Investigation On The Microstructure And Properties Of Mo-12Si-8.5B-xZr Alloys

The Mo-1 2Si-8.5B-xZr(0,1,2,3;4 at.%)alloys were fabricated by mechanical alloying(MA)and hot pressing sintering(HP)technique.The microstructure change of powders during the mechanical alloying process was investigated.It was also studied that the Zr content effected microstructure,mechanical properties and oxidation resistance at high temperature.The strengthening and toughing mechanisms and oxidation resistance mechanisms of the Zr containing Mo-Si-B alloys were discussed as well.It has been found that the mechanical alloying process could not only refine the particles but also dissolve Si and Zr atoms into Mo to strengthen the alloy.The results of sintered alloy indicate the containing Mo-12Si-8.5B alloys are consisted mainly of ?-Mo,Mo3Si and Mo5SiB2 phases.The grain is refined by adding Zr.There are the 26?150 nm second phase particles such as Mo2Zr and ZrO2 distributed in the grain interior and boundaries.With increasing Zr content,the volume fraction of ?-Mo-phase decreases.When adding the 0?1 at.%content of Zr,the alloys possess a continuous matrix with homogeneously distributed intermetallic particles.llowever,when the Zr content increase to 3?4 at%,the alloy appears to form isolated a-Mo particles in the Mo3Si/Mo5SiB2 matrix.Mo-12Si-8.5-xZr alloys have higher density.Microhardness and compression strength are imprived with increasing Zr content.The effeets on the volume fraction and distribution of?-Mo for the fracture toughness of alloys are obvious.When the alloy possesses a high volumc fraction of a-Mo and a continuous ?-Mo matrix,it will have a excellent fracture toughness.In comparison with Mo-12Si-8.5B alloy,the flexural strength value of the containing Mo-12Si-8.5B decrease obviously.Fracture surlface which was observed in bending experiments indicate that Mo-12Si-8.5B alloy is mixed transgranular fracture and intergranular fracture.When adding Zr,the alloy is mainly trsgranular cleavage fracture associated with significant sections and only a little intergranular fracture.Zirconium containing alloys exhibit poor oxidation resistance properties at 1000?1200 ?At 1300 ?,adding 1 at%amount of Zr to Mo-12Si-8.5B alloy can reduce the weight loss in the instant oxidation stage effectively and shorten time to reach the stable oxidation stage.When adding 3?4 at.%amount of Zr to alloys,the aggregate ZrO2/ZrSiO4 particles on oxide scale surface will impede the flowing borosilicate glass.What's more,the interfacial strength among ZrO2/ZrSiO4,SiO2 and MoO2 phase in internal oxide scale is decreased due to the different coefficients of thermal expansion.At the moment it appears stress concentration resulting in crack easily between the phase interface,thereby increasing the oxygen channels from the outside to the base and forming a severe oxidation.