Plastic Behavior Of High Strength Ultrafine-grained Metallic Materials

Grain refinement is the most effective way to strengthen and toughen metallic materials at the same time,but when the grain size is reduced to a certain critical value,although the strength is still improved,the plasticity is significantly reduced,which restricts the application of ultrafine grained metals.There have been many studies on this phenomenon,but there are some deficiencies in the existing theories to explain the phenomenon of plastic reduction of ultrafine grained metals.In this paper,the effect of grain size on the plastic behavior of metals and alloys with different crystal structures(pure iron,low carbon steel,Ti_43.5Ni_6.5Fe alloy,405 ferritic stainless steel and 304austenitic stainless steel)is studied.The critical grain size leading to plastic reduction is obtained and its mechanism is analyzed.The main conclusions are as follows:μm1)The samples with different grain size were prepared by large plastic deformation and recrystallization annealing.The grain size and characteristics of the samples were analyzed by SEM and TEM.The results show that the average grain size of pure iron sample is 0.53-8.17μm;the minimum average grain size of Ti_43.5Ni_6.5Fe alloy sample is0.16μm and the maximum grain size is 6.5μm.The minimum grain size of low carbon steel,405 ferritic stainless steel and 304 austenitic stainless steel is about 1μm,and the maximum average grain size is about 6.5μm.In addition,it is found that the smaller the grain size is,the higher the proportion of hexagonal grains is,and the more the angle between 120°grain boundaries is.2)The results of tensile test at room temperature show that the strength of the five kinds of ultra-fine grained metals increases with the grain refinement,while the plasticity increases first and then decreases with the grain refinement.For example,when the grain size of ultrafine pure iron is refined from 8.17μm to 0.53μm,the tensile strength increases from 380 MPa to 910 MPa,while the elongation after fracture first increases from 18%at 8.17μm to 22%at 2.50μm,and then decreases to nearly 0%at0.53μm.3)The results show that the critical grain size of pure iron,low carbon steel,405stainless steel and 304 stainless steel is 2.50μm,3.04μm,3.09μm and 2.02μm respectively.It can be seen that the critical grain size of pure metal is smaller than that of solid solution alloy,and that of FCC metal is smaller than that of BCC.This is mainly due to the influence of alloy elements and crystal structure on dislocation behavior.4)The results show that the work hardening rate has no relation with elongation and grain size.Low work hardening rate（such as:pure iron is 2.50μm;low carbon steel is 3.04μm）can also have excellent plasticity,while the work hardening rate of samples with poor plasticity can also be very high.The work hardening rate of 405 ferritic stainless steel and 304 austenitic stainless steel decreased with the refinement of grain size;with the decrease of grain size,the work hardening rate of Ti_43.5Ni_6.5Fe alloy changed little.5)The effect of aging on the plastic deformation of ultra-fine grained 405 ferritic stainless steel was investigated subsequently.The results show that the plastic deformation was affected by aging significantly.When the grain size is less than 1.8μm,the plasticity of the sample remains unchanged compared with that before aging,but the strength increases by 200 MPa;when the grain size is 1.8μm,the plasticity of the material is 18%as before aging,but the tensile strength increases from 650 MPa to 800MPa.The critical grain size of 405 ferritic stainless steel after aging is 1.74μm,which is mainly due to the high density ofα’phase in the aging sample.α’phase is rich in Cr element,has high hardness,and is coherent with matrix,which can effectively block dislocation movement and accumulate dislocation without obvious stress concentration.