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

Abstract

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 (NbSS) and (Nb, Me)5 Si3 silicide (Me = Mo, W, or Ti), while large primary NbSS particles appeared besides the eutectic mixture as a result of replacing Nb by W. The directionally solidified samples consisted of coarse NbSS and (Nb,Me)5 Si3 silicides, and the microstructure showed a slight orientation in the direction of growth. The maximum compressive ductility (ɛmax) 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 (σ0.2) and the specific 0.2 pct yield compressive strength (σ0.2S) (σ0.2 divided by the density of sample) at elevated temperatures were markedly improved by the W addition. The directionally solidified samples always showed higher σ0.2 and σ0.2S 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 σ0.2 and σ0.2S values; even at 1770 K, σ0.2 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 \((\dot \varepsilon _m )\) of 1.4×10−7s−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 NbSS and silicide matrix.