Evidence of transformation bursts during thermal cycling of a Pu-Ga alloy

Abstract

The thermodynamics and kinetics of the fcc (delta) to monoclinic (alpha-prime) phase transformation and its reversion in a plutonium-gallium alloy have been studied using differential scanning calorimetry, resistometry, and dilatometry. Under ambient conditions, the delta phase is metastable in a Pu-2.0 at% Ga alloy. Thermal cycling to below the ambient temperature results in a partial transformation to the alpha-prime phase; this transformation is compositioninvariant and exhibits martensitic behavior. Because this transformation results in an unusually large 25% volume contraction that cannot be fully accommodated by purely elastic adjustments, the transformation mode is expected to involve burst formation of individual alpha-prime particles. However, upon cooling, these individual bursts were not resolved by the above techniques, although signals corresponding to the overall accumulation of many alpha-prime particles were observed. On the other hand, upon heating, signals from differential scanning calorimetry, resistometry, and dilatometry showed a series of discrete changes occurring in periodic increments beginning at approximately 32°C. These features correspond to the cooperative reversion of many alpha-prime particles to the delta phase; they appear to be the result of an interplay between the autocatalytically driven reversion of a cascade of individual martensite units, and self-quenching caused by small changes of temperature and/or stress accompanying each individual transformation burst. The heat of the delta/alphaprime transformation is estimated to be about +4 kJ/mole.

Orientation Relationship, Habit Plane, Twin Relationship, Interfacial Structure, and Plastic Deformation Resulting from the δ → α′ Isothermal Martensitic Transformation in Pu-Ga Alloys

“…In order to investigate this, we are planning a number of hot-stage experiments using both optical microscopy and transmission electron microscopy. Past experiments and analyses [18] have strongly suggested that the plates do indeed keep crystallographic registry and no new delta grains are formed. If, in fact, new crystals were formed, dilatometry and resistivity measurements during repetitive thermal cycling would change.…”

Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. % Ga

“…Similarly, the inset of Figure 1b shows that the position of the endothermic peak reaches a maximum for T COND = 25 °C. Also evident in several of the DSC curves are spikes, which have previously been attributed to transformation bursts [8].…”

Opportunities in Plutonium Metallurgical Research*

“…The monoclinic α′ phase differs from the α phase in that Ga remains trapped in the lattice, which leads to expanded lattice parameters. Although the δ " α' transformation is generally characterized as an isothermal martensitic transformation, it can also be induced and characterized with continuous cooling experiments using techniques such as electrical resistivity, dilatometry, and differential scanning calorimetry (DSC) [17,36,47,48]. The observation that the martensite start (M s ) temperature is a strong function of the Ga content has been shown by Faires et al [46], Orme et al [15], Deloffre et al [49], Hecker et al [47], and others [45].…”

“…Adler and Olson have reported that the M s temperature is a function of grain size; the smaller the grain size, the lower the M s temperature [50]. Blobaum et al have recently shown that the α′ → δ reversion occurs via a burst martensitic mode [17]. The observation of sharp spikes in DSC traces and steps in dilatometry data suggest that the reversion of α′ particles occurs in cascades, with each individual particle reverting nearly-instantaneously and triggering the reversion of neighboring particles.…”

“…Substantial progress has been made in our understanding of the phase diagram [13,14], but details of the thermodynamics and kinetics of low temperature and high-pressure transformations remain unresolved. Advances have been made in our knowledge of the δ " α' isothermal martensitic transformation [15][16][17][18][19], but still much is unknown about the atomistic mechanisms and kinetics of this transformation during temperature or pressure excursions. Even after 32 years, the origin of the double-C kinetics in the published timetemperature-transformation (TTT) diagram [15] remains essentially undetermined.…”