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 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.

Nonisothermal and Isothermal Oxidation Behavior of Nb-Si-Mo Alloys

“…[7] through [9], and are presented in Table IV. The results in Table IV indicate the following: (a) the minimum activity of Si for the formation of SiO 2 is almost one order of magnitude less than that of Nb for the formation of Nb 2 O 5 , and (b) the minimum activity of Mo required for the formation of MoO 2 is significantly higher than those of Nb and Si for their respective oxides by several orders of magnitude.…”

“…The refractory Nb-Si-based alloys with high melting temperatures are of interest as candidate materials for aero-engine applications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] The microstructure of the Nb-Si-based alloys consists of hard and brittle intermetallic Nb 5 Si 3 and relatively ductile Nb-solid solution (Nb ss ) phases. The Nb 5 Si 3 exhibits excellent high-temperature strength retention and creep resistance, [1] but poor ductility and fracture toughness.…”

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

“…This effort is for balancing strength and toughness. Successful improvement in strength has been achieved as high as 600 MPa even at 1770 K by alloying, [9,31] frequently accompanied by thermomechanical processing such as directional solidification [7,8,9] and hot isostatic pressing (HIP). [3,4,5] Extensive studies show that the bending strength of the as-cast Nb-10Si alloy is 400 MPa at 1470 K and is 250 MPa for an extruded specimen.…”

“…Further alloying by Mo and W of the Nb-Si binary formed compositions of Nb-10Mo-10Ti-18Si and Nb-10Mo-15W-10Ti-18Si, which have an outstanding strength level. Even at 1770 K, the compressive yield strengths of the Nb-10Mo-10Ti-18Si and Nb-10Mo-15W-10Ti-18Si [7][8][9]31] that were prepared by arc melting and directional solidification are 550 and 650 MPa, respectively. The elements W and Mo are most efficient for improving high-temperature strength, which is mainly attributable to the solid-solution strengthening of the Nb ss by W and Mo.…”

“…By directional solidification and hot-extrusion treatment, for instance, the Nb-10Si alloy shows an increased fracture toughness from 9.6 MPa 1/2 of the as-cast case to 14 MPa 1/2 of the directionally solidified case, [18] then reaches 22 MPa 1/2 of the hot-extruded case. [4,5] Although a breakthrough in the high-temperature strength has been made in the Nb-Si-based in-situ composites, [7][8][9]31] the relationship between toughness, composition, and microstructure is still unknown. A systematic investigation of the properties of the Nb-Si alloys should be carried out prior to design based on the actual requirements of service, to understand the balance of fracture toughness and strength.…”