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Study of the effect of Al, Cr and Sn additions on the microstructure and properties of Nb silicide based alloys P Tsakiropoulos, K Zelenitsas and N Vellios Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, England, UK ABSTRACT The effects of Al, Cr and Sn on segregation, microstructure, phase stability and hardness of Nb-24Ti-18Si-5X (X = Al, Cr, Sn, at%) alloys were studied. The microstructure of the as cast alloys with Cr, Al and Sn respectively contained (Nb,Ti)ss, Nb3Si, αNb5Si3 and C14-NbCr2 Laves, (Nb,Ti)ss and βNb5Si3 and (Nb,Ti)ss, Nb3Sn and Nb5Si3. The microstructures of the heat treated alloys with Al and Cr (1500 oC/100 h) contained (Nb,Ti)ss and αNb5Si3 and the alloy with Sn (1200 oC/100 h) contained (Nb,Ti)ss, Nb3Sn and αNb5Si3. Compared with Al and Cr, alloying with Sn enhanced the stability of the as cast microstructure, caused strong macrosegregation of Si and Ti, suppressed the segregation of Ti in the (Nb,Ti)ss that was promoted by Al and Cr, had the strongest effect on the macrohardness of the cast and heat treated alloys and on the vol% of the Nbss. All three alloying additions promoted the transformation of βNb5Si3 to αNb5Si3 during heat treatment and decreased the hardness of Nb5Si3 in the as cast alloys with Sn having the strongest effect and Al the weakest. After the heat treatment the hardness of Nb5Si3 increased in the alloys containing Cr and Sn and decreased in the Al containing alloy with Cr having the strongest effect. INTRODUCTION In the most advanced gas turbine engines (GTEs) the turbine entry temperature (TET) exceeds the melting temperature of Ni based superalloys. Further improvements in the performance of GTEs to meet operational targets require even higher TET. Research on Nb silicide based alloys has demonstrated that materials based on the Nb-Si-Ti system with transition (Cr, Hf, Mo, Ta, W) and free electron metal (Al, Sn) and/or metalloid (B, Ge) additions and microstructures consisting of a bcc Nb based solid solution (Nbss) with different volume fractions of Nb5Si3 and Nb3Si silicides and C14 NbCr2 Laves phase can offer attractive mechanical properties [1]. The balance of properties has been achieved with solid solution strengthening the Nbss and changes of the properties of intermetallics via substitution of Nb and Si by transition metals and free electron metals and metalloids [2]. The production of Nb silicide based alloys poses many challenges. Control of microstructures can be difficult due to the formation of metastable phases [3, 4] and the macrosegregation of Si which is strongly affected by the alloy chemistry and can lead to significant variation in vol% of Nbss and Nb5Si3 in the cast alloy. The stability of the microstructures can also vary significantly depending on alloy chemistry and processing [5, 6]. The Nb silicide based alloys are susceptible to contamination by interstitial elements [7, 8] and their oxidation resistance depends critically on the vol% of Nbss. Alloy