4.06.4.1 Introduction and History of Mo and Mo Alloys

Molybdenum and its alloys are the perennial candi­dates for refractory metal alloy use in irradiation environments, due in part to their high melting temperature (2896 K), good thermal properties, high-temperature strength, and lower induced radioactivity (as compared to tantalum). The density of molybdenum (10.28 gcm~3) is also significantly lower than that ofTa and W, though greater than Nb. But like other refractory metal alloys, Mo can pres­ent difficulties in fabrication, low-temperature duc­tility, and low-temperature embrittlement from radiation damage. The TZM (Mo—0.5%Ti—0.1% Zr) and Mo-Re alloys were examined as part of the SP-100 and JIMO/Prometheus space reactor pro­grams, respectively, and offer additional benefits of improved high-temperature strength over the pure metal.5,19 Molybdenum and its alloys have also been examined for plasma facing and diverter components in fusion reactor designs due to the relatively low sputter yield, high thermal conductivity, and thermal compatibility with other structural materials.5-27,29-63 In addition, because ofthese benefits, Mo has also been examined for use as a grazing incident metal mirror in fusion diagnostic port designs.64,65

As in all other refractory metals, the mechanical properties are influenced by impurity concentra­tions, particularly through grain boundary weaken­ing. However, improvements in Mo ductility are achievable through grain refinement, impurity con­trol, and the addition of Re or reactive elements such as Ti and Zr. An upper limit to the acceptable level of C was also found to improve grain boundary strength. Low-carbon arc-cast molybdenum (LCAC-Mo) is one such example, in which oxygen impurities are reduced to tens of ppm, nitrogen to <10 ppm, and carbon to <100 ppm.66 Higher levels of C will result in reduced fracture toughness, unless additional reac­tive alloy additions are present in the alloy. The TZM alloy also incorporates a small level of carbon to produce Ti — and Zr-carbide strengthening.

Improvements in ductility and toughness through the ‘rhenium effect’ have been observed in Mo for some time,67-69 and generally occurs when Group Via metals are alloyed with elements from Group Vila and Villa metals.70,71 Explanations for this phe­nomenon range from enhanced mechanical twinning, reduced resistance to dislocation glide, reduction of oxygen at grain boundaries, and increased interstitial oxygen solubility. , Critical evaluation6 of the initial work that had suggested a maximum ten­sile ductility near 11-13 wt% Re78,79 was found to be inconclusive because of inadequate control of O and C impurity levels in the earlier studies. Higher con­centration alloys with 40-50 wt% Re have also been examined for use in the radiation environments. Alloys with Re concentrations up to 41-42% are single-phase solid-solution a-Mo, while those at higher levels incorporate the a-Re2Mo phase. Com­mercially available alloys include Mo-41Re and Mo-47.5Re (sometimes referred to as Mo-50Re).

Recently, introduction ofoxide dispersion strength­ened (ODS)-Mo through the incorporation of lantha­num oxide particles has been examined.80- 2 These alloys show great resistance to recrystallization and high-temperature deformation while maintaining low ductile-to-brittle transition temperatures (DBTT) partly because of their refined grain structure.83-85

The radiation effects database for Mo and its alloys is limited to scattered scoping examinations, which show little overlap in the experimental vari­ables such as material purity, alloying level, material thermomechanical history, irradiation conditions, and postirradiation test conditions. Where available, infor­mation on the physical and mechanical property changes to LCAC-Mo, TZM, Mo-Re alloys, and ODS-Mo will be reviewed.