## 1 Introduction
[PERSON] and [PERSON] [34] reported another _metastable_ phase, \(\gamma^{\text{s}}\), in a 22.2 at.% Nb alloy that was quenched from the \(\gamma\) phase region and annealed for 300 h at 1200 \({}^{\circ}\)C. The structure was studied by a single-crystal XRD pattern and identified as a distorted cubic structure. The \(\gamma^{\text{s}}\) phase might be interpreted as a doubled BCC cell in two directions due to periodic displacement of central atoms in the unit cells. [PERSON] et al. [35] studied the structure and thermodynamic properties of cubic and tetragonal phases of pure uranium and U-Mo alloys. They focused on the metastable \(\gamma^{0}\) phase that is formed in U-Mo alloys at low temperature. Upon heating, the tetragonal structure transforms into the cubic \(\gamma^{\text{s}}\) phase [34]. \(\gamma^{\text{s}}\) phase transforms to \(\gamma\) phase with a quasi-body-centered cubic (\(q\)-BCC) lattice at elevated temperatures, according to [PERSON] et al. [35]. [PERSON] et al. [35] mentioned that the structure of _metastable_ uranium alloys at low temperature does not exactly match the structure of \(\gamma\)-uranium. A following sequence of shear transformations between metastable phases takes place upon quenching from the high-temperature homogeneous \(\gamma\) solid solution [7; 8; 36; 34]:  
\[\gamma\rightarrow\gamma^{\text{s}}\rightarrow\gamma^{0}\rightarrow\alpha^{ \prime\prime}\rightarrow\alpha^{\prime}. \tag{3}\]  
One of the most important conclusions of Ref. [35] is that \(\gamma^{\text{s}}\) has a quasi-body-centered cubic (\(q\)-BCC) lattice. The local positions of uranium atoms in \(\gamma\) correspond to \(\gamma^{\text{s}}\), but the orientations of the central atom displacements are disordered. At a low level of disorder (i.e., at low values of temperature or Mo concentration), the system exhibits long-range correlations in the \"anisotropy direction\", and formation of \(\gamma^{0}\) takes place. At a high level of disorder, long-range correlation in the \"anisotropy direction\" is disturbed, and the formation of \(\gamma^{\text{s}}\) with \(q\)-BCC structure occurs. In this terminology [35], the \(\gamma^{8}\)-phase is similar to the para-elastic state. It resembles the transformation between neptunium phases (\(\beta\)-Np \(\rightarrow\) BCC-Np), which has been described in a similar manner [37]. All of the studied structures of the U-Mo alloys have areas of stability on the \(T-x\) diagram [35]. Thus, in the atomic simulation the change of temperature results in the spontaneous phase transition. Figure 4 shows the calculated phase diagram for U-Mo alloys presented in Ref. [35]. Here, three phases are considered: \(\gamma^{0}\), \(\gamma^{8}\) and \(\gamma\). Figure 4: The \(T\)-\(x\) U-Mo diagram showing areas of the phase stability calculated for different structures: the \(\gamma^{0}\)-phase and \(\gamma\)-phase [35]. The phase stability area for the \(\gamma^{0}\)-phase is bounded by solid blue line. The blue shaded area shows estimations for stability of the \(\gamma^{8}\)-phase. The black dashed curve shows the evaluated line of the phase transition between \(\gamma^{0}\) and \(\gamma\) phases in the U-Nb alloy [16]. Figure 3: The metastable U-Nb binary alloy phase diagram [29], developed by [PERSON] et al. and reproduced with permission from [16], with martensite start, \(\mathrm{M_{S}}\), and martensite finish, \(\mathrm{M_{F}}\), temperatures indicated for the metastable phases [29]. Composition points are plotted (squares) at room temperature. Alloys with 0.75–3 wt.% Nb (1.9 at.%–7.34 at.% Nb) show a single \(\gamma\leftrightarrow\alpha^{\prime}\) transition, whereas alloys with 4–10 wt.% Nb (9.65 at.%–22.16 at.% Nb) show two transitions: \(\gamma\leftrightarrow\gamma^{0}\). and \(\gamma^{0}\leftrightarrow\alpha^{\prime\prime}\).