Brittleness intermetallics

Welding (qv) of titanium requires a protected atmosphere of iaert gas. Furthermore, parts and filler wire are cleaned with acetone (trichloroethylene is not recommended). The pieces to be welded are clamped, not tacked, unless tacks are shielded with iaert gas. A test sample should be welded. Coated electrodes are excluded and higher purity metal (lower oxygen content) is preferred as filler. Titanium caimot be fusion-welded to other metals because of formation of brittle intermetallic phases ia the weld 2oae. 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

The high reactivity of boron with many metals and the formation of brittle intermetallic phases. 

Various jet engine and car engine components have been identified for application of TiAl-based alloys. It is particularly noteworthy that large sheets, which are suitable for superplastic forming and joining by diffusion bonding, can be produced from these intrinsically brittle intermetallic compound alloys. 

Explosive bonding" and cladding. In this method, the controlled energy of a detonating explosive is used to create a metallurgical bond between tantalum and the base metal. A copper intermediate layer is used when tantalum is clad onto carbon steel in order to avoid brittle intermetallic formation at the interface. The limitations of the explosive bonding are as follows ... 

When an intermetallic forms a distinct interfacial layer or otherwise high localized concentration, it is a rigid structural element, prone to cracking and brittle fracture failure on flexing. Therefore, even though the solder joint is composed of a ductile Sn-Pb composition, its overall strength and endurance is dominated by the brittle intermetallic composition and its thickness and structure within the solder joint. 

Only a few coating systems have been found to prevent the reflectory metals from oxidation. In the case of Mo, Pt-cladded components with a diffusion barrier interlayer on the basis of alumina, which prevents the formation of brittle intermetallic molybdenum-platinum... 

Formation of brittle intermetallic phases is likely with many metals and must be avoided. Surfaces to be heated above 300°C should be protected by an inert gas such as argon or helium to prevent embrittlement. It is critical to ensure that the metal is clean prior to welding. An acid pickle wash is recommended. For ambient-temperature pickling, a typical solution is 25 to 35% HF, 25 to 33% HNO3. 

Embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 560 and 980 C (1050 and 1800 °F). Sigma-phase embrittlement results in severe loss of toughness and ductility, and can make the embrittled material susceptible to intergranular corrosion. See also sensitization. 

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