The Influence of Interstitial Alloying Elements on the Phase Stability and Fracture Toughness of V 3 Au
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THE INFLUENCE OF INTERSTITIAL ALLOYING ELEMENTS ON THE PHASE STABILITY AND FRACTURE TOUGHNESS OF V 3 Au M. A. KASSEM, Y. FAHMY AND C. C. KOCH Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina 27695 ABSTRACT Additions of the interstitial elements oxygen and carbon to the A15 structure V 3 Au intermetallic compound resulted in the formation of the L' 2 perovskite structure. The composition of the L'12 phase is about 17.5 at.% oxygen. Analysis of x-ray diffraction line intensities indicated that the interstitial stabilized phase was L'12 perovskite structure rather than the L12 , Cu 3Au structure type. The microhardness of the L'12 phase is about 20% lower than that of the A15 phase and its fracture toughness is 2 to 3.5 times greater. The mechanism for the improved fracture toughness in the L'12 phase is the subject of continued research. INTRODUCTION It is well known [1] that interstitial elements (H, 0, N, C, B) can often stabilize a crystal structure with a close-packed partial structure of metal atoms (fcc or hcp) even though the metallic element itself does not have a closepacked structure (eg. bcc Nb -+ fcc Nb sublattice in NbN and NbC). Atomic size factor effects are apparently important in these interstitial compounds as reflected in Higg's empirical rule that systems with radius ratios of the metalloid atom to the metal atom in the range of 0.41 to 0.59 form phases with close-packed metal partial structures. When the radius ratio exceeds 0.59, more complicated structures are formed. While size is an important factor in interstitial compounds, the electronic structure, bonding, also plays a role. Metalloid elements have been found to stabilize close-packed structures in some compounds, and in at least one instance with good ductility reported in the material [2]. Inoue et al [2] found that the addition of carbon (- 8 at.%) to (Fe, Ni)3 Al alloys stabilized an ordered fcc (LI2 ) structure in rapidly solidified material which exhibited excellent strength and ductility. Inoue and coworkers had also observed the L12 phase in Fe3 A1-C alloys [3] but in this case the LI 2 phase was brittle. Von Philipsborn and Laves [4] have observed the stabilization of the LI structure by the addition of interstitial 0, N, or C to A-15 structure Ti Au and V3 Au. About 16 at.% 0 was required to stabilize L12 in Ti 3 Au but only 3 at.% 0 was needed in V3 Au for complete transformation to the L12 structure. The transformation was irreversible; the L12 phase was stable even after melting, indicating that it was the stable phase, unlike the metastable LI2 phases in the Fe-Al-C systems [2,31 which decomposed on annealing. No mechanical properties for the L12 , Ti3 Au and V 3 Au materials were reported. Our interest in interstitial element stabilized intermetallics involves the possibility of improving the fracture toughness of intermetallics with high melting temperatures by changing their complex crystal structures (eg. A15) to simpler ordered structures (eg. L12 or L'12 ). V 3 Au was
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