Grain Boundary Deformation Research Articles

Wear behavior of novel CVD-coated wiper inserts’ with various chip-breakers and resulting surface integrity during dry drilling of Ti-6Al-4V

In this paper, a comprehensive investigation in terms of tool wear/life, chip morphology, surface roughness/damage, diametric error, burr height, and microstructure is conducted by using an innovative integration of peripheral wiper and central stepped inserts when drilling Ti-6Al-4V alloy in a dry-cutting environment. Two geometries of peripheral wiper inserts termed as LM (groove width-1000 mm, groove depth-65 mm) and GT (groove width-720 mm, groove depth-20 mm) are evaluated at two levels of cutting speeds (50 m/min and 60 m/min) and three levels of feed rates (0.10 mm/rev, 0.15 mm/rev, and 0.20 mm/rev) while keeping the geometry of central stepped insert constant. The GT wiper configuration is found to be more suitable in terms of tool life, drilling maximum number of holes (75) at a cutting speed of 50 m/min and feed rate of 0.10 mm/rev which is ∼3-fold higher than that of the LM geometry. With respect to majority of the drilled hole attributes, better performance of LM geometry is observed in comparison to its GT counterpart due to its improved chip breakability. In addition, all holes are oversized ranging from 30 µm to100 µm. Minor plastic deformation of grain boundaries in the machined sub-surface is noticed to a depth of 10–15 µm in the direction of cutting.

Intrinsic tensile brittleness of tilted grain boundaries and its shear toughening

In the endeavors of working with microstructures in polycrystalline metals for better strength and ductility, grain boundaries (GBs) are placed at the front burner for their pivotal roles in plastic deformation. Often the mechanical properties of polycrystalline metals are governed by mutual interactions among GBs and dislocations. A thorough comprehension of GB deformation is therefore critical for the design of metals of superb performance. In this research, we investigated the mechanical behavior of symmetric tilt grain boundaries in face-centered cubic (F.C.C.) nickel, which may be subject to tension, shearing, and mixing-mode load using molecular dynamics simulations. We observed that (1) there exist four types of micro deformation mechanisms in GBs, and illustrate at the atomistic scale their distinctions and their dependence on the activation of lattice slip in the crystal; (2) GBs are intrinsically brittle under tension but exhibit ductile behavior during shearing. Shifting from pure tension with increasing shear component during mixing-mode load leads to GB toughening; and (3) there lacks conceivable dependence of GB tensile strength on tilted GBs, in contrast to a relatively rough trend of greater shear strength in GBs of large misorientation. GB energy shows no direct connection with GB strength, as broadly reported in existing literature. This research enhances our mechanistic understanding of GB plasticity in crystalline metals, and points to a potential way of making strong-yet-tough polycrystalline metals through GB engineering: in addition to GB structure manipulation, tuning the loading mode of GBs may open another avenue for their better performance.

Shear response of 〈110〉 asymmetric tilt grain boundaries in BCC-Fe

The present study investigated the effect of shear load on the 110 asymmetric tilt grain boundaries of body-centered cubic iron with molecular dynamics simulations. Various mechanisms like grain boundary (GB) sliding, shear-coupled migration, GB migration with rotation, GB decomposition, and GB deformation with dislocations emission were observed. Low-angle tilt GBs migrated with the help of two dislocation mechanisms, gliding and climbing. Two types of twin nucleation mechanisms were noticed in high-angle tilt GBs with varying tilt angles (η). For 16° ≤ η ≤ 112° and η ≥ 130° cases, dynamic self-adjustment of the atoms at the GBs caused GBs decomposition with the formation of twin boundaries (TBs). Newly nucleated TBs migrated under further applied shear load. For 112° < η < 130° case, TBs nucleated from the GB on either side of the misfit dislocations due to their localized core structure on the GB plane.

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

The comparison of densification mechanisms and microstructure evolution among TiB2 ceramics sintered under different levels of high pressure

TiB2 shows significant advantages in high-tech industry but it is hard to fulfill densification. High pressure sintering is an effective method to obtain TiB2 ceramics. This study reports the sintering condition of TiB2 ceramics under different levels of high pressure from 150 MPa to 15 GPa. The results show that the densification temperature reduced with the applied pressure. The densification mechanism of TiB2 ceramics sintered under 150 MPa during the final holding time at 1900℃ was dominated by the mixture of dislocation motion and grain boundary sliding. When the applied pressure achieves at 1.5 GPa, the densification mechanism was transformed into the plastic deformation of grains and grain boundary diffusion, a number of small angle grain boundaries, dislocation cells, substructures and straight grain boundaries were observed. Finally, the densification mechanism was transformed into plastic deformation of grains and grain boundary while the driving force increased to 15 GPa. The dislocation and substructures density increased with pressure, leading to the grain refinement. Our study compared the densification and microstructure evolution of TiB2 ceramics sintered under high pressure, proving the high pressure sintering could regulate the microstructure and refine grains.

Deformation and boundary motion analysis of a faceted twin grain boundary

In this article, molecular dynamics simulations are used to understand how a nickel bicrystal with faceted incoherent Σ3 grain boundaries responds to uniaxial tensile loading. The deformation response is studied over a wide range of temperatures (100 – 900 K) and strain rates (107 – 1010 s−1). The dislocation extraction algorithm and common neighbor analysis are employed to identify the deformation mechanisms. Our results reveal that the yield stress decreases with temperature and increases with strain rate; whereas the elastic modulus decreases with temperature and is independent of strain rate. Furthermore, incipient plasticity is detected ahead of the yield point at lower temperatures and lower strain rates. Interestingly, the incoherent twin grain boundaries are quite mobile under the uniaxial tensile loading at lower temperatures and lower strain rates. But this mobility decreased at higher temperatures and higher strain rates, thereby, confirming this faceted grain boundary's non-Arrhenius (anti-thermal) migration behavior even under mechanical loading. From a deformation perspective, the incoherent twin facet of the grain boundary served as the major source for stacking fault formation at lower temperatures and higher strain rates. However, with the increase in temperature, the stacking faults became shorter and originated from both the incoherent twin facet and the tips of coherent twin facet. These results are in qualitative agreement with the experimental results documented in the literature.

Maximizing the yield stress via synergistic optimization of grain sizes and solute concentrations in extremely fine nanograined metals: A molecular dynamics study

Molecular dynamics simulations are employed to investigate the grain boundary (GB)-segregation-induced strengthening and determine the maximal yield stress (MYS) in extremely fine nanograined Cu–Ag alloys, with variable grain sizes ranging from 5.2 to 17.9 nm alongside diverse solute concentrations. We discover that deformation mechanisms and yield stresses are adjustable through tailoring GB stability. The yield stress increases upon increasing solute concentration, up to a maximum (namely MYS) at a critical solute concentration for each grain size. The MYS shows a maximum of 3.70 to 3.72 GPa at a critical grain size of 7.3 to 10.3 nm. The strengthening and softening mechanisms related to GB segregation are uncovered, based on the model calculations and the dislocation analyses. Beyond the critical grain size, GB segregation suppresses GB-mediated processes while allowing dislocation activities, thereby causing a continuous strengthening. In contrast, below the critical grain size, the GBs, although being saturated, cannot withstand the resistance for GB deformation, which leads to a softening. This work presents a strategy to maximize the yield stress via synergistic optimization of grain sizes and solute concentrations, and provides insights into the design of ultrahigh-strength materials.

Temperature effect on the transition between grain boundary migration and sliding

Grain boundary (GB) dynamics plays an important role in the mechanical and physical properties of nanocrystalline metals. In this study, we investigate the temperature effect on GB deformation transition using molecular dynamics simulations of [100] symmetric tilt GBs in Au bicrystals. Different deformation behaviours were revealed in GBs with the same structures at varied temperatures. GB sliding occurs as the predominant deformation mechanism at elevated temperatures, while GB migration dominates at low temperatures. The temperature-dependent critical stresses for GB migration and GB sliding were compared, revealing that temperature alters the critical GB misorientation at which GB migration transitions to GB sliding. Our study provides new insight into GB-mediated plastic deformation in nanocrystalline materials.

The Atomic Observation of the Structural Change Process in Pt Networks in Air Using Environmental Cell Scanning Transmission Electron Microscopy.

The structural change in Pt networks composed of multiple chain connections among grains was observed in air at 1 atm using atomic-resolution environmental cell scanning transmission electron microscopy. An aberration-corrected incident electron probe with a wide convergence angle made it possible to increase the depth resolution that contributes to enhancing the signal-to-noise ratio of Pt network samples in air in an environmental cell, resulting in the achievement of atomic-resolution imaging. The exposure of the Pt networks to gas molecules under Brownian motion, stimulated by electron beams in the air, increases the collision probability between gas molecules and Pt networks, and the Pt networks are more intensely stressed from all directions than in a situation without electron irradiation. By increasing the electron beam dose rate, the structural change of the Pt networks became significant. Dynamic observation on an atomic scale suggested that the structural change of the networks was not attributed to the surface atomic-diffusion-induced step motion but mainly caused by the movement and deformation of unstable grains and grain boundaries. The oxidized surface layers may be one of the factors hindering the surface atomic step motion, mitigating the change in the size of the grains and grain boundaries.

Consequences of solute partitioning on hardness in stabilized nanocrystalline alloys

Phosphorus segregation at grain boundaries (GBs) in Ni–P nanocrystalline (NC) thin films has been demonstrated to impede grain growth at elevated temperatures. In this study, we investigate the impact of phosphorus segregation in Ni-1at.%P and Ni-4at.%P alloys on their hardness by using nano-indentation and post-mortem transmission electron microscopy to characterize deformation-induced microstructural changes. The NC film exhibited approximately 20% higher average hardness values when phosphorus was in solution, as a result of the as-deposited processing, compared to films that underwent subsequent thermal annealing to stabilize the grain size by partitioning the phosphorus to the GBs. Under various strain rate loading conditions, the deformation was determined to be GB-mediated control. The effects of phosphorus partitioning on intragranular and GB deformation mechanisms were evaluated using atomistic simulations. While larger grain sizes shift deformation accommodation to intragranular mechanisms, the reduction of the GB fraction with increasing grain size can increase the available solute concentrations in the boundaries leading to active GB-mediated deformation. This interplay between solid solution strengthening, solute partitioning, grain size, global and local solute concentration, and deformation is discussed in detail.

Chemical inhomogeneity inhibits grain boundary fracture: A comparative study in CrCoNi medium entropy alloy

Grain boundary (GB) fracture is arguably one of the most important reasons for the catastrophic failure of ductile polycrystalline materials. It is of interest to explore the role of chemical distribution on GB deformation and fracture, as GB segregation becomes a key strategy for tailoring GB properties. Here we report that the inhomogeneous chemical distribution effectively inhibits GB fracture in a model CoCrNi medium entropy alloy compared to a so-called ‘average-atom’ sample. Atomic deformation kinematics combined with electronic behavior analysis reveals that the strong charge redistribution ability in chemical disordered CrCoNi GBs enhances shear deformation and thus prevents GB crack formation and propagation. Inspects on the GBs with different chemical components and chemical distributions suggest that not only disordered chemical distribution but also sufficient “harmonic elements” with large electronic flexibility contribute to improving the GB fracture resistance. This study provides new insight into the influence mechanism of GB chemistry on fracture behavior, and yields a systematic strategy and criterion, from the atoms and electrons level, forward in the design of high-performance materials with enhanced GB fracture resistance.

Misorientation-dependent transition between grain boundary migration and sliding in FCC metals

Grain boundaries (GBs) play a crucial role in the mechanical behaviors of nanocrystalline materials. The dynamics of GBs depend on a variety of intrinsic and extrinsic factors, including GB misorientation, inclination, curvature, metal type and loading history. Controlling GB geometry and deformation behavior thus provides effective means to tailor the mechanical properties of nanostructured materials. However, the relationship between deformation mechanism and GB geometry is still lacking, largely due to the vast amount of metastable GB structures and deformation paths. Here, the relation between GB misorientation (θ, a critical degree of freedom for GB structure) and GB deformation mechanisms was studied by large-scale molecular dynamic simulation in face-centered cubic (FCC) metals. With increasing GB misorientation, the GB deformation mechanism transitions from migration to sliding and further to migration again. The critical transition thresholds (critical misorientations) vary with metal types. An energetic model that links deformation mechanism with GB structure was further developed with a focus on predicting the critical GB misorientation at which GB sliding supersedes GB migration. The influence of metal type on the transition threshold is well captured. We finally extend this model to general GBs with complex atomic structures and compare the theoretical predictions with data reported in the literatures and obtained from the present work. Our findings shed lights on the misorientation-dependent GB deformation mechanisms and can be utilized in GB engineering that pursues high-performance nanocrystalline metals.

Interrupted in-situ EBSD study of crystallographic characteristics and deformation micro-mechanism in TiZrHfNb refractory high entropy alloys during uniaxial tensile deformation

Compared with the phase transformation and twinning, the single-slip, cross-slip and the interaction between slip and grain boundary in metastable refractory high entropy alloys (RHEAs) still need to be explored in sufficient detail. In this work, the deformation mechanisms of two metastable RHEAs (TiZrHfNb0.4 (denoted as Nb0.4) and TiZrHfNb0.6 (denoted as Nb0.6) RHEAs) during tensile deformation were thoroughly investigated by interrupted in-situ EBSD. The results revealed that the ultimate tensile strength (UTS) of the Nb0.4 sample is higher than that of the Nb0.6 sample. This implies that the UTS can be optimized by reducing the content of Nb element in TiZrHfNb RHEA. For the Nb0.4 sample, the deformation mechanism is dominated by the deformation of grain boundaries firstly and then by the cross-slip behavior. The cross-slip system is {112}〈111¯〉/{123}〈111¯〉 slip system. In addition to the cross-slip, the kinking bands also were found to be another important deformation mechanism during the tensile. For the Nb0.6 sample, the deformation mechanism is dominated by the deformation of grain boundaries and the single-slip behavior. The slip system is {112}〈111¯〉 slip system. The difference in the slip behavior mainly contribute to the difference in the UTS between the two types of samples. This work is helpful to enrich the understanding on deformation and contribute to better design strategies of high strength RHEAs.

Shear direction induced transition mechanism from grain boundary migration to sliding in a cylindrical copper bicrystal

Control over grain boundary (GB) motion ways provides an effective mean for tailoring the mechanical and physical properties of nanocrystalline metals. However, shear direction induced transition from GB migration to sliding in nanocrystalline metals has remained elusive. In this work, we used molecular dynamics (MD) simulations to track the dynamic process of a representative Σ11(113) high-angle GB in a cylindrical copper bicrystal under controllable shear directions parallel to the GB plane. When the shear direction changed from the [332¯] direction to the [11¯0] direction (the shear angle θ increased from 0° to 90°), the dominant GB deformation mechanism switched from GB migration to sliding. The critical orientation (critical shear angle) of the transition was [85¯1¯] (75°). Atomistic observations indicated that the disconnection and the surface step nucleation provoked the GB migration and sliding, respectively. This shear direction induced transition was also found in other 〈110〉 symmetrical and asymmetric tilt GBs, as well as nano polycrystalline metals. Analytical models revealed that the transition mechanism originated from the competition between the nucleation energies of the disconnection and surface step. These insights may contribute to the ongoing search for favoring manageable design of nanocrystalline metals by GB-mediated plastic deformation.

Superior high temperature creep resistance of a cast Al–Mg–Ca-Sc alloy with multi-scale hierarchical microstructures

The creep behavior of an Al–Mg–Ca-Sc alloy was investigated in the temperature range of 225–300 °C and applied stresses of 50–80 MPa in tension tests. Superior creep resistance was achieved as compared with those of the reported heat-resistant Al alloys. This improvement is attributed to the multi-scale hierarchy of the microstructure, i.e., Al4Ca eutectic phase network along grain boundaries (GB) and a high density of dispersed nanoscale Al3(Sc, Zr) and Al6Mn precipitates in the matrix. These nanoscale precipitates hinder dislocation motion during creep and maintain high thermal stability. The network of Al4Ca along grain boundaries plays an important role in preventing GB deformation and fracture. A stress exponent of 5 is found when introducing a threshold stress into the power-law equation, which indicates a creep mechanism of lattice diffusion controlled dislocation climb.

Measurement of grain boundary strength of Inconel X-750 superalloy using in-situ micro-tensile testing techniques in FIB/SEM system

Grain boundaries (GBs), known as two-dimensional defects, are omnipresent in polycrystalline metallic alloys and thus influence a wide range of mechanical properties under different environmental conditions like irradiation and corrosion. Therefore, quantifying the strength of individual GBs is critical for understanding the degradation of mechanical properties of materials under different conditions. In this study we developed an efficient approach for the fabrication of micro-tensile specimens with a GB almost perpendicular to the tensile direction, which is expected to advance the development of individual GB tensile testing at micro or nanoscale in a wide scope of materials. An in-situ cantilever micro-tensile testing method was developed and used to quantify the strength of a ∑3 GB in Inconel X-750 with the combination of finite element modeling. The average ultimate tensile strength (UTS) of a non-irradiated ∑3 GB is estimated at around 1.4 GPa, comparable to that of a neutron-irradiated ∑3 GB with a dose of ∼1.5 dpa (1.3 GPa). Moreover, the in-situ push-to-pull micro-tensile testing technique developed in this work provides valuable insights into the high-angle GB deformation and fracture behavior. This method generates qualitatively similar ductility behavior before and after neutron irradiation as the bulk material testing. However, the ductility and UTS values obtained from this method are different from bulk measurements due to vastly different specimen dimensions.

Impact characteristic and crack propagation mechanisms of large-size molybdenum alloy single crystal and polycrystal

The impact characteristic and crack propagation mechanism for large-size Mo3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo3Nb single crystals is lower than that of Mo3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.

Tracking the sliding of grain boundaries at the atomic scale.

Grain boundaries (GBs) play an important role in the mechanical behavior of polycrystalline materials. Despite decades of investigation, the atomic-scale dynamic processes of GB deformation remain elusive, particularly for the GBs in polycrystals, which are commonly of the asymmetric and general type. We conducted an in situ atomic-resolution study to reveal how sliding-dominant deformation is accomplished at general tilt GBs in platinum bicrystals. We observed either direct atomic-scale sliding along the GB or sliding with atom transfer across the boundary plane. The latter sliding process was mediated by movements of disconnections that enabled the transport of GB atoms, leading to a previously unrecognized mode of coupled GB sliding and atomic plane transfer. These results enable an atomic-scale understanding of how general GBs slide in polycrystalline materials.

Study on the effects of H on the plastic deformation behavior of grain boundaries in nickel by MD simulation

It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress.

Aspects of Superplasticity of Metals

The assumption that stable deformation depends on the strain rate is verified. In case of stable deformation, local deformation in a weak cross-section leads to hardening of the metal. Its effect exceeds the effect of reducing the cross-sectional area, as a result of which the deformation affects other sections as well, while the deforming force increases. At low strain rates that are characteristic of superplasticity, the strain inside the grains cannot be really great. During hot deformation, there is resistance to intragrain deformation, intergrain sliding, and accommodation of grain boundaries. The phenomenon of superplasticity is investigated and fixed under stable and unstable deformation, which leads to contradictory results. It is shown numerically that the function of deformation resistance and stable deformation rate increases only when the deformation is carried out with acceleration. The effect of hardening of the surface layer of metal grains on the deformation parameters is described.