High temperature oxidation behavior of the Nb-W-Cr system and response of boron additions
Niobium based super alloys are necessary to fill the rising need in the aerospace industry for materials that can perform for extended times extreme environments at elevated temperatures near the melting point of nickel. Unfortunately niobium has very low oxidation resistance at elevated temperatures based on several studies that have been undertaken to remedy this problem resulting in several complex alloys that have yet to offer performance near that of nickel super alloys. In this study the oxidation behavior of several compositions including a single phase and a two phase alloy containing the NbCr laves phase of the Nb-Cr-W system will be evaluated as well as the system's response to boron additions. Oxidation samples were subjected to a short term oxidation (STO) treatment of 24hrs at temperatures ranging from 700 to 1400°C. A long term cyclic oxidation (LTO) study was also carried out to 168hrs (one week) in 24hr intervals at temperatures ranging from 1200 to 1400°C. Another set of samples were subjected to a heat treatment in an argon inert atmosphere for 24hrs at 1400°C. Oxidation products were characterized using scanning electron microscope (SEM), back scatter electron imaging (BSE), x-ray diffraction (XRD), and x-ray photo spectroscopy (XPS) analyses techniques. SEM and BSE imaging techniques revealed the presence of chromium rich and tungsten depleted particles dispersed in the bulky solid oxidation products. XPS and XRD techniques revealed four oxides present in all oxidation products: Nb2O 5, Cr2O3, WO3, and in most cases NbCrO 4. Long term oxidation data was found to indicate that the addition of boron was detrimental to the oxidation resistance of the alloys over long periods of time. The single phase alloys with a boron addition were found to have undergone chromium segregation in response to the 0.1wt% boron addition after the argon inert atmosphere heat treatment. This study reports the oxidation behavior of various alloys from the Nb-Cr-W alloy system and confirms the chromium segregation in the single phase alloys in response to boron additions. ^
Portillo, Benedict I, "High temperature oxidation behavior of the Nb-W-Cr system and response of boron additions" (2008). ETD Collection for University of Texas, El Paso. AAI1453826.