Abstract
Nb-Si-Cr-Al-Mo base in situ composites with nominal compositions Nb-18Si-5Al-5Cr-5Mo (JG1), Nb-24Ti-18Si-5Al-5Cr-5Mo (JG2), Nb-24Ti-18Si-5Al-5Cr-2Mo (JG3), Nb-24Ti-18Si-5Al- 5Cr-2Mo-5Hf (JG4) and Nb-24Ti-18Si-5Al-5Cr-2Mo-5Hf-5Sn (JG6) were prepared by arc-melting followed by heat treatment. The effects of Mo, Ti, Hf and Sn additions on phase selection, phase transformations, microstructure and oxidation behaviour in the as cast and heat-treated in situ composites were investigated using XRD, SEM, and EPMA. All the alloys are classified as hyper-eutectic alloys since the betaNb5Si3 phase was formed in all the as cast microstructures. After heat treatment, the betaNb5Si3 partly transformed to the alphaNb5Si3 in the presence of Ti. The addition of Ti stabilised the betaNb5Si3 to lower temperatures and increased the solubilities of A1 and Cr in the Nb88. The lattice parameter of the Nb88 increased after heat treatment. This is attributed to the redistribution of solutes between the Nbss and the intermetallic phases and in particular the reduction in Si content. The NbaSi phase was formed in the as cast Hf-free alloys (JGl, JG2 and JG3). However, this phase was unstable at 1500°C and decomposed by a eutectoid transformation to Nb88 and alphaNb5Si3. The eutectoid decomposition was enhanced considerably by the Ti addition. The formation of Nb3Si was suppressed by the Hf addition. In the presence of Ti and Hf, there were Ti and Hf segregations in the Nb5Si3 phase. The Ti solubility in the Hf-rich Nb5Si3 was higher than in the Nb5Si3. The structure of the Nb5Si3 phase was mainly affected by the Hf addition, which prompted the formation of the alphaNb5Si3. The Hf-rich regions in the 5-3 silicide probably corresponded to the alphaNb5Si3, which was stable up to 1500° C in the presence of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb5Si3 and Nbss, respectively. The addition of Sn had a significant effect on the Nbss, essentially leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in the Nbss increased considerably whilst the Cr solubility decreased. The Sn addition promoted the formation of the Si-rich 014 Laves phase and stabilised this phase at 1300°C. The solubility range of Si in the C14 Laves phase formed in alloys of this study was ~ 6.6 to 10.5at.%. For the Ti containing alloys, titanium nitride formed near the surface of the specimen during heat treatment. Alloy Nb-18Si-15Cr (JG5) was studied to clarify the disputed three-phase fields (Nbss-1-0144- alphaNb5Si3 or Nbss-CrNbSi-l-alphaNb5Si3) near the Nb corner. The C14 Laves formed in both the as cast and heat treated samples. The results confirmed that the C14 Laves phase in the binary Nb-Cr system was stabilised by Si to lower temperatures.