Fabrication And Properties Of Ultrafine-grained AZ31 Magnesiun Alloys Strengthened With Ti Dispersions

Magnesium alloys have low density, high specific strength and specific stiffness, strong anti-collision capability, good recycling properties and many other outstanding advantages. Therefore, they have very essential applied value in the fields of aerospace, automotive, optical instruments. However, the poor mechanical strength seriously limits the structural applications of Mg alloys at elevated temperatures. Aiming to develop a new category of Mg alloy which cannot be prepared by melt casting and is featured by low-cost, low-density, high thermal stability and high strength, we suggest that the process of mechanical milling is employed to introduce the light-weight, high strength, and high thermal stability Ti dispersions to the solid solution strengthening AZ31 Mg alloys to produce the fine-grained Mg alloys strengthened with ultrafine Ti dispersions, by means of the technology which combines powder metallurgy and deformation processing. Firstly, the ultrafine Ti dispersion strengthened nanocrystalline AZ31 magnesium alloy powders were prepared by mechanical milling, and then Ti dispersion strengthening fine-grained AZ31 magnesium alloy bar can be obtained through hot pressed and extruded at vacuum.In this paper, the microstructure evolution of mechanical milling process was studied during the preparation of Ti dispersion strengthening nanocrystalline AZ31 magnesium alloy powders. The variation of matrix grain size, the distribution of Ti dispersion and powder morphology in the mechanical milling process were investigated. After milling for 110 h, the crystallite size of the Mg phase of the alloy with 9 wt.%, 18 wt.% and 27 wt.% Ti additions was refined to 108 nm, 86 nm and 66 nm, respectively.Huang Peiyun compaction equation was used to study the compression characteristics of AZ31 Mg-Ti composite powders. Mechanical milling is effective methods for synthesizing novel materials, which has nanocrystalline matrix and refining dispersions, and improve the mechanical properties of materials. The AZ31 Mg alloy with different Ti additions exhibited larger yield stress, which reached above 293 MPa at room temperature and 60 MPa at 300°C. Therefore, it is a tempting method to prepare Ti dispersion strengthening AZ31 Mg alloy by mechanical milling.A study on the grain growth kinetics was carried out by the isothermal and isochronal annealing treatments. The results showed that the grain growth kinetics could be well described by the kinetic equation, Dn-D0n=kt. The grain growth index n of the alloy with 9 wt.%, 18 wt.% and 27 wt.% Ti additions was 6, 7 and 8, respectively. The activation energy for grain growth was calculated to be 107 k J/mol,120 k J/mol and 147 k J/mol, respectively.Ti dispersion strengthening fine-grained AZ31 Mg alloy bar with grain size of 800 nm can be obtained through hot pressed and extruded at vacuum. The tensile yield stress at room temperature of the alloy with 9 wt.%, 18 wt.% and 27 wt.% Ti additions was 194 MPa,214MPa and 244 MPa, their elongation was 12.17%, 12.66% and 11.88%, respectively. The AZ31 Mg alloy with different Ti additions exhibited larger compressive yield stress, which reached above 249 MPa at room temperature. The compressive yield stress of AZ31 Mg alloy with 27 wt.% Ti additions reached about 337 MPa, which was almost two times larger than that of the as-cast AZ31 Mg alloy, and the tensile stress reached about 524 MPa.The mechanical properties of Ti dispersion strengthening fine-grained AZ31 Mg alloy bar at high temperature was studied. The AZ31 Mg alloy with different Ti additions exhibited larger tensile yield stress, which reached above 60 MPa at 300°C, which was almost 2 times of the as-cast AZ31 Mg alloy. In particular, the tensile yield stress of AZ31 Mg alloy with 27 wt.% Ti additions reached about 78 MPa at 300°C, which was almost 2.5 times of the as-cast AZ31 Mg alloy. The elongation at 300°C of the alloy with 9 wt.%, 18 wt.% and 27 wt.% Ti additions was 12.62%, 13.34% and 12.39%, respectively. Therefore, the tensile properties of AZ31 Mg alloy at hightemperature have been significantly improved. Moreover, the AZ31 Mg alloy with different Ti dispersions exhibited higher compressive yield stress, which reached above 60 MPa at 300°C. In addition, the compressive yield stress of AZ31 magnesium alloy increased with the increase of Ti contents.