Features of the Solid Solution (Mo0.9,Cr0.1)Si2 Formation Depending on the State of Initial Mixture

The regularities of solid state synthesis of the solid solution (Mo0.9,Cr0.1)Si2 in vacuum have been investigated in the temperature range 400-1200°C depending on the dispersity and energetic state of the init ial powders, namely molybdenum, chromium and silicon. The energetic state of the in itial mixture was established to be a determin ing factor which affects the principal features of solid state interaction whereas an increase in dispersity only influences the temperature of the interaction start. When non-activated initial mixtures and ones mechanically activated in a p lanetary mill with low number of drum revolutions were used, the solid solution formation proceeded owing to diffusion of silicon into metals through successive formation of lower and higher molybdenum-based silicide phases followed by their interaction. Mechanical activation in a planetary mill with high number of drum revolutions was accompanied by not only decrease in particle size but also changes in the energetic state of the reaction mixture, which resulted in changing the regularities of the solid solution formation. Herein solid solutions on the basis of two higher molybdenum silicide phases, tetraand hexagonal modifications, were formed with further polymorphic transition of the unstable high temperature hexagonal β-MoSi2 phase into the low temperature tetragonal α-MoSi2 phase. It has been established that temperature of the beginning of interaction decreases by 100°C as compared with non-activated initial mixtures and temperature o f the end of the process depends on the amount of accumulated energy: under low energy mechanical activation the process is complete at 1200°C , while a h igh energy activation decreases this temperature by 200-400°C depending on the duration of activation.


Introduction
Th e t rans it ion metal s ilicid es refer to refracto ry co mpounds with melt ing temperatu re close to that of borides and carbides. Their high thermal and chemical stabilities remain up to h igh temperatures, which makes them p ro mising as materials for refractory coatings, various parts for aircraft and space engineering as well as for metallurg ical and chemical industry [1][2][3][4][5]. A mong all of the silicides, MoSi 2 based materials are most known for high temp eratu re ap p licatio n . Th ey ty p ically h av e go od mechanical properties and high heat and oxidation resistance at high (above 900℃) temperatures. Ho wever, a nu mber of shortcomings at lower temperatures block their commercial applicat ion. A key shortco ming is drastic o xidat ion and d is in teg ratio n at 80 0-90 0℃. So me p o s itiv e res u lts co n cern in g th e avo id an ce o f t he effect o f th e lo w temp eratu re o xid at ion h av e b een ach ieved lat ely . In particular, co mposite materials with ceramic addit ives from ZrO 2 , Al 2 O 3 , SiC, etc. have been created. Also, a promising solution of this problem consists in using solid solutions of mo lybdenum silicides added with chromiu m, niobiu m, titanium, tungsten and other refractory elements. The use of chromiu m as a basic additive to MoSi 2 increases the oxidation resistance of this material in the temperature range 600-900℃ [6][7][8][9].
Investigations into the structure and physicochemical properties in complex silicide systems such as Cr-Me-Si, where Ме stands for Ti, Ta, Re, and Mn [10][11][12], have shown that solid solutions on the basis of silicides exhib it better physicochemical properties co mpared to the related individual silicides. The broad region of the existence of solid solutions, with in wh ich the crystalline and electron structures are the same , guarantees a high reproducibility of the operation parameters. Herein solid solutions on the basis of higher silicide phases have the most stable properties.
The use of nanosized powders of solid solutions of silicides as materials for coatings and volumetric heaters ma rked ly increases their high temperature resistance due to the format ion of fine-grained protective film of scale. In spite of good enough clarification of mechanical alloying in various systems [13][14][15] as well as the regularit ies of formation of nanosized indiv idual silicides, in particular silicides of mo lybdenum and t itaniu m, via high energy mechanical treat ment [16][17][18][19][20][21][22], the available in formation on production of nanopowders of solid solutions of refractory compounds, especially silicides, is poor.
This work is devoted to investigation into the regularities of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 format ion through a solid state synthesis depending on the dispersity and energetic state of the initial mixture after milling.
According to the literature [10], the region of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 existence spreads up to 46 mol% CrSi 2 . In this work, therefo re, the init ial mixture was calculated in v iew o f production of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 , na mely (mass%): 58.45 Mo; 3.52 Cr, and 38.03 Si. Besides, some e xt ra silicon was added, whose amount was selected tentatively to compensate losses of silicon during the mill discharge as the lightest and most inclined to oxidation element among the other components. A homogeneous component distribution was reached via milling in the steel drum of planetary mill «Pu lverizette-6» in an ethanol mediu m. The powder part icle size did not exceed 10 µm.
To investigate the effect of the dispersity and the energetic state of the initial mixtures on the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 format ion, different conditions of mechanical activation were selected which were provided by different types of planetary mills used. In particu lar, mechanical act ivation was performed in a high-energy planetary mill «AIR-0.015» (« Gid rotsvetmet», Russia) in an argon mediu m at a planetary d isk revolution nu mber of 548 rpm with an acceleration o f 25 g as well as in a planetary mill «Pulverizette-6» at a disk revolution nu mber of 200 rp m. The mass powder/balls ratio was 1:10. Milling in «Pulverizette-6» was performed under the conditions, which provided a decrease in the particle size of the in itial powder with no change in its composition. Milling in «AIR-0.015» was conducted using an impulse reg ime, which made it possible to avoid an uncontrolled SHS (self-propagating high temperature synthesis) reaction. The amount of the initial mixture was the same in all experiments and equal to 50 g. The steel dru ms and 10 mm milling balls were prior charged with silicon to prevent contamination of the products with the reactor material. The energy transferred to the powder under milling was estimated on the basis of the specific energy, that is, the energy acquired by 1 g powder after one ball collision [23][24], taking into account the time of milling. A criterion for choice of the milling time was minimal change in the phase composition of in itial mixture.
Solid state synthesis of both non-activated and previously mechanically activated reaction mixtures (3 g samples pressed under 0.5 MPа) was conducted in a vacuum furnace SNV-1.3,1/20-I1 at ~ 1·10 -3 Pа in the temperature range 400-1200℃ and 2 h isothermal hold ing. The average rate of temperature rise to the isothermal holding was 10 K/ min.
The synthesis products were examined by an XRD analysis using a diffractometer DRON-3 under filtered Co radiation. The angles were determined with an error o f ± 0.05 deg. The size of crystallites was calculated using the Sherrer formu la. The phases were identified following the powder diffract ion database file ICDD-JCPDS PDF-2. The error of the lattice parameter calcu lation was ± 0.001 n m. The specific surface area was determined using thermal desorption of nitrogen with an error of 5-10 %.
The particle size and elemental co mposition of the powders produced by a low temperature synthesis were determined with help of a transmission electron microscope JEM 2100 S (Japan).
Standard techniques of chemical analysis were used for determination of the elemental co mposition of the final product and estimation of the amount of iron milled into it.

High Temperature Interaction of Non-acti vated Powders of Molybdenum, Chromium and Silicon in Vacuum
The regularities of the (Mo 0.9 ,Cr 0.1 )Si 2 format ion in the course of solid state interaction of the initial co mponents in the temperature range 400-1200℃ were investigated at an isothermal holding of 2 h. The corresponding XRD findings for the synthesis products are listed in Table 1.
These data evidence that the solid state interaction of the initial co mponents begins at 700℃, when the lower mo lybdenum silicide appears, whose reflection intensities change extremely with a ma ximu m at 800℃. The interaction is accompanied with reduction in the mo lybdenum lattice parameter, wh ich may be assigned to diffusion processes and dissolution of chro miu m in molybdenum (ato mic radius of chromiu m is sma ller than that of mo lybdenum by 10 %). Below 700℃ no phase transition is fixed and the reflection intensity is unchanged.
The further temperature rise is accompanied with the beginning of format ion at 800℃ of the tetragonal phase Мо 5 Si 3 , which is present in the interaction products up to 1100℃. At 1000℃ the higher molybdenum silicide of the tetragonal modification, α-МоSi 2 , is formed. Thus, in the temperature range 1000-1100℃ the basic products are Мо 5 Si 3 and α-МоSi 2 along with traces of Si, whereas at 1200℃ only α-МоSi 2 is identified. The change in the α-МоSi 2 lattice parameters (а decreases and с markedly increases) indicates the format ion of the interstitial solid solution (Mo x, Cr y )Si 2 . The data of chemical analysis demonstrate high reproducibility of the elemental composition (58.5 Мо; 3.74 Cr; 37.6 Si, mass%), which practically corresponds to the calculated formu la (Mo 0.9, Cr 0.1 )Si 2 ; contamination with iron does not exceed 0.1 mass%. The absence of chromiu m silicide phases in the temperature range studied draws a conclusion that the solid state interaction proceeds through successive format ion of interstitial solid solutions on the basis of molybdenum silicide phases owing to the dissolution of chromiu m. This is well consistent with the equilibriu m diagrams o f the Mo-Cr-Si system [10], according to which the isostructural silicides Cr 3 Si-Mo 3 Si and Cr 5 Si 3 -Mo 5 Si 3 form a continuous series of solid solutions.

Low Temperature Interaction of Previously Mechanically Acti vated Powders of Molybdenum, Chromi um and Silicon
The effect of the dispersity of the init ial mixture and the energy acquired by it on the regularit ies of the solid solution (Mo x ,Cr y )Si 2 format ion was studied using an XRD and TEM analyses. Table 2 lists the XRD data and the results of determination of the specific surface energy upon milling.
As seen, a high-energy milling leads to the powder refining, which is accompanied by change in the diffraction peaks and the phase composition of reaction mixture.
For mixtures 1-3 subjected to mechanical activation in the mill «Pulverizette-6», pro longation of milling fro m 600 to 3000 min results in changing the peak intensity and broadening the peaks for all o f the components used. First of all, this may be connected with powder refining: the specific surface area monotonously increases from 3.7 m 2 /g (600 min milling) tо 4.5 m 2 /g (3000 min milling).
The dynamics of changing in the XRD patterns fro m the reaction mixtures subjected to milling in the planetary mill «AIR-0.015» demonstrates the significant influence of the acquired energy on the reactivity (Fig. 1).
The 30 min milling does not cause any change in the phase composition of reaction mixture; only some d istortion of the diffraction peaks is observed, which is particularly characteristic for silicon, which is more brittle co mpared to mo lybdenum and chro miu m and is therefore affected by mechanical processing to a greater extent. Its energetic state is more strained owing to refining and disordering of the crystalline lattice.
Prolongation of milling to 45 min leads to change in the phase composition. The mo lybdenum peaks ma rkedly broaden, and reflections fro m silicon and chromiu m are absent. In addition, nuclei of mo lybdenum disilicide of two modifications, name ly low temperature α-MoSi 2 and high temperature β-MoSi 2, appear practically in equal quantity.  The absence of silicon peaks may be related to silicon amorphousness relative to XRD examination caused by the refinement in particle size and deformation of the crystalline lattice during milling. In order to clarify this point, the particles that formed a s mall cloud at the mo ment of powder discharge were specially examined.  indicates that 45 min milling (mixtu re 5) prov ides the reaction mixture with the energy required for part ial mechanosynthesis. The increase in the energy acquired by mixtu res 6 and 7 thanks to prolongation of milling to 60 and 90 min, respectively, provides the necessary conditions for intensification up to the completeness of solid state interaction, wh ich is confirmed by the XRD patterns. The content of mo lybdenum in the synthesis products monotonously reduces, whereas that of the α-MoSi 2 phase increases. The change in the α-MoSi 2 lattice parameters (а=0.321; с=0.783 n m) evidences to the formation of the solid solution (Mo x, Cr y )Si 2 . The calcu lation of crystallite sizes using the Sherrer fo rmula (15 n m for α-MoSi 2 and 55 nm for Mo) confirms a nanostructural state of the initial mixtu re. The specific surface area o f the latter is 3.05 m 2 /g.
The TEM micrographs fro m mixture activated for 90 min (mixture 7) manifest a homogeneous distribution of the initial elements (Fig. 3). a b c d Figure 3. The surface of agglomerate (а) and maps of distribution of silicon (b), chromium (c) and molybdenum (d) over it for the reaction mixt ure mechanically activated for 90 min   TEM micrographs of the reaction mixture (Fig. 4) confirms its nanostructural state and demonstrates that it is composed of agglomerates of polycrystals. The data of micro -XRD analysis revealed the presence of free silicon (light-grey zone), which is amorphous towards XRD examination: the XRD patterns contain no silicon reflections beginning fro m a milling time of 45 min ( Table 2).
The format ion of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 through solid state interaction of mechanically act ivated reaction mixtures in vacuu m was investigated in the temperature range 400-1200℃. The phase composition of the synthesis products was determined using an XRD method (Table 3, Figs. 5 and 6).  The findings for the reaction mixtures that underwent a long milling (1800 and 3000 min) in the planetary mill «Pulverizette-6» suggest that the main features of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 formation do not depend on the time of mechanical activation. The beginning of solid state interaction is fixed at 600℃, that is, 100℃ lower than that for non-activated powders (700℃, Table 1). Then the interaction runs through successive formation of lower silicide phases based on molybdenum, na mely Mo 3 Si and Mo 5 Si 3 . At 900℃, there appears a molybdenum disilicide based phase, α-MoSi 2 , and at 1000℃ the interaction products are a mixture of the all above phases and the initial co mponents. Finally, temperature rise to 1200℃ provides the formation of an α-MoSi 2 based solid solution. All this ma kes it possible to draw a conclusion that mechanical activation of reaction mixtu re in the planetary mill «Pu lverizette-6» is accompanied by refining the initial co mponents, which is confirmed by the increase in the specific surface area of the reaction mixture fro m 2.0 (in itial mixture) tо 3.9 (upon 1800 min milling) and 4.5 m 2 /g (upon 3000 min milling). The energy acquired thanks to the format ion of new surfaces and the change in the crystalline lattice of the in itial co mponents due to format ion of various defects of structure (dislocations, vacancies, etc.) leads to lowering the temperature of the beginning of interaction with no change in the main features of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 format ion. Herein the process of interaction proceeds by a diffusion mechanism similar to non-activated initial mixtures .As for milling in the «AIR-0.015» mill, at any phase composition of mechanically activated reaction mixture, which depends on the energy acquired during milling, the appearance of the dissipative β-MoSi 2 phase is registered at 600℃. Pro longation of milling results in the acquired energy increasing, which, in its turn, increases the mixture reactivity and the amount of the β-MoSi 2 phase (Fig. 5).
According to the XRD patterns fro m the products of interaction at 600℃ of reaction mixture 4, they contain all the initial co mponents, that is Mo, Si and Cr, plus β-MoSi 2 nuclei. In the temperature range 700-800℃, chro miu m peaks disappear and an α-MoSi 2 based phase is formed, the amount of which gradually increases. At 900℃ the interaction yields the solid solution (Mo х ,Cr у )Si 2 on the basis of the both (α and β) modifications of molybdenum disilicide, and at 1000℃ the β → α poly morphic transition occurs.
The solid state interaction of reaction mixtures activated in the planetary mill «AIR-0.015» for 60 and 90 min (mixtures 6 and 7) is different fro m that for mixture 4, wh ich is due to a higher acquired energy and d ifferent co mposition of mechanically act ivated reaction mixtures containing α-MoSi 2 nuclei, mo lybdenum and XRD-amorphous silicon (Fig. 6). As formation of silicides through a solid state synthesis is determined by silicon-in-meta l d iffusion [25], the availability of much silicon in an XRD-amorphous state ensures its higher reactivity co mpared to crystalline silicon and leads to the formation of solid solution on the basis of mo lybdenum d isilicide of both modifications at 700 (mixtu re 7) and 800℃ (mixture 6) depending on the milling duration. The size of polycrystals calculated using the Sherrer formu la is 20 n m for α-MoSi 2 and 15 n m for β-MoSi 2 . The rise in synthesis temperature by 100℃ stimulates the poly morphic β → α transition (mixtu re 7).
The product of synthesis at 800℃ of the reaction mixture 7 milled for 90 min was a brittle b riquette easy to disintegrate. An SHS reaction was not observed even in the case of big initial mixture (to 50 g), which may be associated with phenomenon of "blocking": high-active particles of the initial co mponents become separated with big enough amount of the α-MoSi 2 fo rmed. The size of the powder particles, polycrystals, calculated using the Sherrer formu la did not exceed 45-50 n m. The chemical analysis showed reproducibility of the elemental composition, mass%: 58.1 Мо; 3.3 Cr; 36.7 Si; 1.6 Fe, which corresponds to the formula (Mo 0.9 ,Cr 0.1 )Si 2 . The presence of iron is due to contamination of the powder with the mill material during mechanical activation.
The powder of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 produced under the selected conditions was examined using a TEM microscope. The micrographs are presented in Figs. 7 and 8.
The (Mo 0.9 ,Cr 0.1 )Si 2 powder produced from mechanically activated reaction mixtu re was composed of polycrystal agglomerates sizing to 100 n m whose crystallites did not exceed 20 n m. The carried out investigation into synthesis of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 depending on the energetic state of the reaction mixtures made it possible to establish a number of features of the solid state interaction between the init ial components which determine the difference in phase transitions and, as a result, in runs of the process and its products.
In the case of non-activated powders, the solid solution formation p roceeds by a diffusion mechanism through successive formation of solid solutions on the basis of lower silicides, (Mo,Cr) 3 Si and (Mo,Cr) 5 Si 3 (within 800-900℃) and the higher mo lybdenum silicide phase α-(Mo,Cr)Si 2 (at 1000℃) followed by their interaction/homogenizat ion in the temperature range 1100-1200℃ (Table 1). The formation of the solid solution fro m reaction mixtures that were prev iously mechanically activated for 1800-3000 min in the planetary mill «Pulverizette-6» is characterized by the same regularities as for non-activated mixtures except for the shift (by 100℃) of the start of interaction towards lower temperatures due to the increase in the specific surface area and the structure defectiveness of the initial co mponents. Despite the decrease in the interaction start temperature, the solid solution homogenization was co mplete in the same temperature interval, 1100-1200℃. Th is draws a conclusion that only increase in the specific surface area fro m 2.3 (non-activated powders) tо 4.5 m 2 /g (3000 min milling) and the acquired energy are not sufficient for a change in the mechanis m of solid state interaction.
The solid state interaction in the course of lo w temperature synthesis in vacuum of p reviously mechanically act ivated mixtu res in the planetary mill «АІR-0.015» is quite d ifferent. In our opinion, the change in the regularities of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 formation may be related to the silicon structure peculiarit ies in mixtures 4-7, where, according to the XRD and TEM data, silicon is amorphous towards XRD analysis (mixtures 5-7) or partially amorphous (mixture 4). Upon 30 min milling, reaction mixtures contain all of the in itial co mponents, whereas upon 60 and 90 min milling, the α-MoSi 2 phase is fixed along with mo lybdenum, chromiu m and XRD amorphous silicon (Table 2), which indicates the fact that under the selected conditions of intense milling, the chemical reaction yielding mo lybdenum disilicide prevails over diffusion process as MoSi 2 is the most energetically preferable co mpound (heat of MoSi 2 formation is three times higher than that of lower silicide phases [25]).
It was established that at any phase composition of init ial mixtu re activated in the mill «АІR-0.015», the β-MoSi 2 phase is fixed at 600℃ (Tab le 3). Here the formation of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 occurs without format ion of intermediate lower silicide phases. The acquired energy significantly affects the reactivity of the initial mixture, which, in its turn, determines the final temperature for phase transformations.
Having compared the processes running in planetary mills «Pulverizette-6» and «АІR-0.015», one can suggest that mechanical processing in «Pulverizette-6» acco mpanied with chaotic co mbination of collisions and abrasion proceeds mostly at the expense of abrasion, whereas in the mill «АІR-0.015», under the conditions of the impulse milling proposed by the authors, the action of collisions dominates, which leads to amorphization of free silicon. Herein the peculiarities of system defectiveness may be different.
To sum up, the investigation carried out has shown up that the formation of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 predominantly depends on the reactivity of initial mixture, that is, on the energy acquired by the mixture owing to the milling and also on the phase composition and state of silicon. According to the calculations performed, the acquired energy is proportional to the number of revolutions of the mill dru ms. In this work, the number of dru m revolutions in the «АІR-0.015» mill was greater than that in «Pulverizette-6» mill by three t imes. At practically the sa me specific surface area (4.5 m 2 /g for «Pulverizette-6» (3000 min milling) and 4.2 m 2 /g for «АІR-0.015» (90 min milling)), the regularities of the solid solution format ion are quite different. Also, it should be noticing that the energy acquired owing to milling determines the formation of the dissipative phase β-MoSi 2 and α-MoSi 2 and their solid state interaction. The higher this energy, the sooner the final phase α-MoSi 2 is formed.

Conclusions
The energy acquired by the initial reaction mixture during mechanical activation was established to be a determining factor among the others affecting the process of solid state interaction, whereas the role of the dispersity exhib its itself only in the change in the interaction start temperature.
Formation of the solid solution (Mo 0.9 ,Cr 0.1 )Si 2 through solid state interaction of non-activated initial powders occurs owing to diffusion processes via intermediate stages of formation of solid solutions based on lower and higher mo lybdenum silicide phases and their subsequent interaction.
Mechanical activation in a planetary mill «Pulverizette-6» is accompanied with refining the initial co mponents; the energy acquired by them permits a decrease in the temperature of the interaction start by 100℃; herein the regularit ies of format ion of dissipative intermed iate phases and the final temperature of the solid solution format ion, 1200℃, are the same as for non-activated powders.
Mechanical activation in a h igh-energy mill «АІR-0.015» allo ws one to produce high reactivity mixtures, which intensifies the solid state interaction. Under the following heat treatment, solid solutions on the basis of higher mo lybdenum silicide phases of two modifications, tetragonal α-MoSi 2 and hexagonal β -MoSi 2 , are formed with further transformation of the unstable high temperature β-MoSi 2 phase into the low temperature α-MoSi 2 phase.