Multi-phase interaction accompanied with the phase transformation process in U-21 at%Nb alloy quantified by in-situ neutron diffraction during heating

•Multi-phase interaction accompanied with phase transformation has been quantitatively revealed in U-21 at%Nb alloy.•Thermal-induced lattice strain for α-U phase is strongly anisotropic while that for γ-Nb and γ-U phases is isotropic.•The γ-Nb phase possesses always the soft response while α-U and γ-U phases show the hard response.•Mechanical instability occurs in the phase transformation window (PTW), where fluctuation-like behavior appears in micro-strain response.•The micro-strain would have a significant effect upon the phase transformation process as well as PTW.