Intertwiner Spaces Research Articles

Interactive effects of shock loading parameters on the substructure and mechanical properties of nickel and stainless steel

The dislocation density, dislocation cell size and intertwin spacing have been measured in shock-loaded nickel (about 35 μm grain size) along with the dislocation density, intertwin spacing, twin-fault volume fraction and α′ martensite volume fraction in shock-loaded type 304 stainless steel having grain sizes of 14, 38 and 89 μm. The three stainless steel grain sizes and the nickel sample were simultaneously shock loaded in a single experimental sandwich stack of sheet specimens. Single shock loading events were performed at 150, 300 and 450 kbar at 2 μs pulse duration and 450 kbar at 6 μs pulse duration, while a single sandwich was repeatedly shock loaded three times at 150 kbar at 2 μs pulse duration to investigate the effects of multiply shocked substructures. The dislocation density in the stainless steel decreased without exception as the grain size increased, and the α′ martensite volume fraction increased with increasing grain size. The α′ martensite also increased with increasing shock pressure to some extent with increasing (longer) pulse duration. The residual 0.2% offset yield stress followed a Hall-Petch type relation when considering cell size d and intertwin spacing Δ in nickel ( σ∼d −1, σ∼Δ − 1 2 ), while a Hall-Petch type relation was also followed in stainless steel for grain size D and intertwin spacing Δ ( σ∼D − 1 2 , σ∼Δ −1/2 ). It was shown that dislocation cells, twins or twin-faults and martensite all contribute to subgrain refinement and residual shock strengthening. Multiply loaded nickel and type 304 stainless steel exhibited microstructures and residual properties different from single events but were not representative of either additive shock stress or pulse duration. The initial microstructure was observed to influence the residual shock microstructure strongly as expected on the basis of work-hardening considerations. Multiply shocked 304 stainless steel contained a high density of martensite as compared with (equivalent) single shock events (150 kbar, 2 μs), and this was due to the shock loading of existing (initial) twin faults and the interactions of these faults to nucleate and grow martensite within such twin-fault bundles.

The contribution of deformation twins to yield stress: The Hall-Petch law for inter-twin spacing

Deformation twins in shock-loaded metals contribute to the residual yield stress through a Hall--Petch law form sigma approx. ..delta../sup -//sup 1/2/, where ..delta.. is the inter-twin spacing. This contribution is unclear because no evidence of dislocation pile-ups was found. Pile-ups appear to be absent in most materials.

Shock induced hardness in α-Iron

This investigation deals with the introduction of structural defects into thick specimens of α-iron by the passage of shock waves, and the quantitative determination of the hardness distribution due to these defects. The hardness induced by the shock wave sharply decreases with distance from the plane of entry, and the decrease is associated with the increase of the average inter-twin spacing and the decrease of the average dislocation density. The relation between the hardness, H, and the parameters of the substructure is expressed by the following equation: H = H∗ + kλ−12 + αGb Λ where H∗, k, α, G and b are the material's constants, λ is the average inter-twin spacing and Λ is the average dislocation density. It was also found that a linear relationship exists between Λ12 and λ−1 in shock-treated α-iron.