DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN ULTRA-HIGH TEMPERATURE Nb-Si-Ti-Hf-Cr-Al ALLOY

The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), eutectic colonies of Nba, plus (Nb, Ti)3 Si/(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss+ (Nb, Ti)3 Si/(Nb, Ti)5 Si3 eutectic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.

Cooperative effects of Mo,V and Zr additions on the microstructure and properties of multi-elemental Nb-Si based alloys

Eight multi-elemental Nb-Si-based alloys with various Mo,V and Zr contents were prepared by vacuum non-consumable arc melting.The cooperative alloying effects of Mo,V and Zr on the arc-melted and heat-treated microstructure,mechanical properties as well as oxidation resistance at 1250°C of the alloys were evaluated systematically.The results show that except for adding Mo solely,additions of Mo,V and Zr change the microstructure from eutectic to hypereutectic.The additions of Mo,V and Zr suppress the formation ofα(Nb,X)5 Si 3(“X”represents the alloying elements that substitute for Nb in the lattices),whilst promoting the formation ofγ(Nb,X)5 Si 3.The heat treatment at 1450°C for 50 h promotes the formation of(Nb,X)3 Si phase in the Zr-containing alloys.Alloying with either Mo or Zr improves,and their composite additions more obviously improve the compressive yield strength at 1250°C as well as the microhardness ofγ(Nb,X)5 Si 3.The room temperature fracture toughness of the alloys is enhanced by sole and composite additions of V and Zr,while it is deteriorated by the addition of Mo.The sole addition of Mo,V or Zr improves the oxidation resistance at 1250°C,the composite additions of V with Mo/Zr(especially V-Mo-Zr)degrade the oxidation resistance at 1250°C.

Progress in Nb-Si ultra-high temperature structural materials:A review

Nb-Si based alloys have the greatest potential to replace nickel-based superalloys and become a new gen-eration ultra-high temperature structural material with service temperature of 1200-1400℃ due to their high melting point,high stiffness,low density,and attractive specific mechanical properties at elevated temperatures.This article reviewed the recent progress in the development of Nb-Si based refractory al-loys.Special attention was paid to the property characteristics,preparation techniques,alloying effects,oxidation resistance and fabrication of Nb-Si based alloy components.Thereafter,the trend of develop-ment in the future was forecasted,including:Optimization of the composition and microstructure of Nb-Si based alloys to achieve a balance of mechanical properties;Comprehensive properties evaluation for Nb-Si based alloys to accumulate sufficient data for engineering applications;Development of practical preparation technology for Nb-Si based alloy components with complex shapes;New multilayer compos-ite coatings for Nb-Si based alloy;Joining technology of Nb-Si alloy to itself and to superalloy.

Improvement and application of Y2O3 directional solidification crucible

In order to satisfy the drastic temperature change and high chemical activity in direc- tional solidification of Nb Si based alloys, Y2O3 crucible is demanded to possess high thermal shock resistance and erosion resistance. This paper improved the sintering degree and density of Y2O3 crucible by optimizing the sintering temperature and time, and its practical application per- formance was investigated. Y2O3 grains gathered with the increase of sintering temperature and time, and the contact area enlarged, resulting in the open pores being changed into closed pores. The higher density caused the improvement of erosion resistance of Y2O3 crucibles. However, exces- sive density weakened the thermal shock resistance. Considering high-temperature strength, erosion resistance, thermal shock resistance and costs, optimum sintering temperature and time of Y2O3 directional solidification crucible were 1800 ℃ and 120min, respectively, and the porosity was 20%. Improved Y2O3 crucible has been successfully applied to directional solidification of Nb Si based alloys, and significantly reduced the oxygen contamination. Slight interaction occurred between Hf and Y2O3, but no obvious dissolution, penetration or erosion was found, showing good erosion resistance and thermal shock resistance.