College Park, Maryland      June 6 - 10 , 2004

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WP53: Annealing induced structural changes and microcracking in Mo-Mo₃Si

X.-L. Wang (Spallation Neutron Source, Oak Ridge National Laboratory; Metals and Ceramics Division, Oak Ridge National Laboratory), J. H. Schneibel (Metals and Ceramics Division, Oak Ridge National Laboratory), Y. D. Wang, A. D. Stoica (Spallation Neutron Source, Oak Ridge National Laboratory), J. W. Richardson (Intense Pulsed Neutron Source, Argonne National Lab)

An outstanding issue in the processing of Mo-Mo₃Si intermetallic composites is that microcracks develop after annealing at high temperatures [1], which significantly degrade the mechanical properties of the composite materials. The formation of microcracks is difficult to understand from the view point of differential thermal stresses that develop during cooling, which were estimated to be ~ 100 MPa for both phases. It is unlikely such a level of residual stress would cause microcracking at any stage during cooling. In order to determine the nature of microcracking in Mo-Mo₃Si, we have conducted a systematic study of Mo-Mo₃Si composites using a combination of in-situ neutron diffraction, composition analysis, and scanning electron microscopy.

In-situ neutron diffraction measurements at 1300ºC revealed a significant increase of the lattice parameter in the α-Mo phase as a function of annealing time, whereas the lattice parameter of the Mo₃Si phase shows a corresponding decrease. At the same time, the diffraction peak widths of both phases increased, unexpectedly, with increasing annealing time, giving evidence that plastic deformation occurred at high temperature. X-ray and neutron diffraction measurements made before and after annealing confirmed that the changes in lattice parameters are irreversible, i.e., due to plastic deformation. Because the as-cast materials were obtained off-equilibrium by fast cooling, the α-Mo phase is known to be super-saturated with Si. The in-situ neutron diffraction data therefore suggest that high-temperature plastic deformation during annealing was due to diffusion of Si atoms from α-Mo to Mo₃Si. This finding was corroborated by microscopy studies which demonstrated that the microcracks almost always started at the interface of α-Mo and Mo₃Si grains and grew into Mo₃Si. Furthermore, crack density measurements via microscopy indicated that as the annealing time increased more cracks were formed. Based on these experimental observations, it is clear that mcirocracking in Mo₃Si occurred at high temperature. Quite possibly, the diffusion of Si during high-temperature annealing created a highly inhomogeneous microstructure at the interface where cracks were initiated when the elastic energy exceeded the tensile limit of the Mo₃Si phase.

This research was supported by the Division of Materials Science and Engineering, U.S. Department of Energy. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy.

1. J. H. Schneibel, C. T. Liu, D. S. Easton, and C. A. Carmichael, "Microstructure and mechanical properties of Mo-Mo3Si-Mo5SiB2 silicides," Mater. Sci. Eng. A261, 78-83 (1999).

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