Excellent Tensile Toughness Research Articles

Superior Yield Strength and High‐Tensile‐Toughness Matching of Medium‐Mn Steel with Gradient Structure Developed by Cyclic Torsion

A multiphase‐gradient‐structured medium‐Mn steel with dislocation/stacking fault (SF) density gradient, austenite fraction gradient, and grain size gradient is designed and fabricated by cyclic torsion. It exhibits higher yield strength (YS) (942 MPa), ultimate tensile strength (1295 MPa), and tensile toughness (290 mJ mm−3) than its homogeneous counterpart (672, 1273 MPa, and 278 mJ mm−3, respectively). High‐density‐interlaced SFs are generated at the surface region because of the change in strain path and stress state during cyclic torsion. These massive interactive SFs promote the persistent martensite transformation of the retained austenite in the surface region within the entire uniform strain range. Therefore, in addition to the central layer, the surface region in the gradient structures provides strong strain hardening. Moreover, the continuous martensite transformation in the surface region allows the gradient structure to be retained even after tensile failure, which is beneficial for improving the heterogeneous‐deformation‐induced (HDI) hardening of the gradient‐structured sample during tensile deformation. Thus, the combination of active and persistent transformation‐induced plasticity effect and HDI hardening contributes to its excellent tensile toughness and ductility. Quantitative analysis indicates that HDI strengthening and dislocation strengthening play a dominant role in the sample's ultrahigh YS.

The effects of atomic arrangements on mechanical properties of 2H, 3C, 4H and 6H-SiC

As a typical polytypic compound, silicon carbide has various atomic arrangements; however, the role of atomic arrangements in the plastic deformation mechanisms remains unclear. In this article, the relationship between atomic arrangements and mechanical properties in single crystal and polycrystalline SiC is revealed by first-principle calculations and molecular dynamics simulations. We found that the twin boundary can delay the fracture of the single crystal 6H-SiC chemical bonds, could account for its excellent tensile toughness. The plastic deformation of polycrystalline SiC is dominated by dislocation nucleation and propagation under compressive strain, and by intergranular fracture under tensile strain. In addition, when the temperature exceeds 1200 K, the yield stress of polycrystalline SiC gradually stabilizes. Hexagonal SiC (2H, 4H and 6H) have stronger strain resistance than cubic one (3C) due to the hierarchical resistance of twinning and stacking fault structures to external stress. These results can provide insight into the relationship between atomic arrangements and stress response in SiC materials.

Structuring poly (lactic acid) film with excellent tensile toughness through extrusion blow molding

We reveal a convenient and feasible processing technique to blow polylactide (PLA) film with excellent tensile toughness, which was: PLA granules were isothermally crystallized and then were blown at a temperature lower than the complete melting temperature. The initial crystalline state of PLA granules was changed by adjusting isothermal crystallization temperature, which was a key point to influence the condensed structures and mechanical properties of films. The results showed that a lower annealing temperature was beneficial for tension toughness of films. When the temperature was set at 90 °C, the elongation at break of film reached 114% and 127% along transverse direction (TD) and machine direction (MD), respectively. The mechanical performances of films were related with their condensed structures. The residual crystals effectively induced tensile crystallization and mesophase during blow molding. Crystals, acting as physical linked points, increased the stress transfer. Cohesive entanglement was an important factor causing PLA film embrittlement, which was suppressed by mesophase.