Impact of compressive pressure on the phase stability and transformation behaviour of δ-phase stabilised plutonium alloys

• Significant α′-phase is induced in Pu-1 at% Ga under compression of <300 MPa.
• No α′-phase is detected in Pu-2 at% Ga alloy following compression of <300 MPa.
• On thermal cycling α′-phase is detected in increasing amounts up to a plateau.
• Increasing the compression to 600 MPa induces α′-phase to form in Pu-2 at% Ga alloy.
• α′-phase induced in Pu-2 at% Ga follows both direct and indirect δ-phase reversion.

The phase stability of δ-phase stabilised alloys is typically assessed as a function of time by periodically monitoring the specimens using thermal analytical methods. The specimens are subjected to repeat thermal cycling through phase transformations prior to and following conditioning, designed to test the degree of stability of the alloys.This work examines the impact of thermal cycling on the δ-phase stability of Pu-1 at% Ga and Pu-2 at% Ga alloys subjected to compressive stresses. Compressive pressure in the range of 100–300 MPa induced a significant percentage of α′-phase in the Pu-1 at% Ga alloy specimens, whereas no detectable levels of α′-phase were recorded in specimens of the Pu-2 at% Ga alloy. However, on thermally cycling the specimens, α′-phase was detected in the Pu-2 at% Ga alloy as well. Furthermore, a significant increase in α′-phase was recorded as both alloys were subjected to an increasing number of thermal cycles. The α′-phase increase reached a plateau after a given number of cycles for each of the alloys. The transformation of the α′-phase back to the δ-phase in specimens of both alloys followed an indirect path via the β-phase and γ-phase. Increasing the compressive pressure up to ∼600 MPa generated higher levels of α′-phase in the Pu-1 at% Ga alloy specimens and resulted in significant levels of α′-phase being recorded in the Pu-2 at% Ga alloy specimens. The transformation of the α′-phase back to the δ-phase appeared to follow an indirect path in the Pu-1 at% Ga alloy. This work shows new evidence that in the Pu-2 at% Ga alloy, the transformation followed both the indirect and direct (α′–δ phase) path.