The Effect To The Mechanical Property Of Nanocrystalline Copper Because The Solid Solution Of Tantalum And The Grain Boundary Segregation

Nanocrystalline(nc)materials have received enormous attention in the past decades,more and more people are put into the research of nc.It is well known that this class of materials displays not only outstanding mechanical properties,such as high tensile strength and high strain rate sensitivity,but also enhanced physical properties,e.g.magnetic properties.These superior characteristics stem from the ultrafine grain size or the high fraction of atoms on grain boundaries,the so-called size effect.But such high fraction of grain boundaries also comes with an inherent instability.Many nc materials of interest for both fundamental study and engineering application are now recognized as substantially out of equilibrium and undergo rapid evolution to coarser structures even at modest temperatures.Such coarsening tendencies impede the use of these materials at ambient temperature,especially the use as structure materials which always serve a very long period.The coarsening tendencies of nc materials are as the result of the intensive activities of grain boundaries,such as GB sliding,GB migration and GB overturn.Although these GB activities are necessary for improving the plastic deformation,we have to cease it for stabilizing the structure and maintaining the superior characteristics of nc materials.Alloying with immiscible elements is a promising approach.Immiscible elements tend to strongly segregate to GBs,in analogy to microemulsions where the addition of surfactant is used to stabilize interfacial area,reducing the free energy and slowing down the grain growth.In addition,immiscible elements can effectively pin GBs.Cu-Ta is such an suitable immiscible system.The individual element of this system has different crystalline structure and negligible mutual solubility in the solid state.The diameter of Tantalum atom is much bigger than Copper atom,which make them tend to segregate in the GBs of nc Copper.In this experiment,we used magnetron sputtering to synthesize nc Copper and Cu-Ta alloy(Ta 5%)films on glass substrate.The thickness of these films are about 6 ?m.The immiscible alloy system is compared with a pure copper film synthesized at the same condition.The crystallographic structure was analyzed using an x-ray diffractometer(XRD).The Surface morphology was observed by transmission electron microscopy(TEM).The mechanical properties were performed by Nanoindentation testing system.The results is that nc copper and Cu-Ta have similar structure and grain size.So the solid solution of tantalum can not change the structure of nc Copper.Nc Cu-Ta alloy exhibits superior hardness and creep resistance than copper at modest temperature.So the solid solution of tantalum can improve the mechanical properties of nanocrystalline copper.The nc copper and Cu-Ta alloy samples are annealing at some temperature for some time.The crystallographic structure was analyzed using an x-ray diffractometer(XRD).The Surface morphology was observed by transmission electron microscopy(TEM).The mechanical properties were performed by Nanoindentation testing system.The results is that the tantalum atoms of nc Cu-Ta alloy are segregate to the grain boundary,nc Cu-Ta alloy exhibits superior hardness and creep resistance than copper at modest temperature.So the grain boundary segregation of tantalum can improve the mechanical properties and thermodynamic stability of nanocrystalline copper.