Plutonium is considered a weak material compared to most other structural metals. Also, because of its low melting temperature, even room temperature may show up effects of high homologous temperatures. The mechanical properties are very sen­sitive to impurities, temperature, crystal defects, anisotropy, and phase transforma­tion. Thus, high-temperature application of pure plutonium is not possible. The properties vary with the allotropes. The elastic constants for alpha-plutonium are Young’s modulus: ~82.7-97GPa, shear modulus: ~37.2-43.4GPa, and Poisson’s ratio (0.15). Figure 7.13a shows a comparison of stress-strain curves of alpha-

plutonium and Pu-1.1 at% Ga (delta-phase) alloy. Tensile yield strength and tensile strength vary between ~310 and ~380MPa. Compressive yield strengths are approximately in the range of 345-517 MPa. Elongation to failure and reduction in area are less than 1% at room temperature. The mechanical strength of poly­crystalline plutonium (both a — and b-phases) is quite sensitive to temperature, as illustrated in Figure 7.13b. The upper range shows the trend in ultimate tensile strength and the lower boundary shows the trend in yield strength. There is a con­siderable scatter in the data available for alpha-plutonium, and thus the data are shown in the form of a band. The delta phase has very low strength. Also, the strength of epsilon phase (BCC lattice structure) is very low as the diffusivity is fast through the BCC crystal lattice (more open structure).

Merz and Nelson [9] demonstrated that the tensile behavior of polycrystalline alpha-plutonium is much more sensitive to strain rate at the proximity of ambient temperature than the first thought. This finding is illustrated in Figure 7.14a. Fine­grained (grain size of 1-3 pm) extruded alpha-plutonium at room temperature and with a strain rate of 7 x 10-4s-1 shows good ductility (see Figure 7.14b). This pro­vided the first evidence that deformation mechanisms that can operate at the higher end of the alpha-phase temperature range and into the beta-phase range (i. e., at lower homologous temperatures) could be grain boundary sliding in addi­tion to dislocation glide. For example, grain boundary sliding was shown to play an important role in the deformation of fine-grained alpha phase. At 108 ° C, it exhib­ited superplastic elongation of ~218%, as shown in Figure 7.14c.