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Rare Metals2019年第7期

Mechanical alloying behaviors of Mo-Si-B-based alloy from elemental powders under different milling conditions

Tao Yang Xi-Ping Guo

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University

作者简介：*Xi-Ping Guo,e-mail: xpguo@nwpu.edu.cn;

收稿日期：24 August 2017

基金：financially supported by the National Key R&D Program of China (No. 2017YFB0702903);the National Natural Science Foundation of China (Nos.51431003 and U1435201);the Research Fund of State Key Laboratory of Solidification Processing, China (No.143-TZ-2016);

Mechanical alloying behaviors of Mo-Si-B-based alloy from elemental powders under different milling conditions

Tao Yang Xi-Ping Guo

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University

Abstract：

Elemental powder mixtures with the composition of Mo-12Si-10B-3Zr-0.3Y(at%)were milled in a planetary ball mill using hardened stainless-steel milling media under argon atmosphere.Effects of milling time,milling speed,process control agent,ball-to-powder ratio and milling ball size on the mechanical alloying processes were investigated from the points of morphology,internal structure,grain size,microstrain,phase constituent and issolution of solute atoms.It is shown that under all conditions,the microstructural evolutions of mechanically milled powder particles are similar.The morphological evolution can roughly be pided into five stages:inpidual particle,irregular blocky composite particle,flakeshaped particle,agglomerate and single particle.The internal structure generally undergoes five stages:inpidual particle,coarse lamellar structure,fine lamellar structure,non-uniformly mixed structure and plum-pudding structure.Regardless of exceptional cases,the grain size of Moss decreases and its microstrain increases with the increase in milling time.Si and Zr atoms are dissolved into Mo gradually with the progress of milling.However,the evolutionary rates change significantly with milling conditions.The most significant influencing factor among different milling conditions is the input power from the mill to the powders,which plays a decisive role in the milling process.

Keyword：

Mechanical alloying; Milling parameter; Milling energy; Mo-Si-B;

Received： 24 August 2017

1 Introduction

It is well known that the hotter the turbine rotor inlet temperature is,the higher the efficiency of gas-turbine engine is.Current Ni-based superalloys can function at about 1150℃,which is close to 90%of their melting temperatures.As a result,it is necessary to develop a new structural material with higher working temperature.MoSi-B-based alloys consisting of Moss (molybdenum solid solution),Mo₃Si and Mo₅SiB₂ (T2) are very promising ultrahigh-temperature structural materials due to their high melting temperatures (about 2000℃),excellent oxidation resistance,outstanding high-temperature mechanic al properties and moderate room-temperature fracture toughness [ 1, 2] .Alloying elements (Zr [ 3, 4, 5, 6] ,Y [ 7, 8, 9] ,La [ 10] ,Ti [ 11] ,Cr [ 12] ,Fe [ 13] ,Nb [ 14] and Hf [ 15] ,etc.),especially Zr and Y,have been proven to be beneficial to the improvenment in mechanical properties or oxidation resistance of Mo-Si-B alloys.It has been showwm that appropriate addition of Zr decreases the brittle-to-ductile transition temperature (BDTT) and increases the roomtemperature strength and toughness [ 5, 6] ,high-temperature creep resistance [ 4, 5] and oxidation resistance [ 3] .The addition of Y₂O₃ can pin grain boundaries and therefore limit grain growth during sintering processes [ 7, 8] .Besides,Majumdar et al. [ 9] have reported a substantial improvement in the oxidation resistance of 0.2 at%Y alloyed Mo-9Si-8B (hereafter all compositions are given in at%unless otherwise statedl) at 800 to 1000℃.As a result,an alloy with a composition of Mo-12Si-10B-3Zr-0.3Y is preliminarily designed to obtain excellent comprehensive performance.

Although arc-melting [ 16] or directional solidification [ 17] can be used to fabricate this kind of alloys,powder metallurgy techniques [ 8, 18, 19] are adopted more widely due to their highmelting temperatures.Besides,to lower the sintering temperature and acquire fine microstructure with a continuous Moss matrix,raw powder mixtures are always mechanically milled before sintering [ 8, 14, 18, 19] .It is well known that mechanical alloying (MA) parameters,such as milling speed [ 20, 21] ,ball-to-powder ratio(BPR) [ 21, 22] ,milling time [ 23] ,milling ball size [ 24] and process control agent (PCA) [ 25] ,have significant effects on the mechanical alloying (MA) processes.However,for this kind of alloys,early researches have been mainly focused on the MA processes of Mo-Si binary systems with the composition around MoSi₂,and it has been revealed that MoSi₂ can easily be synthesized by a mechanically induced self-propagating reaction (MSR) [ 26, 27] .Abbasi and Shamanian [ 28] and Yamauchi et al. [ 29] have studied the MA behaviors of Mo-Si-B ternary systems.But the original composition of their powder mixtures was mainly Mo-12.5Si-25B and their focus was the synthesis of Mo₅SiB₂ phase.Bakhshi et al. [ 30] tried to synthesize Mo-Si-B multiphase alloy powders with a composition of Mo-14Si-10B.The results did not show any solid solution of Mo or formation of related intermetallics after MA.Kruger et al. [ 18] studied the MA behaviors of Mo-B,Mo-Si,and Mo-Si-B powdlers systematically,while for Mo-Si-B systems,they mainly studied the structural evolution of powder mixtures with different compositions (Mo-4Si-2B,Mo-6Si-5B,Mo-13Si-12B and Mo-9Si-8B) under a given milling condition.In our previous work [ 31] ,the MA mechanism of Mo-12Si-10B-3Zr-0.3Y powders was investigated under a given milling condition in detail;however,the effects of milling parameters on the MA behaviors of these alloy powders have seldom studied.

As a result,the main purpose of this paper is to reveal the effects of milling parameters including milling speed,PC A,milling time,BPR and milling ball size on the MA behaviors of Mo-12Si-10B-3Zr-0.3Y powder mixtures from the points of morphology,iuternal structure,phase constituent,grain size refinement,microstrain and dissolution of solute atoms.

2 Experimental

Elemental powder mixtures of Mo,Si,B (amorphous state),Zr and Y of 99%purity or better with a composition of Mo-12Si-10B-3Zr-0.3Y and a total mass of 25 g were mechanically milled in a high-energy planetary ball mill under protective atmosphere using stainless-steel vials and milling balls.Table 1 shows the details of processing conditions and corresponding abbreviations.

The phase constituents of milled powders were characterized by X-ray diffraction (XRD,PANalytical XPert PRO) using Cu Kot radiation operating at 40 kV.Microstructural and compositional analyses were conducted by using a scanning electron microscope (SEM,Tescan MIRA 3) equipped with an energy-dispersive X-ray spectrum analyzer (EDS,Inca X-sight).To expose the internal structures,powder particles were mounted by epoxy resin,ground up to 2000-grit SiC paper and then polished by Al₂O₃ abrasive paste.

The grain size and microstrain of Mo or Moss grains were calculated on the basis of Williamson-Hall method [ 32] :

whereβis the full width at half maximum (FWHM) of the diffraction peak in rad subtracting the influence of instrumental line broadening,θis the diffraction angle in degree,λis the wavelength of the X-ray in nm,d is the grain size in nm andεis the microstrain.

Cohen method using Nelson-Riley function as the extrapolation function was adopted to calculate the lattice constant of Mo or Moss.

3 Results

3.1 Morphological evolution of powders

To better understand the evolutionary characteristics of particle morphology,the morphological images of the original powder particles are shown in Fig.1.It can be found that the agglomeration degrees of Mo and B particles are very high and that their primary particle sizes are both less than 1μm.However,the Mo primary particles show a near-spheroidal shape,while the B powder particles have a polyhedral structure.The Si particles have an irregular blocky shape and their size is larger than 50μm.Both Zr and Y particles have a particle size of about 20μm.The former shows obvious traces of deformation,while the latter adsorbs some relatively small particles on their smooth surface.

Figure 2 shows the morphological evolution of powder particles milled under Condition S-300 for different time.It can be seen that after 2-h milling,inhomogeneous deformation occurrs (Fig.2a),i.e.,partial Mo particles are deformed intensively,while most Mo particles still remain the original morphological characteristics.It is easy to recognize the brittle Si particles with clear edges,but their particle size is reduced significantly to about 5μm by fragmentation.With milling time increasing to 5 h,the powder particles are deformed extensively and lose the original morphological characteristics completely.Some irregular blocky composite particles form,which implies that the deformation and cold-welding mechanisms predominate,and consequently,the average particle size increases obviously (Fig.2b).After 15-h milling,the thickness of composite particles decreases significantly through the micro-forging action of milling media.The previous blocky composite particles turn into flake-shaped ones (Fig.2c).With milling time increasing to 20 h,the flake-shaped particles break up into smaller fragments.It is spontaneous for these fragments to gather mutually to reduce the system energy.At this moment,there appear some large and irregular agglomerates (Fig.2d).Afterward,under the continued action of milling media,the previous primary particles are gradually spheroidized(Fig.2e').But it should be noted that the agglomeration degree of particles after 40 h milling is still serious(Fig.2e,e').

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Table 1 Details of processing conditions and corresponding abbreviations

Under each condition,milling time being 1,2,5,10,15,20,30 and 40 h;corresponding product being labeled as C/T,where C and T representing milling condition and time,respectively;for example,S-300/20 representing powder mixture milled for 20 h under Condition S-300

Fig.1 Secondary-electron (SE) morphological images for original powder particles:a Mo,b Si,c B,d Zr and e Y

Fig.2 SE morphological images of powder particles milled for different time under Condition S-300:a 2 h,b 5 h,c 15 h,d 20 h and e,e'40 h

With the other parameters unchanged and just increasing the milling speed to 500 r·min^-1,the morphological evolution of powder particles goes through a similar but quicker progress (Fig.3).That is,after 2-h milling,irregular blocky composite particles (Fig.3a),which are similar to those of Sample S-300/5 (Fig.2b),have already formed.The 5-h milling (Fig.3b,b) results in a similar particle morphology to that of Sample S-300/40 (Fig.2e,e').After40-h milling,different with that at 300 r·min^-1,powder particles milled at 500 r·min^-1 have abetter dispersity with more homogeneous size distribution and smoother surfaces.At this moment,the powder particles mainly exist as single ones rather than agglomerates (Fig.3c).Making a comparison between Fig.3b'and c',it can be deduced that under the severe action of milling media,the boundaries among primary particles in the previous agglomerates vanish and the primary particles merge with each other gradually.Finally,the agglomerates transform into single ones.

Figure 4 shows SE morphological images of powder particles milled for different time under Condition P-2.It can be found that,with the other parameters keeping unchanged as those under Condition S-500,the addition of2 wt%stearic acid (SA) postpones the morphological evolution significantly.After 10-h milling,blocky and flake-shaped composite particles can still be observed(Fig.4a).Only with milling time increasing to 20 h,the powder particles are extensively fragmented and agglomerated (Fig.4b,b).Besides,the present powder particles show more serious agglomeration degree than those of Sample S-500/5.This can be attributed to that the SA molecules adsorbed on the powder particle surfaces lower their surface energy,consequently,reduce the minimum limit of the primary particle size and then result in more serious agglomeration.After 40-h milling,it can be found that although the agglomeration degree of powder particles is still high,the primary particles have been significantly spheroidized,and correspondingly,their size increases a little (Fig.4c,c').It can also be deduced that during this period under the action of milling media,the previous smaller primary particles combine with each other to form larger ones.

Figure 5 shows the morphological images of powder particles milled under Conditions R-10 and B-20 for 5 and40 h to reveal the influences of BPR and milling ball size on the morphological evolution of powder particles.Considering the morphological evolutionary characteristics of powder particles under Condition S-500,it can be found that decreasing BPR from 15:1 to 10:1 delays the morphological evolution process.After 5-h milling,there remain a lot of blocky composite particles (Fig.5a).With milling time increasing to 40 h,as shown in Fig.5b,the powder particles show a near-spheroidal shape and moderate dispersity.But the present particles have poorer size uniformity than that of Sample S-500/40.Increasing the milling ball size from 10 to 20 mm has no significant influence on the evolutionary rate of morphology.After 5-h milling,the powder particles (Fig.5c) have a morphological feature similar to that of Sample S-500/5 (Fig.3a).However,the agglomeration degree of the present particles is higher.After 40-h milling,the powder particles (Fig.5d)show a good dispersity and homogeneous size distribution similar to those of Sample S-500/40.

Fig.3 SE morphological images of powder particles milled for different time under Condition S-500:a 2 h,b,b'5 h and c,c'40 h

Fig.4 SE morphological images of powder particles milled for different time under Condition P-2:a 10 h,b,b'20 h and c,c'40 h

Fig.5 SE morphological images of powder particles milled under Condition R-10 for a 5 h and b 40 h and Condition B-20 for c 5 h and d 40 h

From above description,it can be concluded that the morphological evolutions of powder particles under different processing conditions obey a similar rule,which is schematically described in Fig.6.Initially,the brittle components get fragmented,and the ductile components are deformed by collisions of ball-powder-ball or ballpowder-inner wall of the vial.Since the original ductile particles are soft,they have a large tendency to weld together to from irregular blocky composite particles,which always leads to an increase in particle size.The brittle particles are trapped in or embedded in these composite particles.Afterward,the previous blocky composite particles are easily flattened to flake-shaped ones,especially in the case of adding PCA due to its lubrication effect.Owing to repeated plastic deformation,powder particles at this time show high work-hardening degree.Continued milling will lead to the fragmentation of these work-hardened particles and a consequent reduction in particle size.It is spontaneous for these fragments to gather together to form agglomerates.With further milling,the agglomerates and their primary particles are spheroidized gradually.In some cases,the action of milling media is so severe that the primary particles combine with each other and the agglomerates transform into single particles.The difference in morphological evolution under various milling conditions is just the evolutionary rate,which ranks from great to small as follows:Conditions S-500,B-20,R-10 and S-300.For Condition P-2,the rate appears slowly first and fast afterward.

3.2 Internal structural evolution of powders

Figure 7 shows the cross-sectional back-scattered electron(BSE) images of powder particles milled under Conditions S-300 and S-500 for different time.Coinciding with the morphological characterization,the cross-sectional observation also shows that under Condition S-300 after 2-h milling the particles are separated from each other and the deformation degree is relatively low (Fig.7a).Withmilling time increasing to 5 h,there appear some composite particles with a lamellar structure,where the ductile components (Mo,Zr and Y) are flattened as lamellae and the brittle ones (Si and B) are dispersed at the interlamellar interfaces (Fig.7b).After 20-h milling,the lamellar structure forms extensively,and the lamellar thickness reduces significantly (Fig.7c).But it is worth noting that the lamellae are still easy to be recognized from each other(Fig.7c).With the increase in milling time,the lamellae get further refined and convoluted.After 40-h milling,the previous typical lamellar structure has vanished (Fig.7d).But from the enlarged view (Fig.7d),it can be seen that the internal structure of powder particles at this time is still heterogeneous.However,with the milling speed increasing to 500 r·min^-1,the plum-pudding composite particles comprising of homogeneous Moss matrix and evenly dispersed brittle particles have formed after 40-h milling(Fig.7e,e').

Fig.6 Schematic description of morphological evolution of powder particles during ball-milling process:a fragmentation of brittle components and deformation of ductile ones,b formation of irregular blocky composite particle,c transformation into flake-shaped composite particles,d fragmentation of flake-shaped composite particles and consequent agglomeration,e spheroidization of agglomerates and primary particles and f formation of single particles

From the observation of the internal structures of powder particles milled under five different conditions for different time,it can be concluded that the internal structural evolution obeys a similar path,which is schematically described in Fig.8a.In the initial milling stages,the brittle components are fragmented,and the ductile components are deformed.However,at this time,the elemental components are separated from each other.Afterward,composites with a coarse lamellar structure form.Continued milling refines the lamellae significantly.With further milling,the lamellar structure vanishes,but the internal structure is still heterogeneous,which can be called as nonuniformly mixed structure.When the MA degree is high enough,the plum-pudding composite particles consisting of homogeneous Moss matrix and evenly dispersed brittle particles will form.Figure 8b summarizes the respective internal structural evolution of powder particles milled under different conditions.It can be seen that,with the other milling parameters unchanged,increasing the milling speed from 300 to 500 r·min^-1 can accelerate the internal structural evolution of powder particles significantly.While keeping the milling speed at 500 r·min^-1,the addition of 2 wt%SA results in that the initial evolutionary rate (up to 20 h) of internal structure reduces to as slow as that of Condition S-300.Afterward,further milling makes an accelerated evolutionary rate of the internal structure.By comparing the results under Conditions R-10 and B-20with those of S-500,it can be found that reducing BPR and increasing milling ball size both delay the internal structural evolution to some extent.Similar to the morphological evolution of powder particles,the evolutionary rate of internal structure decreases in the order of Conditions S-500,B-20,R-10 and S-300.For Condition P-2,the rate appears slowly first and fast afterward.

Fig.7 Cross-sectional BSE images of powder particles milled under Condition S-300 for a 2 h,b 5 h,c,c'10 h and d,d'40 h and e,e'Condition S-500 for 40 h

Fig.8 a General and b specific description of internal structural evolution of powder particles during milling processes

3.3 Phase constituents of powder particles

Figure 9 shows XRD spectra of powder mixtures milled under various conditions for different time.It can be seen that under each condition,the intensity of Mo or Moss diffraction peaks decreases and their FWHMs are broadened gradually with the increase in milling time.However,the diffraction peaks of Si and Zr vanish after milling for different time under each condition.Specifically,the Si diffraction peaks can still be observed after 30-h milling under Condition S-300,while under Condition S-500 the diffraction peaks of Si and Zr disappear completely after5-h milling.At the same milling speed,the Si and Zr diffraction peaks are still obvious after 20-h milling with the addition of 2 wt%SA,while further milling to 30 h leads to the complete disappearance of both diffraction peaks.For Conditions R-10 and B-20,the time for the disappearance of Si and Zr diffraction peaks is 15 and 10 h,respectively.

Further observation of Fig.9 shows that except for a small amount of MoSi₂ detected at some milling stages,the final milling products are all Moss.Our previous work has proven that it is easier to form Moss than to form an amorphous phase for the current composition [ 31] .Only weak diffraction peaks of MoSi₂ are observed in Samples S-300/40,R-10/20 and B-20/5,while the diffraction peaks of MoSi₂ under Condition P-2 changes from weak to strong and then fades away with the increase in milling time.Obviously,the amount of MoSi₂ forming under Condition P-2 is qualitatively higher than that under other conditions.It seems that S A has a promotion effect for the formation of MoSi₂.To verify this hypothesis,it was studied the MA behavior of powder mixtures with the composition of MoSi₂ under Conditions S-500 and P-2.The corresponding XRD spectra of the powders milled for different time under these two conditions are shown in Fig.10.It can be seen that a large amount of a-MoSi₂ has formed after 10-h milling under Condition S-500.With further milling,α-MoSi₂ gradually transforms intoβ-MoSi₂.However,only a small amount ofα-MoSi₂ has formed even after 20-h milling under Condition P-2.Only after further milling to40 h,extensiveα-MoSi₂ has formed under this condition.Therefore,it can be concluded that the addition of S A does not prompt but suppresses the formation of MoSi₂.In Mo-12Si-10B-3Zr-0.3Y powder mixtures,Si content is far below the stoichiometric content of Si in MoSi₂ (about66.7%).However,the compositional heterogeneity of powders,especially in the initial milling stage,guarantees the formation possibility of non-equilibrium MoSi₂.Under the action of activation energy provided by milling media,MoSi₂ tends to form in some Si-rich regions of powder particles.With the proceeding of milling process,the previously generated non-equilibrium MoSi₂ reacts with the surrounding Mo matrix to form the equilibrium Moss gradually.As a result,the larger amount of MoSi₂ during milling process under Condition P-2 should be attributed to the inhibiting effect of S A on the consumption of generated MoSi₂ rather than the promoting effect on the formation process.

Fig.9 XRD patterns of powder mixtures milled for different time under different Conditions:a S-300,b S-500 [31] ,c P-2,d R-10 and e B-20

Fig.10 XRD patterns of Mo-66.7Si powder mixtures milled for different time under Conditions a S-500 and b P-2 (to highlight weak diffraction peaks,intensity axes being expressed in logarithmic form)

3.4 Grain size and microstrain of Moss

In order to calculate the grain size,micro strain and lattice constant of the mechanically alloyed Moss,the FWHMs and diffraction angles of the diffraction peaks of Moss should be obtained first.In the present work,Pearson-VII function was adopted to fit the XRD data.For example,Fig.11 shows the treated XRD profile of Sample S-300/40.

The grain size and microstrain of Moss versus milling time under different conditions are shown in Fig.12a,b,respectively.It should be noted that some calculation results have not been listed due to larger fitted errors.Figure 12a shows that except for Condition P-2,the grain size of Moss under other conditions all shows a rapid decrease in the initial stages and then decreases slowly.However,the refining rates change obviously with milling conditions.It can be seen that increasing milling speed can accelerate the refining rate of Moss grains for the whole milling duration significantly.After 40-h milling,the Moss grain size under Condition S-300 is just about 54.3 nm,while the value under Condition S-500 decreases to about6.5 nm.With the addition of 2 wt%SA,the refining rate is remarkably reduced.For example,the grain size of Sample P-2/20 (131.5 nm) is even larger than that of S-500/5(118.3 nm).However,it is worth noting that there remains a relatively high refining rate for further milling after 20 h under Condition P-2.It can also be seen that compared with Condition S-500,the decrease in BPR from 15:1 to10:1 or increase in milling ball size from 10 to 20 mm reduces the refining rate.

Figure 12b shows that the evolutions of Moss microstrain change significantly with milling conditions.Under Condition S-300,the micro strain increases continuously with the increase in milling time.However,for Conditions S-500 and B-20,the microstrain increases at first and decreases afterward.Based on Hall-Petch relation,if the Moss grain size is less than 30 nm,the impact pressure provided by the milling balls under these two conditions is insufficient to deform the Moss further.The previously generated dislocations in the Moss lattice are consumed by the formation of new grain boundaries,recovery and so on,which leads to a sharp decrease in the microstrain [ 31] .With the addition of 2 wt%SA,the microstrain increases extremely slowly during the first 20-h milling but shows a rapid increase afterward.For Condition R-10,the microstrain shows a near-linear increase initially and reaches a stable state finally.

Fig.11 Fitted curve and experimental XRD data for Sample S-300/40

3.5 Dissolution kinetics

A main research interest about MA should be the dissolution of solute atoms.Apparently,compositional analyses are direct characterization methods.However,due to the compositional heterogeneity of powder particles,especially in the initial milling stage,and the spatial resolution limitation of measuring instruments,it is difficult to reveal the dissolution kinetics of solute atoms exactly by the compositional analyses.Fortunately,dissolution is always bound to the change of the lattice constant as the result of the atomic radius difference between the solute and solvent atoms.In our previous work [ 31] ,pure Mo powders were also milled under Condition S-500.A constant Mo lattice parameter means that the effect of cold work on lattice parameter can be neglected.Therefore,lattice constant analysis was adopted as the main method for the characterization of the dissolution of solute atoms,which was assisted by compositional analyses.It has been proven that B is immiscible with Mo [ 31] .Besides,Y can also be neglected because of its very low content.Therefore,the original quinary system (74.7Mo-12Si-10B-3Zr-0.3Y)can be approximated by a ternary system with removal of B and Y,and the residual powder composition (74.7Mo-12Si-3Zr) is normalized as 83.3Mo-13.4Si-3.3Zr.The dissolution of Zr increases the lattice constant due to its larger atomic radius than that of Mo,while the dissolution of Si exerts an opposite effect as a result of its smaller atomic radius.Figure 13a shows the evolutions of Moss lattice constants versus milling time under different milling conditions.It is clear that the lattice constant of Moss increases at first and decreases afterward with the proceeding of MA process under all conditions.It can be deduced that in the initial stages,the expansion effect of Zr dissolution has taken the lead.Afterward,the contraction effect of Si dissolution surpasses the expansion effect caused by Zr dissolution as a result of the low content of the original Zr.Consequently,the lattice constant decreases gradually.Taking a careful observation of Fig.13a,it can be found that there is an incubation period for the dissolution of solute atoms under each condition.During this period,the microstructure is refined adequately and there generate sufficient defects for the fast diffusion and solution of solute atoms.However,the incubation time changes significantly with the milling conditions and is roughly (the milling time needed for the significant change of lattice constant) estimated to be about 20,5,20,15 and 10 h for Conditions S-300,S-500,P-2,R-10,and B-20.

Fig.12 Evolutions of a grain size and b microstrain of Moss under five different conditions (error bars representing fitted errors)

Fig.13 a Evolutions of Moss lattice constants under different milling conditions and b Si and Zr contents in Moss after 40-h milling under different conditions (error bars representing fitted errors)

Figure 13b shows Si and Zr contents in Moss after 40-h milling under different conditions.(For each sample,five points were measured.) It can be seen that Zr contents in Moss are similar and all approach the nominal composition(3.3 at%) after 40-h milling under each condition,while Si contents change with the processing conditions.For Conditions S-500 and B-20,Si contents both approach the nominal composition (13.4 at%).However,for the other conditions,Si contents still show some gaps from the nominal value and decrease in the order of Conditions R-10,P-2 and S-300.Generally,the Moss lattice constants show good agreements with its corresponding compositions.

4 Discussion

As mentioned above,the MA processes of powder particles and the effects of the milling parameters are illuminated in detail from the points of morphology,internal structure,phase constituent,grain refinement,microstrain and dissolution kinetics of solute atoms.It can be concluded that under each condition,the MA paths are similar,but the MA rates are different.Generally,the MA rates rank from great to small as follows:Conditions S-500,B-20,R-10 and S-300.For Condition P-2,the MA rate appears slowly first and fast afterward.

Essentially,MA is an energy-transfer process from the mill to powders [ 21, 33, 34, 35, 36] .As a result,the distinction of MA rates under different conditions should be ascribed to the difference of input powers.Burgio et al. [ 37] studied the power transferred from the mill to the powders during MA process and described it as follows:

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Table 2 Specific parameters for calculation of input power and corresponding calculation results

where P is the total power transferred from the mill to the system in W,φ_b is the yield coefficient accounting for the mutual interference of milling balls,K is a constant,M_b is the total mass of milling balls in kg,ω_p is the absolute angular velocity of the plate in rad·s^-1,ω_v is the absolute angular velocity of the vial in rad·s^-1,R_p is the distance between the center of the plate to the center of the vial in m,R_v is the radius of the milling vial in m (the values for R_p and R_v are fixed at 0.12 and 0.036 m in the present work,respectively) and d_b is the diameter of the milling balls in m.The specific values for the relevant parameters and the corresponding calculation results are listed in Table 2.

It can be seen that both Conditions S-500 and P-2 have the maximum input power.However,from above analyses,it can be concluded that the latter has a significantly slower MA process at first.This can be attributed to the lubrication action of SA.Under Condition P-2,during the microforging processes,the trapped powder particles would like to slide over each other rather than be deformed,which can reduce the effective input energy significantly.However,with the progress of milling process,the SA decomposes gradually,and then the MA efficiency is significantly enhanced.From the analyses of grain size,microstrain and lattice constant,it can be inferred that this transformation is likely to take place after 20-h milling.For theother conditions,the calculation results show a great agreement with the MA rates.That is,a higher input powwer always corresponds to a faster MA rate.Therefore,it can be concluded that the essential difference among different milling conditions is the input power,which plays a key role in MA process.

5 Conclusion

Milling processes under five different conditions follow a similar evolutionary route.With the progress of MA,the morphological evolution can roughly be pided into five stages:inpidual particle,irregular blocky composite particle,flake-shaped particle,agglomerate and single particle.The internal structure generally undergoes five stages:inpidual particle,coarse lamellar structure,fine lamellar structure,non-uniformly mixed structure andl plum-pudding structure.The grain size of Moss decreases continuously with the increase in milling time.Except for the cases of milling longer than 20 h under conditions P-2and S-500,the microstrain of Moss increases continuously with increase in milling time.The solute atoms (Si and Zr)are dissolved into Mo gradually with the increase in milling time.After 40-h milling,Zr contents in Moss under each condition all reach the nominal value,while the dissolved Si contents reach the nominal value only under conditions S-500 and B-20.

The distinct of MA behaviors of powder mixtures among different milling conditions is the MA rate,which ranks from great to small as follows:Conditions S-500,B-20,R-10 and S-300.For condition P-2,the rate appears slowly first and fast afterward.Calculation results show that a higher input power always corresponds to a faster MA rate.For Condition P-2,the addition of SA can reduce the MA process remarkably due to its lubrication action.However,with MA proceeding,SA gradually decomposes,leading to the acceleration of MA rate.