Ductile-to-Brittle Transition in MoSi 2
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DUCTILE-TO-BRITTLE TRANSITION IN MoSi 2 S. R. SRINIVASAN, R. B. SCHWARZ, and J. D. EMBURY Center for Materials Science, Mail Stop K-765 Los Alamos National Laboratory, Los Alamos, NM 87545, USA
ABSTRACT We have studied the mechanical behavior of two fine grained MoSi 2 alloys containing 0.61 at% (0.19 wt%) and 0.29 at% (0.09 wt%) oxygen, respectively. By preparing these alloys in almost identical fashion, their only difference was their oxygen content. The mechanical behavior was studied by four-point flexure tests in unnotched specimens between 8000C and 14000C. We interpret the mechanical behavior data in terms of Davidenkov diagrams which describe the dependence of the apparent ductile-to-brittle transition temperature on the SiO 2 content in the alloys. INTRODUCTION MoSi2 and MoSi 2-based composites have potential for high-temperature structural applications due to their excellent oxidation resistance and high melting temperature (20300C). In addition, MoSi 2 has a density of 6.24 g cm- 3, lower than that of Ni-based superalloys. Two main problems of MoSi 2 have slowed this development: a) lowtemperature brittleness and b) low strength at high temperatures (T > 12000C) [1,2]. The literature reports the ductile-to-brittle transition temperature (DBTT) of MoSi 2 ranging from 900 to 13000C [1,3,4,5]. In considering this data, attention must be given to the experimental method and the stress state used to determine the transition temperature. The intrinsic DBTT can be clearly defined for a single crystal deformed in simple shear, as the temperature at which the stress necessary to nucleate and glide dislocations becomes equal to the cleavage stress. For polycrystalline specimens, however, the comparison of plasticity and fracture is obscured by phenomena such as (a) a lack of a sufficient number of glide systems to accommodate polycrystalline plasticity (von Mises criterion), (b) dependence of flow and fracture stresses on grain size, (c) diffusional grain boundary sliding, and (d) the effect of intergranular phases on grain boundary sliding. In addition, tests are usually done in conditions other than pure shear (e.g., superimposed hydrostatic stress), which may further obfuscate the DBTr determination. Thus, the criterion for defining the DBTT from tests in polycrystals becomes highly subjective. In spite of these complications, various authors have defined the DBTe' in polycrystalline MoSi 2 from the observation of plastic deformation during four-point bend tests. Results from compression tests on single crystal MoSi 2 by Kimura et al. [6] show only 0.3 to 0.6% fracture strain at 13000C depending on the orientation of the specimen. The strain to fracture is about 2% at 14000C. Such a small fracture strain at 13000C, even in compression, suggests that ductility is very much limited and supports the contention in ref. 5 that the DBT" is closer to 13000C. However, similar experiments by Umakoshi et Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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