Effect of deformation on ThO 2 particle morphology in W-1 Wt Pct ThO 2 materials
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TUNGSTEN-THORIA alloys are unique oxide d i s p e r sion-strengthened materials. In the f i r s t place, the relative particle-to-matrix p r o p e r t y r a t i o s of this s y s tem are the inverse of most o t h e r conventional o x i d e dispersion-strengthened systems. ThO2 is elastically much softer, has a l o w e r melting p o i n t , and has a much h i g h e r coefficient of t h e r m a l expansion than does the tungsten m a t r i x , see Table I. Secondly, thoria in tungsten is relatively unstable as suggested by the r a t h e r low value of 40 kcal per mole for the activation energy of the slowest step in the o v e r a l l process of reduction of ThO2, thorium diffusion to the surface, and thorium evaporation.1 Thirdly, in contrast t o other oxide dispersion systems like Ni-ThO2,2 the particle size r a n g e in W-ThO2 m a t e r i a l s is exceptionally l a r g e3-7 (0.005 ~m t o over 10 ~m). Finally, and perhaps most importantly, the l a r g e ThO2 part i c l e s have been observed3-7 to deform concomitant with the m a t r i x when a significant amount of plastic strain has occurred. In light of the observed l a r g e thoria particle deformation, one would expect the dispersion character, i . e . , the size, shape, and distribution of the t h o r i a particles, t o eventually change as a result of m e c h a n ical working. Due t o the congeries of evidence relating the dispersion character t o the mechanical and physical behavior of dispersion-strengthened m a t e r i a l s ,8 one would also expect the behavior of W-ThO~ alloys t o change as the ThOz dispersion morphology changes. However, because of the relatively s m a l l n u m b e r of l a r g e , (>1 ~tm) deformable ThO2 particles in W-ThO2 materials and b e c a u s e of the unique nature of the W-ThO2 system itself, it is not c l e a r at what point the changes in the dispersion brought about by l a r g e particle deformation and fracture are sufficient t o induce significant alterations in m a t e r i a l behavior. The p u r p o s e of this work was t o follow quantitatively the morphology of the ThO2 dispersion as a function of T. E. DUNHAM is Research Metallurgist, Refractory Metals Laboratory, Lamp Metals and Components Department, General Electric Co., Cleveland, Ohio. Manuscript submitted April 2, 1971. METALLURGICAL TRANSACTIONS
deformation from ingot t o w i r e , in o r d e r t o c h a r a c t e r ize the changes which o c c u r , and subsequently use the information obtained to predict qualitatively how t h e s e changes might affect the mechanical and physical behavior of W-ThO2 alloys. I) EXPERIMENTAL PROCEDURE A) Materials and Processing A tungsten 1 wt pct t h o r i a powder was made by adding a thorium nitrate solution to ammonium paratungstate crystals and subsequently f i r i n g and reducing the mat e r i a l to m e t a l powder. The powder was pressed, presintered, sintered, swaged, and d r a w n t o wire in a m a n n e r described by Smithells. 9 Samples of the powder, ingots, rods, and w i r e s were taken during the processing f
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