| Owing to excellent properties,such as high melting point,superior hardness and optimal corrosion resistance,the newly-developed Mo-Si-B alloy exhibits high potential of application in aerospace,energy,power and other fields as a high temperature structural material.Currently,the Mo-Si-B alloys suffer from the limitation of the negative correlation between the room temperature fracture toughness and high temperature strength.In addition,only a few reports are available on the friction and wear behavior of the Mo-Si-B alloys.In order to meet the requirements of high temperature structural engineering applications,it is required to have excellent mechanical properties,along with optimal oxidation resistance,friction and wear behavior.In this study,the microstructure and properties of the Mo-Si-B alloys were improved by using La2O3 based rare earth materials.Specifically,the effect of different ?-Mo phase content and La2O3 doping extent on the microstructure,mechanical properties,high temperature oxidation,friction and wear behavior of the Mo-Si-B-La2O3 alloys was studied.In addition,the mechanisms of enhancement of strength and toughness as well as anti-oxidation and anti-friction wear were also revealed by studying the microstructure and properties of the Mo-Si-B-La2O3 alloys.The multiphase Mo-Si-B-La2O3 alloys with different ?-Mo phase content and extent of La2O3 doping were designed and synthesized.First,the Mo-La2O3 alloy powder was prepared by using the liquid-liquid doping method,and the Mo-Si-B mixed powders containing La2O3 were subsequently obtained by mixing with Si and B powders in different stoichiometric ratios.Consequently,the multi-phase Mo-Si-B-La2O3 alloys with main crystalline phase of ?-Mo,Mo3Si,Mo5SiB2 or Mo3Si,Mo5SiB2 and Mo5Si3 were prepared using mechanical alloying and hot-press sintering methods.The microstructure of the prepared alloys conformed that the liquid-liquid doped Mo-La2O3 alloy powder ensured the uniform distribution of the doped La2O3 particles in the grains of ?-Mo,Mo3Si and Mo5SiB2 or at the phase interface and grain boundaries.The Vickers hardness,fracture toughness,bending strength and compressive strength of the multiphase Mo-Si-B-La2O3 alloys were studied.The results show that the hardness and compressive strength of the alloy are gradually enhanced with the Si and B contents,while the flexural strength and fracture toughness are observed to decrease.No yield deformation or plastic deformation is observed under the applied load,thus,only exhibiting the linear elastic deformation stage.The Mo-Si-B-La2O3 alloys are observed to break suddenly with the elastic deformation of the specimens reaching the maximum value.The fracture morphology of the samples exhibits the typical fracture characteristics of the brittle materials,i.e.mixed transgranular and intergranular fractures.On the one hand,the La2O3 particles in the metal ?-Mo phase are observed to hinder the dislocation movement,thus,strengthening the alloy.Also,the pull-out energy of the La2O3 particles consumes a part of the fracture energy,thus,inducing the transgranular fracture of the ?-Mo phase and dissipating the crack propagation energy,thereby,improving alloy toughness.However,with the augmentation of the Si and B contents,the Mo3Si,Mo5SiB2 and MosSi3 intermetallic compounds in the alloy increase gradually,whereas the content of the ?-Mo phases with superior ductility decreases correspondingly.The lower extent of metal phase cannot completely enwrap the La2O3 particles,which results in a significant increase of the La2O3 particles in the intermetallic compounds or at the interfaces.The hard-brittle second phase at the interface leads to a reduction in the binding property,which makes it easy to produce cracks,along with inducing stress concentration and dissociation along the interface under the action of the external force.The high temperature oxidation resistance of the multiphase Mo-Si-B-La2O3 alloys was also studied.The results indicate that,the oxidation weightlessness of the Mo-10Si-7B-La2O3 and Mo-12Si-8.5B-La2O3 alloys with a small extent of Si and B contents follows a typical straight line path,while the constant temperature oxidation process of the Mo-14Si-9.8B-La2O3 and Mo-25Si-8.5B-La2O3 alloys with a high extent of Si and B contents includes the rapid weightlessness during the early oxidation stage,along with the relatively stable behavior during the middle and late oxidation stages at 1000? and 1100? respectively.On increasing the La2O3 content,the oxidation weight loss of each alloy is observed to decrease,thus,exhibiting a superior oxidation resistance.The morphology and composition of the oxide film of each alloy exhibited obvious difference after the oxidation test.The oxide layer on the surface of the Mo-10Si-7B-La2O3 alloy was composed of oxides formed perpendicular to the surface of the alloy,whereas no borosilicate glass phase was formed.The surface oxide layers of the Mo-12Si-8.5B-La2O3 alloy and Mo-14Si-9.8B-La2O3 alloy demonstrated the bilayer structure of the outer borosilicate glass phase and inner oxide layer.Only a small amount of fine oxide particles were observed to form on the surface of the Mo-25Si-8.5B-La2O3 alloy.The analysis of the antioxidant mechanism indicates that the La2O3 particles inhibit the volatilization of MoO3 by pinning,along with preventing the diffusion of Mo4+,Si4+and B3+through grain boundaries.These particles,thus,reduce the oxidation rate and promote the formation of borosilicate glass phase,leading to the improved oxidation resistance of the Mo-Si-B-La2O3 alloys.The friction and wear properties of the multiphase Mo-Si-B-La2O3 alloys were subsequently studied.It is noted that the friction coefficient of the Mo-Si-B alloy increases with the La2O3 content,and the volume wear rate of the Mo-Si-B alloy containing the ?-Mo phase increases gradually.On the other hand,the volume wear rate of the Mo-25Si-8.5B alloy first increases,followed by a decrease.The friction coefficient of the Mo-Si-B-0.3La2O3 alloy decreases on increasing the load,whereas it first decreases and subsequently increases on enhancing the sliding speed.On the other hand,the volume wear rate increases with load and sliding speed at room temperature.The high temperature friction and wear tests indicate that the friction coefficient of the Mo-Si-B-0.3La2O3 alloy decreases and volume wear rate increases with increasing the temperature,while keeping the friction load and speed as constant.The wear mechanism of the alloy has been discussed based on the wear rates of ?-Mo,Mo3Si,Mo5SiB2 and Mo5Si3,along with their volume fraction in the Mo-Si-B-La2O3 alloy under different loading and sliding speed conditions.At room temperature,the alloy wear is observed to be predominantly follow the mild abrasive and furrow mechanisms at low sliding speed and load.On increasing the sliding speed and load,mild abrasive wear transforms to fatigue and abrasive wear.At high sliding speed and high load,a variety of wear mechanisms,including fatigue,abrasive and exfoliation wear take place on the alloy surface.At high temperature,the alloy is dominated by the oxidation and furrow wear at first.After shedding of the oxide particles and initiation of cracks,the wear begins to change to abrasive and fatigue wear,with finally transforming to exfoliation wear. |
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