Phase transformations in PuGa 1 at.% alloy: Influence of stress on δ → α′ martensitic transformation at low temperatures

Careful experiments are reported about the influence of applied stress (both compressive and tensile) on the formation of alpha' martensite in PuGa 1 at.% at low temperatures. For that purpose, original sample-holders have been designed to apply controlled loadings at low temperature during in situ X-ray diffraction (XRD) analysis and dilatometry experiments. Based on the Patel and Cohen model together with an original transformation pathway involving particular combinations of partial dislocations to approach an Invariant Plane Strain condition, the mechanical driving force and the resulting change in M-s temperature have been computed for the different experimental conditions. First, very good agreement is achieved between experiments and calculations concerning the variation of M-s with applied stress. Although strong asymmetry is observed between compression and tension, it is shown that alpha' can still appear under tension when accounting for the proper orientation of the plates. Second, in situ XRD during several isothermal treatments at various applied stresses provide additional elements to complete our understanding of the transformation process. In particular, the kinetics can be explained by two features specific to autocatalytic processes that are emphasised by the applied stresses: (i) stress induced nucleation of new plates that gives rise to cobweb spatial patterns starting from the specimen surface; (ii) strain induced nucleation associated with the necessary plastic relaxation of the matrix, demonstrated by the broadening of X-ray diffraction peaks. The occurrence of this plastic relaxation with the increase in internal stresses can also explain the saturation of the kinetics below complete transformation.