| Because of their low density and high specific strength,magnesium and its alloys are increasingly used in various fields such as automotive,aerospace,and biomedicine.However,the poor thermal stability and low absolute strength of magnesium alloy greatly limit its wider application.Particle reinforced magnesium matrix composites has become an effective means of strengthening magnesium alloy due to its high mechanical properties at room temperature and high temperature.Ti has the characteristics of high melting point,high elastic modulus,good plasticity and low miscibility with magnesium,which makes Ti an ideal choice for the reinforcement phase of magnesium matrix composites.In this paper,AZ61 powder was used as matrix and metal Ti particles as reinforcing phase.Nanocrystalline AZ61 powder and nanocrystalline AZ61-Ti powder were prepared by mechanical ball milling.Under the same ball milling process,the addition of Ti particles can promote the refinement of particle size and grain size of the final AZ61 powder.As ball milling progresses,the distribution of Ti particles becomes more even around the AZ61 powder particles.Then the discharge plasma sintering was carried out.Under the sintering condition of “450℃ +70MPa+10min”,the stress indexes of mixed crystal AZ61 and mixed crystal AZ61-Ti sintered billet were all between 1 and 2 through densification curve analysis,and the densification process was a combination of diffusion and grain boundary slip.The mixed crystal AZ61-Ti sintered billet was annealed at 300°C～400°C for30min～300min.According to the microstructure evolution of the sintered billet during annealing,the grain growth law of the coarse-grained structure and fine-grained structure of the sintered billet was revealed.The results showed that the grain size of the coarsegrained region increased with the prolongation of annealing time and the increase of annealing temperature.The dynamic equation of grain growth in coarse-grained region was established.The grain growth indexes of the coarse-grained structure at 300°C,350°C and400°C were 16,20 and 33,respectively,and the activation energy of grain growth was1293.3k J/mol.The grain size of the fine-grained region was maintained at about 1 μm during the annealing process at 300°C and 350°C,and only increased from 0.96 μm to 1.25μm after annealing at 400°C for 300 min.It showed that mixed crystal AZ61-Ti sintered billet had excellent thermal stability.The mixed crystal AZ61,mixed crystal AZ61-Ti,and the samples annealed at 300°C and 350°C for 10～40h were subjected to room temperature compression experiments.The mechanical properties of the sintered billet under room temperature compression were obtained,and the optimal annealing treatment parameter was determined as “300°C+30h”.The ultimate compressive strength,compressive yield strength and fracture strain were 387.3MPa,284.8MPa and 6.05%,respectively.The yield strength of mixed crystal AZ61-Ti sintered billet was 74.7MPa higher than that of the micro-crustal pure AZ61 sintered billet,in which fine grain strengthening and thermal mismatch strengthening played a dominant role.At the same time,the work hardening ability of the sintered billet was analyzed,and the results showed that the addition of Ti particles improved the work hardening ability of the mixed crystal AZ61 sintered billet.In addition,by observing the fracture morphology after compression fracture,it was revealed that the cleavage fracture was the main fracture mode of the mixed crystal AZ61-Ti sintered billet. |
PDF Full Text Request