Mechanical properties and microtructure of sputter-deposited Nb 5 Si 3 /Nb microlaminates
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Crystalline Nb5Si3/Nb microlaminates were fabricated to a thickness of 20 /xm by depositing the materials onto elevated temperature (750 °C) substrates. Modulation wavelengths of the microlaminates were varied (A = 40 and 200 nm) while holding their silicide volume fraction constant to assess the effect of layer thickness on the composite properties. X-ray and selected area diffraction confirmed that both the metal and silicide layers exhibited a polycrystalline structure in the as-deposited microlaminates. Nanoindentation measurements of both microlaminates indicated that calculated elastic modulus values were similar to the values obtained by the rule-of-mixtures (ROM). Nanohardness values of the microlaminates increased with decreasing wavelength in a manner described by the Hall-Petch relationship. Vickers hardness (Hv) measurements were also found to be a function of the modulation wavelength, decreasing from 7.32 GPa at A = 40 nm to 3.04 GPa at A = 200 nm. Even with a Nb volume fraction of 50%, the A = 40 nm microlaminate and the monolithic Nb 5 Si 3 film exhibited similar Vickers hardness values of 7.5 GPa. These results show the significant role of modulation wavelength on the hardness, compressive strength, and toughness characteristics of a microlaminate composite.
I. INTRODUCTION Refractory metal silicides such as MoSi2 and Nb5Si3 are being developed to replace nickel and cobalt superalloys and to potentially satisfy stringent performance requirements of turbine engines and hypersonic vehicles at temperatures in excess of 1000 °C under oxidizing conditions.1"17 While these intermetallic compounds exhibit a good combination of high temperature strength and creep resistance between 1000 and 1600 °C, they are inherently brittle at low temperatures. Several microstructural designs have been investigated to improve the toughness while maintaining the high-temperature strength characteristics. Three key design approaches include (i) reinforcing intermetallic matrix composites18"21 with fibers or whiskers (e.g., SCS-6/MoSi2 and Al 2 O 3 /MoSi 2 ) (ii) alloying additions to introduce toughening constituents (e.g., W-MoSi 222 and two-phase Nb-Nb 5 Si 323 ' 24 material alloy), and (iii) multilayer composite with alternating layers of strengthening and toughening materials (e.g., Nb5Si3 and Nb layers). Of these microstructural designs, multilayer composites potentially offer an architecture that can be tailored to obtain a desired combination of hightemperature properties by optimizing layer thickness for a selection of materials. This paper presents a Nb 5 Si 3 /Nb microlaminate system and evaluates the effect of layer thickness on the mechanical properties of the composite. Metal/intermetallic (or ceramic) microlaminates have been processed using vacuum hot pressing of J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
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foils, physical vapor deposition techniques, as well as casting and solid-state reaction methods. For example, the Nb 5 Si 3 /Nb laminates have been f
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