Effect of Grain Size and Deformation-induced Grain Refinement on the Residual Strength of Shock-loaded Metals and Alloys

Abstract

The effects of grain size and grain refinement by the formation of dislocation cells and deformation twins on the residual strength of shock loaded molybdenum, nickel, and type 304 stainless steel have been investigated by simultaneously shock loading various grain sizes of thin sheets of these materials in separate experiments. Pressures ranging from 15 to 55 GPa were employed at a constant pulse duration of 2 μs. Residual microstructures were observed by transmission electron microscopy and grain sizes, D, dislocation cell sizes, d, and inter-twin spacings, Δ, were related to the residual hardness (H) and 0.2% offset yield stress (σ). The grain size and grain size refinement parameters imposed gradients of plastic deformation with a wavelength equal to the spacings D, d, or Δ. The hardness of shock-loaded Mo was observed to be characterized by (H − HO) = KD−1/2, where K was observed to vary in different grain size ranges (and to be smaller for smaller grain sizes; D < 4 μm). There was no significant subgrain refinement in Mo because no noticeable dislocation cells resulted. Shock loaded Ni on the other hand formed dislocation cells which decreased in size with increasing shock pressure. Above about 30 GPa twins also formed in nickel, increasing in volume fraction with increasing shock pressure. In the pressure range 0 to 30 GPa, (σ − σo) = KD−1/2 + k′d−1 while above 30 GPa (σ−σO) = kD−1/2 + K″d−1 + K‴ Δ−1/2; where the K values represent different constants. In addition, it was observed that (H − HO) α D−1/2 up to 55 GPa shock pressure. Only deformation twins were observed to occur in 304 stainless steel and the residual hardness was related by (H − HO) α D−1/2. The yield stress was related by a Hall-Petch equation of the form (σ − σO) = KD−1/2 + K′Δ−1/2.