Microstructures of Machined Chips in Pure Titanium, Pure Iron and 0.83%C Steel

Abstract

Microstructures were examined on the chip specimen of pure titanium with close-packed hexagonal (cph) structure (α-Ti, the shear strain, γ, is ~22), and those of pure iron (α-Fe, γ ≈ 7.5) and 0.83%C steel which had originally pearlite structure (γ ≈ 7.5) with body-centered cubic (bcc) structure by the FE-SEM/ EBSP method and optical microscopy. The hardness of the chip specimens was also measured and compared to that of the original materials. UFGed materials could be produced at relatively small shear strain (~7.5) in pure iron and 0.83%C steel. The average grain diameter of the chip specimen was slightly larger in the 0.83%C, because lamellar cementite phase in original pearlite structure hindered the formation of submicron grains. The hardness of the chip specimens increased with increasing shear strain, and the hardness of the chip specimen with γ ≈ 7.5 (391 Hv) was ~4 times as much as that of the original material (93.9 Hv) in pure iron. However, it was impossible to produce the ultra-fine grained materials by machining of α-Ti even at γ ≈ 22. According to the experimental results obtained so far, the number of slip systems (crystal structure) as well as shear strain seems to be one of the important factors controlling the generation of equiaxed submicron grain structure. Higher stacking fault energy is favorable for cross-slip or climb of dislocations in dynamic recovery which probably governs the generation of submicron grains.