Fig. 1. Schematic illustration of the thermomechanical processing route.

Fig. 2. EBSD results of the heterogeneous structured Zr-2.5Nb alloys rolled at different temperatures: (a) 815 °C, (b) 835 °C, (c) 860 °C, and (d) 880 °C. (a1)-(d1) are the band-contrast (BC) maps showing the evolution of αp and α' with increasing hot-rolling temperature, (a2)-(d2) are the inverse pole figure (IPF) maps exhibiting the orientation distribution, (a3)-(d3) are the misorientation angle distribution (MAD) histograms showing the evolution of MAD with increasing hot-rolling temperature, and (a4)-(d4) are the intervariant misorientation angle in one self-accommodating group showing that the variants were misoriented by the specific 63.3 ° in one group. (step size: 0.1 μm).

Fig. 3. (a) Inverse pole figure (IPF) map of 860 °C HR sample showing the distribution of 60 ° and 90 ° misorientation of α' martensite variants, (b) part of the shelf-accommodating lamellar group in (a) and corresponding {13 $\bar{4}$1} pole figure (PF) map. (step size: 0.1 μm).

Fig. 4. TEM images of the heterogeneous structured Zr-2.5Nb samples showing the evolution of DM and TM with increasing hot-tolling temperatures: (a) 815 °C, (b) high-resolution electron microscopy image of the twin in (a) along the [1 $\bar{2}$10] zone axis, (c) 835 °C, (d) 860 °C, (e) 880 °C, and (f) evolution of average twin thickness with the increase of HR temperatures. The insert in (a) is the selected area electron diffraction pattern (SADP) of {10 $\bar{1}$1} twins. (TM: twin martensite, DM: dislocation martensite, TB: twin boundary).

Fig. 5. Evolution of (a) size and (b) fraction of DM, TM lamellae, and αp grains with increased HR temperature in the heterogeneous structured Zr-2.5Nb alloys. (DM: dislocation martensite, TM: twin martensite).

Fig. 6. Mechanical behavior of HR heterogeneous structured Zr-2.5Nb alloys. (a) Engineering stress-strain curves (The insert table lists the yield strength, ultimate tensile strength, and total elongation of the present Zr-2.5Nb alloys). (b) True stress-strain and strain hardening rate curves of heterogeneous structure Zr-2.5Nb alloys. Comparison of (c) yield strength and (d) ultimate tensile strength of the present heterogeneous structured Zr-2.5Nb alloys with other reported pure Zr [8,18,[55], [56], [57]] and Zr-Nb alloys [28,29,[58], [59], [60]].

Fig. 7. Typical TEM images of 860 °C HR Zr-2.5Nb samples at the strain of (a, b) 2.5% and (c, d) fracture: dislocations in (a) the αp phase and (b) the α' phase at the strain of 2.5%; dislocations in (c) the αp phase and (d) the α' phase at the strain of fracture.

Fig. 8. SEM fractographs of HR Zr-2.5Nb alloys at different temperatures: (a) 815 °C, (b) 835 °C, (c) 860 °C, and (d) 880 °C.

Fig. 9. SEM observation of voids nucleation sites in the heterogeneous structured Zr-2.5Nb alloys, taking the 860 °C hot-rolled sample as an example.

Fig. 10. (a) Illustration of the measurement method of θ and δ. (b) Schematic diagram of the twin shear in a twin martensite plate.

Fig. 11. Grain size dependence of CRSS for dislocation slip and twinning. Both the prismatic <a> slip and the basal <a> slip cannot alone achieve the uniform deformation of polycrystalline Zr-2.5Nb alloys according to the Von-Mises criterion, thus, twinning or pyramidal <c + a> slip has to be activated to accommodate the deformation process, and the grain size dependence of dislocation slip and twinning makes a critical grain size (dc = 3.3 μm) for the transition of deformation mechanisms. When, d > dc, deformation twinning was activated and twin martensite becomes the main martensite type; when d < dc, the pyramidal <c + a> slip was activated, dislocation can achieve the uniform deformation of Zr-2.5Nb alloys and dislocation martensite becomes the main martensite type.

Fig. 12. Loading-unloading-reloading (LUR) tensile tests of the heterogeneous structured Zr-2.5Nb alloys. (a) Typical LUR curves of 815 °C and 860 °C HR Zr-2.5Nb alloys samples. (b) Schematic for calculating back stress. (c) Flow stress and back stress at different strain levels in 815 °C, 835 °C, 860 °C, 880 °C HR samples. (d) Change of the ratio values of back stress to flow stress in different temperatures HR samples when true strain is 1.5%, 2.5%, 3.5%, and 4.5%, respectively.

Fig. 13. Strength contribution of each component structure in the heterogeneous structured Zr-2.5Nb alloys and the comparison between the calculated and experimental yield strength values. (TM: twin martensite, DM: dislocation martensite)