Experimental Studies On The Stress Relaxation Behaviors Of Thin Metallic Wires

The creep problem of materials is ubiquitous in engineering practice.Creep deformation has a significant impact on the safety of engineering structures.Meanwhile,with the development of science and technology,micro-scale materials have been widely used in practical industries.Therefore,understanding the creep properties of microscale materials is becoming increasingly significant.In this paper,the stress relaxation behaviors of micro-scale metallic wires are experimentally investigated.The main results are summarized as follows:(1)A strain gauge cantilever force sensing system was developed.Tensile stress relaxation experiments were performed on micro-scale wires with different diameters at room temperature.The experimental results show that the stress relaxation behaviors of micro-scale wires exhibit both size and stress dependences.It is relvealed that with the same initial stress,the stress relaxation increases with the decreasingly diameter;for a given diameter,the stress relaxation accelerates with the increase of the initial stress.We discussed and proposed that the interaction between dislocations and grain boundaries is the dominant deformation mechanism during the tensile stress relaxation of micro-scale wires.The size effect phenomenon is explained by the mechanism of dislocations exhausted from the free surface and the grain boundary.TEM observations indicate that the grain boundaries mainly act as obstacles to hinder the movement of dislocations during the stress relaxation.Furthermore,the activation volume model related to the sample size was established and the experimental results were predicted.(2)A temperature-controlled micro-tensile platform was successfully built.Tensile and stress relaxation experiments were performed on micro-scale copper wires with different diameters under different temperature fields.The tensile results show that for a same diameter sample,as the temperature increases,the yield strength decreases,the ductility decreases,and the fracture strength decreases.For a given temperature,size effect is not observed on the tensile mechanical properties for wires with different diameters.The stress relaxation results show that for a given diameter,the stress relaxation increases with the increasingly temperature;at a same temperature,the stress relaxation accelerates with the decrease of the diameter.The analysis results show that as the increases of temperature and decreases of diameter,the activation volume decreases,the strain rate increases,the strain rate sensitivity coefficient increases,and the evolution of dislocations increases.We deeply discussed and re-confirmed that the interaction between dislocations and grain boundaries is the dominant deformation mechanism.(3)The influences of initial stress,loading rate and sample size on the torsional stress relaxation behaviors of micro-scale wires were investigated.The experimental results show that before and after the elastic limit,the torsional stress relaxation of the micro-scale wire increases with the increasingly initial stress;an extreme value near the elastic limit is observed.Stress relaxation increases with the increasingly loading rate.At the same time,it is observed that the torsional stress relaxation behavior of micro-scale wires with different diameter exhibit size dependence.It is revealed that the stress relaxation increases as the diameter decreases.The experimental results were analyzed by using thermally activated mechanisms.The dominant deformation mechanism in the torsional stress release of micro-scale wires is discussed,and the dislocation cross-slip is determined as the dominant mechanism.The effect of geometrically necessary dislocations on the torsional stress relaxation behavior is accounted for the size effect.(4)The thin wire samples with different grain sizes were prepared by different heat treatment processes.And corresponding microstructures were characterized by FIB measurement.Then tension and torsion tests were conducted on the sample with different grain sizes.The experimental results show that the yield strength decreases with the increase of grain size.Tensile and torsional stress relaxation tests were performed on samples with different grain sizes.The results show that both the tensile and torsional stress relaxation process decrease with the decreasingly grain sizes.The experimental results were analyzed by using thermally activated mechanisms.The analysis results show that with the increase of grain size,the apparent and physical activation volume increases,the stress rate decreases,the strain rate sensitivity coefficient decreases,and the evolution of dislocation becomes slower.By considering the contribution of geometrically necessary dislocations to inhomogeneous deformation,the differences between tensile and torsional stress relaxation behavior were explained.