A study of the high temperature oxidation and microstructure of niobium silicide-based alloys with the additions of boron and germanium

Niobium silicide-based alloys are a potential candidate martial to supplant nickel-based super alloys for use in the most demanding parts of aircraft engines. This thesis describes research on the development of boron and germanium containing niobium silicide-based alloys. The combined effect of boron and germanium is assessed with focus on the oxidation resistance and oxide scales. Literature regarding phase equilibria, mechanical properties and oxidation is reviewed.
Four alloys were manufactured; a base alloy to facilitate comparison, KZ5 (Nb-18Si-24Ti-5Cr-5Al), and three novel boron and germanium containing alloys, JW1 (Nb-18Si-24Ti-5Cr-5Al-5Ge-5B), JW2 (Nb-18Si-24Ti-5Cr-5Al-5Ge-10B) and JW3 (Nb-18Si-24Ti-5Cr-5Al-10Ge-5B). As-cast and heat-treated (1300oC/1500oC) states of each alloy were characterised.
In the as-cast condition, solid solution (Nb) was present in all alloys. D88 (Nb5Si3Bx) and NbCr2 were stabilised in boron and germanium containing alloys. T2 (Nb5(SiB)3) was found in JW1 and in small amounts in JW2. NbB was found in JW2 and in small amounts in JW3. Heat-treatments resulted in the conversion of NbB to T2, the partial conversion βNb5Si3 to αNb5Si3 in KZ5, and the elimination of (Nb) in JW3. The amount of (Nb) was reduced by B and Ge additions. Macrosegregation of the high germanium alloy (JW3) was more severe, and macrosegregation of the high boron alloy (JW2) was less severe, when compared to JW1.
Boron and germanium additions are shown to improve oxidation resistance, with total mass gains decreased by up to 79% and 50% at 1000oC and 1200oC, respectively. At 1000oC, JW series alloys performed better than KZ5. Heat-treatment had a significant influence on oxidation. At 1200oC, KZ5 performed worse than JW alloys, where JW2 and JW3 oxidation resistance was generally equivalent, and superior to JW1. Ge-rich layers were observed, but no borosilicate glassy layers, believed to be due to competing phase stability. Spallation of oxide scale was eliminated by boron and germanium additions. This work provides valuable results to aid future alloy design with a balance of properties through boron and germanium additions.