Mechanical properties of As-cast and directionally solidified Nb-MoW-Ti-Si in-situ composites at high temperatures

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 = 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 = 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 (epsilon(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 (sigma(0.2)) and the specific 0.2 pct yield compressive strength (sigma(0.2S)) (sigma(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 sigma(0.2) and sigma(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 sigma(0.2) and sigma(0.2S) values; even at 1770 K, sigma(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 (epsilon(m)) of 1.4 X 10(-7)s(-1) 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.