Microstructural Evolution and Mechanical Properties of a Multicomponent Nb-16Si-22Ti-2Al-2Hf-2Cr Alloy Prepared by React
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INTRODUCTION
THE NbSS/Nb5Si3 in-situ composites have great potential to replace nickel-based superalloys in the hottest section of future aircraft engines due to their high melting point 2793 K (2520 C), high stiffness, low density, and attractive specific mechanical properties at elevated temperatures.[1–7] With respect to the toughness, which is also an important parameter for ultrahigh temperature structural materials,[8,9] the incorporation of the ductile and well-deformable NbSS phase within the stiff and creep-resistant Nb5Si3 intermetallic compound matrix can help balance the mechanical properties of NbSS/Nb5Si3 hybrid composite materials, making them strong yet ductile at elevated temperatures. In addition, the combined manipulation of microstructure engineering,[10,11] alloying,[12–15] and thermal mechanical processing[16–19] has been frequently used to balance the creep strength, ambient temperature fracture toughness, and oxidation resistance of NbSS/Nb5Si3 in-situ composites. The NbSS/Nb5Si3 in-situ composites have been successfully fabricated by vacuum arc melting,[8,9] direcW. LIU, Ph.D. Student, Y.M. FU, Master Degree Student, and J.B. SHA, Professor, are with the School of Materials Science and Engineering, Beihang University, Xueyuan Road No.37, Beijing 100191, P.R. China. Contact e-mail: [email protected] Manuscript submitted April 18, 2012. Article published online December 14, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
tional solidification,[10,20] hot extrusion,[21] and powder metallurgy technology.[11,12] Outstanding high temperature strength, room temperature ductility, and toughness have each been achieved. The binary Nb-10Si in the as-cast condition has a fracture toughness of 9.6 MPa m1/2, which increases to 14 MPa m1/2 and 20 to 22 MPa m1/2 when prepared by directional solidification casting and hot extrusion, respectively.[22] In comparison with the as-sintered Nb-10Si-2Fe alloy, an improvement in the fracture toughness by approximately 90 pct was obtained in the powder sintered, hotextruded, and heat-treated sample.[21] Nb-10Ti-18Si alloyed with 10 at. pct Mo and 15 at. pct W and prepared by directional solidification has a yield strength greater than 650 MPa even at 1773 K (1500 C),[20] which to the authors’ knowledge is probably the highest strength of the Nb-Si-based alloys that has been reported so far, while the strength of an arcmelted comparable alloy is approximately 350 MPa.[20] Recently, some research studies have elucidated the influence of high energy ball milling on phase transformation in Nb-Si-based alloys.[12,13] The supersaturated Nb(Si) solid solution could be formed at low ball-topowder weight ratio and short milling times, where increasing the weight ratio or milling time resulted in the formation of a nanocrystalline and amorphous mixture. The effect of powder metallurgical parameters (milling time, sintering temperature, etc.) on the microstructure and mechanical properties of Nb-Si binary VOLUME 44A, MAY 2013—2319
and Nb-Si-Ti(Fe) ternary alloys has also been i
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