Date of Award

2009-01-01

Degree Name

Doctor of Philosophy

Department

Metallurgical and Materials Engineering

Advisor(s)

Shailendra K. Varma

Abstract

The use of refractory metals in high temperature applications has been attractive for several years. Systems based on niobium represent a potential new generation of refractory materials systems because of their low density, high melting points, and strength at elevated temperatures. Despite these desirable characteristics, the widespread use of niobium-based alloys is limited because its oxidation resistance is still inadequate for structural applications. Understanding the oxidation kinetics of Nb-based alloys and the role of the different phases in the oxidation process is of crucial importance to improve the oxidation resistance of these materials.

The effect of Cr content on the microstructure and oxidation behavior of Nb-10W-XCr alloys with four different compositions has been investigated. Experiments were conducted for 24 hours in air at 900, 1100, and 1300ºC under isothermal and cyclic conditions. The oxidation kinetics were evaluated in terms of weight change per unit area with respect to exposure time. The phases present in the alloys and the oxide scales were characterized by XRD, SEM and EDS.

Alloy's microstructure consists of Nb solid solution phase surrounded by a network of NbCr2 Laves phase. An improvement trend for the oxidation resistance with increase of the intermetallic phase has been observed at 1300ºC and oxidation kinetics follows a parabolic behavior. At 900ºC, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO4 and Nb2O5 and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.

Language

en

Provenance

Received from ProQuest

File Size

128 pages

File Format

application/pdf

Rights Holder

Maria del Pilar Moricca

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