Mehmet Uz

Thermotransport of carbon in two-phase V-C and Nb-C alloys

Thermotransport experiments were performed on two-phase V-C and Nb-C alloys for various times, temperature ranges, and compositions. A one-phase region develops in the hotter portion of the sample accompanied by an increase in carbide concentration in the colder region. A mathematical equation in which the direction of transport is described by the sum of the heat of solution,A―H, and the heat of transport,Q*, was fitted to the experimental data for all of the alloys studied. No evidence was found to support the physical motion of the carbide particles predicted by some models. On the assumption that local equilibrium exists between the particles and the matrix, points on the V-C solvus were determined from the temperature corresponding to the one-phase/two-phase interface in the different experiments. The solid solubility for carbon in vanadium was determined to be log C (at. pct C) = 2.918 - 4536/T with a heat of solution of 86.8 ± 2.9 kJ/mol.

Processing and microstructure of Nb‐1% Zr‐0.1% C alloy sheet

A systematic study was carried out to evaluate the effects of processing on the microstructure of a Nb‐1 wt. % Zr‐0.1 wt. % C alloy sheet. The samples were fabricated by cold rolling different sheet bars that were single‐, double‐ or triple‐extruded at 1900 K. Heat treatment consisted of one‐ or two‐step annealing of different samples at temperatures ranging from 1350 K to 1850 K. The assessment of the effects of processing on microstructure involved characterization of the precipitates including the type, crystal structure, chemistry and distribution within the material as well as an examination of the grain structure. A combination of various analytical and metallographic techniques were used on both the sheet samples and the residue extracted from them. The results show that the relatively coarse orthorhombic Nb2C carbides in the as‐rolled samples transformed to rather fine cubic monocarbides of Nb and Zr with varying Zr/Nb ratios upon subsequent heat treatment. The relative amount of the cubic carbide...

Thermal stability of the microstructure of an aged Nb‐Zr‐C alloy

The effects of thermally aging with and without an applied stress on the microstructure of a Nb‐Zr‐C alloy containing 0.9 wt % Zr and 0.06 wt % C were studied. Chemical analysis, metallographic examination, energy dispersive x‐ray spectra of the bulk material, and chemical and x‐ray analyses of the phase‐extracted residue were used to characterize the microstructure. The samples examined were from a creep strength study involving hot and cold working, and various combinations of exposure to temperatures ranging from 1350 to 1755 K with and without applied load for times as long as 34 000 plus hours. The results showed that the initial microstructure consisted primarily of orthorhombic precipitates of Nb2C which were partially or completely transformed to face‐centered cubic carbides of Nb and Zr, (Zr, Nb)C, upon prolonged exposure to elevated temperatures. Furthermore, it was found that the microstructure of the alloy is extremely stable owing to the very finely distributed precipitates throughout its mat...

Characterization of the Microstructure of Nb‐lwt.%Zr‐0.1wt.%C Tubes as Affected by Thermomechanical Processing

Microstructure of Nb‐lZr‐0.1C tubes were characterized as affected by extrusion temperature of the tube shell and its thermomechanical processing to tubing. Two tube shells of about 40‐mm outside diameter (OD) and 25‐mm inside diameter (ID) were extruded 8:1 from a vacuum arc‐melted ingot at 1900 and 1550 K. Two different OD tubes of ∼0.36‐mm wall thickness were fabricated from each tube shell by a series of 26 cold drawing operations with two in‐process anneals. The microstructure of tube shells and the tubing before and after a 2‐step heat treatment were characterized. Residue extracted chemically from each sample was also analyzed to identify the precipitates. The results concerning the effect of the initial extrusion temperature and subsequent processing on the microstructure of the tubes are presented together with a review of results from similar work on Nb‐lZr‐0.1C sheet stock.

Change in direction of carbon thermotransport in Nb-V system with alloying

Thermotransport of carbon in a complete series of Nb-V alloys was studied using a radioactive tracing technique. The heat of transport Q* of carbon in Nb and V is negative; i.e., thermotransport is toward the hotter regions. Marked changes in the value of Q* result from alloying of the two metals, however, and an unusual reversal in its sign, i.e., a reversal in thermotransport direction, occurs at ∼1.5 at. pct V on the Nb-rich end, and again at ∼25 at. pct Nb on the V-rich end of the system. It increases rather sharply from -54.8 kJ/mol in pure niobium to +7.6 kJ/mol at 2.5 at. pct V, and reaches a maximum of +43.6 kJ/mol at approximately 25 at. pct V. Further increase in the V content results in a more gradual decrease in Q* to its value of -42.3 kJ/mol in pure vanadium. Possible explanations of these results based on current theories of thermotransport are presented, and the implications from an engineering standpoint are examined.

Atomic mass transport of carbon in the one-phase Nb-1Zr-C system under a temperature gradient

Mass transport behavior of carbon in the one-phase Nb-1wt%Zr-C system under a temperature gradient was studied using a radioactive tracer technique. Each sample was heated in an ultrahigh vacuum chamber under a pressure of the order of 10−9 Torr. Temperature gradients in different ranges were induced in separate samples to determine any possible effects of temperature on the phenomenon. Results showed that the heat of transport Q∗ of carbon in the Nb-1Zr-C system is −15.2 ± 0.2 kcalmol−1, representing carbon migration from the colder to the hotter regions of the sample. This value is greater in magnitude than Q∗ of carbon in the Nb-C system (−13.1 kcal mol−1) and is independent of temperature within the range of about 1500–2000 K.

Atomic mass transport of carbon in two-phase Nb-1.0Zr-0.1C alloy under a temperature gradient

Atomic mass transport behavior of carbon in two-phase Nb-lZr-0.1C alloy under a temperature gradient was investigated using a radioactive tracer technique. Samples were heated in different temperature ranges for various times to study the effects of temperature and time on the development of carbon concentration profiles. The direction of carbon transport in the Nb-lZr0.1C alloy heated below the solvus of carbon is toward the colder regions, which is a reversal from that in the Nb-lZr-C solid solution alloys. A one-phase region develops in the hotter portion of an initially two-phase sample, and the concentration profile therein approaches that of steady state upon prolonged heating.