We analyze unsteady plastic flow modes in cutting of metals using high-speed imaging of the deformation zone, in situ. For metals which exhibit high levels of strain hardening, the commonly assumed steady (smooth) flow is inherently unstable. Instead, the cutting is characterized by unsteady sinuous flow, with large-amplitude folding, that is triggered by a plastic buckling instability linked to the material microstructure. A mechanochemical effect caused by Al-Alcohol chemical reaction on workpiece surface, which is coupled to the unsteady flow mode, is highlighted. Experimental results reinforce the hypothesis pertaining to flow stability governing the deformation mode and chip type.