DUCTILE-BRITTLE TRANSITION IN REFRACTORY METALS.

This report is presented in two sections the first of these deals with the precision analysis of the divergent beam technique which forms the basis of a stress-strain analysis. The second section deals with the application of the divergent beam method to the ductile-brittle transition in tungsten. In the first section an X-ray method is described which permits a precise determination of the interplanar spacings in a crystal. The method is capable of disclosing small differences in interplanar spacings even between various sets of hkl planes of the same form and, therefore, serves as a basis for a stress-strain analysis. The results of the stress-strain analysis at 298K and 77K presented in the second section of this report show a close correlation between the maximum shearing strains obtained experimentally and the resolved shear stress factor. The build-up of shearing strains is attributed to dislocation multiplication and interaction on active slip planes. It is shown that the absolute strain values obtained are too large and it is also shown that this is due not to inaccuracy of measurements but to inadequacy of sampling of orientations. This sampling is now being improved by tilting the specimen surface relative to the probing X-ray beam. Author