Experimental and numerical studies on strength and ductility of gradient-structured iron plate obtained by surface mechanical-attrition treatment

By means of the surface mechanical-attrition treatment (SMAT), a nano-crystalline surface layer of about 25 mu m was prepared on the iron plate. The grain size exhibits a gradient change from the nano-crystalline surface to the coarse-grain matrix. The nano-layer has been studied by the optical microscopy (OM), transmission electrical microscopy (TEM), and X-ray diffraction (XRD). The microhardness was measured. The tensile properties have been acquired by the designed and implemented tensile process test on the SMATed and coarse-grain samples, accompanying with numerical simulation based on the hypothesis of a gradient-structured material. The fracture morphology was observed by scanning electron microscopy (SEM). The highly strengthened surface-nano crystallization leads to an increase in both the yield strength and ultimate tensile strength, however, appreciable decrease of the uniform elongation. The fracture of the gradient-structured iron plate shows a lamellar structure, which looks like lamellar tearing in the vertical direction of the surface. The numerical simulation effectively presents the stretching process, showing the evolutionary characteristic of gradient-structured material. Failure begins within coarse-grain side of the sample and ends at the nano-crystalline surface.