CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters ^½ (MPa·m ^½ ) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m ^½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.

Titanium gets some twins: Introducing a network of nanotwins is known to improve the properties of face-centered cubic metal alloys. Zhao et al . used gentle compression in liquid nitrogen to introduce a similar network of nanotwins in hexagonal closed-packed titanium. By starting with a titanium that has little oxygen, the twin network can be more easily established. The nanotwinned network improved the yield strength by 50% and ductility by 20% at room temperature. The cryogenic properties were even better, with a yield strength of two gigapascals and a tensile ductility of 100% before failure. —BG

Screw dislocations: A hard case to crack: The motion of dislocations or defects in a metal influences its strength and toughness. If these defects can be “pinned” by adding alloying elements, it should be possible to create a stronger alloy. It was thought that there shouldn't be much of an interaction between screw dislocations and any alloying elements. However, Yu et al. show that for α-Ti, the profound hardening effect of oxygen is due to the strong interactions with the core of the dislocations. First-principles calculations reveal that distortion of the interstitial sites at the dislocation core creates a very strong but short-range repulsion for oxygen atoms. Science , this issue p. 635

Stretching diamond to the limit: Diamond is thought of as being unbendable, but thin samples can actually deform elastically. Applying relatively large amounts of strain to diamond may shift its electronic properties, which is of interest for a number of applications. Dang et al. elastically stretched micrometer-sized plates of diamond along different crystallographic directions. These relatively large samples show that deep-strain engineering can be accomplished in more uniform diamond specimens and may have a large impact on the electronic properties. Science , this issue p. 76