The Effect Of Grain Boundary Segregation Tantalum Atom On The Creep Of Nanocrystalline Copper At Room Temperature

Nanocrystalline metal materials can have much higher strength, according to Hall-Petch relationship. Nanocrystalline metal materials can have much higher creep rate and further can have good plasticity on the basis of Coble creep formula. Hence, nanocrystalline metal material has attracted increasing attention in academia as a potential star with higher strength and plastic. With the development of preparation technology, nanocrystalline metal materials with different grain sizes have been obtained. Through the mechanics research, we found they did have the ultra-high strength, compared with coarse crystal, but their plastic will decrease severely. This is related to their unique deformation mechanism. Besides, the thermal stability of nanocrystalline metal material will decrease, leading to grain growth even at room temperature. Therefore, the synthesis of nanocrystalline metal with high thermal stability and even thermodynamical stability have become a hot spot of academic research. To enhance their stability, nanocrystalline metal Cu-Ta was prepared by Magnetron sputtering. Grain boundary segregation could easily be seen, because Ta atom was much larger than Cu atom. So that grain boundary segregation Ta has a pinnig effect on grain boundary so that the grain boundary can be limited to moving and it can enhance the thermal stability of nanocrystalline metal. The specific work is as follows:1. The nanocrystalline Cu and Cu-Ta films with different grain sizes less than 100nm were prepared by changing the magnetron sputtering parameters.The thickness of the films is 2 urn, respectively, all the grain size and distribution are single uniform, the isometric grains are divided by large angle grain boundaries.2. The relationship between strain rate sensitivity and creep strain rate of nanocrystalline Cu and its grain sizes were studied by nano indentation apparatus. It was found that the hardness and grain size of the nanocrystalline Cu with the grain size between 18 nm to 89 nm are fully conformed to the Hall-Petch relationship. Also, the nanocrystalline Cu has a very high strain rate sensitivity that increased with the decrease of the grain size. Nanocrystalline Cu room temperature creep rate will be increased significantly with the decrease of the grain size, conforming to the Coble creep rule.3. The same grain size of the nanocrystalline Cu and Cu-Ta films with the different heat treatment are to study the effect of grain boundary segregation Ta on thermal stability of the nanocrystalline Cu. Experiments found that due to the Ta atom ratio diameter is larger than Cu atom ratio diameter, Ta as doping elements prone to form the grain boundary segregation. Grain boundary segregation Ta elements can significantly improve the thermal stability of the nanocrystalline Cu, but only on the dynamics of slow rather than stop on thermodynamics.4. Compare the creep strain rate of the nanocrystalline Cu and Cu-Ta films at room temperature to study the effect of grain boundary segregation Ta on the creep strain rate of the nanocrystalline Cu by grain boundary diffusion.Experiment found that the grain boundary segregation Ta elements can obviously increase the hardness of nanocrystalline Cu, but the strain rate sensitivity and creep rate were reduced.However, by further tempering, increasing the degree of grain boundary segregation cant be further improve the thermal stability of the nanocrystalline Cu. Instead,the creep rate will likely rise.