Paper Title
EFFECTS OF CARBON CONTENTS ON MICRO STRUCTURE AND PROPERTIES OF Nb-Si BASED ALLOYS

Abstract
The development of modern aerospace technology requires the engines have a larger thrust-weight ratio, which requires high temperature structural materials with higher temperature bearing capability. Nb-Si based alloy with good high temperature performance has higher specific strength than other refractory metal silicides, but its toughness is relatively poor and it is easy to be oxidized at high temperatures. Adding alloying elements to the Nb-Si alloy system can fundamentally alleviate this problem. Nb-Si based ultrahigh temperature alloys have gradually developed to multi-elemental system alloys with more than six elements. So far, there have been no studies on the influence of adding carbon element to the microstructure and mechanical properties of multi-elemental Nb-Si based ultrahigh temperature alloys. Therefore, in order to optimize the alloy compositions, it is particularly important to systematically explore the influences of carbon contents on the microstructure, phase constituents and properties of multi-elemental Nb-Si based ultrahigh temperature alloys. In the present presentations, the Nb-Si based alloys adding element carbon were prepared by vacuum non-consumable arc melting. The microstructure, nano indentation micro hardness, room temperature fracture toughness, compressive strength at 1250℃ and oxidation resistance of the alloy were evaluated. The results show that adding carbon in multi-elemental Nb-Si based alloys can make the eutectic point of the alloy move to the direction of lower silicon content, and increase the hypereutectic degree. Alloying with carbon inhibit the formation of (Nb, X)3Si phase and promote the formation of γ(Nb,X)5Si3 phase, and precipitate (Ti,X)C in the alloys. The dissolved carbon(mainly in γ(Nb,X)5Si3) has the effect of solid solution strengthening and improve the nano indentation hardness of phases. Alloying with carbon increase the compressive strength at 1250 ℃ of the alloys. The high temperature compressive strength of 4C alloy is 31.8% higher than that of 0C alloy due to the solid solution strengthening, the increase of primary silicide contents and (Ti,X)C precipitation. The addition of carbon can improve the fracture toughness at room temperature, the KQ value of 3C alloy was 13.5 MPa•m1/2, which is the highest value among the six alloys. Alloying with carbon reduces the oxidation resistance of the alloy at 1250 ℃, and both oxide film thickness and porosity of the alloy increase. Keywords - Nb-Si Based Alloy; Carbon alloying; Phase constituents; Fracture Toughness; Compressive strength; High-Temperature Oxidation Resistance; Nano Indentation Hardness