STRENGTH AND TOUGHNESS OF BERYLLIUM NIOBIUM INTERMETALLIC COMPOUNDS

Bend and compression strengths, fracture toughness, and high-temperature microhardness of Be-Nb intermetallic compounds were measured at temperatures up to 1200-degrees-C. Be12Nb and Be17Nb2 materials exhibited brittle behavior at temperatures below 1100-degrees-C in bending and below 800-degrees-C in compression. Hot isostatically pressed (HIP) Be12Nb had the highest low-temperature strengths (250 MPa in bending and 2750 MPa in compression) resulting from its greater fracture toughness (K(IC)=4 MPa M1/2) compared with the other Be-Nb materials, vacuum hot pressed (BBP)Be12Nb, and HIP Be17Nb2, which had K(IC)=2.5 MPa m1/2. Measured strengths for the HIP Be12Nb were more than twice that measured for the VHP Be12Nb or for VHP Be17Nb2. The HIP Be12Nb also exhibited good high-temperature mechanical properties, having a bend strength of 250 MPa at 1200-degrees-C, compared with less than 100 MPa for the VHP Be12Nb. However, intergranular embrittlement was observed at intermediate temperatures, reducing the HIP Be12Nb bend strength and fracture toughness below those measured for the other materials. HIP Be17Nb2 exhibited poor low-temperature properties, but high-temperature bend strengths of 740 MPa at 1100-degrees-C and 400 MPa at 1200-degrees-C were measured. Strength in compression was similar for all materials above 800-degrees-C, decreasing sharply to about 600 MPa at 1000-degrees-C and to 200 MPa at 1200-degrees-C. Microhardness and indentation creep tests also revealed similar high-temperature behavior among the materials. Power-law creep exponents ranging from 4.1 to 6.6 and activation energies of 220-290 kJ mol were measured for the beryllides, with the HIP Be12Nb having the highest activation energy for creep.