Helium embrittlement

Under certain conditions, embrittlement can be enhanced by the presence of the helium bubbles (helium embrittlement). The accepted view is that this embrittlement is the result of stress-induced growth of helium gas bubbles at the grain boundaries. The bubbles eventually link up and cause intergranular failure. 

Helium embrittlement is another structural material damage mechanism. Helium is produced in (n,a ) reactions. Trace quantities of boron and nitrogen in structural materials can make a significant contribution to the helium production. Consideration must also be given to (n,a) reactions in isotopes which are formed as a consequence of irradiation for example Ni-58 (n,y) Ni-59 (n,a)... 

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

Substantially different fracture modes are observed between aged tritium-loaded and unloaded steel specimens. Helium-3 is vastly less soluble in metals than is hydrogen (tritium) helium pockets (bubbles) form with high internal pressures. Hydrogen embrittlement also contributes to this effect. 

In this test, a thin disk of the metallic material is pleiced as a membrane in a test cell and subjected to helium pressure until it bursts. Because helium is inert, the fracture is caused by mechanical overload no secondary physiced or chemical action is involved. An identical disk is placed in the same test cell and subjected to hydrogen pressure until it bursts. MetaUic materials that are susceptible to gaseous hydrogen embrittlement will fracture at a pressure that is lower than the helium burst pressure materials that are not susceptible will fracture at the same pressure for both hydrogen and helium. 

The ratio between the helium burst pressure, Ph, and the hydrogen burst pressure, Ph2, is a measure of the susceptibility of the material to gaseous hydrogen embrittlement or... 

It should be noted that irradiation of steels in thermal reactors also produces embrittlement, though in this case the main contributor of helium is the (n, a) reaction of slow neutrons with boron impurity in the steel. Because of the high thermal absorption cross section of the boron, however, this burns out at fairly moderate exposures, leading to saturation of the effect. 

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. 

Carburisation Carbon deposition can induce the precipitation of coarse internal carbides that embrittle the Ni-Cr-Mo alloys at low temperature [10], Moreover, the creep strain is strongly affected by carburisation [11, 12]. However, it is worth noting that the creep rupture lifetime of Hastelloy X under carburising helium is not far below that in air [13]. 

Will define baseline composition of helium that will embrittle refractory alloy of choice Will define alteration of gas composition due to release of impurities from superalloys Tensile specimens used to understand if mechanical properties significantly Impacted... 

Embrittlement (metallurgy) The reduction in fracture toughness of a material by the addition of impurity atoms, such as hydrogen or helium in high strength steel, or mercury or indium in aluminum. 

---