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To improve the high-temperature strength of Nb-Mo-Ti-Si in-situ composites, alloying with W and a directional solidification technique were employed. The alloy composition of Nb-xMo-10Ti-18Si (x ϭ 10 or 20) was used as the base, and Nb was further replaced by 0, 5, 10 and 15 mol pct W. For samples without W, the as-cast microstructure was a eutectic mixture of fine Nb solid solution (Nb SS ) and (Nb, Me) 5 Si 3 silicide (Me ϭ Mo, W, or Ti), while large primary Nb SS particles appeared besides the eutectic mixture as a result of replacing Nb by W. The directionally solidified samples consisted of coarse Nb SS and (Nb,Me) 5 Si 3 silicides, and the microstructure showed a slight orientation in the direction of growth. The maximum compressive ductility ( max ) at room temperature decreased with increasing W content and was in the range of 0.8 to 2.3 pct, in contrast to the Vickers hardness (HV), which increased with W content. The 0.2 pct yield compressive strength ( 0.2 ) and the specific 0.2 pct yield compressive strength ( 0.2S ) ( 0.2 divided by the density of sample) at elevated temperatures were markedly improved by the W addition. The directionally solidified samples always showed higher 0.2 and 0.2S values than the as-cast samples. At elevated temperatures, the directionally solidified sample with 10 mol pct Mo and 15 mol pct W had the highest 0.2 and 0.2S values; even at 1770 K, 0.2 was as high as 650 MPa. The directionally solidified materials alloyed with W exhibited excellent compressive creep performance. The sample with 10 mol pct Mo and 15 mol pct W had a minimum creep rate ( m ) of 1.4 ϫ 10 Ϫ7 s Ϫ1 and retained steady creep deformation at 1670 K and an initial stress of 200 MPa. Under compression, the damage and failure of these in-situ composites were dominated by decohesion of interfaces between the Nb SS and silicide matrix.

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This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys-Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si-prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nb ss ) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nb ss and a eutectic of Nb ss /Nb 5 Si 3 (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa 1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa 1/2 . It is suggested that toughness is supplied by the metallic Nb ss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nb ss , intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nb ss /Nb 5 Si 3 microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nb ss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nb ss cleavage. 1m 1m