Dislocation structure of plastic relaxation waves in polycrystals and alloys under intense shock wave loading

Synergetics of the interaction of mobile and immobile dislocations in the formation of dislocation structures in a shock wave. Effect of the stacking fault energy

“…The purpose of this work is to analyze the influence of the stacking fault energy on the critical stress of the transition from a cellular distribution of dislocations in a shock wave to a uniform distribution of dislocations with stacking faults. As was done in the previously per formed analysis of the influence of the grain size and density of precipitates on the aforementioned transi tion [7], the present analysis is based on the kinetic equation for the density of dislocations and on the critical conditions of the formation of their cellular distribution [14]. This paper is organized as follows.…”

“…In [7,14], the influence of the pressure, grain size, and volume density of precipitates on the transition from a cellular dislocation structure to a uniform dis tribution of dislocations behind the shock wave front was analyzed using the model equation for the disloca tion density in the following form [15,16]: (1) where ρ(y, t) is the density of mobile dislocations; y is the coordinate along the normal to the dislocation slip plane; t is the time; u is the dislocation velocity; D m is the coefficient of the diffusion of screw segments of dislocation loops through the double cross slip mech anism; λ m and δ f ρ 1/2 are the mean free paths of dislo cations between the events of their multiplication at obstacles of the nonstrain and strain (forest of disloca tions with density ρ f = ρ) origins, respectively; δ f ≈ 10 ⎯2 is the coefficient determining the intensity of the latter process; and h a is the characteristic distance of the annihilation of screw segments of dislocation loops through the cross slip mechanism [17]. In equation (1), the parameters ξ and β determine the type of dis location structure formed in the material.…”

“…In equation (1), the parameters ξ and β determine the type of dis location structure formed in the material. For the parameters ξ < 1 and β < 1, the propagation of a shock wave results in the formation of a spatially homoge neous dislocation structure; for ξ > 1 and β > 1, this process leads to the formation of an inhomogeneous (cellular) dislocation structure [7,14]. The specific (1), as applied to the problem of the origin and propagation of plastic shock waves, is that some of the coefficients and parameters of this equation depend on the pressure in the shock wave.…”

“…With an increase in the radiation intensity and, hence, in the pressure and strain rate, the dislocation structure formed in the shock wave develops in the fol lowing sequence: uniform dislocation density distri bution-cellular dislocation structure-uniform dislo cation density distribution with stacking faults, and, finally, at pressures above 60 GPa, the formation of a strain induced structure consisting of microtwins [1][2][3][4]. In polycrystals, the character of the dislocation structure in a shock wave is determined by the grain size [5][6][7], whereas in alloys, the dislocation structure depends on the presence of precipitates in the material [7,8] and on the stacking fault energy [9]. As the pres sure increases above 1-10 GPa, the generation of geo metrically necessary dislocations at the shock wave front becomes a significant factor [4,10].…”

“…Above this pressure, there appears a uniform distribution of dislocations with stacking faults. In polycrystals, according to [7], the cellular dislocation structure is not formed when the grain size is less than ≈1 μm. In particular, this structure is not formed in nanocrystalline nickel [5].…”