Strength and Toughness of Be 12 Nb, Be 17 Nb 2 and Two-Phase Be 12 Nb-Be 17 Nb 2
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STRENGTH AND TOUGHNESS OF Be12Nb, Bei 7 Nb 2 AND TWO-PHASE Be 1 2 Nb-Be 1 7 Nb 2 Stephen M. Bruemmer, Bruce W. Arey and Charles H. Henager, Jr. Pacific Northwest Laboratory, Richland, WA ABSTRACT Bend strength, compression strength, and fracture toughness of niobium beryllide intermetallic compounds have been assessed at temperatures from ambient to 1200 0C. Hot-isostatically-pressed (HIP) Be12Nb showed significantly improved lowand high-temperature mechanical properties over vacuum-hot-pressed (VHP) material. Strengths at 20 0C were 250 MPa in bending and 2750 MPa in compression with a fracture toughness of -4 MPa4m, much higher than previously measured for this compound. High-temperature (; 1000*C) mechanical properties were also improved with bend strengths of 250 MPa at 1200 0C as compared to only 70 to 100 MPa for the VHP material. However, severe pest embrittlement was detected in the HIP material at temperatures between 650 and 1000 0C. HIP Be17Nb2 exhibited poorer low-temperature strength, but much better hightemperature strength, than the HIP Be12Nb. Bend strengths were one-half that measured for Be12 Nb at 20 0C, but were several times greater at 1100 to 1200 0 C. Fracture toughness for Be17Nb2 changed little with temperature below 1100°C, exhibiting values of 2.2 to 2.5 MPa•/m. Compression strengths for Be1 7Nb2 were similar to those measured for VHP Be12 Nb, consistently lower than for HIP Be12 Nb. No improvement in mechanical properties were observed for the VHP two-phase, Be12Nb + Be1 7Nb2 heat. Behavior was similar to VHP Be12 Nb at temperatures below 11 00 0C, with low strengths and fracture toughnesses over this temperature range. However, strengths increased at temperatures above 11 00°C. Results indicate that a two-phase microstructure may improve mechanical properties, but that variables such as grain size and oxide dispersion have a dominant influence on behavior. INTRODUCTION Refractory metal beryllides, such as Bel 2X, Bel 3X and Be17 X2 , have significant hightemperature strength-to-weight benefits over most other intermetallics, metallic alloys or ceramics [1-8]. Much of these benefits stem from their very low densities (2.7 to 5.1 g/cm3), but beryllides also exhibit high elastic moduli and excellent oxidation resistance. As with many high-melting intermetallic compounds, the structural application of beryllides is limited by low-temperature brittleness due to their complex crystal structures. Classical slip deformation processes appear to be restricted until temperatures are reached where diffusion processes become rapid (about one-half the melting temperature). However, mechanisms of embrittlement are essentially unknown since direct measurements of deformation and fracture behavior in these compounds are extremely limited. The most studied of the beryllide compounds has been the body-centered-tetragonal Be1 2X (Mn12Th structure type, 14/mmm space group, D174h Schoenflies notation). Mat. Res. Soc. Symp. Proc. Vol. 273. @1992 Materials Research Society
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Recent work [7,8] has evaluate
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