Fcc Stainless Steel Research Articles

A critical discussion of elasto-visco-plastic self-consistent (EVPSC) models

Two recently proposed elasto-visco-plastic self-consistent (EVPSC) models, based on solving for stress or stress increment, are discussed, and their results are compared with the ones of the visco-plastic self-consistent (VPSC) approach. The advantages and shortcomings of the models are demonstrated, with emphasis on how the grain responses are affected by the time increment and inclusion-medium interaction strength used. Experimental data obtained for an FCC stainless steel is used as the benchmark system for performing the simulations and assessing the reliability of the results. Predictions of stress-strain response of the aggregate, standard deviations in intergranular stress and strain rate, along with the evolution of internal lattice strain are presented and compared, with special focus on their dependence on the incremental approach chosen. We find that the chosen time increment may significantly affect predicted grain responses. To avoid such effect a method based on using two separate time increments is proposed, one for approximating the stress rate and another for actual integration of stress, strain, and state variables. The proposed method successfully eliminates the dependence of results on the time increment used, thus allowing more reliable computations while preserving the computational efficiency of the elasto-visco-plastic model.

Void and helium bubble interactions with dislocations in an FCC stainless steel alloy: anomalous hardening and cavity cross-slip locking

The critical stress for cutting of a void and He bubble (generically referred to as a cavity) by edge and screw dislocations has been determined for FCC Fe0.70Cr0.20Ni0.10—close to 300-series stainless steel—over a range of cavity spacings, diameters, pressures, and glide plane positions. The results exhibit anomalous trends with spacing, diameter, and pressure when compared with classical theories for obstacle hardening. These anomalies are attributed to elastic anisotropy and the wide extended dislocation core in low stacking fault energy metals, indicating that caution must be exercised when using perfect dislocations in isotropic solids to study void and bubble hardening. In many simulations with screw dislocations, cross-slip was observed at the void/bubble surface, leading to an additional contribution to strengthening. We refer to this phenomenon as cavity cross-slip locking, and argue that it may be an important contributor to void and bubble hardening.

Mechanism and Prediction of Hydrogen Embrittlement in fcc Stainless Steels and High Entropy Alloys.

The urgent need for clean energy coupled with the exceptional promise of hydrogen (H) as a clean fuel is driving development of new metals resistant to hydrogen embrittlement. Experiments on new fcc high entropy alloys present a paradox: these alloys absorb more H than Ni or SS304 (austenitic 304 stainless steel) while being more resistant to embrittlement. Here, a new theory of embrittlement in fcc metals is presented based on the role of H in driving an intrinsic ductile-to-brittle transition at a crack tip. The theory quantitatively predicts the H concentration at which a transition to embrittlement occurs in good agreement with experiments for SS304, SS316L, CoCrNi, CoNiV, CoCrFeNi, and CoCrFeMnNi. The theory rationalizes why CoNiV is the alloy most resistant to embrittlement and why SS316L is more resistant than the high entropy alloys CoCrFeNi and CoCrFeMnNi, which opens a path for the computationally guided discovery of new embrittlement-resistant alloys.

Line-length-dependent dislocation mobilities in an FCC stainless steel alloy

Using molecular dynamics simulations, we compute the mobility of an edge dislocation in a random FCC Fe0.70Ni0.11Cr0.19 alloy over temperatures from 100 to 900 K and shear stresses up to 600 MPa. Dislocation mobilities are shown to be intrinsically length-dependent when the line length is below a minimum value, with shorter lines having a reduced mobility. We show that this minimum line length is sensitive to both temperature and stress. We develop a drag model with terms accounting for solute, phonon, and singular drag mechanisms, and fit the model to MD data in the length-independent regime. Using the drag parameters from this fit, we implement a kinetic Monte Carlo (kMC) model for dislocation motion in the solute-drag-dominated regime. The kMC model is then used to explain the length dependence of the mobility and further characterize the statistical solute environment experienced by the dislocation. Our methods and findings are readily extensible to other crystal structures (e.g., HCP, BCC).

Strengthening mechanisms in nanostructured copper/304 stainless steel multilayers

Nanostructured Cu/304 stainless steel (SS) multilayers were prepared by magnetron sputtering. 304SS has a face-centered-cubic (fcc) structure in bulk. However, in the Cu/304SS multilayers, the 304SS layers exhibit the fcc structure for layer thickness of ≤5 nm in epitaxy with the neighboring fcc Cu. For 304SS layer thickness larger than 5 nm, body-centered-cubic (bcc) 304SS grains grow on top of the initial 5 nm fcc SS with the Kurdjumov-Sachs orientation relationship between bcc and fcc SS grains. The maximum hardness of Cu/304SS multilayers is about 5.5 GPa (factor of two enhancement compared to rule-of-mixtures hardness) at a layer thickness of 5 nm. Below 5 nm, hardness decreases with decreasing layer thickness. The peak hardness of fcc/fcc Cu/304SS multilayer is greater than that of Cu/Ni, even though the lattice-parameter mismatch between Cu and Ni is five times greater than that between Cu and 304SS. This result may primarily be attributed to the higher interface barrier stress for single-dislocation transmission across the {111} twinned interfaces in Cu/304SS as compared to the {100} interfaces in Cu/Ni.

Numerical simulations of hydrogen–dislocation interactions in fcc stainless steels. : part I: hydrogen–dislocation interactions in bulk crystals

The basic equations for the coupling between stress and hydrogen diffusion are presented. A numerical simulation method is proposed. It is based on the diffusion equation including a hydrostatic stress gradient term, and on a discretisation of the hydrogen concentration field. The simulation scheme is applied to elementary dislocation configurations in order to compute the influence of solute hydrogen on dislocation interactions. A simple general expression is derived for the hydrogen screening effect on the pair interactions between co-planar dislocations. This expression is used to investigate the influence of diffusing hydrogen on the cross-slip probability of screw dislocations. First numerical results are presented. They are discussed in relation with the modelling of transgranular stress corrosion crack propagation.

Numerical simulations of hydrogen–dislocation interactions in fcc stainless steels. : part II: hydrogen effects on crack tip plasticity at a stress corrosion crack

The discrete simulation method for hydrogen–dislocation interactions is applied to the study of Stress Corrosion Cracking (SCC). We recall the main results of the experimental study of the fracture micro-crystallography in austenitic stainless steels, along with the successive stages of the Corrosion Enhanced Plasticity Model. Numerical simulations allow the assessment of the critical parameters affecting the model stages. Solute hydrogen promotes the formation of dense dislocation pile-ups, and a ‘zigzag’ type of fracture along alternating slip planes at the SC crack tip. We provide an analytical expression for the stress field of a dilatation line in the vicinity of a crack, from which we derive all the hydrogen–crack–dislocation elastic interactions terms. Diffusing hydrogen also has a marked pinning effect on a dislocation source at a crack tip. This effect exhibits a strong dependence on the crystal orientation. These results are discussed from the viewpoint of SCC fracture mechanisms.

Microstructures and Mechanical Properties of Nanostructured Copper-304 Stainless Steel Multilayers Synthesized by Magnetron Sputtering

ABSTRACTNanostructured Cu/304 stainless steel (SS) multilayers were prepared by magnetron sputtering at room temperature. 304SS has a face-centered cubic (fcc) structure in bulk. However, in the Cu/304SS multilayers, the SS layers exhibited fcc structure for layer thickness of less than or equal to 5 nm. For 304SS layer thickness larger than 5nm, bcc 304SS grains were observed to grow on top of the initial ≈ 5 nm of fcc SS. The maximum hardness of Cu/304SS multilayers was ≈ 5.5 GPa (factor of two enhancement compared to rule of mixtures hardness) achieved at a layer thickness of 5nm, with a decrease in hardness with decreasing layer thickness below 5 nm. The hardness of fcc/fcc Cu/304SS multilayers (layer thickness ≤ 5 nm) is compared with Cu/Ni, another fcc/fcc system, to gain insight on how the mismatch in physical properties such as lattice parameters and shear moduli of the constituent layers affect the peak hardness achieved in these nanoscale systems.

Comparison of residual microstructures associated with impact craters in fcc stainless steel and bcc iron targets: the microtwin versus microband issue

Deformation microtwins characteristic of Neumann bands dominate the residual microstructures beyond a dynamic recrystallization and highly deformed regime below the crater wall of impact craters in polycrystalline bcc iron targets impacted by 3.2 mm diameter iron projectiles at velocities ranging from 0.5 to 3.8 km s −1. Corresponding impact craters in polycrystalline fcc 304L stainless steel targets impacted by 3.2 mm diameter stainless steel projectiles at velocities ranging from 0.5 to 3.9 km s −1 were observed to have a more limited dynamic recrystallization zone at the crater wall followed by a highly deformed transition into a region of mostly microtwins, but with some intermixing of grains containing microbands.

The annealing behavior of implanted nitrogen in fcc stainless steel

Post-implantation annealing of N-implanted 304 stainless steel at 400 °C has been investigated by conversion electron Mossbauer spectroscopy, X-ray diffraction and Auger electron spectroscopy. After a 1 h anneal, the near complete dissolution of the as-implanted Fe2N-like nitride phase results in a 9 at.% N fcc solid-solution phase. After the final anneal (64 h), N has diffused to a depth of about 2 μm and remains in solid solution with an average content of 4 at.%. An average N diffusion coefficient at 400 °C is estimated to be ∼10-12-10-14 cm2/s, depending on anneal time, too small to explain the deep penetration observed in high-flux, high-dose N-implanted stainless steel. The present results provide additional evidence for beam controlled N migration where Cr plays an important role.

Growth-specific carbide precipitation at annealing twins in 304 stainless steel

Here the authors report some unique and specific growth features for lamellar M{sub 23}C{sub 6} precipitates in aged type 304 stainless steel which are not only directional with regard to {l_brace}111{r_brace} trace directions, but also very specific with regard to their alignment with specific annealing twin boundaries -- boundary geometry and crystallography. This growth specificity has not been reported previously in connection with coherent twins in FCC stainless steels.

Vacancy formation energies in bcc and fcc stainless steels by positron annihilation

The temperature dependence of the positron annihilation peak coincidence rate has been measured in bcc and fcc stainless steels. Trapping model analysis of the data shows that the vacancy formation energies in the bcc stainless steels are about 20% lower than those in the fcc stainless steels. The vacancy formation energies calculated from current theoretical models are about 10–15% larger than those determined experimentally.