Micromechanical Deformation Research Articles

Study on micro-deformation in micro-grinding of selective laser melting FeCoCrNi high-entropy alloy based on molecular dynamics

Microscopic deformation behavior is an important part of the study of micro-grinding processing. This paper investigates the removal mechanism in the micro-grinding of selective laser melting FeCoCrNiAl0.5 high-entropy alloy. The micro-grinding process is analyzed through simulation using the molecular dynamics simulation method. The study examines how the thickness of the undeformed chip and the grain size affect the microdeformation behaviors of shear strain, micro-grinding force, the evolution of dislocations, and microstructural transformation. The experimental results confirm that the molecular dynamics simulation model accurately predicts the micro-deformation behavior of FeCoCrNiAl0.5 high-entropy alloy during micro-grinding. The micro-deformation mechanism involves dislocation slip and twinning deformation, while the removal mechanism mainly includes the elimination of axial and dendritic crystals. During micro-grinding, the grinding force experiences small to large fluctuations, and the total length of dislocations gradually increases. As the undeformed cutting thickness increases, the micro-grinding force also gradually increases. Simultaneously, the type of dislocations and laminar dislocation nuclei increase, leading to growth in the total length of dislocations. The number of high shear strain atoms, the micro-grinding force, and the degree of fluctuation in the single crystal high-entropy alloy are higher than those in the polycrystalline material, but the total dislocation length is significantly smaller.

Bending properties and deformation micromechanisms of near-α titanium alloy sheets/foils considering initial texture characteristics and size effect

Clarifying the bending deformation behaviour of metal foils is essential for meeting the lightweighting requirements of metal honeycomb structures. This study selected Ti65 alloy specimens with various thicknesses and initial α-phase textures to conduct room-temperature three-point bending experiments. A detailed comparative analysis was performed on the bending deformation behaviour of Ti65 alloy sheets and foils. It reveals that in the foil specimens, the weakening effect of surface layer grains is significant. Under the influence of size effects, the springback angle of Ti65 alloy foils with coarse α-phase grains decreases markedly. Additionally, through various methods, including global Schmid factor analysis, lattice rotation analysis, and crystal plasticity finite element method simulations, the specific relationship between α-phase texture and the bending performance of Ti65 alloy foils was thoroughly investigated. The rolling process influences the bending performance of Ti65 alloy by determining the α-phase texture. Unidirectionally rolled Ti65 alloy products exhibit a transverse α-phase texture, which predisposes them to form transverse texture α-phase macrozones during bending. In contrast, cross-directionally rolled Ti65 alloy foils possess a basal α-phase texture biased toward the transverse direction. This not only helps avoid the formation of macrozones but also ensures coordinated deformation across the foil, resulting in superior bending performance.

Justification of the Changes in Work Hardening Exponent Based on Micro-Mechanisms of Deformation in a Plain Carbon Steel

Justification of the Changes in Work Hardening Exponent Based on Micro-Mechanisms of Deformation in a Plain Carbon Steel

Microdeformation and strengthening mechanism of 3D printed TC4/TC11 gradient titanium alloy subjected to tensile loading

3D printing of heterogeneous alloys holds promising application potentials in industry, especially in the field of aerospace. However, the relationship between the microstructure and mechanical properties of heterogeneous gradient titanium alloys produced by this technique, especially the micromechanical properties and mechanisms within the gradient zones, remains largely unexplored. In this study, we investigated the tensile micromechanical properties and deformation mechanism of 3D printed TC4/TC11 titanium alloy using the digital image correlation method in combination with scanning electron microscopy. The results show that this heterogeneous gradient titanium alloy possesses superior tensile properties. Furthermore, we elucidated the influence of microstructure and grain size on the alloy’s mechanical properties. Notably, an increased presence of secondary α phases and refined grain size were found to reduce the likelihood of microcrack initiation and propagation. Interestingly, all samples fractured on the TC4 side, with the microcracks predominantly extending at a 45° angle to the tensile direction. This observation underscores the necessity for optimizing the fabrication process of the TC4 alloy. Additionally, the strengthening mechanism of the gradient-bonded region of the alloy was also explored. The heterogeneous deformation-induced strengthening was identified as a key factor enhancing the strength of the dual alloyed zone. Collectively, this study provides valuable insights for optimizing the 3D printing process of heterogeneous gradient titanium alloys. It also lays a solid foundation for future investigation in this burgeoning field.

Tensile Deformation Mechanism of an In Situ Formed Ti-Based Bulk Metallic Glass Composites.

Ti-based bulk metallic glass composites (BMGMCs) containing an in situ formed metastable β phase normally exhibit enhanced plasticity attributed to induced phase transformation or twinning. However, the underlying deformation micromechanism remains controversial. This study investigates a novel deformation mechanism of Ti-based BMGMCs with a composition of Ti42.3Zr28Cu8.3Nb4.7Ni1.7Be15 (at%). The microstructures after tension were analyzed using advanced electron microscopy. The dendrites were homogeneously distributed in the glassy matrix with a volume fraction of 55 ± 2% and a size of 1~5 μm. The BMGMCs deformed in a serrated manner with a fracture strength (σf) of ~1710 MPa and a fracture strain of ~7.1%, accompanying strain hardening. The plastic deformation beyond yielding was achieved by a synergistic action, which includes shear banding, localized amorphization and a localized BCC (β-Ti) to HCP (α-Ti) structural transition. The localized amorphization was caused by high local strain rates during shear band extension from the amorphous matrix to the crystalline reinforcements. The localized structural transition from BCC to HCP resulted from accumulating concentrated stress during deformation. The synergistic action enriches our understanding of the deformation mechanism of Ti-based BMGMCs and also sheds light on material design and performance improvement.

Study on fatigue properties with composite waveform and variable amplitude of TC21 titanium alloy pulsed laser-arc hybrid welded joints

TC21 titanium alloy is widely used in the manufacturing of key components such as aerospace industry, its welded structure in actual service is often subjected to the cyclic loading with composite waveform and variable amplitude and leads to fatigue failure. Therefore, in this work, the composite waveform and variable amplitude fatigue properties of TC21 titanium alloy pulsed laser-arc hybrid welded joints were investigated in this paper, and the micro-deformation mechanism and crack initiation mechanism of welded joints under different initial maximum cyclic stresses were revealed. The results indicated that the micro-deformation mechanism was the competition between twinning and dislocation slip in the α′ phase at low stress. When the initial maximum cyclic stress increased, the generation and slip of dislocation dominated, and the main distribution of dislocations gradually transitioned from the α′ phase to the β phase. This caused the phase transition in the β phase to the α phase, which increased the energy at the α/β phase interface and reduced the resistance of fatigue microcrack initiation accordingly, thus promoting fatigue failure. In addition, pores were the main factor of fatigue failure for welded joints. Therefore, in this work, the influence of defect size, location and shape on the fatigue life of welded joints was considered to develop a fatigue life prediction model based on the control parameter Z-parameter and equivalent stress amplitude. The predicted composite waveform and variable amplitude fatigue life of TC21 titanium alloy welded joints was within the triple error band.

Comparing microstructural and micromechanical deformation of the TMJ disc in two anterior disc displacement models.

Anterior disc displacement (ADD) has been used to establish temporomandibular joint disorder (TMD) models. Based on whether preserve of the retrodiscal attachment, the modelling methodologies include ADD with dissecting the retrodiscal attachment (ADDwd) and ADD without dissecting the retrodiscal attachment (ADDwod). This article aims to determine which model better matches the micromechanical and microstructural progression of TMD. Through meticulous microscopic observations, the microstructure and micromechanical deformation of the TMJ discs in ADDwd and ADDwod rabbit models were compared at 2 and 20 weeks. Scanning electron microscopy and transmission electron microscopy showed that collagen fibres became slenderized and straightened, collagen fibrils lost diameter and arrangement in the ADDwd group at 2 weeks. Meanwhile, nanoindentation and atomic electron microscopy showed that the micro- and nano- mechanical properties decreased dramatically. However, the ADDwod group exhibited no significant microstructure and micromechanical deformations at 2 weeks. Dissection of the retrodiscal attachment contribute in the acceleration of disease progression at the early stage, the devastating discal phenotype remained fundamentally the same within the two models at 20 weeks. ADDwod models, induced stable and persistent disc deformation, therefore, can better match the progression of TMD. While ADDwd models can be considered for experiments which aim to obtain advanced phenotype in a short time.

The influence of high strain rates on mechanical properties and microstructure of CuCrZr alloy under strong current carrying condition

The extreme condition of high-speed metal deformation under intense current flow is commonly encountered in the realms of national defense, military, and industry. However, there has always been a lack of effective research method for the micro deformation mechanism of metal under this extreme condition. We have developed a high current and high strain rate tensile (HCHST) testing platform based on RC circuits, which is used to research the micro-deformation mechanisms of CuCrZr alloy under this extreme condition. The dynamic tensile tests of CuCrZr alloy at different high strain rates (1401s−1, 2957 s−1, 5503 s−1, 8012 s−1) under the high current density (6550 A/mm2) condition were conducted using the HCHST experimental platform. The mechanical properties and microstructure of CuCrZr alloy under the HCHST were analyzed by combining TEM and EBSD. The research results indicate that the microhardness of the alloy in the HCHST #1 environment (6550 A/mm2,1401s−1) is almost the same as the original state. This is attributed to the strong current promoting the movement of dislocations and reducing the accumulation of dislocations. Under condition like HCHST#1, the dislocation annihilation rate and dislocation formation rate of CuCrZr alloy almost reach equilibrium. At higher strain rates, the dislocation multiplication rate is much higher than the dislocation annihilation rate caused by strong current. This imbalance leads to dislocation entanglement and the formation of dislocation cells, resulting in an increase in the microhardness of the material. Additionally, EBSD was used to analyze the changes in grain size, average volume fraction of LAGBs, average KAM value, and texture of the samples, further verifying the dislocation evolution and microhardness changes of copper alloys under HCHST conditions. Our research results provide strong theoretical for the application of CuCrZr in the field of electrical thermal mechanical coupling.

Effect of wire diameter compression ratio on drawing deformation of micro copper wire

A crystal plasticity finite element model was developed for the drawing deformation of pure copper micro wire, based on rate-dependent crystal plasticity theory. The impact of wire diameter compression ratio on the micro-mechanical deformation behavior during the wire drawing process was investigated. Results indicate that the internal deformation and slip of the drawn wire are unevenly distributed, forming distinct slip and non-slip zones. Additionally, horizontal strain concentration bands develop within the drawn wire. As the wire diameter compression ratio increases, the strength of the slip systems and the extent of slip zones inside the deformation zone also increase. However, the fluctuating stress state, induced by contact pressure and frictional stress, results in a rough and uneven wire surface and diminishes the stability of the drawing process.

Crystal plasticity finite element simulations on extruded Mg-10Gd rod with texture gradient

The mechanical properties of an extruded Mg-10Gd sample, specifically designed for vascular stents, are crucial for predicting its behavior under service conditions. Achieving homogeneous stresses in the hoop direction, essential for characterizing vascular stents, poses challenges in experimental testing based on standard specimens featuring a reduced cross section. This study utilizes an elasto-visco-plastic self-consistent polycrystal model (ΔEVPSC) with the predominant twinning reorientation (PTR) scheme as a numerical tool, offering an alternative to mechanical testing. For verification, various mechanical experiments, such as uniaxial tension, compression, notched-bar tension, three-point bending, and C-ring compression tests, were conducted. The resulting force vs. displacement curves and textures were then compared with those based on the ΔEVPSC model. The computational model's significance is highlighted by simulation results demonstrating that the differential hardening along with a weak strength differential effect observed in the Mg-10Gd sample is a result of the interplay between micromechanical deformation mechanisms and deformation-induced texture evolution. Furthermore, the study highlights that incorporating the axisymmetric texture from the as-received material incorporating the measured texture gradient significantly improves predictive accuracy on the strength in the hoop direction. Ultimately, the findings suggest that the ΔEVPSC model can effectively predict the mechanical behavior resulting from loading scenarios that are impossible to realize experimentally, emphasizing its valuable contribution as a digital twin.

Unveiling the deformation micro-mechanism for mechanical anisotropy of a CoCrFeNi medium entropy alloy

The equiatomic Cr-Co-Fe-Ni medium-entropy alloy has the face-centered cubic structure. Single crystals of this alloy were tested by in-situ micropillar compression along different loading axes under scanning electron microscope. The transmission electron microscopy characterization and molecular dynamics simulation were incorporated for quantitative analysis of the effects of different crystal orientations on the deformation mechanisms. The <001>-oriented pillar not only exhibited extensive deformation-induced nano twinning, but also has been identified for the first time to undergo the FCCHCP phase transformation at room temperature. The strain localization tendency of <011>-oriented samples was confirmed through uniaxial tests to interpret the significant serration on stress-strain curves. The prominent strain hardening of <111>-oriented pillars was attributed to intense intersection between slip planes as evidenced by the extra density of Lomer-Cottrell locks. Such a high hardening rate has caused subsequent kinking of pillars. Functional division of different regions of kink band was conducted based on Orowan model. In principle, multi-principal element alloys can theoretically be designed and developed to combine a variety of excellent properties, which is an important class of candidate structural materials for advanced engineering systems. These findings provide promising guidance for understanding the mechanical anisotropy and application of these alloys.

Microstructure-Based Modeling of Deformation and Damage Behavior of Extruded and Additively Manufactured 316L Stainless Steels.

In this study, we investigated the micromechanical deformation and damage behavior of commercially extruded and additively manufactured 316L stainless steels (AMed SS316L) by combining experimental examinations and crystal plasticity modeling. The AMed alloy was fabricated using the laser powder bed fusion (LPBF) technique with an orthogonal scanning strategy to control the directionality of the as-fabricated material. Optical microscopy and electron backscatter diffraction measurements revealed distinct grain morphologies and crystallographic textures in the two alloys. Uniaxial tensile test results suggested that the LPBFed alloy exhibited an increased yield strength, reduced elongation, and comparable ultimate tensile strength in comparison to those of the extruded alloy. A microstructure-based crystal plasticity model was developed to simulate the micromechanical deformation behavior of the alloys using representative volume elements based on realistic microstructures. A ductile fracture criterion based on the microscopically dissipated plastic energy on a slip system was adopted to predict the microscopic damage accumulation of the alloys during plastic deformation. The developed model could accurately predict the stress-strain behavior and evolution of the crystallographic textures in both the alloys. We reveal that the increased yield strength in the LPBFed alloy, compared to that in the extruded alloy, is attributed to the higher as-manufactured dislocation density and the cellular subgrain structure, resulting in a reduced elongation. The presence of annealing twins and favorable texture in the extruded alloy contributed to its excellent elongation, along with a higher hardening rate owing to twin-dislocation interactions during plastic deformation. Moreover, the grain morphology and defect state (e.g., dislocations and twins) in the initial state can significantly affect strain localization and damage accumulation in alloys.

Basal and prismatic slip in hafnium

Metals with hexagonal crystal structure are found in many safety-critical applications, from titanium alloy fan blades in jet engines to magnesium alloys in the automotive industry. They also exist in nuclear reactors, for example hafnium alloy control rods whose structural integrity is critical to nuclear safety. However, mechanistic understanding of the micromechanical properties and deformation behaviour of Hf remains elusive. To aid in this understanding, we performed in situ scanning electron microscopy single crystal micropillar compression tests of a commercial Hf alloy, where the samples were aligned to activate 〈a〉 prismatic and 〈a〉 basal slip systems. We then employed transmission electron microscopy to examine the dislocation structure in the deformed pillars. These two slip systems exhibited distinctly different behaviour, where prismatic slip is planar and basal slip is wavy. Prismatic slip is the easiest deformation mode, while basal slip is at least twice as difficult. Generally, the characters of prismatic and basal slip in Hf show similarities to those in some other hexagonal metals such as Ti and Zr, except for the higher relative difficulty of basal slip. Our results provide fundamental knowledge and parameters that can be used for modelling degradation and predicting performance of Hf, and shed light on the general behaviour of basal and prismatic slip systems in low-c/a-ratio hexagonal metals.

Effect of three-stage heat treatment on mechanical properties and micro-deformation mechanism of TC4 titanium alloy welded joint

The influence of three-stage heat treatment on the microstructure and mechanical properties of pulsed laser-arc hybrid welded TC4 titanium alloy joints was studied by experiments and molecular dynamics calculations. The results showed that after the three-stage heat treatment, a large number of fine and diffusely distributed secondary α phases were found in the welded joint, which increased the content of the phase boundary. The TEM characterization and calculation results show that the phase boundary had a strong hindering effect on the dislocation slip. As a result, under three-stage heat treatment conditions, the hardness of the welded joint was increased by 50%, and the yield strength and tensile strength of the welded joints were increased by 24.3% and 13.9%, respectively. The impact toughness of the base metal significantly improved after heat treatment, which was 31.6% higher than that before heat treatment due to the increase in the equiaxed α phase from the weld metal to the base metal. A large number of stacking faults were generated during the tensile deformation of the welded joint before heat treatment due to the low stacking fault energy of the α phase, which could effectively release the stress and hinder the dislocation slip. After heat treatment, the stacking fault of the welded joint decreased owing to the obvious increase in stacking fault energy. The micro-cracks propagated rapidly along the lamellar α phase to fracture. Therefore, the plasticity of the welded joint decreased, although the strength was improved after heat treatment.

Micro-mechanisms of anisotropic deformation in the presence of notch in commercially pure Titanium: An in-situ study with CPFEM simulations

This study investigates the impact of notch severity and initial texture on the micro-mechanisms during low strain deformation in commercially pure Titanium, exploiting in-situ electron back scatter diffraction (EBSD) experiments and crystal plasticity finite element model (CPFEM). In-situ tests were performed at different strain steps of un-notched and notched samples of transverse direction (TD) and rolling direction (RD) orientations. CPFEM, based on initial EBSD microstructures, predicted profuse prismatic slip traces and early activation of prismatic slip in notched sample, with RD orientation exhibiting higher activity. Further, CPFEM results revealed early activation of high CRSS slip systems as well as evidence of early twin activity at notch tip due to severely localized plastic deformation and steep strain gradient, as observed by GND maps causing higher stress at grain boundaries. At the notch tip, digital images correlation (DIC) at microscale indicated strain localization at 45∘ and 90∘ to the tensile axis for TD and RD orientations, respectively. Furthermore, 2.5D and 3D CPFEM confirmed distinct strain patterns at notch tip: TD orientation exhibited combined basal and prismatic slip influences, while RD orientation displayed dominant prismatic slip systems at localized strain. The model also successfully predicted anisotropic surface roughness, contributing to early necking in RD orientation.

Effect of a subsurface void on the micromechanics of ductile metal indentation using remeshing

Near-surface voids and pores are generated in metal processing operations as diverse as additive manufacturing and powder processing, but their effect on indentation hardness has not been explored outside homogenized frameworks. Here we model the micromechanics of near-surface void deformation up to closure in a wedge-indentation field in ductile metal, and reveal the considerable softening effect of voids on the indentation hardness. Both symmetrically and eccentrically located voids, at various depths below the free surface, are studied. Notably, the extent of apparent reduction in the elastic modulus due to a void is much smaller than its effect on apparent hardness, e.g. 6.5% against 55% for the same void. Critical to the simulations is an adaptive remeshing finite element (FE) framework that allows accurate capture of processes like void closure and void-wall self-contact. The simulations reveal the subsurface plastic strain, strain-rate, and velocity fields with high fidelity, and their radical differences from the radial indentation field in a void-free specimen. These differences include the presence of localized pockets of high and low strain, and the initial accommodation of material displaced by the indenter by a corresponding reduction in void area. Unlike voids under uniform compression, the void-area evolution in indentation shows a characteristic sigmoidal pattern of reduction with indentation depth for all but the smallest voids. Interestingly, the indented surface profile can exhibit a one-sided pile-up feature which is diagnostic of the presence of an eccentrically located sub-surface void. Our remeshing scheme is versatile, as exemplified by its ability to model the extreme deformation associated with two nearly closed-spaced voids under an indenter. Our work shows that void-closure simulations in related applications like forming could benefit from the adoption of a remeshing framework.

Deformation behavior of the hybrid S-phase layer formed on a stainless-steel substrate by thermochemical heat treatment under mechanical loading

The hybrid S-phase layer was formed on the surface of an austenitic stainless steel by the thermomechanical heat treatment, which was carried out at a low temperature (475 °C). The thickness of the layer depends on the methane (CH4) gas content in the heat treatment chamber. The microstructure of the layer contains plenty of microstructural defects, such as slip bands, twins, stacking faults, and entangle dislocations, as confirmed by detail electron microscopy investigations. The S-phase layers were subjected to nanoindentation and nanoscratching to investigate their micro-mechanical properties and deformation aspects; and compared against the substrate (bulk austenitic) material. This layer exhibit about 6 folds increase in hardness (11.65–14.65 GPa) to that of bulk austenitic (2.33 GPa), whereas the Young's modulus increased by 1.70 times (124.84–142.34 GPa for the S phase layers and 83.25 GPa for bulk material). This increase in hardness caused relatively less plastic deformation in the layer, compared to bulk austenitic under identical loading conditions. This induced different sub-surface deformation aspects, not only in terms of deformation depth, but also in microstructural aspects, as reviled through a transmission electron microscopy investigation on the residual nanoindentation imprint and scratch tracks.

Experimental–numerical analysis of silicon micro-scratching

The continuous decrease in feature size in the production of electronic chips has two major consequences for silicon wafers: (i) processing conditions for slicing and grinding operations need to be optimized and (ii) surface damage imposed by wafer handling and transport needs to be minimized. Cultivating a deeper understanding of the subsurface implications of contact, in particular during scratching, is necessary to achieve these goals. The active subsurface micro-deformation mechanisms, including phase transformations and crack formation, can only be measured post-scratch. On the other hand, numerical models are free of experimental constraints and can therefore serve an instrumental role in uncovering the active deformation mechanisms during scratching. However, model identification and validation with experimental data is essential for reliable simulation results. Therefore, in this paper, a numerical scratch model consisting of a continuum particle methodology combined with a large strain inelastic constitutive model is exploited, whereby the parameters are identified from indentation data. The numerical–experimental results are next compared for a range of micro-scratch tests. The experimental micro-scratch device was mounted inside a scanning electron microscope to perform in situ scratch experiments. The real-time observations at the scratched surface allowed to assess and exclude possible experimental artifacts. The simulations demonstrate the ability to investigate the evolution of the phase transformations underneath the scratched surface and relate this to the hydrostatic pressure. Notably, at the simulated subsurface phase boundary, hydrostatic tension was found which is expected to play an important role in median crack formation, as often reported in the literature. For the simulation and experiments, the steady-state segments are compared and it was found that the model accurately reproduces the substantial surface recovery due to the Si-II → Si-a phase transformation. Additionally, the simulated and experimental surface cross-sectional profiles for normal scratching loads of 30, 40 and 50 mN are in adequate agreement.

Effect of grain size on fatigue strength of 304 stainless steel

Abstract In this study, three types of 304 stainless steel samples with different strengths were prepared by refining the grain size through rolling. The microstructure of the samples was observed by electron microscopy. The influence of grain size on the static tensile properties and fatigue strength of the material is mainly attributed to changes in the plastic deformation fracture mechanism and micro-deformation mechanism. In addition, a new fatigue strength prediction model is proposed based on the influence of tensile strength and work-hardening capacity. Compared with the staircase method and Basquin formula models, the proposed model maintains the accuracy of fatigue strength prediction while reducing the cost of fatigue experiments. This provides a new approach for predicting the fatigue strength of specific materials and improving anti-fatigue design capabilities.

Lattice Rotation and Deformation Mechanisms under Tensile Loading in a Single-Crystal Superalloy with [001] Misorientation.

This study investigates how deviation angles close to the [001] orientation affect the tensile properties and deformation behavior of a nickel-based single-crystal superalloy at room temperature. The research focuses on samples with deviation angles of 3°, 8°, and 13° from the [001] orientation and examines their strength and ductility. We employed scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) to explore the deformation micro-mechanisms at varying angles. Findings reveal that strength decreases and ductility increases as the deviation angle widens within the [001] vicinity. The study emphasizes that <110> octahedral slip-driven crystal slip and rotation are crucial for understanding tensile deformation. The deformation differences in samples at varying angles are attributed to the differential engagement of mechanisms. Specifically, at lower angles, reduced ductility and increased strength are due to short lattice rotation paths and work hardening causing superlattice stacking faults (SSFs) to slip in two directions on the {111} plane within the γ' phase. As the angles increase, the lattice rotation paths extend, and Shockley partial dislocations (a/6<112>) accumulate in γ channels. This process, involving SSFs moving in a single direction within the γ' phase, results in higher ductility and reduced strength.