Hardening (metallurgy)

About: Hardening (metallurgy) is a research topic. Over the lifetime, 25584 publications have been published within this topic receiving 376012 citations.

A new mechanism of work hardening in the late stages of large strain plastic flow in F.C.C. and diamond cubic crystals

Abstract: A new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c.c. and diamond cubic crystals. The mechanism is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls. Hardening of the cells is through the build-up of long range misfit stresses that result when the primary dislocation flux cuts trough the geometrically required dislocation density of the cell walls that is associated with the lattice misorientations between cells. Experiments show that these misorientations increase monotonically with increasing strain. There is no recovery in the cells. At the end of Stage III, hardening in the cell walls saturates, but the hardening due to misfit stresses in the cells continues unabated, giving rise to the rate independent hardening of Stage IV. Eventually this hardening is also terminated in Stage V when the misfit stresses inside cells reach a critical level that triggers rate dependent stress relaxation in the cells by secondary glide processes. The new mechanism makes successful predictions for Stage IV processes, including: hardening rate, plastic resistance levels, the gradual increase in hardening rate with plastic resistance, the residual lattice strains on unloading that can be measured with X-ray peak distortions and broadening, and for the Baushinger effect.

A one-dimensional theory of strain-gradient plasticity : Formulation, analysis, numerical results

Abstract: This study develops a one-dimensional theory of strain-gradient plasticity based on: (i) a system of microstresses consistent with a microforce balance; (ii) a mechanical version of the second law that includes, via microstresses, work performed during viscoplastic flow; (iii) a constitutive theory that allows • the free-energy to depend on the gradient of the plastic strain, and • the microstresses to depend on the gradient of the plastic strain-rate. The constitutive equations, whose rate-dependence is of power-law form, are endowed with energetic and dissipative gradient length-scales L and l, respectively, and allow for a gradient-dependent generalization of standard internal-variable hardening. The microforce balance when augmented by the constitutive relations for the microstresses results in a nonlocal flow rule in the form of a partial differential equation for the plastic strain. Typical macroscopic boundary conditions are supplemented by nonstandard microscopic boundary conditions associated with flow, and properties of the resulting boundary-value problem are studied both analytically and numerically. The resulting solutions are shown to exhibit three distinct physical phenomena: (i) standard (isotropic) internal-variable hardening; (ii) energetic hardening, with concomitant back stress, associated with plastic-strain gradients and resulting in boundary layer effects; (iii) dissipative strengthening associated with plastic strain-rate gradients and resulting in a size-dependent increase in yield strength.

Deformation mechanisms of face-centered-cubic metal nanowires with twin boundaries

Abstract: This letter addresses the issue of deformation mechanisms and mechanical tensile behavior of the twinned metal nanowires using atomistic simulations. Free surfaces are always the preferential dislocation nucleation sites in the initial inelastic deformation stage, while with further plastic deformation, twin boundary interfaces will act as sources of dislocations with the assistance of the newly formed defects. The smaller the twin boundary spacing, the higher the yielding stresses of the twinned nanowires. Twin boundaries, which serve both as obstacles to dislocation motion and dislocation sources, can lead to hardening effects and contribute to the tensile ductility. This work illustrates that the mechanical properties of metal nanowires could be controlled by tailoring internal growth twin structures. (c) 2007 American Institute of Physics.

Cyclic hardening properties of hard-drawn copper and rail steel

Abstract: A cyclic hardening law due to Armstrong and Frederick (CEGB Report RD/B/N731, 1966) has been extended to describe plastic strain accumulation (ratchetting) in hard-drawn copper and rail steel. The four parameters of the theoretical model were determined from a single uniaxial test on each material, in which unequal tension and compression were applied. Using these parameters the model was found to give good predictions of the ratchetting rate measured in non-proportional cycles of tension-torsion-compression, which are representative of the stress cycles experienced by surface elements in rolling and sliding contact.