Effect of Accumulative Roll Bonding on Microstructure and Mechanical Properties of Layered AX10-ZK60 Magnesium Alloy Composites

Introduction to Accumulative Roll Bonding

Accumulative roll bonding (ARB) is a severe plastic deformation (SPD) technique that produces bulk metallic composites by repeatedly rolling, cutting, stacking, and re-rolling dissimilar sheets. The process is particularly attractive for magnesium alloy systems because it enables the creation of layered composites with tailored mechanical properties without introducing high-temperature phases that could degrade the base alloy. The study of AX10-ZK60 layered composites via ARB addresses a significant materials engineering challenge: combining the high strength of ZK60 (a rare-earth-containing magnesium alloy with excellent creep resistance) with the good formability and lower cost of AX10 (a low-alloyed magnesium alloy).

Microstructural Evolution During ARB

The ARB process subjects the layered composite to repeated cycles of plastic deformation, cutting, and stacking, resulting in progressive grain refinement and increasing interfacial area between the dissimilar layers. In the as-received state, AX10 exhibits a typical wrought microstructure with average grain size of approximately 40–60 μm, while ZK60 shows a coarser grain structure with rare-earth particles distributed at grain boundaries. After multiple ARB cycles, the grain refinement follows a predictable progression: initial stages involve grain elongation and dislocation accumulation, intermediate stages show subgrain formation and partial dynamic recrystallization, and advanced stages achieve nanocrystalline or ultrafine-grained structures in the deformed regions.

The interface between AX10 and ZK60 layers undergoes significant microstructural transformation during ARB. The dissimilar deformation behavior of the two alloys (due to differences in flow stress and strain rate sensitivity) generates interfacial shear stresses that promote dynamic recrystallization at the interface. This results in a narrow transition zone (typically 5–20 μm wide after 3–5 ARB cycles) characterized by refined grains and enhanced bonding strength. The interface bonding quality is critical for composite performance, and insufficient bonding can lead to delamination during subsequent forming operations.

Mechanical Property Enhancement

The mechanical property improvement in AX10-ZK60 ARB composites follows the Hall-Petch relationship, with yield strength increasing as grain size decreases. After 3–5 ARB cycles, the ultimate tensile strength typically reaches 380–420 MPa, representing a 50–70% improvement over the as-received state. However, this strength gain comes at the cost of ductility, with elongation decreasing from 8–12% to 2–5%. The trade-off between strength and ductility is a fundamental challenge in ARB processing, and optimization requires careful control of reduction ratio per cycle (typically 60–70%), total number of cycles, and rolling temperature.

The layered architecture of the composite also introduces a texture evolution that significantly influences mechanical anisotropy. Magnesium alloys exhibit strong basal texture due to their hexagonal close-packed (HCP) crystal structure, and the ARB process modifies this texture through repeated rolling and stacking. The resulting texture affects the tension-compression asymmetry (TCR) of the composite, which is a critical consideration for applications involving cyclic loading.

Engineering Implications and Challenges

From a practical standpoint, the ARB process for magnesium alloy composites faces several challenges. First, the rolling force required increases with each cycle due to work hardening, necessitating progressively more powerful rolling mills. Second, surface quality degrades with repeated rolling, requiring intermediate surface treatment (such as polishing or chemical etching) to ensure good bonding in subsequent cycles. Third, the cutting operation between cycles introduces free edges that may serve as crack initiation sites during service.

The temperature control during ARB is another critical parameter. Rolling at room temperature maximizes grain refinement but may cause excessive work hardening and reduced ductility. Warm rolling (150–250°C) promotes dynamic recrystallization and maintains better ductility, but may reduce the degree of grain refinement. The optimal rolling temperature depends on the desired balance between strength and ductility for the specific application.

Summary

The ARB processing of AX10-ZK60 layered magnesium alloy composites offers a promising route to high-strength lightweight structural materials with tailored mechanical properties. The technique leverages the complementary strengths of the two base alloys while achieving significant grain refinement and interface bonding through severe plastic deformation. However, the practical implementation requires careful optimization of process parameters to balance strength, ductility, and manufacturability, and the resulting composites must be evaluated for their performance under realistic loading conditions that may include temperature cycling, fatigue, and corrosion exposure.