Effect Of Grain Size And Strain Rate On Plastic Behavior Of Pure Copper

Strength and ductility are the basic mechanical properties of structural materials.Severe plastic deformation such as equal channel angular pressing(ECAP)significantly increases the strength of material by means of grain size refinement.On the other hand,changing deformation conditions also have a significant impact on the mechanical behavior of the material.For example,the flow stress of the material is obviously increased at high strain rate deformation.The surface of polycrystalline materials wil become rough during plastic deformation,and this phenomenon is called "orange peel effect".Up to now,the law of its formation has been understood clearly in metals with grain size as low as 18?m under quasi-static deformation conditions.However,when the grain size is refined to submicron/nanometer scale and under high strain rate deformation,does the "orange peel effect" still exist?If it does,whether this effect obeys the same rule as that for coarse-grained counterpart and quasi-static strain rates?The answers to these questions play an important role in the development of "orange peel effect" related plasticity theory in the range of ultrafine crystalline/nanocrystalline and the plastic forming and application of these new materials.On the other hand,Strain rate Sensitivity(SRS)is an important parameter to characterize the plastic behavior of materials.It has been shown that the SRS of FCC metal will increase significantly with the decrease of grain size.This phenomenon can be perfectly explained by the thermal activation dominant deformation mechanism.However,when the strain rate exceeds?103s-1,the dominant mechanism of plastic deformation gradually changes from thermal activation to dislocation drag.Under the domination of dislocation drag mechanism,is the relationship between SRS and grain size identical to the relationship at low strain rate or not?It's really worth looking forward to.In particular,SRS is more important for plastic deformation at high strain rate.In order to solve these problems,FCC oxygen-free high-conductivity copper was chosen in this article.The ultrafine-grained copper prepared by ECAP was subseqently annealed to achieve samples with different grain size.Quasi-static and dynamic tensile tests was conducted to study the grain size dependence of "orange peel effect" and SRS at different strain rate.The major innovative conclusions are obtained as follows:1.Oppositing to the general trend that SRS increases with decreasing in grain size at low strain rate,this study shows that the SRS of pure copper increases monotonously and significantly with grain size at high strain rate.SRS is strain rate related,especially at high strain rate,SRS increases significantly.This is the immediate reason for the opposite trend of grain size dependence of SRS at high and low strain rates.2.The reason of the inverse trend of relationship between SRS and grain size as strain rate increases is that the deformation mechanism gradually changes from thermal activation at low strain rate to dislocation drag at high strain rate.This transformation of deformation mechanism is confirmed by the microstructure observation based on the local misorientation analysis.Furthermore it proves that the domination of dislocation drag mechanism is strengthened with increasing grain size.According to the constitutive model established in this paper,SRS consists of two parts:thermal activation component and dislocation drag component.Under the dominantion of thermal activation mechanism,thermal activation component,which is independent of strain rate and increases with decreasing in grain size,is generally small.Under the domination of dislocation drag mechanism,the dislocation drag component increases rapidly with strain rate,resulting much higher value than that of the thermal activation component,and also it increases significantly with increasing in grain size.Therefore,the combined effect of the two components lead to the opposite trend of grain size dependence of SRS at low and high strain rate.3.In the range of 90?m to 0.5?m in grain size,the "orange peel" of deformed copper shows self-affine surface morphology,the identical fractal dimension is independent of grain size,while the correlation length is proportional to grain size.The essence of "orange peel effect" is that the different plastic deformation characteristics between different grains results in the up and downs between adjacent grains.Refining grain not only reduces the surface roughness Ra,but also decreases the surface roughing rate."orange peel effect" is strongly strain rate dependent,and the surface roughness introduced at high strain rate was significantly larger than that introduced at low strain rate.It is indicated that the difference in plastic deformation characteristics of grains will intensify as strain rate increases.4.At high strain rate deformation,the variation of flow stress with grain size of pure copper still follows the Hall-Petch relationship.The value of K is proportional to the logarithm of the strain rate.Meanwhile,the uniform elongation increases monotonously with grain size at high strain rate tensile deformation.