Stable metallic glasses may be produced, com­monly in intermetallic compounds. Interest in the

irradiation properties of this class of materials resulted from preliminary tests that showed that these materials actually became more ductile upon irradiation.69 Other intermetallic compounds have been shown to become amorphous upon irradiation. Although semiconductors such as Si and Ge are susceptible to amorphization under irradiation, the phenomenon is almost exclusively restricted to inter­metallic compounds.70 To mention only a few, Zr3Al, Mo3Si, Nb3Ge, and Fe2Mo are compounds that have been studied in the amorphous state. Results and a detailed review of mechanisms and theories of amor — phization have been published by Motta.70 In simple terms, the lattice disruption and defect generation from irradiation disrupts long-range order in the system. Thermal annealing competes with the disor­dering so that there is a critical temperature above which amorphization is not possible. Figure 30 shows a plot of the irradiation exposure necessary for amorphization as a function of temperature for Zr3Fe.71 The critical temperatures and the necessary exposures are both functions ofthe material as well as the impinging particle. Once formed, the amorphous phases are stable under irradiation, but the critical temperatures are typically lower than would be expe­rienced for structural materials in nuclear systems. They are of interest, however, because some inter­metallic phases, such as Fe2Mo and Fe3B found in commercial alloys, become amorphous under irradiation.70,72 In the example of Zr3Fe, the critical temperature under argon ion irradiation is approxi­mately 250 °C, a temperature too low for most, but not all, reactors.

The intermetallic alloys that can be produced in the amorphous state before irradiation are of more interest as potential structural materials, although they remain in the category of research interest at the present time. In addition to the increase in duc­tility upon irradiation, the absence of a crystalline structure with interacting dislocations was further incentive to investigate the irradiation properties of this class of materials. Metallic glasses containing boron, such as Fe40Ni40B20 and (Mo.6Ru.4)82B18 are a few examples, with the former receiving the most attention in terms of mechanical properties.69,73-75 Amorphous alloys are complex systems where changes in free volume and segregation into clusters of differing composition result in changes in behavior as irradiation proceeds. Investigation of the Fe-Ni-B alloy has shown that ductility first decreases and then increases with increasing fluence due to the compet­ing effects of free volume and formation of regions of boron-depleted and boron-rich clusters.73 For suffi­ciently high fluences, the result is severe embrittle­ment. In the case of alloys based on the intermetallic Zr3Al, very severe embrittlement upon irradiation is attributed to the formation of new amorphous

phases.76

Even though a crystal structure is absent, the atoms may be dislodged from their locations, creating additional free volume. Without the bonds present from a crystal lattice, the low binding energy results in high displacement levels for fluence levels that what would be considered low in crystalline alloys. Fluence levels in the range of 1016—1021 ncm~2 have been investigated resulting in displacement levels exceeding 100 dpa. However, simply having similar displacement levels does not permit a true compari­son with crystalline materials. Much research is nec­essary before this class of materials becomes of commercial importance.

1.04.8 Conclusions

The relationship between the irradiation-induced microstructure and tensile properties has been briefly presented using representative classes of alloys. The austenitic stainless steels are an important class of alloys, and they are less complex than the martensitic steels. In the unirradiated condition, the austenitic alloys are primarily hardened by dislocation reactions leading to conventional work hardening, and the martensitic steels are hardened by phase transforma­tions requiring careful heat treatments. The primary irradiation effects are similar, but they influence microstructure and, therefore, behavior in different ways. Both types of alloys have important applica­tions in the nuclear field. Helium embrittlement might be the most important, considering the use of alloys in a neutron environment at high tempera­tures. For the proper conditions, helium can nearly always cause catastrophic failure. Repair welding of alloys with as little as 1-10 appm helium can lead to severe intergranular cracking.

The refractory metals are useful for space reactor application because of their liquid metal compatibil­ity and their high-temperature strength. Space reac­tors can lose heat only by thermal radiation, necessitating high temperatures. However, this class of alloys is most susceptible to embrittlement by interstitial impurities, and synergism of impurities with irradiation-induced defects is an area that must be addressed further.

Amorphous alloys are a research curiosity in that they have interesting properties with respect to irradiation but little application at the present time. Any new class of alloys must be understood before it can be engineered, so