Showing papers on "Grain boundary strengthening published in 2015"

Grain-size dependent mechanical behavior of nanocrystalline metals

Abstract: Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 106 s−1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d>1 μm, 1 μm

Deformation microstructures, strengthening mechanisms, and electrical conductivity in a Cu–Cr–Zr alloy

Abstract: The ultrafine-grained microstructures, mechanical properties and electrical conductivity of a Cu–0.87%Cr–0.06%Zr alloy subjected to multiple equal channel angular pressing (ECAP) at temperatures of 473–673 K were investigated. The new ultrafine grains resulted from progressive increase in the misorientations of strain-induced low-angle boundaries during the multiple ECAP process. The development of ultrafine-grained microstructures is considered as a type of continuous dynamic recrystallization. The multiple ECAP process resulted in substantial strengthening of the alloy. The yield strength increased from 215 MPa in the original peak aged condition to 480 MPa and 535 MPa after eight ECAP passes at 673 K and 473 K, respectively. The strengthening was attributed to the grain refinement and high dislocation densities evolved by large strain deformation. Modified Hall–Petch analysis indicated that the contribution of dislocation strengthening to the overall increment of yield stress (YS) through ECAP was higher than that of grain size strengthening. The formation of ultrafine grains containing high dislocation density leads to a small reduction in electrical conductivity from 80 to 70% IACS.

Study of dynamic recrystallization in a Ni-based superalloy by experiments and cellular automaton model

Abstract: The hot compressive behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rates of 0001–1 s−1 and deformation temperatures of 920–1040 °C It is found that the dynamic recrystallization is the main softening mechanism for the studied superalloy during hot deformation The deformation temperature and strain rate have a significant influence on the dynamically recrystallized grain size Based on the experimental results, an inverse power law equation is established to describe the relationship between the dynamically recrystallized grain size and the steady-state flow stress A cellular automaton model with probabilistic state switches is established to simulate the dynamic recrystallization behaviors of the studied superalloy The flow stress and the dynamically recrystallized grain size can be well predicted by the established model Then, the dynamic recrystallization kinetic and the evolutions of the average grain size and grain boundary fraction are studied based on the simulated results The simulated results show that the dynamic recrystallization is initially heterogeneous, and gradually becomes homogeneous with the increase of the volume fraction of dynamic recrystallization With the increase of strain, the average grain size decreases, while the grain boundary fraction increases Furthermore, the average grain size and the grain boundary fraction remain relatively constant when the deformation is under a steady state

The Contribution of Constitutional Supercooling to Nucleation and Grain Formation

Abstract: The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al. On that basis, Winegard and Chalmers further considered ‘supercooling and dendritic freezing of alloys’ where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

High-temperature stability and grain boundary complexion formation in a nanocrystalline Cu-Zr alloy

Abstract: Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were created with mechanical alloying and their thermal stability was studied from 550–950°C. Annealing drove Zr segregation to the grain boundaries, which led to the formation of amorphous intergranular complexions at higher temperatures. Grain growth was retarded significantly, with 1 week of annealing at 950°C, or 98% of the solidus temperature, only leading to coarsening of the average grain size to 54 nm. The enhanced thermal stability can be connected to both a reduction in grain boundary energy with doping as well as the precipitation of ZrC particles. High mechanical strength is retained even after these aggressive heat treatments, showing that complexion engineering may be a viable path toward the fabrication of bulk nanostructured materials with excellent properties.

Hall–Petch Breakdown in Fine-Grained Pure Magnesium at Low Strain Rates

Abstract: The effects of grain size and strain rate on tension behavior at ambient temperature were investigated for several extruded magnesium with an average grain size in the range between 1 and 20 μm. In quasi-static strain rate regimes (1 × 10−2 to 10−4 s−1), the activation volume was ~ 20b 3 (b is the Burgers vector), which suggested that the major contribution of deformation was cross-slip and/or multiple slips. In contrast, in low strain rate regimes (<1 × 10−4 s−1), the ductility increased with grain refinement, and the maximum elongation-to-failure was 230 pct for a grain size of 1.2 μm and a strain rate of 1 × 10−5 s−1; such ductility was never observed in magnesium at room temperature. In addition, the activation volume was also reduced to ~5b 3. Observations of the deformed surface revealed plentiful traces of grain boundary sliding. This mechanism played an important role in deformation. As a result, while the yield strength was aligned on the Hall–Petch relation in quasi-static strain rate regimes, an inverse Hall–Petch effect was observed in low strain rate regimes.

Friction Stir Processing of a High Entropy Alloy Al0.1CoCrFeNi

Abstract: High entropy alloys are a new class of metallic materials with a potential for use in structural applications. However, most of the studies have focused on microhardness and compressive strength measurements for mechanical properties determination. This study presents the tensile deformation behavior of a single-phase, face-centered cubic Al0.1CoCrFeNi high entropy alloy (HEA). Friction stir processing was carried out to refine the grain size. Scanning electron microscopy and electron backscatter diffraction were carried out for microstructural examination. The grain size of the alloy was on the order of millimeters in the as-received condition. The average grain size after friction stir processing of the alloy was 14 ± 10 micrometers. The mechanical properties were determined through microhardness measurement and mini-tensile tests. The friction stir processed alloy showed a total elongation of ~75% for the mini-tensile sample used and yield strength of 315 MPa. It is an exceptional combination of strength and ductility. Friction stress was determined to be 174 MPa and the Hall–Petch coefficient was 371 MPa (µm)1/2. Such a high value of Hall–Petch coefficient suggests that grain boundary strengthening can be a very effective strengthening mechanism for the HEA Al0.1CoCrFeNi.

Inverse pseudo Hall-Petch relation in polycrystalline graphene.

Abstract: Understanding the grain size-dependent failure behavior of polycrystalline graphene is important for its applications both structurally and functionally. Here we perform molecular dynamics simulations to study the failure behavior of polycrystalline graphene by varying both grain size and distribution. We show that polycrystalline graphene fails in a brittle mode and grain boundary junctions serve as the crack nucleation sites. We also show that its breaking strength and average grain size follow an inverse pseudo Hall-Petch relation, in agreement with experimental measurements. Further, we find that this inverse pseudo Hall-Petch relation can be naturally rationalized by the weakest-link model, which describes the failure behavior of brittle materials. Our present work reveals insights into controlling the mechanical properties of polycrystalline graphene and provides guidelines for the applications of polycrystalline graphene in flexible electronics and nano-electronic-mechanical devices.

A Unified Model for the Prediction of Yield Strength in Particulate-Reinforced Metal Matrix Nanocomposites.

Abstract: Lightweighting in the transportation industry is today recognized as one of the most important strategies to improve fuel efficiency and reduce anthropogenic climate-changing, environment-damaging, and human death-causing emissions. However, the structural applications of lightweight alloys are often limited by some inherent deficiencies such as low stiffness, high wear rate and inferior strength. These properties could be effectively enhanced by the addition of stronger and stiffer reinforcements, especially nano-sized particles, into metal matrix to form composites. In most cases three common strengthening mechanisms (load-bearing effect, mismatch of coefficients of thermal expansion, and Orowan strengthening) have been considered to predict the yield strength of metal matrix nanocomposites (MMNCs). This study was aimed at developing a unified model by taking into account the matrix grain size and porosity (which is unavoidable in the materials processing such as casting and powder metallurgy) in the prediction of the yield strength of MMNCs. The Zener pinning effect of grain boundaries by the nano-sized particles has also been integrated. The model was validated using the experimental data of magnesium- and titanium-based nanocomposites containing different types of nano-sized particles (namely, Al2O3, Y2O3, and carbon nanotubes). The predicted results were observed to be in good agreement with the experimental data reported in the literature.

Structural impact on the Hall–Petch relationship in an Al–5Mg alloy processed by high-pressure torsion

Abstract: The evolution of microstructure and microhardness was studied in a commercial 5483 Al–5Mg alloy processed by high-pressure torsion (HPT) under a pressure of 6.0 GPa up to 10 turns. Significant grain size refinement was observed even after 1/4 turn, and additional processing led to a further grain size reduction and a shift in the distribution of grain boundary misorientation angles towards higher values. An essentially fully homogeneous microstructure was reached after 10 turns with a final grain size of ~70 nm, a saturation Vickers microhardness of Hv≈240 which was attained at and above equivalent strains of ~150, a relatively narrow grain size distribution and a fraction of ~80% of high-angle grain boundaries. Analysis shows the Hall–Petch plot deviates from the conventional linear relationship for samples processed through small numbers of turns, but after 3 or more turns there is a direct correlation between the results obtained in HPT processing and coarse-grained samples.

Thermodynamic Grain Size Stabilization Models: An Overview

Abstract: Grain boundaries in a nanocrystalline microstructure produce an increase in the excess free energy of the system. Grain growth is a consequence of the thermodynamic driving force reducing this excess. Thermodynamic stabilization is an approach based on eliminating the driving force by suitable alloy additions that can produce a metastable equilibrium state at the nanoscale grain size, as opposed to kinetic stabilization where the grain growth mobility is restricted by pinning and/or drag mechanisms. The present paper reviews and compares various models proposed for thermodynamic stabilization.

Coupling effect of intergranular and intragranular particles on ductile fracture of Mo–La2O3 alloys

Abstract: Mo– x La 2 O 3 ( x =0, 0.5, 1.0, 1.5, and 2.0 wt%) alloys were prepared by using the solid–solid mixing/doping method. Addition of La 2 O 3 particles effectively refines the grains and significantly elevates the recrystallization temperature of the Mo alloys. Both intragranular and intergranular La 2 O 3 particles are formed in the alloys, with size and volume fraction increasing with La 2 O 3 additions. Room temperature (RT) tensile testing results show that the Mo–La 2 O 3 alloys with 0.5–1.0 wt% La 2 O 3 additions have high strength (100 MPa above the pure Mo) and simultaneously great elongation (2 times of the pure Mo). Elongation will be reduced when La 2 O 3 addition beyond 1.5 wt%. The strengthening mechanisms of the Mo–La 2 O 3 alloys are mainly intragranular particle strengthening and grain boundary strengthening. The ductile fracture is predominantly controlled by the microcrack nucleation at intergranular particles. However, high temperature (HT) tensile testing results show that the Mo–La 2 O 3 alloys with 1.0–1.5 wt% La 2 O 3 additions have the superior strength–elongation combination. Microvoids (dimples) are formed after intragranular particle debonded. The HT deformation mechanisms involve the microvoid coalescence in the grain interior and the intergranular microcrack propagation. The ductile fracture depends on a competition between the intergranular and intragranular damage development. The present experimental results provide in-depth insight into the coupling effect of intergranular and intragranualr particles on the ductile fracture of Mo–La 2 O 3 alloys at RT and HT, respectively.

Strengthening mechanisms in a Zr-modified 5083 alloy deformed to high strains

Abstract: The effect of extensive grain refinement through equal-channel angular processing (ECAP) at 300 °C on the mechanical properties was examined in a Zr-modified 5083 aluminum alloy. It was shown that any increase in the yield stress (YS) by increasing the number of pressings was attributed to additional contributions from three strengthening mechanisms: (i) grain boundary strengthening related to the size of the crystallites bounded by deformation-induced high-angle boundaries (HAB), (ii) dislocation strengthening related to highly increased dislocation density and (iii) solute strengthening related to promoting dynamic strain aging (DSA) by intense plastic straining. Grain size strengthening is the primary contributor to the increase in the YS as a result of ECAP. Dislocation strengthening plays an important but minor role in strengthening the alloy after ECAP. No ECAP effect on the dispersion strengthening was observed. However, a dispersion of nanoparticles was found to cause an increase in the YS through grain refinement and the accumulation of lattice dislocations during ECAP. It was found that the deformation-induced low angle boundaries (LABs) play an insignificant role in the strengthening of the material.