STRUCTURAL CONSIDERATIONS IN DEVELOPING REFRACTORY METAL ALLOYS
Progress made in applying advanced techniques and structural concepts to the problem of strengthening the Group VI-A metals, Cr, Mo, and W, is examined. At low temperatures, T/T/sub m/ < 0.2, dislocations and grain boundaries play an important role. High strength values snd the marked temperature and strain-rate dependence of the vield stress exhibited by the Group VI-A metals in this range are consistent with the idea that dislocation movement is rate controlling and thermally activated. Dislocation-mobility considerations also point to the existence of high stresses in advance of a moving crack, and, consequently, a low level of crack-propagation resistance. A second factor contributing to the brittleness of the Group VI-A metals is grain-boundary weakness caused by interstitial-atom impurities. Studies of rhenium additions to the Group VI-A metals suggest that the brittleness tendency can be inhibited by alloying. There is evidence that alloying with rhenium influences dissolved interstitial atoms, changes the morphology of dislocations and grain-boundary oxides, promotes twinning, and affects the density of states at the Fermi level. At intermediate temperatures, 0.2 < T/T/sub m/ < 0.4, dislocation-interstitial atom impurity interactions contribute to strengthening. At high temperatures, T/T/sub m/> 0.4, stable dispersions of fine particles are potent strengtheners of the Group VI-A. Small nonmetailic additions, i.e., dope, acting in solution or possibly at the grain boundaries may also contribute to the stability of non-sag tungsten filaments. (auth) Experimental work in the initial stage of a long range research program on SAP, in the field of the ORGEL project, is described. The principal improvement on the semi-finished products was obtained by a degassing thermal treatment under vacuum (about twenty hours at 600 to 625 deg C, under 10/ sup -4/ to 10/sup -5/ mm Hg) of the powders after cold compaction. The main consequences of this special treatment on SAP are following ones: an important decrease of the residual gas content, which becomes lower than 10 ppm, and consequently, an excellent high temperature (600 deg C) stability and a considerable improvement of the welding properties. Some important characteristics of the four SAP grades, mechanical properties at room and at high temperature, fatigue and creep resistance, pressure tests, and hot stability are given along with the actual tolerances obtained in the fabrication of fuel element and pressure tubes. A very large dispersion of the results must probably be attributed to the nonhomogeneity of the powders. (auth)
- Research Organization:
- Battelle Memorial Inst. Defense Metals Information Center, Columbus, Ohio
- DOE Contract Number:
- AF33(616)-7747
- NSA Number:
- NSA-17-029316
- OSTI ID:
- 4712358
- Report Number(s):
- DMIC-182
- Resource Relation:
- Other Information: Orig. Receipt Date: 31-DEC-63
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BRITTLENESS
CHROMIUM
CHROMIUM ALLOYS
CONTROL
CRACKS
DEFECTS
DEFORMATION
DENSITY
DISLOCATIONS
DISPERSIONS
ENERGY LEVELS
FERMI LEVEL
GRAIN BOUNDARIES
GRAIN SIZE
HIGH TEMPERATURE
IMPURITIES
LOW TEMPERATURE
MOLYBDENUM
MOLYBDENUM ALLOYS
MOTION
RHENIUM ALLOYS
SOLUTIONS
STABILITY
STRESSES
TEMPERATURE
TENSILE PROPERTIES
TUNGSTEN
TUNGSTEN ALLOYS
WIRES