Deformation behavior of a two-phase Mo–Si–B alloy

Intermetallics 15 (2007) 687e693 www.elsevier.com/locate/intermet

Deformation behavior of a two-phase MoeSieB alloy K.S. Kumar*, A.P. Alur Division of Engineering, Brown University, 182 Hope Street, Providence, RI 02912, USA Available online 13 November 2006

Abstract MoeSieB alloys are being considered as possible candidates for high-temperature applications beyond the capabilities of Ni-based superalloys. In this paper, the high-temperature (1000e1400  C) compression response over a range of quasi-static strain rates, as well as the monotonic and cyclic crack growth behaviors (as a function of temperature from 20  C to 1400  C) of a two-phase MoeSieB alloy containing a Mo solid solution matrix (Mo(Si,B)) with w38 vol% of the T2 phase (Mo5SiB2) is discussed. Analysis of the compression results confirmed that deformation in the temperatureestrain-rate space evaluated is matrix-dominated, yielding an activation energy of w415e445 kJ/mol. Fracture toughness of the MoeSieB alloy varies from w8 MPaOm at room temperature to w25 MPaOm at 1400  C, the increase in toughness with temperature being steepest between 1200  C and 1400  C. SeN response at room temperature is shallow whereas at 1200  C, a definitive fatigue response is observed. Fatigue crack growth studies using R ¼ 0.1 confirm the Paris slope for the two alloys to be high at room temperature (w20e30) but decreases with increasing temperature to w3 at 1400  C. The crack growth rate (da/dN ) for a fixed value of DK in the Paris regime in the 900e1400  C range, increases with increasing temperature. Ó 2006 Published by Elsevier Ltd. Keywords: B. Mechanical properties at high temperatures; B. Fatigue resistance and crack growth; D. Microstructure; A. Molybdenum silicides

1. Introduction The demand for materials for elevated temperature applications beyond the realm of Ni-based superalloys has generated significant research interest in multiphase refractory alloys that contain a matrix phase that in principle is capable of providing damage tolerance, and a significant volume fraction of second phase(s) that enhances the creep resistance of the alloy. Thus, multiphase NbeSi-based alloys and MoeSi-based alloys have been the focus of research attention over the past few years; an overview on each of these alloy systems that provides a brief history of their development, summarizes the current understanding and identifies the numerous challenges that lie ahead, was recently published [1,2]. Additional important requirements for such applications are isothermal and cyclic oxidation resistances, and considerable attention has been devoted to improve this property in these refractory

* Corresponding author. Tel.: þ1 401 863 2862; fax: þ1 401 863 7677. E-mail address: [email protected] (K.S. Kumar). 0966-9795/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.intermet.2006.10.008

alloy systems using approaches that include alloying as well as application of coatings [3e6]. Over the past few years, Perepezko and coworkers [7e11] have provided systematic documentation of the phase equilibria and associated microstructures that are present in ternary MoeSieB alloys rich in Mo; furthermore alloying strategies to produce enhanced oxidation resistance over a range of temperatures have also been explored. Some of the findings are highlighted in Ref. [2]. These studies have shown that the addition of B to a binary MoeSi alloy enables the formation of a ternary silicideeboride (the T2 phase, Mo5SiB2) that is in equilibrium with the terminal Mo solid solution, and that the solubility of Si and B in Mo is low at room temperature (that for B being considerably lower relative to Si) and this low solubility is also maintained at high temperatures (>1500  C). The T2 phase has the D8l structure (body centered tetragonal) with 32 atoms per unit cell and exists over a range of compositions (i.e. off-stoichiometric compositions are possible). The two-phase T2 þ Mo solid solution region as well as the three-phase T2 þ Mo solid solution þ Mo3Si region adjacent to it on the B-lean side offer the possibility of

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varying the ratio of matrix phase (Mo solid solution) and intermetallic phase(s) extensively. A review of the mechanical properties of these alloys (twophase and three-phase alloys) was provided in our recent paper [12] and will not be repeated here but it is worth noting that the compression and tension responses as a function of temperature and strain rate of several ternary and quaternary alloy compositions, some in single crystal form but most as polycrystals, have been characterized [12e23], and the cyclic crack growth response of a ternary MoeSieB alloy at ambient temperatures 800  C, 1200  C and 1300  C [24e26] has been reported. Additionally, we reported in detail the fatigue behavior of a ternary two-phase alloy, both in terms of fatigue life and fatigue crack growth response, in the temperature range from 20  C to 1400  C [27] and supported the results with extensive microstructural observations. This paper provides an overview of the results from our ongoing effort on the compression and fatigue behavior of this two-phase MoeSieB alloy in the forged condition and rationalizes the properties obtained by comparing the initial microstructure to the post-deformation microstructure.

and KQ was calculated at each instance and reported as KIC after verifying that the result met the required criteria to be valid. Hour-glass-shaped specimens for SeN tests with dimensions in accordance with ASTM E466-76 were machined with their longitudinal axis in the forging plane and tested at room temperature (in air) and at 1200  C (in vacuum) in a tensionetension mode with R ¼ 0.1 (R ¼ smin/smax) and the resulting fracture surfaces were characterized. Compact tension specimens were used to obtain fatigue crack growth response (da/dN versus DK ) and these were machined with the plane of the specimen coincident with the forging plane. The dimensions of relevance for the compact tension specimen and the allowed dimensional tolerances and permitted inter-relationships between the various dimensions complied with ASTM specifications; fatigue pre-cracks were introduced in these specimens in a compressionecompression mode, and tests were conducted from 20  C to 1400  C in vacuum using an R ¼ 0.1 and frequencies varying from 5 to 10 Hz. Crack growth increments following a specified number of cycles (either 20,000 or 25,000 cycles) at a fixed value of DK were obtained by interrupting the test.

2. Experimental procedure 3. Results Powders of Moe2 wt.%Sie1 wt.% B alloy (Moe6Sie8B in at%) were produced by the plasma rotating electrode process (PREP) and consolidated by hot isostatic pressing (HIPing) in Nb cans at 1760  C using a pressure of 200 MPa. The resulting 120-mm-tall HIPed compact was then subjected to isothermal forging at 1760  C and reduced to a ‘‘pancake’’ with a final thickness of 20 mm and this material in the asforged condition was evaluated for mechanical properties. Cylindrical samples (5 mm diameter  10 mm tall) were electro-discharge machined from the forged compact, with the long axis of the sample parallel to the direction of forging. High-temperature compression tests were conducted at a constant displacement rate, in vacuum (1.33  104 Pa), in the temperature range of 1000e1400  C, and in the strain-rate regime of 104 to 107 s1. Plastic strain in each sample at the end of the test, as measured by change in specimen height, was in the range of 5e7%. A limited number of compression tests were also conducted at 1200  C and 1400  C on a similarly isothermally forged, three-phase MoeSieB alloy (Mo solid solution phase þ T2 þ Mo3Si) with a composition of Moe 3Sie1B (at%), and on a powder-processed off-the-shelf TZM plate for purposes of comparison. Microstructure of the samples was characterized before and after the hightemperature tests by optical, scanning (SEM) and transmission electron microscopy (TEM). Monotonic crack growth studies were undertaken using notched and compressionecompression fatigue pre-cracked three-point bend specimens (27 mm length  6 mm height  3 mm thick). These tests were conducted in air and in vacuum in the temperature range of 20e600  C. Tests at higher temperatures up to 1400  C were conducted only in vacuum, based on the results of isothermal oxidation studies. Loade displacement data were extracted from these experiments

3.1. Microstructure of the as-received material Optical metallograpy and SEM examination combined with X-ray diffraction confirmed that the isothermally forged Moe SieB alloy consisted of a matrix Mo solid solution phase in which was dispersed the T2 phase, as discrete particles for the most part, although local ‘‘spherical islands’’ of highly interconnected T2 phase regions representing a high volume fraction were occasionally observed (Fig. 1a). These local regions that are very rich in T2 content are thought to be a consequence of compositional inhomogeneity in the electrode used in the PREP process. A measurement of the area fraction of T2 phase in the regions where the phase is present as discrete particles provides a value of 0.38; this is in reasonable agreement with phase diagram predictions for this alloy composition. Electrolytic etching (10 g oxalic acid þ 100 ml water, 5 V, fraction of a second) reveals the grain size of the Mo solid solution matrix in the as-forged condition (Fig. 1b) and for the most part, but not always, grain boundaries are pinned by the T2 particles. 3.2. High-temperature compression response At 800  C, the compressive flow behavior of the MoeSieB alloy was only marginally affected by a change in the nominal strain rate from 104 s1 to 107 s1 (Fig. 2a). The flow stress at 4% strain remains very high (of the order of 1050e 1100 MPa) and is comparable to the 0.2% offset yield stress at room temperature (1280 MPa). It is thus reasonable to assume that quasi-static strain rate (104 s1 to 107 s1) has little effect on the material response between 20  C and 800  C. In contrast to the response observed at 800  C, the response

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From the compression test data for the two-phase MoeSie B alloy at different temperatures (1000  C, 1200  C, 1400  C and 1550  C) and strain rates (104 s1 to 107 s1), and recalling that the flow stress at 4% strain corresponds to a steady-state stress, strain rates were plotted as a function of inverse temperature (Fig. 2d) for two narrow stress ranges (e.g. 130e170 MPa and 425e460 MPa). The slope of the straight line fit to the data yields the activation energy (Q) for the deformation process and in the two instances, values obtained for Q were 415 and 445 kJ/mol which compare reasonably well with the activation energy for self-diffusion of Mo (400 kJ/mol e from Ref. [28]). This finding is in agreement with the previous observation that deformation is matrix-controlled. 3.3. Crack growth response to monotonic loading The variation in fracture toughness with temperature is shown in Fig. 3. The room-temperature toughness of the asforged, two-phase MoeSieB alloy is about 8 MPaOm and increases marginally to w9 MPaOm following extended annealing of the as-forged material (1600  C/48 h). With increasing temperature, the toughness increases gradually to 13 MPaOm at 600  C for tests conducted in air and in vacuum. Tests at higher temperatures were conducted in vacuum based on the results of isothermal oxidation studies that confirmed extensive material loss above 600  C in air. The steepest rise in toughness is noted in the 1200e1400  C regime, where it increases from w18 MPaOm to about 25 MPaOm. 3.4. Fatigue life (SeN response)

Fig. 1. Optical micrographs showing (a) the distribution of the T2 particles in the Mo solid solution matrix in the isothermally forged condition with occasional clusters of T2 particles (see inset), and (b) grain boundaries in the solid solution matrix are revealed by electrolytic etching confirming most T2 particles are located at grain boundaries.

from 1000  C to 1400  C regime is strongly strain-rate dependent; thus, for example, at 1000  C, the flow stress drops from about 1100 MPa at the fastest strain rate to about 400 MPa at the slowest rate. An examination of compressive stressestrain data at 1000  C in Fig. 2b confirms that this large decrease in flow stress is due to both, a lowering of the yield stress of the material at the slowest rate as well as a significant decrease in the hardening response during plastic deformation. The variation of the flow stress at 4% strain with strain rate at each of the three test temperatures could be fitted to a power-law and the results are shown in Fig. 2c. At 1000  C and 1200  C, the fit yielded a stress exponent of w7 for all four strain rates examined. At 1400  C, in the regime spanning strain rates from 104 s1 to 106 s1, the exponent obtained was 5.2 whereas, between 106 s1 and 107 s1, the strainrate dependence was significantly higher, yielding an exponent of 2.5, indicating a possible transition in the underlying deformation mechanism(s).

The SeN response of the two-phase MoeSieB alloy was assessed at room temperature and at 1200  C. The results are shown in Fig. 4. The SeN response at room temperature is shallow although the failure stress is high (565e 500 MPa); this behavior is expected for brittle materials showing little fatigue susceptibility. A single initiation site was not evident and fracture propagated through a mix of intergranular and transgranular cleavage modes. In regions where intergranular fracture is observed, the grain boundary facets are of the order of 10 mm or less and equiaxed, suggestive of partial recrystallization. These observations are consistent with those made for the monotonically loaded bend specimens. At 1200  C, the SeN response of the MoeSieB alloys is superior to that at room temperature, with an endurance limit (defined by survival of the specimen after 107 cycles) of w550 MPa. The material is more prone to fatigue failure at this temperature as evidenced by the steeper slope of the Se N curve relative to room temperature. This would be expected since the material is capable of plastic deformation at this temperature. Fracture surface examination of the MoeSieB specimens tested at 1200  C once again revealed a mix of intergranular failure and transgranular cleavage; the grains delineated by the grain boundary facets, however, did not appear as equiaxed as they did following the room-temperature test and this is suggestive of some plastic deformation prior

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to grain boundary separation at the higher test temperature of 1200  C relative to room temperature. 3.5. Crack growth response to cyclic loading (da/dN versus DK ) The da/dNeDK response for the MoeSieB alloy at R ¼ 0.1 and R ¼ 0.2 in air at room temperature is provided in Fig. 5a. There appears to be no visible difference in behavior and therefore all subsequent tests were only conducted at R ¼ 0.1. An examination of Fig. 5a reveals a threshold DK of a little over 5 MPaOm and a steep Paris slope of n ¼ 17; the behavior resembles that of a brittle ceramic. The combined effect of temperature (up to 600  C) and environment (air) on cyclic crack growth behavior of the MoeSieB alloy is seen in Fig. 5b. In the absence of an environmental effect, the crack growth response would be expected to either remain the same or improve with increasing temperature. The results in this temperature regime confirm some dependence of cyclic crack growth response on environment. Thus, the Paris slope increases from 17 at room

temperature to 19 at 300  C in air and to 23 at 600  C in air (Fig. 5c). When the test is conducted at 600  C in vacuum, the Paris slope returns to the value at room temperature of 17 (Fig. 5c). In contrast to the Paris slope, the threshold DK does not change (w5.4 MPaOm) in this test temperature range (20e600  C) for tests conducted in air or vacuum. Tests at higher temperatures (900  C, 1200  C, and 1400  C) were conducted in vacuum. The cyclic crack growth response for this regime is shown in Fig. 5d. The behavior from room temperature to 900  C may be grouped where a marginal increase in Paris slope occurs (from n ¼ 17 at room temperature to n ¼ 11 at 900  C e Fig. 5c), with the threshold DK remaining around 5e5.5 MPaOm. At 1200  C, however, there is a measurable improvement in the crack growth behavior with the Paris slope dropping to w6 and then further down to 3.5 at 1400  C; the threshold DK continues to remain between 5 MPaOm and 6 MPaOm. An examination of the fracture surface of the 1200  C compact tension specimen of the MoeSieB alloy reveals transgranular cleavage (Fig. 6a), and a careful examination of the cleavage facets

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at high magnifications reveals fatigue striations (Fig. 6b) that indicate local plasticity and fatigue failure susceptibility. These observations, suggesting enhanced local plasticity, are consistent with the increased fracture toughness of the Moe SieB alloy from w8 MPaOm at room temperature to w18 MPaOm at 1200  C. 4. Discussion The as-received microstructure in the two-phase Moe6Sie 8B (in at%) alloy consists of a dispersion of relatively coarse 800 R = 0.1

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T2 particles in a more-or-less continuous Mo solid solution matrix. The individual T2 particles are blocky in shape with sizes ranging from 10 to 20 mm and the volume fraction of the T2 phase in the alloy is w35e38 vol%. In the as-forged condition, the matrix contains a moderately high dislocation density and this is likely to influence the early stage flow behavior of the material, perhaps more at 1000  C than at 1400  C. While B in Mo is quite likely an interstitial solid solution strengthener, Si is thought to be a substitutional element; the atomic radii of Mo and Si, however, differ substantially (0.14 nm for Mo and 0.117 nm for Si) and this is at least in part responsible for the low solubility of Si in Mo. The large difference in atomic radius, however, will contribute in an appreciable way to substitutional solid solution hardening. Recent work by Jain et al. [29] confirms this to be the case. Jain et al. [29] evaluated a MoeSieB solid solution alloy in compression at 1000  C and 1200  C and compared its response to those of pure Mo and a two-phase MoeSieB alloy. The potent solid solution strengthening capacity of Si was confirmed and the role of the matrix in strengthening the two-phase alloy was elucidated. With respect to high-temperature crack growth behavior, the monotonically loaded specimens are exposed to significantly less time at temperature (usually in the order of 10e 20 min) than are the cyclically loaded specimens (several hours). The limited room-temperature toughness and its insensitivity to an anneal are attributed to the hardening of the Mo matrix by Si in solid solution (and to a lesser extent by B), to the presence of fairly coarse, angular T2 particles, and to the presence of a recrystallized (at least, partially) matrix microstructure. Although ultra-pure Mo is known to have a ductile-to-brittle transition temperature below room temperature, small levels of interstitials (particularly, N and O) adversely affect ductility (and toughness) and promote intergranular failure and being powder-processed products, these levels though not measured are anticipated to be relatively high and may also contribute to intergranular embrittlement. Minimal changes occur in matrix composition, microstructure or strength up to 1000  C and thus the lack of dramatic improvements in toughness is to be expected. Improvements in toughness at higher temperatures are thought to be a combined consequence of loss in matrix strength and dynamic recovery at the crack tip. Similar arguments may be advanced for cyclic crack growth response as a function of temperature. To understand the underlying mechanisms in cyclic crack growth, Arrhenius-type plots were generated for the two-phase MoeSieB alloy by plotting ln(da/dN ) versus reciprocal temperature in the relevant temperature regimes for specific values of DK in the Paris regime. The slope of such a curve provides apparent activation energy for the process. Details of these calculations are available in Ref. [27]. These results confirmed that in the 20e600  C range, the crack growth response is only minimally affected by air as compared to testing in vacuum. Likewise, apparent activation energies extracted using an Arrhenius-type relationship illustrate grain boundary diffusion dominance in the 900e1200  C regime and volume diffusion dominance in the 1200e1400  C regime [27].

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Fig. 5. Cyclic crack growth response of the two-phase MoeSieB alloy: (a) da/dNeDK in air at room temperature using R ¼ 0.1 and 0.2, (b) da/dNeDK in air at 20  C, 300  C and 600  C using R ¼ 0.1, (c) variation of Paris slope with test temperature in air and in vacuum, and (d) da/dNeDK in vacuum in the temperature range 900e1400  C using R ¼ 0.1.

5. Conclusions An overview of the mechanical properties of a two-phase MoeSieB alloy containing w38 vol% of the T2 phase embedded as discrete particles in a Mo solid solution matrix is presented with the material being in an isothermally forged condition. The main highlights are 1. High-temperature deformation of the MoeSieB alloy is matrix Mo solid solution controlled; the presence of blocky T2 particles is instrumental in providing physical obstruction to slip, thereby substantially increasing the flow stress of the ‘‘composite’’. Post-deformation examination confirmed dynamic recovery in the matrix, particularly at the slower strain rates. In contrast, the T2 phase in the two-phase alloy showed a range of behavior from brittle fracture through plastic deformation to simply elastic deformation. The specific response observed is dependent on test temperature and strain rate. 2. The fracture toughness of the MoeSieB alloy at room temperature is 8 MPaOm and this low toughness is

believed to arise due to intrinsically low matrix toughness and stress concentration from the angular, fairly large brittle T2 particles located at the grain boundaries. Toughness increases gradually with temperature to w17e18 MPaOm at 1200  C, but then sharply to 25 MPaOm at 1400  C. Associated with this increase is a transition in fracture mode with multiple cracking and higher incidences of transgranular cleavage being observed at 1200  C and 1400  C. 3. The SeN response obtained at room temperature for the MoeSieB alloy is typical of a material with a toughness of 8 MPaOm, whereas the SeN response at 1200  C was indicative of the fact that the material was prone to fatigue failure, showing a larger cycle dependency of failure stress; this is in line with a material capable of plastic deformation. The MoeSieB alloy is capable of enduring 550 MPa for 107 cycles. 4. Cyclic crack growth studies at room temperature illustrated a high threshold DK and a steep Paris slope for the MoeSieB alloy. In the 20e600  C regime, when tested in air the MoeSieB alloy did not display significant deterioration in crack growth response relative to tests in

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program monitor. Electron microscopy was performed using the Experimental Shared Facilities that are supported by the NSF-MRSEC on Micro- and Nano-Mechanics of Structural and Electronic Materials (NSF Grant DMR-9632524) at Brown University. References

Fig. 6. Fracture surface from the compact tension specimen tested in cyclic loading at 1200  C: (a) mixture of transgranular cleavage and intergranular fracture, and (b) fatigue striations on the transgranular facets confirming fatigue susceptibility.

vacuum. In the temperature interval 600e1400  C, where all tests were conducted in vacuum, the MoeSieB alloy showed a continuous decrease in the Paris slope with increasing temperature, reaching w6 at 1200  C, and 3.5 at 1400  C. Fatigue striations were clearly observed on the cleavage facets of the MoeSieB alloy tested at 1200  C.

Acknowledgments This effort was supported by the Office of Naval Research (contract no: N00014-00-1-0373) with Dr. S.G. Fishman as the

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