Weldable ductile molybdenum alloy development

Abstract

Molybdenum and its alloys are attractive structural materials for high-temperature applications. However, various practical issues have limited its use. One concern relates to the loss of ductility occurring in the heat-affected weld zone caused by segregation of oxygen to grain boundaries. In this study, a series of arc melted molybdenum alloys have been produced containing controlled additions of B, C, Zr, and Al. These alloys were characterized with respect to their tensile properties, smooth bend properties, and impact energy for both the base metal and welds. These alloys were compared with a very high purity low carbon arc cast molybdenum reference. For discussion purposes the alloys produced are separated into two categories: Mo–Al–B alloys, and Mo–Zr–B alloys. The properties of Mo–Zr–B alloy welds containing higher carbon levels exhibited slight improvement over unalloyed molybdenum, though the base-metal properties for all Mo–Zr–B alloys were somewhat inconsistent with properties better, or worse, than unalloyed molybdenum. A Mo–Al–B alloy exhibited the best DBTT values for welds, and the base metal properties were comparable to or slightly better than unalloyed molybdenum. The Mo–Al–B alloy contained a low volume fraction of second-phase particles, with segregation of boron and carbon to grain boundaries believed to displace oxygen resulting in improved weld properties. The volume fractions of second-phase particles are higher for the Mo–Zr–B alloys, and these alloys were prone to brittle fracture. It is also noted that these Mo–Zr–B alloys exhibited segregation of zirconium, boron and carbon to the grain boundaries.