| Mo-Si-B alloys are considered as promising high-temperature structural materials of gas-turbine engine blades attributing to their high melting temperature,excellent high-temperature creep strength and oxidation resistance properties.However,multiphase Mo-Mo3Si-Mo5SiB2 alloys which are widely studied have a trade-off between the room temperature fracture toughness and high temperature oxidation resistance due to the different properties of the phases in the alloys,which results in the the difficulty in improving their comprehensive properties effectively and therefore limits the application and promotion of Mo-Si-B alloys significantly.This paper intends to improve the room temperature mechanical properties and high temperature oxidation resistance of Mo-12Si-8.5B alloy through the reasonable design of the structure,including the addition of the second phase of ZrB2 and the regulation of ?-Mo grain size.The effects of microstructure evolution on room temperature toughness and high temperature oxidation resistance were investigated systematically,and the relationship of composition,microstructure,mechanical properties and oxidation resistance was established,revealing the various strengthening and toughening effects caused by ZrB2 additions and ?-Mo grain size change,especially clarifying the mechanism of high temperature oxidation resistance.Mo-12Si-8.5B alloy added with 0.2-4.0wt%ZrB2 was composed of submicron ?-Mo,Mo3Si,Mo5SiB2 phases and nano-scaled ZrO2 and ZrBi2 particles produced by oxygen absorption reaction of ZrB2.In Mo-12Si-8.5B-1.0wt%ZrB2 alloy,Mo3Si and Mo5SiB2 were distributed uniformly in ?-Mo matrix,where the second particles were mainly dispersed in ?-Mo grains,partially distributed at grain boundaries,and the grain sizes of each phase were reduced significantly.The fine grain structure,second phase particles and grain boundary purification caused by ZrB2 addition had a good strengthening and toughening effect,especially the toughening effect.Therefore the fracture toughness of the alloy was increased by 25.3%by adding with 1.0wt%ZrB2.Furthermore,ZrB2 improved the high temperature oxidation resistance of the alloys significantly,and especially 1.0-2.5wt%ZrB2 addition could promote the formation of a continuous and passivated silicate glass layer on surface of the alloy quickly,showing excellent oxidation resistance at different high temperatures,and oxidation resistance temperatures of the alloy were widened to 900-1400?.The coarse-grained and fine-grained powders with a composition of Mo-12Si-8.5B-1.0wt%ZrB2 were mixed in different proportions and then hot-pressing sintered.The sintered alloy consisted of micron and sub-micron ?-Mo grains,sub-micron Mo3Si and Mo5SiB2,and nano-scale second particles,exhibiting multi-scale microstructure.The alloy with ?-Mo of 15.7 vot.%micron grains exhibiting the microstructure that micron and sub-micron ?-Mo grains distributed uniformly and formed a continuous matrix,showed a maximum fracture toughness value of 13.1 MPa·m1/2,and it also maintained the high strength.The improved toughness of the alloy was due to the stronger dislocations storage ability and dislocations slip ability of the coarse micron ?-Mo,as well as its excellent crack trapping effect.Furthermore,the crack deflection caused by the sub-micron fine grain structure and the second phase particles also promoted the toughness improvement.When the content of the micron grains of ?-Mo was over 15.7 vot.%,the aggregation of excessive coarse ?-Mo grains caused the continuous ?-Mo matrix to transfer to island distribution,resulting in weak toughening effect.The oxidation resistance of the multi-scaled Mo-Si-B alloy was related to the volume fraction ratio of micron/sub-micron ?-Mo grains and their distribution.When there were more coarse micron?-Mo grains but aggregated unobviously,the alloy exhibited better oxidation resistance.The analysis of oxidation resistance mechanism of Mo-Si-B alloy showed that Zr and B derived from ZrB2 mainly affected the oxidation behavior of Mo-Si-B alloy by promoting the growth and dense of the silicate glass layer.On the one hand,B(B2O3)increased the fluidity of silicate glass,which promoted the quick coverage of silicate glass on the alloy surface at the initial oxidation stage.On the other hand,Zr improved the passivation of the glass scale during the steady oxidation stage by enhancing its viscosity,and thus the passiviting silicate glass scale inhibited the diffusion of oxygen effectively.However,Mo-Si-B alloy designed with suitable fractions of micron/sub-micron ?-Mo grains could further improve the oxidation resistance.The coupled effects of lower oxidation rate of the coarse ?-Mo grains and the shortened lateral flow distance of silicate glass due to fine ?-Mo grains could reduce the initial mass loss effectively. |
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