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Substructure And Mechanical Properties Of Refractory Metals

Report Number: WADD TR 61-181
Author(s): B. S. Lement, D. A. Thomas, S. Weissmann, W. S. Owen, P. B. Hirsch
Corporate Author: Manufacturing Laboratories, Inc.
Laboratory: Directorate of Materials and Processes
Date of Publication: 1961-08
Pages: 272
Contract: AF 33(616)-6838
Project: 7351 - Metallic Materials
Task: 73512
AD Number: AD0266881

Quantitative results on the substructural characteristics of tungsten, molybdenum, tantalum and columbium are being obtained through the coordinated programs of the five participating laboratories. Changes in subboundary spacing of worked materials have been measured microscopically as a function of annealing temperature, and corresponding changes in particle size and lattice strain have been obtained by Fourier analyses of diffraction-line shapes. Combined application of x-ray reflection microscopy, double-crystal diffractometry, and transmission electron microscopy to single-crystal and polycrystalline tungsten has revealed various orders of substructure in terms of size, disorientation, and dislocation arrays; and progressive alterations have been traced through the stages of recovery and recrystallization. Stacking faults and twins have now been found in tungsten and columbium after high-temperature annealing; the stacking faults are believed to result from a lowering of the stacking-fault energy by impurities.

The origin of fibered structures in heavily-drawn b.c.c. wire is now better understood. The complex interfolding of the distorted grains and the formation of deformation bands produce an unexpectedly large number of boundary intercepts per unit of transverse distance. There is also a reduction in the amount of grain boundary area transverse to the fiber axis, which probably decreases the number of sites for the initiation of brittle fracture. An x-ray technique developed for rapid determination of stereographic pole figures is being applied to a detailed study of the deformation and annealing textures in tungsten strip.

Correlations have been found between the substructural characteristics and the mechanical behavior. Recovery-annealing of worked molybdenum strip results in increased ductility which is associated with an increase in x-ray particle size term. Systematic studies on the role of interstitial impurities in tantalum and columbium have been initiated. Measurements of the lower yield stress as a function of grain diameter have confirmed Petch relationships; however, the lattice friction stress was found to be strongly dependent on temperature, impurity content and substructure. Since the substructural detail changes as the annealing temperature is raised to vary the grain size, this may account fr the anomalous values of the Petch slope that have been reported previously

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