Boron-fiber-reinforced ZK60A magnesium composites: Utilizing ultrafine grains and low-temperature superplasticity

This paper highlights the potential of ultrafine-grained (UFG) magnesium alloys for fabricating a high quality boron-fiber composite through isothermal foil-fiber-foil compression in a superplastic mode. High-pressure torsion at room temperature and at 200 °C was used to form UFG states in a commercial ZK60A magnesium alloy. The ranges of temperature and strain rates for achieving superplasticity in the UFG alloy were established; microstructures and textures before and after superplastic deformation were characterized. Based on the achieved results and using finite element simulation of the processing, the precise isothermal compression parameters required to fabricate the ZK60A composite with a UFG matrix reinforced with boron fibers were determined: a temperature of 240 °C and a strain rate of 10 −4 s −1 . The resulting metal-matrix composite, consisting of alternating UFG foil layers and continuous 140 µm boron fibers, exhibited enhanced three-point bending strength and fatigue performance provided by homogeneous UFG matrix, high fiber strength, and strong interfacial bonding. In contrast to conventional laminated or fibrous composites, no cracks penetrating perpendicularly into the material were observed. Furthermore, the influence of matrix–fiber reaction products on fiber degradation during isothermal compression was evaluated. Finally, a model for the fabrication of high quality boron-fiber magnesium-matrix composites via superplastic upsetting is proposed. The core feature of this model is the capability of the UFG matrix (grain size < 1 µm) to flow smoothly into narrow interfiber spaces during superplastic deformation at a moderate temperature of 240 °C. The resulting composite, characterized by a homogeneous UFG matrix and robust interfacial bonding, exhibited superior bending strength and fatigue resistance compared to conventional composites.