys. Several studies on Mg–RE systems have shown that regions with a high supersaturation of solute atoms can serve as favorable sites for dynamic precipitation during creep. RE-containing phases formed within the matrix are effective obstacles to dislocations, hindering grain matrix deformation during creep. To develop creep-resistant alloys, as mentioned above, it is necessary to consider alloying elements that effectively suppress GBS and GMD. Ca is effective in hindering GBS through the formation of thermally stable phases along grain boundaries. Owing to its low solubility in Mg (~0.8 at%), however, Ca contributes a limited strengthening effect against GMD via precipitation within the matrix. In contrast, RE additions provide excellent creep resistance due to their high solubility and thermal stability. Nevertheless, their high cost restricts their widespread use in industrial applications. Given these limitations of Ca and RE additions, the Mg–Sn system has emerged as a promising cost-effective alternative, providing thermal stability. In addition, Sn has higher solubility in Mg (~3.3 at%), enabling the formation of a large volume fraction of strengthening precipitates within the matrix. However, Mg–Sn alloys inherently exhibit a longer incubation period for precipitation compared to Mg–Ca and Mg–RE alloys, resulting in sluggish precipitation kinetics within the matrix. This limitation implies that most solute atoms dissolved in the matrix are unable to contribute effectively to dislocation hindrance during creep deformation. From this perspective, it is worth paying attention to the aging potential of as-cast alloys. A limited age-hardening response of Mg–Sn-based alloys was attributed to the high interfacial energy between the α-Mg matrix and incoherent Mg2Sn precipitates. This results in the formation of coarse precipitates distributed with low number density. This characteristic contrasts with Mg–RE alloys, which show a rapid aging response due to a multi-step aging procedure involving the formation of transition phases along with solid-state transformations. Zn alloying in Mg–Sn-based alloys has been reported to be effective by providing heterogeneous nucleation sites for Mg2Sn, leading to refined precipitates with an increased number density. Subsequent study further indicated that Zn segregates to the interphase boundary between the Mg matrix and Mg2Sn, thereby lowering the interfacial energy and reducing the nucleation barrier for Mg2Sn. However, a fundamental challenge remains in that the incubation time for precipitation remains long compared to Mg–RE-based alloys due to the absence of transitional phases. This study introduces a novel approach by utilizing a transition phase that accelerates the precipitation of the equilibrium Mg2Sn phase, thereby enhancing the age-hardening response and resistance to GMD during high-temperature creep. To achieve this, we identify a transition phase that has orientation relationships with the equilibrium phase, Mg2Sn, acting as an aging accelerator. Furthermore, the unique phase transformation behavior observed within the nucleus and its correlation with the accelerated hardening response are discussed.