On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions

Abstract

In this paper two Nb-silicide-based alloys with nominal compositions (at.%) Nb-12Ti-18Si-6Ta-2.5W-1Hf-2Sn-2Ge (JZ1) and Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (JZ2) were studied. The alloys were designed using the alloy design methodology NICE to meet specific research objectives. The cast microstructures of both alloys were sensitive to solidification conditions. There was macro-segregation of Si in JZ1 and JZ2. In both alloys the βNb5Si3 was the primary phase and the Nbss was stable. The A15-Nb3X (X = Ge,Si,Sn) was stable only in JZ2. The Nbss+βNb5Si3 eutectic in both alloys was not stable as was the Nb3Si silicide that formed only in JZ1. At 800 °C both alloys followed linear oxidation kinetics and were vulnerable to pesting. At 1200 °C both alloys exhibited parabolic oxidation kinetics in the early stages and linear kinetics at longer times. The adhesion of the scale that formed on JZ2 at 1200 °C and consisted of Nb and Ti-rich oxides, silica and HfO2 was better than that of JZ1. The microstructure of JZ2 was contaminated by oxygen to a depth of about 200 μm. There was no Ge or Sn present in the scale. The substrate below the scale was richer in Ge and Sn where the NbGe2, Nb5(Si1-xGex)3, W-rich Nb5(Si1-xGex)3, and A15-Nb3X compounds (X = Ge,Si,Sn) were formed in JZ2. The better oxidation behavior of JZ2 compared with JZ1 correlated well with the decrease in VEC and increase in δ parameter values, in agreement with NICE. For both alloys the experimental data for Si macrosegregation, vol.% Nbss, chemical composition of Nbss and Nb5Si3, and weight gains at 800 and 1200 °C was compared with the calculations (predictions) of NICE. The agreement was very good. The calculated creep rates of both alloys at 1200 °C and 170 MPa were lower than that of the Ni-based superalloy CMSX-4 for the same conditions but higher than 10−7 s−1.

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-5Al-5Cr-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions

“…Similarly, with the research described in [ 14 ], we found it very difficult to make an alloy with composition as close as possible to the nominal one, owing to loss by evaporation of elements with low melting points. In the same way as in [ 14 ], we made up for the elemental losses by slightly increasing the weight of lost elements. After many attempts, the “best” two alloys were selected for further study (see Section 2.2.2 ).…”

“…The microstructure of the bottom of the button was finer ( Figure 5 c) where the (Nb,W) ss and HfO 2 exhibited white contrast. The solubility of Ta in the (Nb,W) ss was high (15.2 at.%) and the Si content was low (1.7 at.%), see Table S3 , compared with the Nb ss in the as cast alloys JZ1 and JZ2 [ 14 ] and JZ3-AC.…”

“…We were interested in Nb-silicide-based alloys of the Nb-Ti-Si-Ta-W-Hf-Ge-Sn-Al-Cr system and used the alloy design methodology NICE [ 4 ] to design/select alloy(s). The reasons for the inclusion of the aforementioned elements in the alloys to be studied were outlined in the Section 1 and Section 2.1 and were discussed in [ 4 , 5 , 13 , 14 , 19 , 26 ]. We expected difficulties in making alloys with composition close to nominal (see Section 2.2.2 ), owing to us using arc melting and alloying elements with a very wide range of melting temperatures (T m Sn = 232 °C, T m Al = 660 °C, T m Ge = 937 °C, T m Si = 1412 °C, T m Ti = 1667 °C, T m Cr = 1857 °C, T m Hf+ = 2227 °C, T m Nb = 2467 °C, T m Ta = 2980 °C and T m W = 3400 °C) and our experience from the research described in [ 14 ].…”

“…Alloy selection was assisted from data about how the alloying additions of Al, Cr, Ge and Sn affect the macrosegregation of Si in RM free alloys (Tables 4–6 in [ 13 ]). The creep target was the same as that used for the design of the two alloys that were studied in [ 14 ]. The oxidation target was considered realistic for Nb-silicide-based alloys (see [ 5 ] and the Section 2.4 in [ 4 ]) and was set bearing in mind the results about the oxidation of the alloy JZ2 [ 14 ].…”

“…The alloy design methodology N iobium I ntermetallic C omposite E laboration (NICE) [ 4 ], which has been developed in our research group, addresses (a) to (f) and was used in the research described in this paper (NICE does not consider fracture toughness, but it can predict (calculate) the vol.% Nb ss , which is important for the toughness of Nb-silicide-based alloys). This research was motivated by four key findings, namely (i) the superior creep of Nb-silicide-based alloys with RM additions compared with Ni-based superalloys [ 5 , 6 , 7 ], (ii) the elimination of pest oxidation when Sn or Ge are in synergy with Al and Cr additions [ 4 , 7 , 8 , 9 , 10 , 11 , 12 ], (iii) the suppression of pest oxidation at intermediate temperature with the simultaneous elimination of scale spallation at a high temperature when Al, Cr, Ge and Sn were all simultaneously present in a Nb-silicide-based alloy without RM additions [ 13 ] and (iv) the improved adhesion of the scale that formed at 1200 °C on the alloy Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (alloy JZ2 in [ 14 ]).…”