Investigation On The Thermal Stability Of Ultra-fine Grained Pure Nickel

Ultra-fine grained(UFG) materials due to its excellent mechanical property raised widely attention. Although ultra-fine grained materials have excellent mechanical properties, at room temperature, ultra-fine grained materials are in the high energy of metastable state,which have a trend to a steady state. It have a significant influence on the mechanical property. This thesis has carried on the system research of the thermal stability of the ultra-fine grained pure nickel,which provides a theoretical support for the selection of working temperature.Equal channel angular pressing(ECAP) at room temperature with rout C was successfully processed to eight passes in pure nickel(99.93%). At the end of the trial after room temperature ECAP, the cold-rolling deformation of a part of the samples were carried out at the reduction of 75%. Annealing behavior of pure nickel after ECAP was investigated by isothermal annealing and isochronal annealing, the microstructure and microhardness evolution after deformation and annealing were studied. The JMAK model was used to analysis the recovery and recrystallization mechanism of the two deformed samples.The Arrhenius equation was used to caculate the apparent activation energies.Based on the nucleation and grain growth model, analysis the difference of grain size after annealing at different temperatures.The results show that the microstructure of the ECAP processed subsequent cold-rolling nickel exhibits discontinuous recrystallization during subsequent annealing. When the annealing temperature is below 280℃,the microhardness slightly decreases,the microstructure changes little. When the annealing temperature is over 280℃, because the dislocation density decreases through recovery and recrystallization,the microhardness decreases markedly and the thermal stability becomes worse.Analysis of recrystallization kinetics show specimens annealed at 300℃ below, two dimensional growth occur.Meanwhile, the specimens anneal at higher temperature(320℃) which is a three dimensional growth. The magnitude of the apparent activation energies for recrystallization is estimated to be～77KJ/mol,which is consistent with the grain boundary diffusion. The microstructures clearly show that with increasing the annealing temperature the microstructures become more homogeneous with smaller average grain size. The DSC results show that the apparent activation energies for recrystallization is estimated to be-81KJ/mol,which is consistent to the Arrhenius results.Through the microstructure of the eight passes ECAP(8P) nickel and the eight passes ECAP subsequent cold-rolling(8P+CR) nickel,8P+CR nickel has a smaller grain size causing that the 8P+CR nickel has a geometric deformation basing on the 8P nickel.The 8P nickel needs longer time to be fully recrystallization than the 8P+CR nickel,at the same time,the 8P nickel reveals smaller grain size. The Arrhenius results show that the apparent activation energies for the 8P nickel is estimated to be-87kJ/mol,which is larger than the 8P+CR nickel.It illustrates that the 8P nickel has a better thermal stability than the 8P+CR nickel.