Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Abstract

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the aNb 5 Si 3 and bNb 5 Si 3 is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb-24Ti-18Si-5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 8C) alloys were the niobium solid solution, (Nb,Ti) ss , the niobium 5-3 silicides, aNb 5 Si 3 and bNb 5 Si 3 , and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of bNb 5 Si 3 , while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the bNb 5 Si 3 phase transformed to aNb 5 Si 3 according to the reaction bNb 5 Si 3 /aNb 5 Si 3 C(Nb,Ti) ss . The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 8C in the two Cr-rich alloys. Ta and Cr retard the bNb 5 Si 3 /aNb 5 Si 3 C(Nb,Ti) ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti) ss , aNb 5 Si 3 and the C14 silicide Laves phase, in the Nb-24Ti-18Si-6Ta-8Cr-4Al alloy. q

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti, Hf and Sn

“…Our density results are in agreement with those reported in Refs. [7,8,23], where the densities of Nb silicide base in situ composites were reported to be in the range 6.6e7.2 g/cm 3 . Table 1 gives the actual chemical composition of each alloy studied in this work, which was determined by EDX, as well as the constituent phases present in the as cast and heat treated alloys, which were identified by XRD.…”

Ab initio investigation of the Nb–Al system

“…With the exception of Hf and Ti most other alloying additions increase the DBTT of Nb [16]. Furthermore, even though the maximum solid solubility of Si in Nb is low, the actual Si content in the latter depends on alloy composition and Nb ss with/out Si content can form in the microstructure [5,8,17]. Thus, in Nb silicide based alloys it is possible to have (Nb, TM, RM, simple metal, metalloid) solid solution, (Nb ss ), where TM is transition metal (e.g., Cr, Hf, Ti), RM is refractory metal (e.g., Mo, Ta, W), and the simple metal(s) and metalloid(s) respectively can be Al, Sn and B and Ge [3][4][5][6][8][9][10][11][12][13][14].…”

Ab Initio Study of Binary and Ternary Nb3(X,Y) A15 Intermetallic Phases (X,Y = Al, Ge, Si, Sn)

“…These elements suppress the formation of the tetragonal (stable) Nb 3 Si [10][11][12] and Sn plays a key role in the elimination of pest oxidation [5,9]. The effectiveness of Sn depends on its concentration and its synergy with other elements in the alloy and on whether the A15-Nb 3 Sn is stable in the microstructure.…”