AZ91 is one of the most broadly used Mg alloys because of its good castability and reasonable mechanical properties. Strengthening AZ91 with carbon short fibers aims to increase tensile and fatigue strength, creep, and wear resistance. One of the proposed applications of reinforced AZ91 is the production of pistons for trucks. Such reciprocating parts are subjected to alternating fatigue loads which can lead to fatigue failure. In this respect, studying the tensile and fatigue behavior of materials subjected to such loading conditions is of great interest. The alternating low-cycle fatigue (LCF) and high-cycle fatigue (HCF) of unreinforced AZ91 and carbon fiber-reinforced AZ91 (AZ91-C) were investigated at 20 °C and 250 °C. Tensile tests were carried out at the same testing temperature to find the appropriate fatigue testing stress and strain for stress-controlled and strain-controlled tests, respectively. The fatigue curves of stress against the number of cycles (S–N) revealed that the composite AZ91-C’s fatigue strength was 55 MPa under HCF, while that of the matrix alloy AZ91 was only 37 MPa at 250 °C. Fracture investigations were conducted on the broken test samples. The fracture approach in the matrix material (AZ91) is mixed ductile/brittle containing fatigue serration, fiber fracture, and separation in the reinforced material (AZ91-C).
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