Precipitation of Mo 5 Si 3 in MoSi 2
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S.A. Maloy, S-Q. Xiao, and A.H. Heuer Case Western Reserve University, Cleveland, Ohio 44106
J. Garrett McMaster University, Hamilton, Ontario L8S 4M1, Canada (Received 10 August 1992; accepted 30 December 1992)
Mo 5 Si 3 precipitates in MoSi2 single crystals have been studied. The orientation relationship between the long rectangular lath-like precipitates and the matrix is [H0] MoSi2 ||[110] Mo5Si 3 and (002)MoSi2||(220)Mo5si3- Two sets of misfit dislocations extend along the long dimensions of the precipitates.
I. INTRODUCTION MoSi2 is being considered as a high temperature structural composite material because of its excellent oxidation resistance up to 1700 °C, its high temperature plasticity and high thermal conductivity, and its compatibility with reinforcements such as SiC and AI2O3.1 In addition to MoSi 2 , two intermediate compounds, M05S13 and Mo 3 Si, also exist in the Mo-Si system2; M05S13 is of interest here. Mo 5 Si 3 and MoSi2 form a eutectic at 1890 °C,3 and both have a body-centered tetragonal crystal structure, a — 0.9642 nm and c = 0.4910 nm for Mo 5 Si 3 (space group, 14/mem),4 and a = 0.3204 nm and c = 0.7848 nm for MoSi2 (74/mmm). 5 Si depletion from MoSi 2 can result in the formation of Mo 5 Si 3 precipitates, because MoSi2 is essentially a line compound; such precipitates could affect the mechanical properties of MoSi 2 . This paper presents a TEM study of such Mo 5 Si 3 precipitates in a MoSi2 single crystal grown from the melt by crystal pulling.
high purity (intrinsic) Ge x-ray energy dispersive spectrometer (EDS) detector and in a JEOL 4000EX high resolution electron microscope with point-to-point resolution of ~0.17 nm. III. PRECIPITATE MORPHOLOGY AND ORIENTATION RELATIONSHIP Precipitates were present in thin foils, and EDS analysis showed that they were higher in Mo and lower in Si than MoSi 2 . Analysis of the diffraction pattern of these precipitates identified them as Mo 5 Si 3 (see Figs. 1 and 2), suggesting that Mo 5 Si 3 precipitates formed due to the evaporation of Si from MoSi2 during crystal growth. Figure 1 and Fig. 2 are low magnification images showing these Mo 5 Si 3 precipitates viewed along (001) and (110), respectively. (Unless otherwise noted, all directions and planes refer to the MoSi2 matrix.) The accompanying selected area electron diffraction patterns (SADP's) include precipitate and matrix reflections and reveal the following orientation relationship:
II. PROCEDURE Arc-melted buttons of stoichiometric MoSi2 were prepared by mixing 99.49% Si (Johnson Matthey, Precision Metals Division, Braxton, Ontario, Canada, L6W 3M8) with high purity Mo (GTE Products, Towanda, PA 18848). The manufacturer's impurity analyses (in ppm) are as follows: Al-10, Ca-3, Cr-14, Cu-6, Fe-47, Mg-3, Mn-6, Na-7, Ni-38, P b - < 5 , Si-153, Sn-14, and K-23. Single crystals of MoSi2 were grown from these buttons by the Czochralski technique at a rate of 300 mm/h under ultrahigh purity argon further purified by flowing through a Ti getter. Crystals were oriented by the Laue ba
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