point but worsens corrosion resistance. To maintain corrosion resistance, gold must be kept above 60%. High-temperature strength and corrosion resistance can be improved by further alloying, e.g., with chromium, palladium, manganese, and molybdenum. Added vanadium allows wetting ceramics. Gold-copper has low vapor pressure. Au-Ni Gold-Nickel. Continuous series of solid solutions. Wider melting range than Au-Cu alloys but better corrosion resistance and improved wetting. Frequently alloyed with other metals to reduce proportion of gold while maintaining properties. Copper may be added to lower gold proportion, chromium to compensate for loss of corrosion resistance, and boron for improving wetting impaired by the chromium. Generally no more than 35% Ni is used, as higher Ni/Au ratios have too wide melting range. Low vapor pressure. Au-Pd Gold-Palladium. Improved corrosion resistance over Au-Cu and Au-Ni alloys. Used for joining superalloys and refractory metals for high-temperature applications, e.g. jet engines. Expensive. May be substituted for by cobalt-based brazes. Low vapor pressure. Pd Palladium. Good high-temperature performance, high corrosion resistance (less than gold), high strength (more than gold). usually alloyed with nickel, copper, or silver. Forms solid solutions with most metals, does not form brittle intermetallics. Low vapor pressure. Ni Nickel alloys, even more numerous than silver alloys. High strength. Lower cost than silver alloys. Good high-temperature performance, good corrosion resistance in moderately aggressive environments. Often used for stainless steels and heat-resistant alloys. Embrittled with sulfur and some lower-melting point metals, e.g. zinc. Boron, phosphorus, silicon and carbon lower melting point and rapidly diffuse to base metals. This allows diffusion brazing, and lets the joint be used above the brazing temperature. Borides and phosphides form brittle phases. Amorphous preforms can be made by rapid solidification. Co Cobalt alloys. Good high-temperature corrosion resistance, possible alternative to Au-Pd brazes. Low workability at low temperatures, preforms prepared by rapid solidification. Al-Si Aluminium-silicon. For brazing aluminium. Active alloys Containing active metals, e.g., titanium or vanadium. Used for brazing non-metallic materials, e.g. graphite or ceramics. Role of elements Some additives and impurities act at very low levels. Both positive and negative effects can be observed. Strontium at levels of 0.01% refines grain structure of aluminium. Beryllium and bismuth at similar levels help disrupt the passivation layer of aluminium oxide and promote wetting. Carbon at 0.1% impairs corrosion resistance of nickel alloys. Aluminium can embrittle mild steel at 0.001%, phosphorus at 0.01%. In some cases, especially for vacuum brazing, high-purity metals and alloys