US3779716A

US3779716A - Tantalum carbide-tantalum fiber composite material

Info

Publication number: US3779716A
Application number: US00218686A
Authority: US
Inventors: R Riley; J Taub
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1972-01-18
Filing date: 1972-01-18
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18

Abstract

The thermal stress resistance of refractory metal carbides is substantially improved by the addition of chopped metal fibers. A composite consisting of 50 vol percent TaC-50 vol percent chopped Ta wire shows approximately a 40 percent improvement in thermal stress resistance over that of pure TaC.

Description

United States Patent [1 1 Riley et al.

[ TANTALUM CARBIDE-TANTALUM FIBER COMPOSITE MATERIAL [75] Inventors: Robert E. Riley; James M. Taub,

both of Los Alamos, N. Mex.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

22 Filed: Jan. 18,1972

211 Appl. No.: 218,686

[52] US. Cl. 29/l82.8, 75/204, 75/DIG. 1 [51] Int. Cl. C22c 29/00 [58] Field of Search 75/DIG. 1, 203, 204;

[56] References Cited UNlTED STATES PATENTS 11/1966 Jens 75/DIG. 1

[ Dec. 18, 1973 3,149,253 9/1964 Luebke 75/D1G. 1 3,479,155 11/1969 Rudy 29/1827 3,114,197 12/1963 De Bois et al. 75/D1G. 1 3,352,650 11/1967 Goldstein et al 75/D1G. 1

Primary Examiner-Carl D. Quarforth Assistant ExaminerE. A. Miller Att0rneyJ0hn A. Horan [57] ABSTRACT 1 Claim, 1 Drawing Figure TANTALUM CARBIDE-TANTALUM FIBER COMPOSITE MATERIAL BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the U. S. ATOMIC ENERGY COMMISSION. It relates to structural materials having improved thermal stress resistance at temperatures in excess of 2,000 C and more particularly to such materials consisting of refractory metal carbide-refractory metal fiber composites.

It is known in the art that structural materials exposed to temperatures in excess of 2,500 C for any substantial period of time, i.e., for more than 60 minutes, rapidly degrade. Refractory metal carbides are obvious candidates for such structural materials. However, they are brittle and readily crack in such temperature regimes. This cracking in turn rapidly leads to complete fracturing which may result in a catastrophic failure of the particular structure. As used within this application, fracturing means the separation of a massive body into two or more smaller bodies.

SUMMARY OF THE INVENTION We have now found that the addition of a chopped metal fiber to the refractory metal carbide will substantially enhance the thermal stress resistance of the carbide. The refractory metal carbide-metal fiber composites may readily be formed through hot pressing. While various refractory metal carbides may be used as the matrix material, it is desirable that the carbide be a monorather than a dicarbide. Various chopped metal fibers such as Ta, Mo, 75 vol percent W-25 vol percent Re, Re, etc., may be used. In particular, chopped rhenium fibers are useful because rhenium does not form a carbide. The limiting factor in the use of a particular metal fiber in the composites of this invention is the formation of the metal carbide-carbon eutectic. It will be apparent that the use of a metal fiber M as a reinforcing material in a metal carbide matrix is undesirable if the temperature of the MC-C eutectic is below that to which the composite will be exposed in its structural use. In the case of rhenium, the limiting temperature regime is that at which the Re-C eutectic occurs. The temperatures at which the various refractory metal carbides form eutectics with carbon are well known in the art.

We have found that a composite consisting of 50 vol percent TaC-50 vol percent chopped Ta wire shows approximately a 40 percent improvement in thermal stress resistance over that of pure TaC. At temperatures in excess of 2,500 C, a composite material of this type will still crack, but the presence of the fibers inhibits the propagation of the cracks completely through the structure so that fracturing is avoided or substantially reduced. As can be seen from the FIGURE, up to about 70 vol percent Ta, TaC-chopped Ta wire composite materials exhibit isotropic thermal characteristics. This indicates that neither the size nor the diameter to length ratio of the chopped metal fibers is critical in the composites of this invention. Indeed, it suggests that the metal need not be present as fibers at all but may serve as well in an irregular particulate form.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE compares the results of steady-state thermal stress testing of various TaC-chopped Ta wire specimens machined in both the parallel and perpendicular to the pressing direction with that ofa pure TaC specimen tested in the parallel direction only.

DESCRIPTION OF THE PREFERRED EMBODIMENT Tantalum wire 20 mils in diameter was chopped to approximately 0.25 inch lengths and blended in desired proportions with TaC powder having 1.5 micron nominal size. The blended material was then hot pressed at l,800 C for a time sufficient to produce a density of about to percent theoretical. Typically, this took about onehalf hour.

Steady-state thermal stress specimens were machined from the 10, 50, 60, and 70 vol percent Ta wire billets in both the parallel and perpendicular to the pressing direction. The FIGURE compares the results of steadystate thermal stress testing on these materials with that of pure TaC specimen tested in the parallel direction only. The percent TaC material was 96 percent of theoretical density. There is clearly a significant increase in thermal stress resistance associated with certain contents of Ta fiber. With the exception of the 30 vol percent TaC-70 vol percent specimens, the material is isotropic. Whereas pure refractory metal carbides have steady-state thermal stress resistances in the range of 500 to 650 C radiator temperature, the 50 vol percent TaC-50 vol percent chopped Ta wire composite material has a steady thermal stress resistance at about 900 C radiator temperature. This is an improvement of about 40 percent over that of TaC alone.

One-inch-o.d. by one-quarter-inch-i.d. by one sixteenth-inch-thick specimens of TaC reinforced with 60 and 70 vol percent chopped Ta wire were thermally shock tested. There was a large difference in the power setting required to initiate a crack and the power setting required to completely fracture the materials. Typ ically, a crack could be made to initiate at a power setting of 801l0 and then to propagate (sometimes across the entire specimen). However, these cracks were very tight and did not significantly degrade the strength of the specimen. Apparently the Ta wires bridging the cracks act in effect as tiny reinforcing bars. With the thermal shock instrumentation used, pure refractory metal carbides require power settings of 60 to 80 for crack initiation. In such carbides, however, only a slight increase in the power setting leads to rapid fracture.

It was only at power settings in excess of 200 that these materials could be fractured. In fact, the withgrain specimens could not be fractured at all, even though, at these power settings, the specimens had so many cracks that they resembled cracked safety glass. Several specimens were finally forced to part into two pieces by melting the Ta wires holding the cracked segments together.

The thermal shock data for these materials are summarized in Table I. As can be seen, two thermal shock indices are given; one pertains to crack initiation and one pertains to fracture. This latter index is more important in terms of the ultimate capability of these materials. Included in Table I for comparison are the results for with-grain RVC graphite. As can be seen, crack initiation was not obtained with RVC; rather fractures through the washer occur.

Room temperature comparative thermal conductivity data were also obtained on these materials. Table II presents a comparison between the experimental and predicted values of the thermal conductivity. The model assumed is a dispersion of Ta wires in a continuous matrix of TaC. The axes of the wires are assumed to be parallel to the radial plane of the pressing, but randomly oriented in that plane. The porosity relationships were those derived for hot-pressed ZrC.

Table I THERMAL SHOCK DATA FOR Ta WIRE REINFORCED TaC COMPOSITES Thermal Shock Index Grain For Crack Material Orientation Initiation For Fracture RVC Graphite 252/255 60 vol Ta wire 40 vol TaC WG 80/100 can't fracture 60 vol Ta wire 40 vol TaC AG 70/80 225 70 vol Ta wire 30 vol TaC WG 80/!00 cant fracture* 70 vol Ta wire 30 vol TaC AG 60/80 200 3 *Melting of Ta fibers occurs before fracture Table II COMPARISON OF PREDICTED AND EXPERIMETAL VALUES OF THERMAL CONDUCTIVITY OF TaC-Ta (WIRE) HOT-PRESSED COMPOSITE BODIES (W/cmK at Table III shows x-ray diffraction data for various TaC-chopped Ta wire composites. The numbers refer to the volume percent TaC or Ta. As pressed means the composite prepared by hot pressing at l,800 C for about one-half hour. Heat treated means exposed to 2,300 C for 10 hours in a hydrogen environment after being hot pressed. It can be seen that the hot pressed composites show some slight reaction of the Ta fibers with the surrounding TaC matrix to produce the lesser carbide Ta C. The heat treated specimens having higher Ta fiber contents show a much more pronounced reaction between the fibers and the surrounding TaC matrix to produce Ta C. The strong presence of the hydride phase indicates that even after l0 hours at 2,300 C a substantial amount of the Ta remains noncarbided. It is not critical to this invention that the fibers remain substantially present as the metal. The lesser metal carbides have physical and mechanical properties that are sufiiciently different than those of the monocarbide matrix that crack propagation is stopped or greatly impeded on reaching them. They thus serve to prevent or greatly inhibit fracturing in substantially the same manner that the metal fibers do.

TABLE III X-RAY DIFFRACTION DATA FOR TaC-CHOPPED Ta WIRE COMPOSITES Phases Present Sample Description Strong Medium Weak 90 TaC-l0 Ta as pressed TaC Ta C TaC-3O Ta as pressed TaC Ta C 50 TaC-50 Ta as pressed TaC Ta,C 40 TaC-60 Ta as pressed TaC Ta C 30 TaC-70 Ta as pressed TaC Ta,C TaC-l0 Ta heat-treated TaC 70 TaC-30 Ta heat-treated TaC Ta C 50 TaC-50 Ta heat-treated Ta C TaH TaC 4O TaC-60 Ta heat-treated Ta,C

TaC

o.1\ 30 TaC-7O Ta heat-treated Ta C TaC TaH

What we claim is:

I. A structural material having an improved thermal stress resistance at temperatures in excess of 2,000 C comprising a composite of 50 volume percent TaC and 50 volume percent chopped Ta fibers.