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Aimed at the difficulty in producing high-quality machined surface due to the existence of brittle-phase SiC in SiC_p/Al composites, this paper used the molecular dynamics simulation and ultra-precision turning test to investigate the material removal process of SiC_p/Al composites at nanoscale, and focused on the machined surface formation mechanism, brittle-ductile transition and tool wear mechanism in single crystal diamond ultra-precision turning of SiC_p/Al composites. The results indicate that high-pressure phase transition is the main reason for the brittle-ductile transition of brittle-phase SiC in SiC_p/Al composites. With the increase of cutting depth, the removal of SiC particles in SiC_p/Al composites experienced from ductile cutting mode to hybrid brittle-ductile cutting mode and finally to purely brittle cutting mode. The SiC-Al interface and soft Al matrix in SiCp/Al composites considerably affected the brittle-ductile cutting mode transition mechanism when machining SiC particles in SiC_p/Al composites. The existence of tensile stress on the uncut chip could induce the peak of brittle SiC crack initiation in the cutting zone. The primary wear mechanisms of SCD tools were abrasive wear originated from hard SiC particles’ scrape and machining induced graphitization.