Mechanical twins in 304 stainless steel after small-charge explosions

Optical, scanning electron, and scanning tunneling microscopy, as well as X-ray diffraction, were employed to detect microstructural modifications caused in AISI 304Cu steel disks by explosions of small charges placed at close ranges. Explosions induced limited or no gross macro-deformation. NSP (plastic) explosive spherical charges of 54.5 and 109 g TNT equivalent mass and explosive-to-target distances in the range from 6.5 to 81.5 cm were used to achieve peak pressures in the 160-0.5 MPa range. Two alloy grain sizes (60 and 32 μm) were tested. Surface optical and scanning electron microscopy revealed partial surface melting, zones with indications of recrystallization, and intense mechanical twinning, which was detected also by X-ray diffraction. In the interior of the samples, twins were exclusively found. They can be seen up to some distance from the explosion-impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For metal object mapping after explosions, an important source of information in forensic science, the maximum charge-to-target distance at which the phenomena disappear has been singled out for each charge and alloy grain size and related to the critical resolved shear stress for twinning.