Stress-assisted Grain Research Articles

Effect of test temperature on deformation microstructure and tensile property of a novel Ni-Co-based superalloy

The tensile behavior, deformation microstructures and failure mechanisms of a novel Ni-Co base precipitation-hardened superalloy, have been investigated at room temperature (RT), 450, 550, 650 and 750 °C. Deformation of the alloy at RT, 450, 550 °C is primarily mediated by dislocation slip and stacking faults while nano-sized deformation twinning is also involved at 650 and 750 °C. The yield and tensile strength decrease with increasing test temperature. In the temperature range of 450–650 °C, however, no obvious reduction in the yield strength and work-hardening rate at low strains is observed, which could be correlated to a high density of stacking faults or deformation twins. Low yield strength and work-hardening at 750 °C, could be related to the softening by thermal activation and the softening of γ′ particles caused by the repeated cutting via stacking faults or twins. However, as the testing temperature is over 450 °C, elongation of the alloy decreases significantly with increasing test temperature, which could be associated with intergranular cracking induced by stress-assisted grain boundary oxidation.

Zinc-Induced Liquid Metal Embrittlement in Austenitic Microstructures

The problem of liquid metal embrittlement (LME) is one of the main concerns to be addressed when developing high-strength automotive steels. Although LME has been extensively investigated over the past decade, the role of LME-induced cracking on the failure mechanism of steel substrates has not been adequately explored. This study investigates the influence of LME cracking on the tensile properties and failure mechanism of an austenitic microstructure. An in-depth analysis of the LME crack propagation path revealed that the crack propagated predominantly along highangle, random grain boundaries. The detailed failure analysis showed that the Zn-coated austenitic steel failed in an intergranular mechanism without any plastic strain being applied to the grains during deformation. The results of the study indicate that stress-assisted grain boundary diffusion is responsible for the premature failure of Zn-coated austenitic microstructures at much lower tensile stresses than that of the uncoated specimen. It was found that the application of thermomechanical conditions characterized by low stress and low temperature, combined with microstructures featuring a low Zn grain boundary diffusion rate, can effectively reduce LME cracking.

On the role of plastic relaxation in stress assisted grain boundary oxidation

The influence of plasticity on the high-temperature stress-assisted grain boundary oxidation of nickel-based superalloys used in applications such as turbine rotor discs is investigated using the method of discrete dislocation plasticity (DDP). The misfit stress fields of nib-shaped intrusions are captured by a continuous distribution of edge dislocations whose extra half planes represent the volumetric misfit of the oxide, which is implemented in a planar formulation of DDP by invoking the linear superposition principle. DDP simulations show that stresses generated by an intrusion several microns or more in size are large enough to generate dislocation pileups with associated stresses at the intrusion interface on the order of 1 GPa, which in turn lead to localized growth and morphology change of the intrusion by stress-assisted diffusion. This morphology change relaxes the compression stress inside the intrusion near the base, and therefore increases the fracture resistances of the intrusion. The effects of applied loading and background plasticity on the growth rate of the intrusion in defect-free and prestrained samples are predicted. It is found that applied tensile stress generally increases grain boundary oxidation, while in prestrained samples the enhancement of the intrusion growth rate by the applied load is insignificant due to dislocation pile-ups ahead of the oxidation process.

Advanced time-resolved characterization of Stress Assisted Grain Boundary Oxidation of 718 Ni superalloy

Nickel superalloys are used for harsh condition application cases as they have high chemo-thermomechanical stability. However, they can suffer from embrittlement due to stress corrosion cracking. This effect is difficult to observe as it can take place at long time scales. Here we propose a feasible experiment to study stress assisted grain boundary oxidation, a phenomenon that has similar mechanisms involved that can take place in laboratory compatible time scales. We observed the event using phase and diffraction contrast tomography while applying monotonic loading of the sample at 650 °C. This initial analysis shows that the experimental setup is a good candidate for the study of such degradation mechanism.

Shear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals?

Grain size reduction is a very efficient way to block dislocation movements and therefore create very strong metals and alloys. Not only grain boundaries are known obstacles for dislocations, but when reaching nanometer dimensions, crystallites usually become dislocation free, which imposes an additional constraint to develop plasticity. A recent effort to understand grain boundaries-based deformation mechanisms has therefore emerged. These mechanisms can be manifold, involving conservative and diffusive processes that are very poorly understood. A first approach consisting in downscaling mechanisms that are documented at large scale such as Coble creep, proved very limited. On the other hand, stress-assisted grain growth or shear-coupled grain boundary migration, that were recently observed in small-grained materials at room or low temperature may provide a crucial step to fully understand dislocation-less plasticity in nanocrystals. As this is a completely new field with many more degrees of freedom, a continuous research effort has to be carried out to link the mechanical properties of nanocrystals to these mechanisms specifically linked to grain boundaries.

Grain growth of nanocrystalline aluminum under tensile deformation: A combined in situ TEM and atomistic study

Nanocrystalline Al thin films have been strained in situ in a transmission electron microscope using two separate nanomechanical techniques involving a push-to-pull device and a microelectromechanical system (MEMS) device. Deformation-induced grain growth was observed to occur via stress-assisted grain boundary migration with extensive grain growth occurring in the necked region, indicating that the increase in local stress drives the boundary migration. Under applied tensile stresses close to the ultimate tensile strength of 450 MPa for a nanocrystalline Al specimen, measured boundary migration speeds are 0.2 – 0.7 nm s−1 for grains outside necked region and increases to 2.5 nm s−1 for grains within the necked region where the local estimated tensile stresses are elevated to around 630 MPa. By tracking grain boundary motion over time, molecular dynamics simulations showed qualitative agreement in terms of pronounced grain boundary migration with the experimental observations. The combined in situ observation and molecular dynamics simulation results underscore the important role of stress-driven grain growth in plastically deforming nanocrystalline metals, leading to intergranular fracture through predominant grain boundary sliding in regions with large localized deformation.

Synergetic deformation-induced extraordinary softening and hardening in gradient copper

A gradient-structured Cu sample composed of grain-size gradient surface layer (GSL) and homogeneous coarse-grained (CG) core was fabricated by surface severe plastic deformation. Microhardness measurements revealed that both tension-induced softening in the top nanograined surface layer and hardening in the subsurface layer of integrated gradient sample were far more pronounced than that of a freestanding GSL, i.e. extraordinary softening and hardening occurred in gradient sample. The synergetic deformation between GSL and CG core produced accumulation of geometrically necessary dislocations and thereby back stress strengthening, resulting in extra flow stress and hardening. The extraordinary softening was attributed to the extra flow stress-assisted grain coarsening in nanograined layer. Furthermore, strain measurements proved that the CG matrix plays critical roles in stabilizing the grain coarsening and homogenizing the strain distribution in nanograined layer, which were favorable for the uniform ductility.

Role of oxygen in enhanced fatigue cracking in a PM Ni-based superalloy: Stress assisted grain boundary oxidation or dynamic embrittlment?

The role of oxygen in enhanced fatigue cracking in an advanced Ni-based superalloy for turbine disc application has been evaluated in fatigue crack initiation and propagation stages along with static oxidation tests. It is found that the grain boundary oxide intrusion has a layered structure. The microstructure- and deformation-dependent grain boundary oxidation dominates the fatigue crack initiation and early propagation processes. As the crack propagates, this contribution arising from oxidation damage may gradually be overtaken by dynamic embrittlement processes until the mechanical damage outstrips the oxygen-related damage, resulting in a transition from intergranular to transgranular crack propagation.

Fatigue-Assisted Grain Growth in Al Alloys

Stress-assisted grain growth at room temperature is known for materials with nanocrystalline grains. For larger grain sizes, the grain growth usually takes place at higher homologous temperatures even under stress. Here we report, for the first time, significant grain growth at room temperature under fatigue loading in microcrystalline grains (≥10 μm) in Al 7075. We demonstrate that this grain growth at room temperature is similar to non-uniform grain growth due to grain rotation and coalescence rather than the thermally and the stress-assisted driven grain growth. We show that the grain growth is associated with the formation of a strong near-Cu {112}<111> texture component as a result of fatigue-assisted deformation. These changes in microstructural features (viz., grain size, grain orientations and texture) are fundamentally important in understanding the cyclic crack induced deformation behavior and for predicting the fatigue lifetime in structural materials.

Stress-assisted grain growth in nanocrystalline metals: Grain boundary mediated mechanisms and stabilization through alloying

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature.

Influences of triple junctions on stress-assisted grain boundary motion in nanocrystalline materials

Stress-assisted grain boundary motion is among the most studied modes of microstructural evolution in crystalline materials. In this study, molecular dynamics simulations were used to systematically investigate the influences of triple junctions on the stress-assisted motion of symmetric tilt grain boundaries in Cu by considering a honeycomb nanocrystalline model. It was found that the grain boundary motion in nanocrystalline models was highly sensitive to the loading mode, and a strong coupling effect which was prevalent in bicrystal models was only observed when simple shear was applied. In addition, the coupling factor extracted from the honeycomb model was found to be larger and more sensitive to temperature change than that from bicrystal models for the same type of grain boundary under the same loading conditions. Furthermore, the triple junctions seemed to exhibit unusual asymmetric pinning effects to the migrating grain boundary and the constraints by the triple junctions and neighboring grains led to remarkable non-linear grain boundary motion in directions both parallel and normal to the applied shear, which was in stark contrast to that observed in bicrystal models. In addition, dislocation nucleation and propagation, which were absent in the bicrystal model, were found to play an important role on shear-induced grain boundary motion when triple junctions were present. In the end, a generalized model for shear-assisted grain boundary motion was proposed based on the findings from this research.

Quantitative in-situ TEM study of stress-assisted grain growth

Abstract

Stress-driven grain growth in ultrafine grained Mg thin film

Significant grain growth, manifest as a doubling of the average grain size, was triggered during high rate Kolsky bar experiments on ultrafine grained Mg. This grain growth is attributed to the high applied stress and is consistent with recent reports of stress-driven grain boundary migration. The experimental results indicate that stress can trigger grain growth, thereby reducing the importance of thermal activation at very high stresses. This also indicates that stress-assisted grain growth is not restricted to small grained fcc metals.

Observations of grain boundary impurities in nanocrystalline Al and their influence on microstructural stability and mechanical behaviour

The exceptional properties of nanocrystalline materials lend themselves to a wide range of structural and functional applications. There is recent evidence to suggest that grain boundary impurities may have a dramatic effect on the stability, strength and ductility of nanocrystalline metals and alloys. In this study, transmission electron microscopy and atom probe tomography were used to characterize specimens deposited at different base pressures, thus providing a direct comparison of impurity content with microstructural stability and mechanical behaviour. Atom probe measurements provide clear experimental evidence of grain boundary segregation of oxygen in samples deposited at higher base pressures. It is proposed that these oxygen atoms pin the boundaries, preventing stress-assisted grain growth and resulting in increased strength and loss in ductility. This study provides the first direct experimental evidence that boundary impurities play a critical role in determining the microstructural stability and deformation behaviour of nanocrystalline metals.

Investigations on the deformation behavior of polycrystalline Cu nanowires and some factors affecting the modulus and yield strength

This paper consists of two parts. In the first part, the deformation behavior of polycrystalline Cu nanowires under tension, bending and torsion is studied by using molecular dynamics simulations. The results show that both free surfaces and grain boundaries can control the plastic deformation. In detail, for polycrystalline Cu nanowires under tension, the stress-assisted grain growth caused by atomic diffusion and grain boundary migration is found, which is thought to be the reason for necking. Then under the bending effect, full dislocations and deformation twins of polycrystalline Cu nanowires are found in the grains. Moreover, the twin boundaries act as obstacles to dislocation motion. Finally, full dislocations and fivefold deformation twins are detected in polycrystalline Cu nanowires in the torsional state. These phenomena are in good agreement with the experimental observations of Liao et al (2005 Appl. Phys. Lett. 86 103112). The second part investigates the effects of sample shape and crystallographic structure on the modulus and yield strength under tension, bending and torsion. The results demonstrate that the modulus (in particular the bending and torsion modulus) can be significantly influenced by both the effects; however, remarkable difference in yield strength can merely be caused by different crystallographic structures (here, different crystallographic structures refer to polycrystalline and single-crystalline structures).

SMIG model: A new geometrical model to quantify grain boundary-based plasticity

Stress-assisted grain boundary migration is a mechanism that has proven active in polycrystals but that relies on a limited number of models. Those models do not apply to general grain boundaries and often fail to reproduce the intensity of the coupling between the migration distance and the produced shear strain. Recently a new geometrical model, entitled the shear migration geometrical (SMIG) model, that is valid for all tilt boundaries has been introduced to account for the low coupling factors observed experimentally. In the present work we propose, on the basis of this model, (i) to determine, for a given tilt grain boundary, the number of possible coupling modes and (ii) to evaluate the shuffling needed to rearrange atoms as the grain boundary migrates. We will show that, for a given grain boundary defined by a misorientation angle and a grain boundary plane, it is almost always possible to find a coupling mode implying the shuffling of up to 20 atoms, supposedly without long-range diffusion. This characteristic is of prime importance in polycrystals where collective grain boundary motions are required to accommodate strain.

Correlations between mechanical stress, electrical conductivity and nanostructure in Al films on a polymer substrate

The relation between mechanical stress, electrical resistivity and film nanostructure is investigated here for 100 nm thick aluminum films deposited on micrometer-thick polysulfone. It is observed that film electrical resistivity increases significantly before mechanical failure occurs, because of the formation of micro-cracks on the film surface. The applied tensile stress also influences the film nanostructure in an irreversible manner, causing plastic rearrangement of the film grains well before macroscopic failure occurs, which suggests stress-assisted grain growth at the nanostructural level.

Roles of grain boundary and dislocations at different deformation stages of nanocrystalline copper under tension

The plastic deformation of nanocrystalline copper subjected to tension has been studied using molecular dynamics simulation. The results show that, in the initial stage, the deformation is mainly boundary-mediated in small grains; while in the late stage, the deformation is accommodated by dislocations in large grains. It is also found that the stress-assisted grain growth occurs owing to atomic diffusion and grain boundary migration. These results are consistent with recent experimental observations.

In situ TEM observations of fast grain-boundary motion in stressed nanocrystalline aluminum films

Free-standing nanocrystalline Al thin films have been strained in situ in a transmission electron microscope at room-temperature. Extensive grain-boundary migration accompanies the in situ loading and has been observed to occur preferentially at crack tips and only in the presence of the applied stress. This grain growth precedes dislocation activity, and measured boundary velocities are greater than can be explained by diffusive processes. The unambiguous observations of stress-assisted grain growth are compatible with recently proposed models for stress-coupled grain-boundary migration. The growth occurs in a faceted manner indicative of preferential boundaries. The fast collapse of small grains with sizes of 30–50 nm demonstrates the unstable nature of a nanocrystalline structure. Clearly observable shape changes testify to the effectiveness of grain-boundary migration as a deformation mechanism, and preferential grain growth at crack tips resulted in efficient crack tip blunting, which is expected to improve the films’ fracture toughness.

Stress‐Driven Surface Topography Evolution in Nanocrystalline Al Thin Films

Stress-assisted grain growth at room temperature is identified as a plastic deformation mechanism in nanocrystalline thin films. Unique surface relief is attributed to the direct application of stress-coupled grain boundary migration theory. The figure shows a false-color SEM image of surface topography and an AFM height profile as a result of stress-assisted grain growth. A strategy for tailoring the mechanical properties of nanostructured metals is shown.