Laser shock on a TRIP alloy: mechanical and metallurgical consequences

The present study concerns an Fe-Ni alloy (30% Ni) which was submitted to laser shocks with an energy density of 10¹⁵ and 10¹⁷ W m^–2. The main metallurgical consequence is the existence of a martensite transformation which appears in an area near to the back face of the sample. This was evidenced by microscopy and by measures of Vickers microhardness. The localization of the transformation can be explained by the reflection of the compression wave in the shape of an expansion wave. This study gives also, according to Meyer's theory, the maximum time necessary to induce the transformation, i.e. 40 ns. The mechanical consequence is the existence of tensile residual stresses at a level close to the strength limit. The stress field was determined by X-ray diffraction and by destructive methods. The shape of the stress field curve is quasi-sinusoidal with a period of about 150 mm; it is linked to the compression wave and to the expansion wave which follows the first one at the surface.