Hierarchical 3D Nanolayered Duplex-Phase Zr with High Strength, Strain Hardening, and Ductility

Nanolayered, bimetallic composites are receiving increased attention due to an exceptional combination of strength and thermal stability not possible from their coarse-layered counterparts or constituents alone. Yet, due to their 2D planar, unidirectional arrangement, they are highly anisotropic, which results in limited strain hardening and ductility. Therefore, like many high-performance, ultrastrong materials of our time, they succumb to the usual strength-ductility trade-offs. Here we present the formation of a novel hierarchical microstructure, comprised of crystals consisting of 3D nanolayered $\alpha /\beta \text{−}\mathrm{Zr}$ networks. By direct comparison with coarse-layered material of the same chemistry, we show that the unusual hierarchical 3D structure gives rise to high strain hardening, high strength, and high ductility. Using TEM analysis and hysteresis testing, we discovered that the 3D randomly oriented biphase boundaries result in progressively dispersive rather than localized slip with increasing strain. Dislocation activity in the $\alpha \text{−}\mathrm{Zr}$ lamellae transitions from single slip to multislip and eventually to multimodal slip as strain increases. The diffusive slip-promoting properties of 3D layered networks can potentially invoke simultaneous high strength, strain hardening, and ductility, and reveal a new target in the microstructural design of high performance structural materials.

Figure 1

(a) Zr-Nb binary phase diagram. (b) Microstructure of the coarse-layered Zr-2.5Nb. (c) Distribution of the $\alpha$ phase thickness in the CL (black) and hierarchical 3D layered Zr-2.5Nb (red). (d) Temperature (red) and strain (black) vs time schedule associated with the DTMPT. (e),(f) Hierarchical microstructure of 3D Zr-2.5Nb.

Figure 2

(a) Duplex phase in hierarchical 3D Zr-2.5Nb. (b) Two phases have a Burgers orientation relationship. (c) High-resolution TEM image of the $\alpha /\beta$ interface.

Figure 3

(a) Engineering stress-strain curves for the CL and hierarchical 3D layered Zr-2.5Nb. (b) Variation of back stress with strain.

Figure 4

(a) and (b) Comparison of the mechanical properties of hierarchical 3D layered Zr-2.5Nb with other Zr alloys [48, 49, 50, 51, 52, 53, 54]. Group I has homogeneous microstructures and group II has heterogeneous microstructures.

Figure 5

Dislocation structures. (a) Tensile tests were interrupted at four critical stresses. (b) At ${\sigma }_e=230$ and (c),(d) at ${\sigma }_e=394\mathrm{MPa}$. (e), (f) At ${\sigma }_e=587$ (g) and (h) at ${\sigma }_e=693\mathrm{MPa}$. (i) Illustration of the slip systems.