발간논문

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The deformation and fracture behavior of W-Ni-Fe heavy alloys are investigated. The W content has been varied between 90 and 98wt. %, while keeping the Ni: Fe weight ratio constant at 1 : 1. The tungsten grain size has been also varied by prolonged sintering up to 44h. Th tensile specimens have been prepared by usual powder metallurgy techniques. The specimens have been sintered at 1450˚or 1500℃ in hydrogen. The stress-strain curve for some specimens show, after the yield point, a decrease of the stress, which resembles Lu¨ders strain. This discontinuous yield behavior becomes more pronouced with higher W content and smaller tungsten grain size of the specimens. The deformation stress increases with higher W content and smaller tungsten grain size. The defermation stress of all the specimens with varying W content and grain size can be correlated with the average matrix thickness. This result is explained in terms of the dislocation pile-up in the matrix. The specimens are fully densified after 15 min to 1h of sintering. In these specimens, cracks are formed at the interface between the tungsten grains when the grain deformation reaches critical levels during the tension test. The number of these inter-granular cracks increases with deformation. The specimens then fracture catastrophically by propagation of the cracks along the grain-matrix interface and through the matrix and tungsten grains. When the specimens are over-sintered for 4 and 8h, porosity increases to about 2% by formation of large irregular pores. The ductility becomes 0 and the strength also decreases sharply. This result is attributed to the stress concentration at the sharp corners of the pores. Upon further sintering, the porosity decreases again with a recovery of the mechanical properties.

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