The “black” layer near the crater surface of a middle-carbon steel targetpenetrated by a high speed copper jet was observed by SEM. Micro-hardnesstest showed that the “black” layer near the crater surface was much harder thanthe matrix. It almost reached to the untempered martensite phase microhardness.The microstructure features divided the penetrated middle-carbonsteel into four layers from the crater surface to the matrix: needle-like martensitelayer, austenite and ultra-fine grained(UFG) ferrite layer, plastic deformationlayer and normal matrix. The phase transformation was caused by the highpressure introduced by the hypervelocity impact and plastic deformation. Andthe microstructure distribution has something relationship to the differentpressure condition and deformation strain and strain-rate because the shockwave decays as it spreads to the deep. XRD patterns confirmed that there werenew phases like austenite in the middle-carbon steel after jet penetration.
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