Effect of boron doping on the mechanical properties of ordered Ni-Mo alloys

HASTELLOY B-2 alloy was found to exhibit environmental embrittlement when t ested in air and hydrogen at ambient temperature after atomic ordering intr oduced by a heat treatment at 700 degreesC for 24 h. Molybdenum in the HAST ELLOY B-2 alloy was probably reactive enough to dissociate water vapor in a ir to generate atomic hydrogen, resulting in hydrogen embrittlement. The pe rcentage of transgranular fracture increased as the test environment change d from hydrogen gas to moist air or water vapor, and vacuum or oxygen. With the addition of 100 wt ppm B, the environmental embrittlement was complete ly eliminated, with the tensile properties independent of both test environ ment and strain rate. The fracture mode remained the same, i.e. ductile dim pling, after B-doping when tested in different environments. The immunity o f the B-doped B2 alloy to environmental sensitivity remained even after lon g-term heat treatment. Auger analysis does not detect any boron segregation at the grain boundaries. The mechanism of boron doping in eliminating the environmental embrittlement in the Ni-Mo alloy is apparently different from that in many L1(2)-type alloys such as Ni3Al and Ni3Si. (C) 2001 Elsevier Science Ltd. All rights reserved.