K.J.M. Blobaum

Grain Boundary Engineering of Copper Shaped-Charge Liners

Grain boundary engineering technology has been successfully transferred from rolling and forging to back-extrusion, and back- extruded copper shaped-charge liners with engineered microstructures have been produced. The grain-boundary engineered SCLs have special boundary fractions of 0.60 and 0.66, compared to 0.48 in a conventionally processed SCL. It is hoped that these engineered SCLs will perform better through a combination of a delayed onset of plastic instability and an improved resistance to void nucleation and coalescence. Previous work demonstrates that SCL performance is affected by microstructure, and there is significant evidence which shows that microstructure influences both the fracture and constitutive responses of a material. For SCLs, constitutive properties which affect both the strength and stability of a material are important to performance. In the work described here, the focus is on improving stability properties such as strain-hardening rate and strain-rate sensitivity through grain boundary engineering. Initial mechanical tests of grain boundary engineered copper indicate that its yield strength is nearly the same as conventionally processed material, even though the engineered sample has a larger grain size. Yield strength certainly affects the strength of an SCL, but its effect on stability, in terms of delaying the onset of plastic instability in a jet, is still under investigation. Although we can measure constitutive properties such as yield strength and strain to failure in the laboratory, the true effects of grain boundary engineering on SCL performance can only be measured in actual tests of the liners. Therefore, conventionally processed and grain boundary engineered SCLs will be fielded in real shaped-charges.

Probing the isothermal (delta)->(alpha)' martensitic transformation in Pu-Ga with in situ x-ray diffraction

The time-temperature-transformation (TTT) curve for the {delta} {yields} {alpha}{prime} isothermal martensitic transformation in a Pu-1.9 at. % Ga alloy is peculiar because it is reported to have a double-C curve. Recent work suggests that an ambient temperature conditioning treatment enables the lower-C curve. However, the mechanisms responsible for the double-C are still not fully understood. When the {delta} {yields} {alpha}{prime} transformation is induced by pressure, an intermediate {gamma}{prime} phase is observed in some alloys. It has been suggested that transformation at upper-C temperatures may proceed via this intermediate phase, while lower-C transformation progresses directly from {delta} to {alpha}{prime}. To investigate the possibility of thermally induced transformation via the intermediate {gamma}{prime} phase, in situ x-ray diffraction at the Advanced Photon Source was performed. Using transmission x-ray diffraction, the {delta} {yields} {alpha}{prime} transformation was observed in samples as thin at 30 {micro}m as a function of time and temperature. The intermediate {gamma}{prime} phase was not observed at -120 C (upper-C curve) or -155 C (lower-C curve). Results indicate that the bulk of the {alpha}{prime} phase forms relatively rapidly at -120 C and -155 C.

Spectroscopic Signature of Aging in δ-Pu(Ga)

Resonant photoemission, a variant of photoelectron spectroscopy, has been demonstrated to have sensitivity to aging of Pu samples. The spectroscopic results are correlated with resistivity measurements and are shown to be the fingerprint of mesoscopic or nanoscale internal damage in the Pu physical structure. This means that a spectroscopic signature of internal damage due to aging in Pu has been established.