Fig. 1. Microstructure characterization of an original cold-drawn W wire. EBSD orientation maps of the (a) longitudinal section and (b) cross section of the cold-drawn W wire. (c) Bright-field two-beam TEM image showing high-density dislocations in the cold-drawn W wire. (d) Higher-magnification image depicting dislocation structures from the rectangular frame in (c).

Fig. 2. Fabrication of W microwires with high-density dislocations and reduced GBs. A wedge-shaped lamella extracted by a microprobe from the cross section of a cold-drawn W wire and characterized by EBSD to indicate GB position and guide the FIB sculpture of W microwires. A typical W microwire and a tensile gripper aligned before testing are presented.

Fig. 3. Side-view TEM images showing GBs inside two representative W microwires (I and II). Microwire I has one HAGB of 35.3° misorientation, suggested by SAED patterns taken from G1 and G2 at the same X and Y tilt angles. Microwire II has one LAGB of 9° misorientation and two HAGBs (HAGB misorientation between G1 and G3 is 39.9°, while HAGB misorientation between G2 and G3 is 40.8°), suggested by SAED patterns taken from G1 + G2 and the SAED pattern taken from G3 at ΔXG3-G1=28.1°, ΔYG3-G1=-28.3° tilting angles.

Fig. 4. Exceptional room-temperature strength-ductility combination achieved in W microwires. Extracted frames show the plastic deformation of microwires (a) I and (b) II, indicating the morphology at the maximum uniform elongation and before ductile fracture. (c) Engineering stress-strain curves of microwires I and II.

Fig. 5. Yield strength versus uniform elongation of W microwires compared with those of cold-drawn W microwires [42], [43], [44], annealed cold-drawn W microwires [43], single-crystal (SC) W microwires [38]. The data from other refractory metallic microwires [38] including niobium (Nb), molybdenum (Mo) and tantalum (Ta) and some existing interconnect metallic nanowires including copper (Cu) [45] and aluminum (Al) [46,47] are also included.

Fig. 6. (a) Bright-field TEM image showing a typical PTP sample. The red rectangle indicates the observation area. (b) A TEM micrograph showing high-density dislocations using a two-beam condition g =$(0~\bar{1}1)$. SAED pattern inset in (b) shows the loading axis is near [110] direction. (c-h) Series of TEM images magnified from the white rectangular zone in (b) showing representative dislocation activities of the W microwire in the corresponding time interval by in situ TEM tensile testing.

Fig. 7. (a) A crack initiates and propagates after necking. (b) A succession of snapshots showing the crack propagation after crack initiation in (a), accompanied by dislocation activities ahead of the crack tip and crack-tip plasticity. The black dotted lines outline the crack, the yellow arrows indicate dislocations ahead of the crack, and the small round circles in (b) indicate the crack embryos.

Fig. 8. in situ TEM tensile testing of a W microwire containing one LAGB and one HAGB. (a) A TEM micrograph showing an overview of all three grains G1, G2, and G3 before tension with the loading axis close along [110]. (b-g) Series of TEM images taken from the white rectangular zone in (a) showing the dislocation activities. The yellow arrows indicate dislocation activities of pre-existing dislocations, and the red arrows indicate the dislocation activities emitted from the GB. (h-j) Successive frames showing dislocation motion and sample deformation in the corresponding time interval.

Fig. 9. Schematic diagram showing the microstructures and deformation mechanisms of the annealed W microwires, deformed W microwires and W microwires developed in this work, respectively. Their mechanical properties (i.e., yield strength versus uniform elongation) are plotted and compared in Fig. 5.