Date of Award

2014-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Lawrence E. Murr

Second Advisor

S K. Varma

Abstract

The effect of adding Re to Nb-based alloys and is intended to analyze in depth the microstructures of Nb based alloys with Re, Si and Cr additions, in atomic percentages. The binary alloys (Nb-5Re, Nb-5Si and Nb-5Cr) reveal the formation of a single phase, NbSS, NbSS+ Nb3Si and NbSS+NbCr2 respectively. The formation of the single phase was confirmed by TEM studies for the Nb-5Re alloy. Addition of Re to form ternary alloys, has helped in the formation of Nb5Si3 and (Nb, Re) Cr2, in Nb-5Re- 5Si and Nb-5Re-5Cr respectively. Quaternary alloy Nb-5Re-5Si-5Cr has Nb5Si3, NbCr2 and NbSS. The oxidation behavior has been studied and the formation of the oxides has been characterized using XRD, SEM, EDS. Nb-Re-Si-Cr-X (Al, B, W) alloy system has been examined at temperatures between 700 and 1400°C in air. The continued work was to develop and discover a new materials system capable of replacing nickel based super alloys. Additions of aluminum were found to provide limited oxidation resistance. A discontinuous layer of Al2O3 and SiO2 was observed to form at all temperatures adapted for this study. Alloy containing aluminum additions were observed to suffer from pest oxidation at intermediate temperatures due to the development of Nb2O5. Poor oxidation resistance at intermediate temperatures for alloys with aluminum additions was attributed to a transformation in the structure of Nb2O5 formed. Pesting was observed at 900oC, consuming the metal completely. Additions of chromium were observed to increase oxidation resistance through the development of a layered oxide structure containing SiO2 and CrNbO4. Internal oxidation layer was observed to develop oxides in the midst of the phases formed. Boron addition has helped in the formation of the 3, 5 silicides, NbSS, and Laves phase. The combination of oxides of Nb2O5, CrNbO4 and SiO2 has helped improve the oxidation resistance of the alloy. Rhenium in this alloy has been a major element in terms of forming Re-oxides which has resulted in the negative weight gain. Alloys with this type of weight gain is advantageous as it refers to nullifying the formation of bulky Nb2O5. There was no metal left at 1400oC. Pesting was no observed at low temperature range. However, spalling was noticed at 1200 and 1300oC. Tungsten additions have helped in the formation of Nb5Si3, Cr2Re3, NbCr2, and NbSS. All the phases formed were intermetallics, except the solid solution. These high temperature phases have helped to resist oxidation to an extent. The formation of a CrNbO4 and SiO2 has helped in lowering the oxidation kinetics. No pesting was observed, spalling of the oxide was noticed only at 1300oC. Oxidation behavior of these alloys were characterized by the weight change per unit surface area method. Oxidation products were characterized by x-ray diffraction and scanning electron microscopy in several modes including backscatter imaging, secondary imaging, energy dispersive x-ray spectroscopy, and x-ray mapping.

Language

en

Provenance

Received from ProQuest

File Size

102 pages

File Format

application/pdf

Rights Holder

Ruth M. Sierra

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