Formation mechanism of the high-speed deformation characteristic microstructure based on dislocation slipping and twinni
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As-annealed commercial pure titanium (grade 1) was selected as a model material whose crystalline structure was hexagonal close-packed. The evolution of the microstructure and micro-orientation induced by high-speed compression was characterized to elaborate the formation mechanism of the high-speed deformation characteristic microstructure in a-titanium. Twinning played a coordinating role for dislocation slipping that was the main plastic deformation mechanism. The high-speed deformation characteristic microstructure of as-annealed commercial pure titanium was an adiabatic shear band (ASB) with an average width of 50 lm at a strain rate of 5400 s1, whose initial grains were 0.5–1.0 lm in size. The formation and extension of ASB were attributed to the interaction between the shear stress and the adiabatic temperature rise. A formation model of ASB in a-Ti was proposed in terms of the formation mechanism of the high-speed deformation characteristic microstructure.
I. INTRODUCTION
Considering the circumstances of high-speed deformation, such as collisions between automobiles, trains, ships, or airplanes and their surrounding objects and the explosive load generated in industrial accidents, military operations, and terrorist attacks, metallic plates became invalid due to the formation of adiabatic shear band (ASB).1 Therefore, it was crucial to improve the antiadiabatic shear ability of material. The first and most important step was to clarify the formation mechanism of the high-speed deformation characteristic microstructure to improve the anti-adiabatic shear ability of the material. For common metallic materials, such as steel, aluminum, and copper, the adiabatic shear behavior has been the focus of a large amount of research. The thermoplastic instable model proposed by Zener and Hollomon has been generally accepted, which revealed the formation mechanism of ASB from the mechanical point of view.2,3 The formation of ASB resulted from mutual competition among strain strengthening, strain rates strengthening, and thermo-softening. ASB was formed at the point where the thermo-softening was stronger than the strain strengthening and strain rate strengthening. From a microstructural point of view, the ASB can be divided into three parts: the center exquiaxed grain zone, transition zone, and matrix. The transition zone is a Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.409
severely deformed area induced by shear stress, which presents as an elongated deformed structure along the direction of shear stress. Additionally, the center exquiaxed grain results from dynamic recrystallization, but it is different from the classic dynamic recrystallization. The classic dynamic recrystallization is controlled by a diffusion mechanism, which can be implemented by diffusion with vacancy migration introduced by deformation defects. However, the velocity of this diffusion mechanism is too low to accomplish the whole dy
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