Effect Of Surface Mechanical Attrition Treatment On Mechanical Properties Of Pure Iron

Multi-scale engineering materials with a gradient structure can reduce some surface failures.A common feature of these materials is the increased strength by forming a hard phase of nano/microstructure on the surface.However,the properties of strength and ductility are contradictory in most materials.Conventional techniques can easily increase the strength of the material,but the elongation is greatly reduced.In this study,pure iron(99.5 wt.%of Fe)with body-centered cubic(BCC)structure was studied.The stacking fault energy(SFE)is of 200 mJ/m~2.By performing a surface mechanical attrition treatment(SMAT),it is desirable to obtain a good match between plasticity and strength.The main research contents include:studying the mechanical properties of pure iron after short-term SMAT process,analyzing the effect of annealing temperature and cryogenic pretreatment on the mechanical properties of SMAT pure iron.It is characterized by tensile test,hardness test,and optical microscopy(OM),scanning electron microscope(SEM),X-ray diffractometry(XRD)and other test methods.The main conclusions are as follows:(1)The samples treated by the local SMAT showed an optimal combination of strength and ductility.The yield strength of the SMAT 60 s sample was increased by about twice,while the elongation was still well retained.The combination of strength and ductility is ascribed to the extra strain hardening behavior and coexistence soft and hard phases.(2)The strength of pure iron subjected to SMAT for 360 s is increased significantly,while elongation would decrease dramatically.It showed the strain softening behavior due to the isolated gradient structure.(3)The thermal stability of the nanograins in the sample SMAT for 360 s was500℃.After annealing in the range of 50℃ to 500℃,the surface hardness value and the yield strength decrease slightly and the ductility increases with increasing temperature.When the sample of SMAT 360 s was annealed at 600℃,the surface hardness value and yield strength decreased significantly,and the elongation increased.(4)The surface hardness of pure iron obtained by SMAT for 90 min at different temperatures is greatly improved,and both decrease with increasing distance from the treated surface.The surface hardness of the CT-SMAT specimen is slightly higher than that of the RT-SMAT specimen,and the gradient of the plastic deformation layer is more obvious.(5)The yield strength of RT-SMAT iron is 201 MPa,and the yield strength of CT-SMAT iron is 252 MPa,which is about 1 to 2 times that of the annealed sample.Compared with the RT-SMAT sample,the yield strength of the CT-SMAT sample is increased to a greater extent,while the elongation remains substantially unchanged.