简介概要

Microstructure and improved mechanical properties of Mo-12Si-8.5B alloys with lanthanum oxide addition

来源期刊：Rare Metals2019年第9期

论文作者：Xiao-Hui Lin Bin Li Lai-Ping Li Jing Liang Xuan-Qiao Gao Xin Zhang

文章页码：848 - 854

摘要：Mo-12 Si-8.5 B alloys with different La₂ O₃ contents were fabricated by mechanical alloying and then hot pressing.The effects of La₂ O₃ on microstructure,room and elevated temperature mechanical properties of Mo-12 Si-8.5 B alloys were studied.The microstructure of Mo-12 Si-8.5 B alloy with La₂ O₃ additions exhibits a continuous α-Mo matrix,where the spherical Mo₃ Si and Mo₅ SiB₂ intermetallic phases are distributed inside the grains and along the grain boundaries.The detailed microstructure shows that some nanoscale La₂ O₃ particles are dispersed mainly in the a-Mo grains and partially in the intermetallics.These La₂ O₃ particles can refine the grain sizes of a-Mo matrix and intermetallic,but the refining effect is limited with the La₂ O₃ addition further increasing.The mechanical testing results show that the La₂ O₃ addition simultaneously improves the compression strength and fracture toughness of Mo-12 Si-8.5 B alloy,due to that theα-Mo matrix is strengthened and toughened at ambient temperature and intermetallics are strengthened at elevated temperatures.The enhancing effect is sensitive to the amount of La₂ O₃ additions,and the Mo-12 Si-8.5 B alloy can obtain a better combination of strength and toughness when the content of La₂ O₃ is 0.9 wt%.

稀有金属(英文版) 2019,38(09),848-854

Microstructure and improved mechanical properties of Mo-12Si-8.5B alloys with lanthanum oxide addition

Xiao-Hui Lin Bin Li Lai-Ping Li Jing Liang Xuan-Qiao Gao Xin Zhang

Refractory Metals Material Research Center,Northwest Institute for Non-ferrous Metal Research

作者简介：Bin Li e-mail:libincailiao@163.com;

收稿日期：15 May 2017

基金：financially supported by the China Postdoctoral Science Foundation (No. 2016M602885);the National Natural Science Foundation of China (Nos.51371141 and 51701162);

Microstructure and improved mechanical properties of Mo-12Si-8.5B alloys with lanthanum oxide addition

Xiao-Hui Lin Bin Li Lai-Ping Li Jing Liang Xuan-Qiao Gao Xin Zhang

Refractory Metals Material Research Center,Northwest Institute for Non-ferrous Metal Research

Abstract：

Mo-12 Si-8.5 B alloys with different La₂ O₃ contents were fabricated by mechanical alloying and then hot pressing.The effects of La₂ O₃ on microstructure,room and elevated temperature mechanical properties of Mo-12 Si-8.5 B alloys were studied.The microstructure of Mo-12 Si-8.5 B alloy with La₂ O₃ additions exhibits a continuous α-Mo matrix,where the spherical Mo₃ Si and Mo₅ SiB₂ intermetallic phases are distributed inside the grains and along the grain boundaries.The detailed microstructure shows that some nanoscale La₂ O₃ particles are dispersed mainly in the a-Mo grains and partially in the intermetallics.These La₂ O₃ particles can refine the grain sizes of a-Mo matrix and intermetallic,but the refining effect is limited with the La₂ O₃ addition further increasing.The mechanical testing results show that the La₂ O₃ addition simultaneously improves the compression strength and fracture toughness of Mo-12 Si-8.5 B alloy,due to that theα-Mo matrix is strengthened and toughened at ambient temperature and intermetallics are strengthened at elevated temperatures.The enhancing effect is sensitive to the amount of La₂ O₃ additions,and the Mo-12 Si-8.5 B alloy can obtain a better combination of strength and toughness when the content of La₂ O₃ is 0.9 wt%.

Keyword：

Composite materials; Mo silicide; Nanoparticles; Microstructure; Compression property; Fracture toughness;

Received： 15 May 2017

1 Introduction

Because of high melting point,excellent high-temperature mechanical properties and oxidation resistance,the MoSi-B alloys as the potential candidate high-temperature structural material have been the focus of research attention [ 1, 2, 3, 4, 5, 6, 7, 8] .Mo-Si-B alloy which consisted of the Mo solid solution phase (α-Mo) and the intermetal lie phases of Mo₃Si and Mo₅SiB₂ (T2) was firstly proposed by Berczik [ 9] and further promoted by Schneibel et al. [ 10, 11, 12, 13, 14, 15] .As the only ductility phase in Mo-Si-Balloy,the content and distribution ofα-Mo phase can significantly affect the deformation and fracture behaviors of alloys at room temperature (RT).In addition,α-Mo phase plays a critical role at elevated temperature,and the current study declared that the strength of Mo-Si-B alloy at high temperature(HT) was mainly influenced by deformation and recrystallization ofα-Mo phase [ 16, 17, 18] .The Mo₃Si and Mo₅SiB₂ phases are very strong which can provide the necessary creep resistance and oxidation resistance of the alloy at high temperature,but the intrinsic room tempera—ture brittleness of those intermetallic phases can decrease the ductility and toughness of Mo-S i-B alloy,which limits the process and application of the alloy.Obviously,there is a competitive relationship between achieving well creep and oxidation resistance at HT and maintaining appropriate ductility and toughness at RT in Mo-Si-B alloy,which is related to the morphology and content of theα—Mo metal phase and intermetallic phases.It is very difficult and limited to balance those properties to meet these requirements of structural materials for ultrahigh temperature application by only controlling the microstructure.Thus,the optimizing performance of each phase is a feasible way.

Some previously research work indicated that the oxide particle doped in molybdenum alloy would improve the mechanical properties of alloy [ 19, 20, 21, 22, 23] .It has been known that the La₂O₃ particle can enhance the strength of Mo via fine-grained strengthening and dispersion strengthening [ 24] .Zhang et al. [ 25] showed that La₂O₃ particles will greatly improve the plane strain fracture toughness (K_IC) of Mo,and the K_IC value of the Mo-1 wt%La₂O₃ alloy reaches 24.76 MPa-m^1/2,which is 2.5 times higher than that of pure Mo.Liu et al.found that La₂O₃ particle doped in MoSi₂ will increase the fracture toughness value by over50% [ 26] .This is because that the particles promote crack bridging and trapping mechanisms during crack propagation.Thus,the La₂O₃ addition could be a feasible method to strengthen and toughen the Mo-Si-B alloy.

The present work systematically studied the microstructure,room temperature and high-temperature compression and fracture properties of Mo-Si-B alloys with different La₂O₃ additions which were prepared by mechanical alloying and then hot pressing.If the ductility and strength of theα-Mo matrix can be improved,it should be possible to reduce the content of theα-Mo phase.That means the content of intermetallics can be properly increased,thereby allowing for improved high-temperature properties such as creep and oxidation resistances.

2 Experimental

The following elemental materials were used in this study:Mo (<2μm) of 99.9 wt%purity,Si (<5μm) of99.8 wt%purity,B (<1μm) of 99.5 wt%purity and La₂O₃ (<100 nm) of 9 9.9 wt%purity.The composition of Mo material is presented in Table 1,and the composition of La₂O₃-doped Mo-Si-B alloy is listed in Table 2.A milling process was carried out for 6 h in a planetary ball mill (QM-3SP2) with a rotational speed of 400 r·min^-1.The dispersive powder mixtures were further processed by mechanical alloying (RetschPM-400MA) for 15 h in a W₂C container with W₂C milling balls,the rotational speed is 300 r·min^-1,the ratio of powder and ball is 1:10,and the W₂C container is filled with argon for resisting the oxygen.After mechanical alloying process,the grain size of mixed powder can reach submicron level.The La₂O_3-doped MoSi-B powder was hot pressed into a round bulk with a dimension ofΦ60 mm×10 mm.The hot pressing process was carried out at 1600℃with 50 MPa for 2 h in vacuum.Then,the round bulk was annealed at 1600℃for 1 h to release the residual stress and homogenize the composition.

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Table 2 Composition of La₂O₃-doped Mo-Si-B alloys

The samples were polished and etched with Murakami’s etchant (10 g potassium ferricyanide,10 g sodium hydroxide,and 80 ml distilled water) for optical metallographic examinations.Optical microscopy (OM) observations were performed by using an OLYMPUS CX71metalloscope.The microstructure of phases was examined with scanning electron microscope (SEM,JSM-6700F).The microstructure,particle distribution and dislocation of the alloy were observed using transmission electron microscope (TEM,JEM-3010) combined with selected area electron diffraction (SAED).The thin foils were obtained using twin-jet electropolishing in a solution of5 vol%butyl alcohol,5 vol%perchloric acid and 90 vol%ethanol at 30 V and-30℃.The compression specimens with a size of 5 mm×5 mm×10 mm were fabricated from the annealed round bulks by electrical discharge machining.Before testing,the surface of specimens was polished flat.Testing at room temperature was carried out in air,and at 1000 and 1300℃in vacuum,with a strain rate of 2.5×10^-4 S^-1.The three-point bending experiments were determined using specimens of 3 mm×4mm×25 mm with a strain rate of 4.2×10^-4 s^-1,and five values were determined for each alloy to obtain a reliable data.The fracture toughness value (K_q) was tested with a strain rate of 4.2×10^-5 s^-1 by using chcvronnotched bend bars (3 mm×4 mm×25 mm) with a notch tip depth of 2 mm.

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Table 1 Composition of Mo material (10^-6)

3 Results and discussion

3.1 Microstructural observation

The optical micrographs of Mo-12Si-8.5B doped with different contents of nanoscale La₂O₃ particle are shown in Fig.1,and detail microstructure is shown in Fig.2.These samples have a completely homogeneous microstructure in which the intermetallic phases of Mo₃Si and Mo₅SiB₂ are distributed on the continuousα-Mo matrix.All alloys show a fine-grained microstructure,and the average grain size is less than 1.5μm which was determined by linear intercept length method.The microstructure is relatively insensitive to the La₂O₃ addition and the additive amount.The relative density of La₂O_3-doped Mo-12Si-8.5B alloy is in the range of 94.3%-95.2%(Table 3),indicating that the La₂O₃addition neglectfully influences the sintering process and the hot pressing could obtain a relative high density of MoSi-B alloy.

Figure 3 shows TEM bright-field images for the microstructure of Mo-12Si-8.5B doped with La₂O₃ particle.One can see that the grain morphology of theα-Mo phase is regular,but those of the intermetallic phase is irregular and near spherical.The white two-way arrows in the image illustrate the grain sizes ofα-Mo and intermetallic.The average grain size ofα-Mo and intermetallic phases for the MSB alloy is bigger than 1μm (Fig.3a,d),while the value decreases to submicron scale (Fig.3b,e)with the addition amount of La₂O₃ up to 0.9 wt%or more.However,the grain refining effect by La₂O₃ becomes not obvious when the addition further increases to 2.5 wt%(Fig.3c,f).It reveals that the La₂O₃ addition can refine simultaneously the grain sizes ofα-Mo and intermetallic inMo-Si-B alloy,but the grain size is weakly dependent on La₂O₃ content.In addition,the intermetallic phases (Mo₃Si and Mo₅SiB₂) mainly distribute at the grain boundaries ofα-Mo phase by stack and dispersion modes (black arrow in Fig.3b,e) and partially distribute on the internal ofα-Mo phase with the size of only 200-300 nm (Fig.3a,b).

Fig.2 SEM image for distribution and morphologies of inter-metallics in MSB alloy

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Table 3 Relative density of Mo-12Si-8.5B doped with different contents of La₂O₃(%)

Figure 4 shows TEM images combined with SAED pattern,illustrating the distribution of the nanoscale La₂O₃particles in the MSB-0.9 alloy.These spherical La₂O₃particles (shown by the arrows) distribute dispersively within theα-Mo,Mo₃Si and Mo₅SiB₂ (T2) phases,which are proved by the corresponding SAED results.Although the size of these particles keeps at only 20-30 nm,the particle aggregation does not happen in the alloy.

Fig.1 OM images of a MSB,b MSB-0.3,c MSB-0.9,d MSB-1.5 and e MSB-2.5

Fig.3 TEM images for microstructure and grain size ofα-Mo phase of a MSB,b MSB-0.9,c MSB-2.5 and intermetallic phase of d MSB,e MSB-0.9,f MSB-2.5

Fig.4 TEM images and inserted SAED patterns of La₂O₃ particles distributed on aα-Mo,b Mo₃Si and c Mo₅SiB₂ phase of MSB-0.9

The Mo-12Si-8.5B alloy in present work has a fine grain size because of the preparation process and La₂O₃particle addition.After mechanical alloying,the mixed powders obtain a fine size and homogeneous composition which can contribute to refining the size of intermetallic phases and avoiding agglomeration of the intermetallic phases during hot pressing process [ 27] .These intermetallic phases with submicron size (Mo₃Si and Mo₅SiB₂)are distributed in the interior and on grain boundaries ofα-Mo phase,which can refine the grain size of matrix via pinning the boundaries ofα-Mo phase (Fig.3).In addition,the compression stress can also limit the grain growth in a certain extent during hot pressing process.The nanoscale La₂O₃ particles not only prevent the recrystallization and growth of theα-Mo phase at high temperature but also supply the nuclei for the Mo₃Si and Mo₅SiB₂ phases to refine the intermetallics by segmenting the reaction zone.Thus,the fine-grained microstructure has been obtained by controlling the preparation process and component in current work.

3.2 Compression property

The compressive strength and yield strength with different La₂O₃ additions at room and high temperatures are shown in Fig.5a-c.Figure 5a shows that as the content of La₂O₃addition increases,the compressive strength and yield strength of Mo-12Si-8.5B alloy have a upward trend and reach a maximum value (2806 and 2721 MPa,respectively) for the MSB-0.9.Then,both of them start to decline until the minimum value (2158 and 2061 MPa,respectively) for the MSB-2.5,which is 11%lower than those for the La₂O₃-free alloy (MSB alloy).This displays some differences in earlier study [ 28] .

Fig.5 Compressive strength and compressive yield strength depending on La₂O₃ addition at a RT,b 1000 and c 1300℃

The compressive strength and yield strength of Mo-12S i-8.5 B alloy responding to different La₂O₃ additions at1000℃are presented in Fig.5b.At the test temperature,the strength of the La₂O₃-doped Mo-12Si-8.5B alloy is higher than that of the La₂O₃-free alloy,and they also have a maximum value when doped with 0.9 wt%La₂O₃,i.e.,the MSB-0.9 alloy.These La₂O₃ particles perform an excellent strengthening effect for Mo-12Si-8.5B alloy under elevated temperature.At 1300℃,as the La₂O₃content increases,the compressive strength lies in the range of 300-400 MPa,and the maximum value is obtained at MSB-0.9 (Fig.5c) as well,while the yield strength presents a slight improvement.The compressive testing results indicate that the La₂O₃ particles can improve the room temperature and high-temperature performances of Mo-12Si-8.5B alloy at the same time.These nanoscale La₂O₃particles distributed in the Mo₃Si and Mo₅SiB₂ phases can strengthen the intermetallic phases,which are major strength providers for the Mo-Si-B alloy at high temperature.In fact,due to the intrinsic brittleness,the high strength intermetallic hardly has plastic deformation ability at ambient temperature.The deformation is only controlled by the ductileα-Mo matrix.Some nanoscale La₂O₃ particles distributed in theα-Mo phase can strengthen the matrix of Mo-Si-B alloy through inhibiting the migration of dislocations (Fig.6) at ambient temperature.But these particles which exist in the intermetallic compound have mixed impacts in terms of strengthening effects and deteriorations in compound property.In current research,the La₂O₃-doped Mo-Si-B alloy can possess high strength for the nanometer-sized La₂O₃ particles which disperse in the intermetallic compound.The La₂O₃ particles can increase the strength of alloy mainly by strengthening theα-Mo matrix at ambient temperature,but at elevated temperatures,these La₂O₃ particles can strengthen not only theα-Mo matrix by inhibiting migration of dislocations and grain boundaries,but also the Mo₃Si and Mo₅SiB₂phases for the deformation ability of those intermetallic compounds can be obviously enhanced.During compressive test at 1000℃,the dislocation activity occurs in the intermetallic compound.These activated dislocations appear to be emitted from theα-Mo matrix/intermetallic compound interface and glided along slip planes within the T2 phase [ 29] .Thus,the particles (Fig.4b,c) which distribute within the Mo₃Si and Mo₅SiB₂ phases could improve the high-temperature stability and performance of these phases,because of the thermal inactivation and the uniform distribution of the La₂O₃ particle itself.

Fig.6 TEM image for propagation and entanglement of dislocations inα-Mo phase because of presence of nanoscale La₂O₃ particles

Figure 5 shows that the strengthening effect is sensitive to the content of La₂O₃ particle.Excessive hard particles existing in alloy may produce more micro-cracks during the deforming process,and more micro-cracks will lead to the decline in the strength of Mo-12Si-8.5B alloy.Meanwhile,excessive La₂O₃ particles will increase the probability of particle agglomerations on grain boundaries,which will bring a harmful effect to the mechanical properties of Mo-12Si-8.5B alloys at ambient and elevated temperatures.

3.3 Flexure strength and fracture toughness

The room temperature flexure strength and fracture toughness values of Mo-12Si-8.5B alloy doped with different contents of La₂O₃ are shown in Fig.7.As the La₂O₃content increases,the flexure strength and fracture toughness values of Mo-12Si-8.5B alloy slightly increase (about10%) firstly,then decrease with more La₂O₃ additions,as compression property trend.It is indicated that excessive La₂O₃ particles are also harmful to the flexure strength and fracture toughness of Mo-12Si-8.5B alloy.

The La₂O₃ particles can simultaneously decrease the grain sizes of theα-Mo phase and intermetallic phases.According to the well-known Griffith flaw,the critical stress (σ_c) required for crack propagation in a brittle material was described by the following expression [ 30] :

where E is the modulus of elasticity,γ_s is the specific surface energy,and a is one half the length of an internal crack (or length of a surface crack).According to Eq.(1),the length of crack (a) is inverse ratio with critical stress(σ_c),which means the shorter of a,the higher ofσ_c.In poly crystalline material,the length of internal crack is mostly corresponding to grain size.Thus,La₂O₃ particle decreases the grain size,meaning that a smaller size of internal crack can be produced.The fracture toughness value of Mo-12Si-8.5B alloy will be improved by controlling the micros true ture scales.Otherwise,a large number of dislocations are pinned around the La₂O₃ particles to greatly decrease the dislocation pile-up at grain boundaries,resulting in the release of stress.This makes the micro-crack more difficult to be created and propagated at the grain boundaries,which also promotes the toughness of the alloy [ 25] .

Fig.7 Room temperature flexure strength and fracture toughness(K_q) values of Mo-12Si-8.5B alloy doped with La₂O₃

4 Conclusion

The microstructure of the Mo-12Si-8.5B alloy shows that the submicron Mo₃Si and Mo₅SiB₂ (T2) phases are located at the grain boundaries and in interiors of a continuousα-Mo matrix.The La₂O₃ particles embedded into Mo-12Si-8.5B alloy can further decrease the grain size ofα-Mo and intermetallic phases.The refinement effect is dependent on the La₂O₃ content.These nanoscale La₂O₃ particles are distributed mainly in the grains of theα-Mo matrix and partially in the intermetallics.The La₂O₃ addition can enhance the compression properties of the Mo-12Si-8.5B alloy under room and elevated temperatures,including compression and yield strengths.Also,it can improve the ambient temperature flexure properties of the alloy which contain the flexure strength and fracture toughness.The Mo-12Si-8.5B alloy can obtain an optimum mechanical performance for the well combination of strength and toughness when the content of La₂O₃ addition is 0.9 wt%.Excessive La₂O₃ addition will bring a harmful effect to the mechanical properties of Mo-12Si-8.5B alloy.

Acknowledgements This study was financially supported by the China Postdoctoral Science Foundation (No.2016M602885) and the National Natural Science Foundation of China (Nos.51371141 and51701162).