Strengthening Effect Of Grain Boundaries Research Articles

Simulation of polycrystal deformation with grain and grain boundary effects

Modeling the strengthening effect of grain boundaries (Hall–Petch effect) in metallic polycrystals in a physically consistent way, and without invoking arbitrary length scales, is a long-standing, unsolved problem. A two-scale method to treat predictively the interactions of large numbers of dislocations with grain boundaries has been developed, implemented, and tested. At the first scale, a standard grain-scale simulation (GSS) based on a finite element (FE) formulation makes use of recently proposed dislocation-density-based single-crystal constitutive equations (“SCCE-D”) to determine local stresses, strains, and slip magnitudes. At the second scale, a novel meso-scale simulation (MSS) redistributes the mobile part of the dislocation density within grains consistent with the plastic strain, computes the associated inter-dislocation back stress, and enforces local slip transmission criteria at grain boundaries. Compared with a standard crystal plasticity finite element (FE) model (CP-FEM), the two-scale model required only 5% more CPU time, making it suitable for practical material design. The model confers new capabilities as follows: (1) The two-scale method reproduced the dislocation densities predicted by analytical solutions of single pile-ups. (2) Two-scale simulations of 2D and 3D arrays of regular grains predicted Hall–Petch slopes for iron of 1.2 ± 0.3 MN/m 3/2 and 1.5 ± 0.3 MN/m 3/2, in agreement with a measured slope of 0.9 ± 0.1 MN/m 3/2. (3) The tensile stress–strain response of coarse-grained Fe multi-crystals (9–39 grains) was predicted 2–4 times more accurately by the two-scale model as compared with CP-FEM or Taylor-type texture models. (4) The lattice curvature of a deformed Fe-3% Si columnar multi-crystal was predicted and measured. The measured maximum lattice curvature near grain boundaries agreed with model predictions within the experimental scatter.

Strengthening mechanism of steels treated by barium-bearing alloys

The deoxidation, desulfurization, dephosphorization, microstructure, and mechanical properties of steels treated by barium-bearing alloys were investigated in laboratory and by industrial tests. The results show that barium takes part in the deoxidation reaction at the beginning of the experiments, generating oxide and sulfide compound inclusions, which easily float up from the molten steel, leading to the rapid reduction of total oxygen content to a very low level. The desulfurization and dephosphorization capabilities of calcium-bearing alloys increase with the addition of barium. The results of OM and SEM observations and mechanical property tests show that the structure of the steel treated by barium-bearing alloys is refined remarkably, the lamellar thickness of pearlitic structure decreases, and the pearlitic morphology shows clustering distribution. Less barium exists in steel substrate and the enrichment of barium-bearing precipitated phase mostly occurs in grain boundary and phase boundary, which can prevent the movement of grain boundary and dislocation during the heat treatment and the deformation processes. Therefore, the strength and toughness of barium-treated steels are improved by the effect of grain-boundary strengthening and nail-prick dislocation.

A nanoindentation study on grain-boundary contributions to strengthening and aging degradation mechanisms in advanced 12 Cr ferritic steel

Nanoindentation experiments and microstructural analysis were performed on advanced 12% Cr ferritic steel having extremely fine and complex martensitic microstructures, to answer unsolved questions on the contributions of grain boundaries to strengthening and aging degradation mechanisms in both as-tempered and thermally aged steels. Interesting features of the experimental results led us to suggest that among several high angle boundaries, block boundary is most effective in enhancing the macroscopic strength in as-tempered virgin sample, and that a decrease in matrix strength rather than reduction in grain-boundary strengthening effect is primarily responsible for the macroscopic softening behavior observed during thermal exposure.

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.

Cyclic deformation behavior and intergranular fatigue cracking of a copper bicrystal with a parallel grain boundary

Abstract Cyclic deformation behavior and intergranular fatigue cracking of a [4 9 16]/[4 9 27] copper bicrystal with a grain boundary (GB) parallel to the stress axis were investigated under constant plastic strain control at room temperature in air. It was found that cyclic saturation stresses of the bicrystal increased with increasing strain amplitude, and were higher than the plateau stress of 28–30 MPa, if modified by an orientation factor ΩB of the bicrystal. Surface observations showed that a GB affected zone (GBAZ) with secondary slip formed owing to plastic strain incompatibility near the GB. As cyclic deformation was continued, fatigue cracks always initiated at the intersection sites of persistent slip bands (PSBs) with the GB. Gradually, they linked each other along the GB, leading to intergranular cracking. However, fatigue crack nucleating along slip bands was not observed as intergranular cracking occurred. It is indicated that intergranular fatigue cracking is the major damage mode even though the GB was parallel to the stress axis. Based on the results above, the GB strengthening effect and intergranular fatigue cracking mechanism were discussed in combination with the GBAZ and the interaction between PSBs and GB.

Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Abstract Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (~200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.