Structure property relationships in molybdenum silicon(2)-molybdenum(5) silicon(3) eutectics.

Abstract

Directionally solidified MoSi$\sb{2}$-Mo$\sb{5}$Si$\sb{3}$ eutectic rods were produced by the Czochralski method. A script lamellar microstructure was produced and an inverse square root dependence upon the pull rate (39-210 mm/hr) was observed for the lamellar spacing (2.6 $\mu$m-1.09 $\mu$m). The lamellae were textured with (001) of Mo$\sb{5}$Si$\sb3$ and $\langle 110\rangle$ of MoSi$\sb2$ parallel to the rod axis and an orientation relationship consisting of (110) MoSi$\sb2//\lbrack 110\rbrack$Mo$\sb5$Si$\sb3$ and (110)MoSi$\sb2$//(002)Mo$\sb5$Si$\sb3$ was observed. The eutectic grows with a lamellar plate morphology inclined at 15$\sp\circ$ relative to the rod axis and periodic branching of the Mo$\sb5$Si$\sb3$ lamellae produced the script lamellar microstructure. Addition of 0.35 atomic percent erbium produced a more fibrous microstructure and an erbium-rich compound, Er$\sb2$Mo$\sb3$Si$\sb4$, formed congruently with the Mo$\sb5$Si$\sb3$-MoSi$\sb2$ during eutectic solidification. A ledge-terrace growth mechanism was proposed to explain the solidified microstructures. Room temperature hardness was found to be a function of microstructural scale and was consistent with the Hall-Petch relationship. This relationship was also found to describe the dependence of hardness on grain size of powder processed MoSi$\sb2$/Mo$\sb5$Si$\sb3$/44.5p composites and monolithic MoSi$\sb2$ data reported in the literature. A more tortuous crack path was observed as the spacing decreased. Consequently a modest increase in fracture toughness was recorded at the finest spacings (2.0 MPa $\sqrt{\rm m}$ at 6.56 $\mu$m to 3.0 MPa $\sqrt {\rm m}$ at 0.93 $\mu$m) compared to monolithic MoSi$\sb2$ (2.85 MPa $\sqrt {\rm m}$). The high temperature deformation behavior was investigated as a function of lamellar spacing over the temperature range 1100$\sp\circ$C-1400$\sp\circ$C and strain rates of 1 $\times$ 10$\sp{-4}$/s to 1 $\times$ 10$\sp{-6}$/s. The experimental creep rate decreased as spacing decreased for a given stress level. A constitutive model for creep which incorporates reinforcement spacing for creeping fibers in a creeping matrix was found to describe the creep behavior. This model proposes $\dot\varepsilon$ $\alpha$ $\lambda\sp{\rm m}$ with m = 1 consistent with that found experimentally. Creep deformation was found to be controlled by dislocation motion in the Mo$\sb5$Si$\sb3$ phase. Creep strength of (314) oriented Mo$\sb5$Si$\sb3$ single crystals was found to be superior to most orientations reported for single crystal MoSi$\sb2$. Deformation was controlled by dislocation motion as a stress exponent of $\approx$6 was recorded with an activation energy of 520 kJ/mol.