PSI - Issue 28

S.A. Atroshenko et al. / Procedia Structural Integrity 28 (2020) 101–105 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussions Three penetration zones are distinguished in a punched target (fig. 1), due to various stages of penetration of the projectile. The first zone, caused by the non-stationary penetration stage, described by Savenkov and Khantuleva (2014), corresponds in depth to ~ 0.5 penetration thickness and, the second ∼ 1/4 thickness (stationary penetration stage), the third (associated with reflection of the shock wave from the free surface) ∼ 1/4 thickness. After shock penetration of the target, signs of transformation of the deformed state and the formation of new structures due to dissipation of the mechanical energy of the impactor were revealed. The formation of the titanium structure is determined mainly by twinning processes. However, each zone has its own characteristics. In the near-surface zone, short twins with pointed ends are mainly observed (fig. 2). Moreover, in a number of grains, the so-called twin networks are observed, i.e. in the same parts of the grains, the twins have a different direction, which indicates the simultaneous activation of various twinning systems. Apparently, in this case, due to the complexity of the stress state, all intersecting slip systems can start working simultaneously and all five twinning planes that exist in titanium alloys can be involved as shown in Liu T.S. and Steinberg M.A. (1952).

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Fig. 2. Alloy structure in the first zone: (a) at the surface of the collision; (b) at a distance of 2.8 mm from the surface of the collision (structureless region); (c) at a distance of 0.6 mm from the frontal surface. Unstructured regions are observed between grains (fig. 2), the same structureless regions are also observed on other parts of the shores of the cavity of the first zone, for example, at a distance of ~ 2.8 mm from the impact surface (fig. 2b) i.e. where the penetration rate of the striker is still quite high. Although the nature of the formation of these regions is generally not clear, it can be assumed that the discovered structurelessness of the titanium alloy (the absence of a grain structure) may be associated with the formation of imperfect crystals and the transition of a part of the material due to some thermomechanical processes to the amorphous state as it described in Kozlov E.V. et al. (2006). The formation of a new structure in the first zone occurs not only directly off the coast of the cavity, but also at a sufficient distance from them (up to 5.5 mm from the coast of the cavity). So, fig. 2c shows the structure formed at a distance of 0.6 mm from the frontal surface and 5.4 mm from the shore of the cavity. It can be seen that grains with twin nets alternate with structureless regions and with grains in which there is only one system of twins of the same crystallographic direction. The microhardness of the investigated titanium alloy in the region of twin nets is HV = 2.64 GPa (with a span of 2.50 - 3.03 GPa), in the structureless regions HV = 2.2 GPa. This behavior of the change in microhardness leads to an unexpected result. So, according to some data, microhardness for a number of materials correlates with yield stress. By analogy with the yield strength, there is a well-known Hall – Petch relation for microhardness as shown in Kamyshanchenko N.V. et al. (2011): H=H 0 +kd -0,5 , where H 0 is microhardness of single-crystal titanium; k is the coefficient; d is the characteristic grain size. Then, based on this ratio and our results on microhardness measurements, the coefficient k should have a negative value. The high value of microhardness in the area of twin nets is due to the high coefficient of strain hardening, which is typical for the operation of intersecting slip systems, and high levels of deformation stress at relatively low values of deformation.