Plasticity without dislocations in a polycrystalline intermetallic.

Dislocation activity is critical to ductility and the mechanical strength of metals. Dislocations are the primary drivers of plastic deformation, and their interactions with each other and with other microstructural features such as grain boundaries (GBs) lead to strengthening of metals. In general, suppressing dislocation activity leads to brittleness of polycrystalline materials. Here, we find an intermetallic that can accommodate large plastic strain without the help of dislocations. For small grain sizes, the primary deformation mechanism is GB sliding, whereas for larger grain sizes the material deforms by direct amorphization along shear planes. The unusual deformation mechanisms lead to the absence of traditional Hall-Petch (HP) relation commonly observed in metals and to an extended regime of strength weakening with grain refinement, referred to as the inverse HP relation. The results are first predicted in simulations and then confirmed experimentally.

Strong texture in nanograin bulk Nd-Fe-B magnets via slow plastic deformation at low temperatures.

Nanograin magnets are potential candidates for ultrastrong magnets with high coercivity and high energy product. However, existing nanograin bulk magnets exhibit modest energy products because desired structures with nanoscale grain size and strong orientation (texture) are often difficult to obtain. This study describes a synchrony of the nanoscale grain size and strong texture in bulk Nd-Fe-B magnets by plastic deformation with slow strain rates at temperatures well below the melting point of the Nd-rich phase. High grain orientation (>90%) has been achieved for nanoscale grain sizes of 80-110 nm, giving the nanograin bulk magnets record high energy products (≥45 MGOe). Loosely-packed thick grain boundaries and creep-like stress-strain curves have been observed. Grain boundary mediated creep-like deformation is proposed as the mechanism for the high-degree nanoscale grain alignment under the deformation conditions without liquid phase. The effect of the strain rate on the texture and magnetic properties has been investigated systematically.