Plastic deformation of directionally solidified ingots of binary and some ternary MoSi2/Mo5Si3 eutectic composites

Abstract The high-temperature mechanical properties of directionally solidified (DS) ingots of binary and some ternary MoSi2/Mo5Si3 eutectic composites with a script lamellar structure have been investigated as a function of loading axis orientation and growth rate in a temperature range from 900 to 1500°C. These DS ingots are plastically deformed above 1000 and 1100 °C when the compression axis orientations are parallel to [11¯0]MoSi2 (nearly parallel to the growth direction) and [001]MoSi2, respectively. [11¯0]MoSi2-oriented DS eutectic composites are strengthened so much by forming a script lamellar microstructure and they exhibit yield stress values several times higher than those of MoSi2 single crystals of the corresponding orientation. The yield stress values increase with the decrease in the average thickness of MoSi2 phase in the script lamellar structure, indicating that microstructure refinement is effective in obtaining better high-temperature strength of these DS eutectic composites. Among the four ternary alloying elements tested (V, Nb, Ta and W), Ta is found to be the most effective in obtaining higher yield strength at 1400 °C.


Introduction
There is an increasing demand for ultrahigh-temperature structural materials that can drastically raise the operating temperature of combustion systems in fossil-fueled power plants so as to improve their thermal efficiency and consequently to reduce fuel consumption as well as climate-warming CO 2 emission. Since the turbine inlet temperature of the most advanced gas turbine combustion systems already exceeds 1600 °C, which is about 200 °C higher than the melting temperature of current Ni-based superalloys, [1] Ni-based superalloy turbine blades are usually used with air cooling. Therefore, a drastic increase in thermal efficiency of these gas turbine combustion systems is unlikely, unless a new class of ultrahigh-temperature structural materials is developed, which can withstand severe environments and exhibit superior mechanical properties and corrosion resistance at high temperatures. MoSi 2 -based alloys and composites have received a considerable amount of attention as promising candidates for such ultrahigh-temperature structural applications due to their high melting temperatures (around 2000 °C), low densities, high thermal conductivities and good oxidation resistance. [2][3][4][5][6][7][8][9][10][11][12] Various MoSi 2 -based composites produced by powder metallurgy routes have been investigated extensively. [2][3][4][5][6]8,9,12] Some of these MoSi 2 -based composites exhibit improved fracture toughness at ambient temperature. However, their high-temperature strength is rather poor mostly because of intergranular SiO 2 layers. [2,4,13] Alternatively, directionally solidified (DS) MoSi 2  eutectic composites containing the Mo 5 Si 3 phase with the tetragonal D8 m structure have been confirmed to exhibit better creep properties than other MoSi 2 -based composites produced through powder-metallurgy routes due mainly to the elimination of high-angle grain boundaries and intergranular SiO 2 layers. [14] DS MoSi 2 /Mo 5 Si 3 eutectic composites have a fine script lamellar microstructure composed of a continuous MoSi 2 matrix and an interconnected network of Mo 5 Si 3 elongated along the growth direction. [15][16][17] Mechanical properties of DS eutectic composites are expected to be affected by microstructural characteristics such as lamellar spacing, orientation relationship and interface morphology as well as alloying elements. Further improvements in high-temperature strengths are thus expected to be highly probable by controlling the growth conditions in the DS process and also by ternary additions. Recently, we have investigated effects of ternary additions on the microstructure and thermal stability of DS MoSi 2 /Mo 5 Si 3 eutectic composites. We have confirmed that DS ingots of binary and various ternary-alloyed MoSi 2 /Mo 5 Si 3 eutectic composites exhibiting a homogeneous and fine script lamellar structure can be obtained by controlling the growth rate during DS processing and by adjusting the amount of ternary additions. [16][17][18][19] These DS ingots with a homogeneous script lamellar structure are approximated as single crystals having an orientation relationship of (110) MoSi2 [15][16][17] Our preliminary study on mechanical properties of binary DS eutectic composites suggests that high-temperature strength depends on the average thickness of the MoSi 2 phase. [20] We also expect that the strength of DS MoSi 2 / Mo 5 Si 3 eutectic composites depends on the crystal orientation in view of the anisotropic deformation behavior of the tetragonal MoSi 2 and Mo 5 Si 3 phases. [7,10,[21][22][23][24][25][26] It is technologically very important to find out the strongest orientation of the DS eutectic composites as well as the most beneficial microstructure (lamellar thickness) of the DS eutectic composites for better high-temperature mechanical properties.
In the present study, we investigate the deformation behavior of DS MoSi 2 /Mo 5 Si 3 eutectic composites of binary and some ternary alloys as a function of temperature and loading axis orientation in order to elucidate effects of the lamellar spacing and ternary addition on their mechanical properties and the anisotropy in high-temperature strength with the possible causes.

Experimental procedures
Rod ingots of MoSi 2 /Mo 5 Si 3 two-phase eutectic composites with nominal compositions of Mo -54 at.% Si (binary), Mo -54 at.% Si -2 at.% X (X = V, Nb, W) and Mo -54 at.% Si -5 at.% Ta were prepared by arc-melting. DS ingots of the two-phase eutectic composites were grown from these rod ingots using an optical floating zone furnace at various growth rates between 5 and 100 mm h -1 under an Ar gas flow. Microstructures of as-grown DS ingots were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with JSM-7001FA and JEM-2000FX electron microscopes (JEOL Ltd., Tokyo, Japan), respectively. Chemical compositions were analyzed by energy dispersive X-ray spectroscopy (EDS) in SEM.
Specimens for compression tests with dimensions of 1.2 × 1.2 × 3.0 mm 3 were cut from the DS ingots by electrical discharge machining (EDM). Compression tests were carried out on an Instron-type testing machine at a strain rate of 1 × 10 −4 s −1 at temperatures ranging from 900 to 1500 °C in vacuum. The loading axis orientations tested are [110] MoSi2 and [001] MoSi2 , which are nearly parallel to and perpendicular to the growth direction of the DS ingots, respectively. [17] Crystallographic orientations of these DS ingots are hereafter referred to those of the MoSi 2 matrix and are indicated with a subscript 'MoSi 2 ' unless otherwise stated.
Deformation microstructures were examined by optical microscopy (OM) and TEM. Specimens for TEM observations were sliced from deformed specimens by EDM, mechanically polished and then Ar ion-milled using a JEOL EM-09100IS ion milling machine operated at 6.0 kV for milling and 2.0 kV for finishing.

Microstructures of the DS ingots
SEM backscattered electron images of a cross-section ((110) MoSi2 // (001) Mo5Si3 ) of DS ingots of binary and some ternary eutectic composites grown at various growth rates are shown in Figure 1. Dark and bright regions in the SEM images correspond to MoSi 2 and Mo 5 Si 3 , respectively. A homogeneous script lamellar structure is observed for most of the DS ingots investigated in this study, except for the binary DS ingot grown at 100 mm h -1 , in which a cellular structure composed of fine and coarse script lamellar structures is observed.  MoSi2 , using test grids drawn on SEM images of the (110) MoSi2 cross-sections, as seen in the right bottom of Figure 1. The average lamellar spacing λ, which is estimated as a sum of the average thickness of both phases, is plotted in Figure 2 as a function of growth rate. The λ value decreases with the increase in the growth rate R, following the relationship of λ 2 R = constant, as proposed by Jackson and Hunt [27]. Figure 3 shows a typical TEM micrograph of a binary DS composites grown at a growth rate of 100 mm h -1 . The thin foil was cut parallel to the (110) MoSi2 // (001) Mo5Si3 cross-section. Many grown-in dislocations are observed to exist in the MoSi 2 matrix but the density of grown-in dislocations in the Mo 5 Si 3 phase is by far smaller. The Burgers vectors of grown-in dislocations in the MoSi 2 matrix are confirmed to be <100> by the contrast analysis in TEM. These dislocations are believed to be introduced during cooling by the mismatch of the coefficient of thermal expansion between the two phases. Similar TEM microstructures are observed for all DS ingots.

[110] MoSi2 orientation
Typical stress-strain curves for [110] MoSi2 -oriented specimens of binary DS eutectic composites grown at growth  and W-alloyed specimens. The difference in temperature variations of stress-strain curves between 1000 and 1400 °C is ascribed to the difference in the deformation mechanisms, as described later in the next section on deformation microstructure observations. Figure 6 shows the temperature dependence of yield stress of [110] MoSi2 -oriented specimens of binary DS eutectic composites grown at growth rates of 10 mm h -1 (homogeneous script lamellar structure) and 100 mm h -1 (cellular structure), together with those of single crystals of MoSi 2 and Mo 5 Si 3 with [110] MoSi2 and [001] Mo5Si3 orientations, respectively, for references [24][25][26]. Values of yield stress for both specimens of binary DS eutectic composites decreases with increasing temperature and are several times higher than those of MoSi 2 single crystals with the corresponding orientation at all temperatures. The yield stress is higher for specimens with a homogeneous script lamellar structure (R = 10 mm h -1 ) than for specimens with cellular structures (R = 100 mm h -1 ) at all temperatures, indicating the importance of the formation of a homogeneous and fine script lamellar structure for better high-temperature strength of DS MoSi 2 /Mo 5 Si 3 eutectic composites. Of importance to note in Figure 6(a) is that the yield stress for the DS MoSi 2 /Mo 5 Si 3 eutectic composite with a fine homogeneous script lamellar structure (R = 10 mm h -1 ) is very high at high temperatures, exceeding 385 MPa at 1400 °C and 290 MPa at 1500 °C. Figure 4(b) shows typical stress-strain curves for [001] MoSi2 -oriented binary DS composite specimens grown at R = 10 mm h -1 . Plastic flow is observed only above 1100 °C, which is 100 °C higher than the onset temperature for [110] MoSi2 -oriented binary DS composite specimens. The yield stress decreases drastically and monotonously with increasing temperature, as shown in Figure 6(b), in which the temperature dependence of yield stress is also shown for single crystals of MoSi 2 with the [001] MoSi2 orientation for reference [24]. Although the value of yield stress of the binary DS composite specimen is much higher than that of the [001]-oriented MoSi 2 single crystal at 1100 °C, the opposite is true at and above 1300 °C.

[001] MoSi2 orientation
This implies that the strength of single crystals of Mo 5 Si 3 is lower than that of single crystals of MoSi 2 for the corresponding orientations at and above 1300 °C. For both specimens, plastic flow is observed only above 1000 °C and premature failure occurs in the elastic region without any appreciable plastic strain below 900 °C. The stress-strain curves for specimens tested between 1000 and 1200 °C generally exhibit a yield drop followed by steady state flow, whereas those above 1300 °C exhibit only nearly steady-state flow from the relatively early stage of plastic deformation.
The variations of stress-strain curves with temperature are similar for [110] MoSi2 -oriented specimens of all of binary and ternary DS eutectic composites. Typical stress-strain curves obtained for [110] MoSi2 -oriented specimens of binary and some ternary DS eutectic composites tested at 1000 and 1400 °C are shown in Figure 5. For binary specimens deformed at 1000 °C, the yield stress (flow stress at 0.2% plastic strain) depends on the growth rate, while the maximum stress reached before the yield drop is almost independent of the growth rate. A similar trend is also observed for V-alloyed specimens. At 1400 °C, in contrast, both the yield stress and maximum stress depend on the growth rate for the binary  above 1300 °C. It is worth noting in Figure 6(b) that the yield stress for the DS MoSi 2 /Mo 5 Si 3 eutectic composite with a fine homogeneous script lamellar structure (R = 10 mm h -1 ) is very high, exceeding 500 MPa at 1400 °C, which is by far higher than any other values reported so far for high-temperature materials.     [25,26] the introduction of dislocations into the Mo 5 Si 3 phase in the script lamellar structure is considered to be assisted by the stress concentration generated by dislocation pile-ups against the interphase boundary in the MoSi 2 matrix. Then, it is reasonable to consider that plastic deformation is initiated in the MoSi 2 matrix and that it propagates into Mo 5 Si 3 after the stress concentration at the interphase boundary due to pile-up dislocations in the MoSi 2 matrix reaches some critical values. As seen in Figure 5, for binary specimens deformed at 1000 °C, the yield stress (flow stress at 0.2% plastic strain) depends on the growth rate, while the maximum stress reached before the yield drop is almost independent of the growth rate. The yield stress may correspond to the stress level at which plastic deformation is initiated in the MoSi 2 matrix, while the maximum stress primarily to the stress level at which plastic deformation starts to propagate into the Mo 5 Si 3 phase. A TEM bright-field (BF) image of a [110] MoSi2oriented binary DS composite specimen deformed to ~1% plastic strain at 1000 °C is shown in Figure 8   in Figure 7. Dislocations marked C are invisible for g = 330 (Figure 9(b)), while they are visible for the other imaging conditions presented in Figure 9. (1)   In addition, the (001) stacking fault plane cannot be a slip plane for partial dislocations with the Burgers vector parallel to <331>. Stacking faults on (001) in as-grown and plastically deformed MoSi 2 have been observed by many researchers. [22,23,[29][30][31]. We have also observed similar stacking faults on (001) formed during high-temperature compression tests of [001]oriented single crystals of WSi 2 with the C11 b crystal structure and proposed a possible formation mechanism, in which the (001) stacking fault is formed by the climb motion of 1/6<331> partial dislocations that originate from a <100> perfect dislocation. [32] Although the formation mechanism of stacking faults on (001) observed in the MoSi 2 matrix of the present DS eutectic composite specimens has yet to be clarified, it is reasonable to consider that the formation of these (001) stacking faults during deformation at 1100 °C (~0.6 T m , T m : melting point) involves a significant diffusion process similarly to the case proposed for WSi 2 single crystals. [32] In

Deformation modes and strain compatibility
If a crystal is sheared by a small amount s by the operation of a slip system whose slip direction and slip plane normal are respectively parallel to the x 0 and y 0 axes of the (x 0 , y 0 , z 0 ) orthogonal coordinate system, the strain tensor ε 0ij of the shear deformation is given by the following equation: [33,34] In  Figure 8(b) shows a TEM BF image of a [110] MoSi2oriented specimen of the binary DS composite compressed to ~2% plastic strain at 1400 °C. In the MoSi 2 matrix, a high density of long and curved <100> dislocations mostly with non-screw characters frequently form dislocation nodes and sub-boundaries (some are marked with arrowheads in Figure 8(b)), indicating the occurrence of dislocation climb in the MoSi 2 matrix at about 2% plastic strain corresponding to a nearly steadystate flow. The dislocation structure in the Mo 5 Si 3 phase does not differ much from that observed in the specimen deformed at 1000 °C. Although the activation of dislocations in both MoSi 2 and Mo 5 Si 3 phases are observed, the dislocation density is much higher in MoSi 2 than in Mo 5 Si 3 , suggesting that plastic deformation occurs dominantly in the MoSi 2 matrix. This may be the reason why both the yield stress and maximum stress vary with the growth rate in marked contrast to the case of 1000 °C.  , of the four are confirmed to be operative experimentally in the present study. On the assumption that these two slip systems are equally activated, the strain tensor for Mo 5 Si 3 is described also by Equation (3). If the plastic strain tensors of the two constituent phases are identical, macroscopic plastic deformation is expected to occur without introducing any cracks at the interphase boundary because no strain incompatibility is developed at the boundary. In [001] MoSi2 -oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites, in contrast, the onset temperature (1100 °C; ~0.6 T m ) for plastic flow is found to be higher than that for [110] MoSi2 -oriented specimens. This is because plastic deformation of [001] MoSi2 -oriented specimens needs the operation of diffusion-controlled deformation processes such as the climb motion of dislocations accompanied by the formation of (001) stacking faults in MoSi 2 . We now consider how plastic deformation of [001] MoSi2 -oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites occurs at temperatures lower than the actually observed onset temperature (1100 °C) for plastic flow without diffusion-controlled processes. The

[001] MoSi2 loading axis
In order to discuss the strain compatibility between the two constituent phases in DS MoSi 2 /Mo 5 Si 3 eutectic composites, the strain tensor for the relevant operative slip systems has to be transformed so as to be described with the (x, y, z) orthogonal coordinate system for the MoSi Tables 2 and 3, respectively. The strain components in Tables 2 and 3 are normalized to their respective absolute value of ε zz . The strain component ε zz corresponds to the normal strain along the compression axis since the loading axis is parallel to the z axis in this case. Under the condition of uniaxial loading along [110] MoSi2 , four {011}<100> equivalent slip systems are equally stressed with the identical Schmid factors of 0.463 and these four slip systems are assumed to be equally activated in MoSi 2 . The strain tensor for the MoSi 2 matrix is then given in the following form. far as the resultant macroscopic strain tensor described by Equation (4) is achieved in each of the constituent phases. However, no apparent plastic flow is observed in [001] MoSi2 -oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites below 1000 °C, indicating that strain compatibility is not actually achieved at the interphase boundary. We believe that the macroscopic strain tensor generated in MoSi 2 cannot be in the form of Equation (4), which needs the simultaneous activation of some of the eight equivalent {013}<331> slip systems. Indeed, [001]-oriented MoSi 2 single crystals have been reported to exhibit extremely high work-hardening followed by early fracture at about 0.5% plastic strain below 1200 °C. [24] This clearly suggests that macroscopic flow accompanied by the multiple activation of {013}<331> slip systems in MoSi 2 so as to satisfy the strain compatibility at the interphase boundary is difficult to achieve in [001] MoSi2 -oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites below 1000 °C and that plastic deformation of this orientation occurs only at higher temperatures where diffusion-controlled deformation processes play an important role. In other words, the difficulty in multiple activation of {013}<331> slip systems in the MoSi 2 matrix is the reason for the higher onset temperature for plastic flow for [001] MoSi2oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites.

Influences of growth rate and ternary addition on high temperature strength
The yield stress of [110] MoSi2 -oriented DS MoSi 2 /Mo 5 Si 3 eutectic composites depends on the growth rate during directional solidification (Section 3.2.1) and is considered to correspond to the stress level at which plastic deformation is initiated in the MoSi 2 matrix (Section 3.   (112)[111] slip systems with a nonzero Schmid factor (0.318) are operative equally, the resultant macroscopic strain tensor becomes identical to that given by Equation (4). Then, strain compatibility at the interphase boundary is expected to be satisfied, as where diffusion-controlled deformation processes play an important role. This is currently under investigation in our group. Figure 12 shows the temperature dependence of yield stress for [110] MoSi2 and [001] MoSi2 -oriented binary DS MoSi 2 /Mo 5 Si 3 eutectic composites grown at 10 mm h -1 together with that for [110] MoSi2 -oriented Ta-doped DS eutectic composites grown at 10 mm h -1 . The temperature dependence of yield stress obtained for some typical high-temperature materials [35][36][37][38][39] are also shown in the figure for comparison.
[001] MoSi2 -oriented DS eutectic composites exhibit yield stress values higher than [110] MoSi2 -oriented DS eutectic composites at all temperatures investigated (above 1100 °C). Surprisingly, the yield stress value obtained at 1400 °C for the [110] MoSi2oriented Ta-doped DS eutectic composite is comparable to that obtained at 1400 °C for [001] MoSi2 -oriented DS eutectic composites, exceeding 500 MPa even at such a high temperature of 1400 °C. The yield stress values of these DS MoSi 2 /Mo 5 Si 3 alloys are much higher than those not only of modern Ni-base superalloys such as CMSX-4 but also of recently developed ultrahigh-temperature structural materials such as ULTMAT Mo-Si-B alloys and Nb silicide-based DS alloys. [35][36][37][38][39] The yield stress values of these DS MoSi 2 /Mo 5 Si 3 eutectic ingots at 1400 °C are comparable to or higher than those of CMSX-4 at ~1050 °C, which indicates the significant advantage of these DS MoSi 2 /Mo 5 Si 3 eutectic alloys for high-temperature structural applications over advanced Ni-base superalloys and recently developed ultrahigh-temperature structural materials. Our preliminary results indicate that these DS MoSi 2 /Mo 5 Si 3 eutectic alloys exhibit excellent creep properties at temperatures exceeding 1300 °C. This will be published soon elsewhere.

Conclusions
High-temperature mechanical properties as well as deformation mechanisms of DS ingots of binary and some ternary MoSi 2 /Mo 5 Si 3 eutectic composites grown at various growth rates have been investigated in the figure. The yield stresses for those with a homogeneous script lamellar structure increase with the decrease in the average thickness of MoSi 2 both at 1000 and 1400 °C with the decrease being more pronounced at 1400 °C. Microstructure refinement of DS MoSi 2 /Mo 5 Si 3 eutectic composites is thus quite effective in obtaining better high-temperature strength.
The trend for the MoSi 2 average thickness dependence of yield stress is very similar for binary and all ternary alloys at both 1000 and 1400 °C, except for that alloyed with 5 at.% Ta tested at 1400 °C. The yield stress at 1400 °C for the 5 at.% Ta-doped alloy is significantly higher than those of binary and other ternary alloys, exceeding that of [001]-oriented single crystals of binary Mo 5 Si 3 . Ta additions are thus found to be very effective in obtaining better high-temperature strength of the DS MoSi 2 /Mo 5 Si 3 eutectic composites. The substantial increase in yield strength observed for the Ta-doped alloy is believed to be due to the solid-solution hardening effects of large and heavy Ta atoms on plastic deformation of the MoSi 2 matrix especially at temperatures   [35][36][37][38][39]