Phase constitution and mechanical properties of carbides in the Ta-C system

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Publication Type thesis
School or College College of Engineering
Department Materials Science & Engineering
Author Hackett, Kenneth Robert
Title Phase constitution and mechanical properties of carbides in the Ta-C system
Date 2009-05
Description Carbides in the Ta-C system were prepared by hot-pressing mixed powders of TaC and Ta in C/Ta atomic ratios of TaCo.6, TaCo.7, TaCu.8, TaCo.9, and TaCi.o at 1800 °C for 2 h. Rietveld analysis was performed on all compositions, and it was found that TaCo.6 was a mixture of 21% hexagonal close packed (HCP) a-Ta2C and 79% rhombohedral ^ - T a ^ - x , TaCo.7 was a mixture of 44.6% rhombohedral ^ - T a ^ - x and 55.4% face centered cubic (FCC) TaCy , TaCo.s and TaCo.9 were approximately 100% FCC TaCy , and TaC 1.0 was approximately 100% TaC. The TaC and TaCy compositions, produced were single phase with C/Ta ratios essentially the same as the starting nominal compositions and densities exceeding 96.5% of theoretical. Grain size decreased with increasing carbon deficiency from 1.0 to 0.8, with a corresponding increase in Vickers hardness from 13.5 to 20.0 GPa. The flexural strength, however, decreased from 650 to 500 MPa due to a decrease in fracture toughness from 5.3 to 3.8 MPaVm. Fracture toughness measured by single-edge-precracked-beam (SEPB) technique was exceptionally high (12.7 ± 0.7 MPaVm) for the a-Ta2C and ^ - T a ^ . x composition with a high weight percent of the ^ - T a ^ . x phase (~83 wt %). This disconnect between the change in hardness and strength indicates that the toughened compositions may exhibit R-curve behavior. Fracture surfaces were highly faceted with multiple cleavage crack planes and steps for TaCo.6 and TaCo.7. It is possible that the cleavage planes correspond to the stacking faults between the FCC and the HCP stacking layers within the stacking sequence ABABCACABCBC of the ^ - T a ^ . x phase bridge cracks in the crack wake with accompanying variations in local carbon concentrations fluctuating between TaCy and a-Ta2C.
Type Text
Publisher University of Utah
Subject Carbides; Tantalum alloys
Dissertation Institution University of Utah
Dissertation Name MS
Language eng
Relation is Version of Digital reproduction of "Phase constitution and mechanical properties of carbides in the Ta-C system" J. Willard Marriott Library Special Collections, TA7.5 2009 .H33
Rights Management © Kenneth Robert Hackett
Format Medium application/pdf
Format Extent 14,676,696 bytes
Identifier us-etd2,105187
Source Original: University of Utah J. Willard Marriott Library Special Collections
Conversion Specifications Orginal scanned on Epson GT-30000 as 400 dpi to pdf using ABBYY FineReader 9.0 Professional Edition.
ARK ark:/87278/s63x8n87
Setname ir_etd
ID 193808
Reference URL https://collections.lib.utah.edu/ark:/87278/s63x8n87