On Kinetics of Grain Refinement and Strengthening by Dynamic Recrystallization

Abstract

The grain refinement and strengthening of an austenitic stainless steel are studied under multiple multidirectional forging at 1073 K up to strains of 4. The structural changes are characterized by the development of discontinuous and continuous dynamic recrystallization (DRX) leading to the grain refinement down to submicrometer level. The new ultrafine grains develop primarily along original grain boundaries and deformation microbands, leading to heterogeneous necklace-like microstructures at intermediate strains followed by rapid expansion of the DRX grains occupying the whole worked sample at large strains. The change in the fraction of ultrafine grains is commonly characterized by a sigmoid-type dependence on strain and can be expressed by a modified Jonson-Mehl-Avrami-Kolmogorov equation. The DRX development is accompanied by a stepwise decrease in the flow stress at reloading in each subsequent forging pass especially in intermediate strains. This stepwise softening results from rapid growth of freshly nucleated grains. In contrast, after forging, both the yield strength and ultimate tensile strength at room temperature increase progressively with an increase in the number of forging passes. The strengthening during DRX development can be attributed to concurrent contribution of work-hardening and grain refinement and can be expressed by a summation of recovered and recrystallized fractions.

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

“…where F UFG is the fraction of ultrafine grains, ε is the total strains, k and n are constants, which depend on material and processing conditions [33][34][35].…”

A Combined Method to Model Dynamic Recrystallization Based on Cellular Automaton and a Phenomenological (CAP) Approach

“…This figure shows the results of samples hot-compressed at the strain rate of 0.1 s -1 and various deformation temperatures. In the case of simulated results, fDRX was obtained by using Equation (9). The results indicate that the proposed model can satisfactorily estimate the microstructure developments.…”

“…One can apply the deformation heterogeneity by coupling the present model with the crystal plasticity models such as the work proposed by Chuan et al [58] by using a rate-dependent crystal plasticity model, or Beygelzimer and Spuskanyuk [59] by utilizing a self-consistent model. The fraction of DRX area was calculated by utilizing the following equation: The fraction of DRX area was calculated by utilizing the following equation: (9) where N DRX and N total denote the number of cells belonging to DRX grains and the number of total cells, respectively. The kinetics of the DRX phenomenon can be evaluated by considering the evolution of recrystallization volume fraction with the deformation time.…”

“…In the case of CDRX, the stored strain energy is released by the formation of subgrains with a progressive transformation of low-angle grain boundaries into high-angle grain boundaries [5,6], while during DDRX, new recrystallized grains are nucleated along the initial grain boundaries and grow towards the area with higher dislocation density [7] to decrease the stored strain energy [8]. In other words, the new DRX nuclei are generated via the bulging mechanism [9] and grow by consuming the work-hardened neighboring grains.…”

Characterization of the Microstructure Evolution of Ni-Based Superalloy at Liquidus Temperature by Electromagnetic Field

“…Generally, equiaxed and spherical grains can be obtained by the application of an electromagnetic field. When the effective nucleation substrates could be ignored, dendrite fragments are thought to be the major reason for the equiaxed grain formation [12]. Therefore, the fragmentation caused by the forced convection is also an important factor for the grain refinement [13,14].…”