Micromechanical Deformation Behavior Research Articles

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.

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.

Characterization of PPS Piston and Packing Ring Materials for High-Pressure Hydrogen Applications.

The widespread adoption of renewable energy hinges on the efficient transportation of hydrogen. Reciprocating piston compressor technology in non-lubricated operation will play a key role, ensuring high flow rates and compression ratios. These systems rely on advanced high-strength sealing solutions for piston and rod packing rings utilizing advanced fiber-reinforced polymers. Polyphenylene sulfide (PPS) polymer matrix composites have seen use in tribological applications and promise high mechanical strength and wear resistance. The presented work describes carbon and glass fiber-reinforced PPS matrix polymers in comparison, which are characterized by complementary methods to investigate their properties and potential for application in reciprocating compressor under non-lubricated operation. Thermo-mechanical and tribological testing was supported by microstructure analysis utilizing advanced X-ray and electron imaging techniques. New insights in micromechanical deformation behavior in regard to fiber materials, interface strength and orientation in fiber-reinforced polymers are given. Conclusions on the suitability of different PPS matrix composites for high-pressure hydrogen compression applications were obtained.

Micromechanical properties and deformation behavior of the constituent phases in 3rd generation complex phase AHSS: In-situ neutron experiment and crystal plasticity simulation

This paper investigated the micromechanical properties and deformation behavior of the constituent phases in 3rd generation (GEN) complex phase advanced high-strength steel (AHSS). The hybrid method was a comprehensive combination of uniaxial tensile test with digital image correlation (DIC), electron backscatter diffraction (EBSD), in-situ neutron diffraction (NED) and crystal plasticity finite element method (CPFEM). Results reveal that the microstructure of 3rd GEN AHSS exhibits a complex phase of ferrite (46.2 %), bainite (36.1 %), martensite (9.4 %) and austenite (8.3 %), which would be promising microstructure in the development of new-generation complex phase AHSS. The significant findings are: (1) excellent ductility is obtained while high strength is preserved despite small martensite volume fraction, which can reduce the tendency of martensite cracking; (2) macroscopic stress-strain relationships and microscopic hardening properties of constituent phases are successfully determined, which are very important for predicting the overall performance of complex phase AHSS; (3) presence of bainite with high volume fraction plays a critical role in inducing plastic strain localization in ferrite, resulting in the heterogeneous stress/strain distribution. It is disclosed that pronounced strain localization is mostly initiated in ferrite grains while the deformation bands are mainly located in ferrite/bainite grain boundaries. Furthermore, strain partitioning between ferrite and bainite results in the high-stress concentration in bainite grains. As a result, these “hot zones”, i.e., ferrite grains, ferrite/bainite grain boundaries and bainite grains, experience high strain, strain gradient and stress, respectively, which can be a primary origin of ductile fracture initiation in the 3rd GEN complex phase AHSS. The findings reported in the present investigation would be of great interest in the development of new-generation complex phase automotive steels.

Thermo-mechanical properties and internal architecture of PI composites for high-pressure hydrogen applications

The efficient transport of Hydrogen is pivotal to the wide scale adaption of energy produced from renewable sources. Reciprocating piston compressors with high strength non-lubricated sealing solutions for piston and rod packing rings will be a key technology to fulfill gas purity and high-pressure gradients demands set by hydrogen applications. Polyimide polymer matrix composites have seen use in tribological applications and promise high mechanical strength and temperature resistance. The presented work describes carbon fiber reinforced Polyimide blends in comparison, characterized by complementary advanced methods to investigate their properties and potential for reciprocating compressor applications. Thermo-mechanical and tribological testing was supported by microstructure analysis utilizing synchrotron tomography and electron microscopy, pinpointing the necessity of dry lubricating additives such as PTFE. New insights in micromechanical deformation behavior of fiber reinforced polymers are given and promising fiber-reinforced polyimide compositions suitable for high-pressure hydrogen compression applications were identified.

Examining the Effect of MnS Particles on the Local Deformation Behavior of 8MnCrS4-4-13 Steel by In Situ Tensile Testing and Digital Image Correlation

In this study, the behavior of MnS particles in a steel matrix is investigated through in situ tensile testing and digital image correlation (DIC) analysis. The goal of this research is to understand the mechanical behavior of MnS inclusions based on their position in the steel matrix. To accomplish this, micro-dog bone-shaped samples are prepared, tensile tested, and analyzed. Macro-mechanical results reveal that the material yields at a stress of 350 MPa and has an ultimate tensile strength of 640 MPa, with a total elongation of 17%. For micro-mechanical analysis, scanning electron microscopy (SEM) images are taken at incremental strains and processed using DIC software to visualize the local strain evolution. The DIC analysis quantifiably demonstrates that the local strain is highest in the ferrite matrix, and while lowest in the pearlite matrix, the MnS particles and the interfaces between different materials experienced intermediate strains. The research provides new insights into the micro-mechanical deformation behavior of MnS particles in a steel matrix and has the potential to inform the optimization of the microstructure and properties of materials containing MnS inclusions.

Micro-mechanical deformation behavior of heat-treated laser powder bed fusion processed Ti-6Al-4V

Industrial implementation of heat-treated Laser Powder Bed Fusion (L-PBF) processed Ti-6Al-4 V components requires a thorough understanding of the plastic deformation mechanisms to predict the part performance in safety-critical environments. Here, we study the micro-mechanical deformation behavior of a heat-treated L-PBF processed Ti-6Al-4 V by in-situ uniaxial tensile loading, during which high-resolution strain fields were monitored by Scanning Electron Microscope (SEM) based Digital Image Correlation (DIC). SEM-DIC revealed: (i) the transformed beta phase accommodates higher strain than the primary alpha phase; (ii) strain accumulation in primary alpha occurs primarily at the interface regions where the Al content is lower; and (iii) needle-shaped secondary alpha precipitate in the transformed beta creates strain localization pathways that bridge the interfacial strain bands. Based on the in-situ deformation behavior, recommendations are made on microstructure tailoring and alloy design to prevent strain localization and enhance the quasi-static mechanical properties of l-PBF processed titanium alloy components.

Many-Scale Investigations of Deformation Behavior of Polycrystalline Composites: II-Micro-Macro Simultaneous FE and Discrete Dislocation Dynamics Simulation.

The current work numerically investigates commercial polycrystalline Ag/17vol.%SnO composite tensile deformation behavior with available experimental data. Such composites are useful for electric contacts and have a highly textured initial material status after hot extrusion. Experimentally, the initial sharp fiber texture and the number of 3-twins were reduced due to tensile loading. The local inhomogeneous distribution of hardness and Young’s modulus gradually decreased from nanoindentation tests, approaching global homogeneity. Many-scale simulations, including micro-macro simultaneous finite element (FE) and discrete dislocation dynamics (DDD) simulations, were performed. Deformation mechanisms on the microscale are fundamental since they link those on the macro- and nanoscale. This work emphasizes micromechanical deformation behavior. Such FE calculations applied with crystal plasticity can predict local feature evolutions in detail, such as texture, morphology, and stress flow in individual grains. To avoid the negative influence of boundary conditions (BCs) on the result accuracy, BCs are given on the macrostructure, i.e., the microstructure is free of BCs. The particular type of 3D simulation, axisymmetry, is preferred, in which a 2D real microstructural cutout with 513 Ag grains is applied. From FE results, 3-twins strongly rotated to the loading direction (twins disappear), which, possibly, caused other grains to rotate away from the loading direction. The DDD simulation treats the dislocations as discrete lines and can predict the resolved shear stress (RSS) inside one grain with dependence on various features as dislocation density and lattice orientation. The RSS can act as the link between the FE and DDD predictions.

Qualitative Investigation of Damage Initiation at Meso-Scale in Spheroidized C45EC Steels by Using Crystal Plasticity-Based Numerical Simulations

This research uses EBSD data of two thermo-mechanically processed medium carbon (C45EC) steel samples to simulate micromechanical deformation and damage behavior. Two samples with 83% and 97% spheroidization degrees are subjected to virtual monotonic quasi-static tensile loading. The ferrite phase is assigned already reported elastic and plastic parameters, while the cementite particles are assigned elastic properties. A phenomenological constitutive material model with critical plastic strain-based ductile damage criterion is implemented in the DAMASK framework for the ferrite matrix. At the global level, the calibrated material model response matches well with experimental results, with up to ~97% accuracy. The simulation results provide essential insight into damage initiation and propagation based on the stress and strain localization due to cementite particle size, distribution, and ferrite grain orientations. In general, it is observed that the ferrite–cementite interface is prone to damage initiation at earlier stages triggered by the cementite particle clustering. Furthermore, it is observed that the crystallographic orientation strongly affects the stress and stress localization and consequently nucleating initial damage.

Investigating the Effect of Cementite Particle Size and Distribution on Local Stress and Strain Evolution in Spheroidized Medium Carbon Steels using Crystal Plasticity‐Based Numerical Simulations

Herein, micromechanical material deformation behavior of medium carbon steel—with varying cementite particle size and distribution—under monotonic tensile load is modeled. The statistical phase morphology data are collected from the microscopy images of 83% spheroidized C45EC steel. Virtual microstructures for nine different cases are constructed using Dream.3D by varying the concentrations of small, large, and bimodal cementite particles on the ferrite grain boundaries. A crystal plasticity‐based numerical simulation model, i.e., DAMASK, is used to simulate the local material deformation behavior. The material model parameters for elastic–plastic ferrite and elastic cementite phases are adopted from the literature and calibrated by modifying the fitting parameters to match the experimentally observed material flow curve. It is observed that the cementite particle size and distribution in the primary phase affect the local mechanical behavior of the material. An analysis of deformation at 20% of global strain has helped in the qualitative understanding of factors affecting the stress and strain concentration points. The evolution of stresses and strains in the least and the most stressed representative volume elements (RVEs) has provided interesting information regarding the path followed by the dislocation movement during deformation. The study has provided insight into the factors affecting the material's global and local deformation behavior.

The effect of δ phase on the microplasticity evolution of a heat-treated nickel base superalloy

The micromechanical deformation behaviour of the commercial Ni-base polycrystalline Ni-base superalloy GH4169 has been investigated in terms of the volume fraction and grain boundary distribution of the δ (Ni3Nb) phase. The main objective of this work was to gain an understanding about the mechanisms of deformation of the superalloy at the grain scale. To that purpose, experimental and microstructural observations are relied upon to construct realistic models of the polycrystalline superalloy microstructure using crystal plasticity concepts. It was found that a volume fraction of the grain boundary δ phase in excess of approximately 3.0% results in a weakening of the room temperature strength, and the detrimental effect increases with increasing the δ phase volume fraction. This study also revealed that the grain boundary δ phase, being stronger than the surrounding grains, impedes the transmission of plastic slip through the grain boundaries, thus enhancing the localisation of plastic slip within the grains. However, the strengthening effect associated with such slip blocking mechanism by the δ phase is considerably lower than the weakening of the deformation resistance of the grains due to the depletion of the meta-stable γ″ precipitates within the grains resulting from the stable δ phase formation.

Micromechanical properties and deformation behavior of Al3BC/6061 Al composites via micropillar compression

Al3BC has been proved to be a promising candidate as reinforcement of Al alloy. In this work, Al3BC reinforced 6061 Al composites were in-situ fabricated through a liquid-solid reaction method followed by hot extrusion, and the micromechanical properties as well as deformation behavior of the composites were investigated via micropillar compression. The Al3BC/6061 composites show a significant improvement of compression strength than the matrix alloy, indicating an outstanding strengthening effect of Al3BC. Compared with the large parallel local slip bands on the post-compressed pillars of unreinforced matrix materials, the wrinkled slip bands were observed and homogenously distribute on the post-compressed pillars of the composites, indicating an impeding effect on the propagation of slip bands and a better slip homogenization caused by Al3BC particles. The strengthening mechanisms were also discussed and microstructural characterizations reveal that the high strength of the composites is mainly attributed to its high dislocation density, grains refinement and load transfer effects caused by Al3BC.

Achieving ultra-high bearing strength of tungsten nanoribbons in a transforming metal matrix

Nanowires (or nanoribbons) were identified as the most potential structural reinforcements for composites due to their ultrahigh strength, but it is difficult to transcribe their ultrahigh strength into the composite. Here, an in-situ W nanoribbbons-NiTi matrix composite was fabricated by mechanical reduction of a W-NiTi ingot, and the micromechanical deformation behavior was studied by in-situ high energy X-ray diffraction during tensile loading-unloading. It is revealed that the large elastic deformation of the W nanoribbons in a NiTi matrix is dictated by and synchronized with the martensitic transformation of the matrix. The bearing strength of the W nanoribbons, up to 11.6 GPa, was achieved in this composite. A significant load transfer took place from the matrix to the nanoribbons, exhibiting the characters of locality and rapidity. The stress fraction carried by the W nanoribbons with the volume fraction of 3% is up to 45% for the strength of the composite.

Micromechanical response analysis of Ti-Ni shape memory alloy undergoing martensitic reorientation and detwinning

The micromechanical deformation behavior of Ti-Ni shape memory alloy in the fully martensitic state is studied using a previously developed constitutive model for martensitic reorientation and detwinning. Considering a microscale two-variant material particle, the interaction between different martensitic habit plane variants in a variant pair is investigated by numerical simulations. It is shown that the variant pairs can be compatible or incompatible while interacting with each other. Furthermore, our numerical analysis shows that the predicted loading response of Ti-Ni shape memory alloy under simple tension depends on the number of existing martensitic variants in a material particle under consideration.

Characterization of Thermal, Mechanical and Tribological Properties of Fluoropolymer Composite Coatings

Perfluoroalkoxy (PFA) is a potential polymer coating material for low-temperature waste heat recovery in heat exchangers. Nonetheless, poor thermal conductivity, low strength and susceptibility to surface degradation by erosion/wear pose restrictions in its application. In this study, four types of fillers, namely graphite, silicon carbide, alumina and boron nitride, were introduced to enhance the thermal, mechanical and tribological properties in PFA coatings. The thermal diffusivity and specific heat capacity of the composites (reinforced with 20 wt.% filler) were also measured using laser flash and differential scanning calorimetry techniques, respectively. The results indicated that the addition of graphite or boron nitride increased the thermal conductivity of PFA by at least 2.8 orders of magnitude, while the composites with the same weight fraction of alumina or silicon carbide showed 20-80% rise in thermal conductivity. The micromechanical deformation and tribological behavior of composite coatings, electrostatically sprayed on steel substrates, were investigated by means of instrumented indentation and scratch tests. The deformation response and friction characteristics were investigated, and the failure mechanisms were identified. Surface hardness, roughness and structure of fillers influenced the sliding performance of the composite coatings. PFA coatings filled with Al2O3 or SiC particles showed high load-bearing capacity under sliding conditions. Conversely, BN- and graphite-filled PFA coatings exhibited lower interfacial adhesion to steel substrate and were prone to failure at relatively lower applied loads.

A multi-scale self-consistent model describing the lattice deformation in austenitic stainless steels

Differently oriented grains within a polycrystalline material exhibit different micro-mechanical lattice responses for a given macroscopic stress due to the local elastic and plastic anisotropy. A physical understanding of the lattice deformation mechanism during plastic flow is still lacking. In this study, a three-dimensional multi-scale self-consistent model is developed to examine the micro-mechanical deformation behaviour of F.C.C. polycrystalline austenitic stainless steels under uniaxial tensile loading at ambient temperature. The model is formulated in a crystal based plasticity framework and takes into account the detailed kinematics of dislocation slip and its influence on the evolution of the dislocation distribution on different crystallographic planes of individual grains within a polycrystalline material. The effect of athermal solute strengthening is also incorporated. Predictions of the microscopic lattice response developed within individual grains are compared with various available experimental results obtained using neutron diffraction (ND). There are significant differences in the way in which the lattice strain evolves with macroscopic strain obtained by different groups. This is explained in terms of the variation in initial residual stress state in the samples tested by these different groups.

Ductile fracture and deformation behavior in progressive microforming

Abstract With the increasing demand for the quality and quantity of miniaturized parts, fabrication of microparts directly using sheet metals is proven to be promising and efficient for mass production. In this process, however, there are many unknowns in terms of size effect and its affected fracture and deformation behavior. This study is thus aimed at investigating the micromechanical damage and deformation behavior in progressive microforming and establishing a systematic knowledge to support the microformed part design, process configuration and tooling design. In detail, a micro cylindrical part is fabricated via shearing process and a multi-level flanged part is produced via progressive micro extrusion and blanking. To explore the effect of material microstructure on the deformation behavior, ductile fracture and the product quality of microformed part, the original sheet metals are annealed under different temperatures. To realize the microforming process, a progressive microforming system is developed and its characteristics are investigated. The effect of grain size on dimensional accuracy, microstructure evolution and fracture behavior in microforming is also studied. The ductile fracture and its induced defects are identified and the damage accumulation is predicted. In the end, the validity and applicability of different fracture criteria in microforming is discussed.

Discrete element modelling of creep of asphalt mixtures

Creep tests on asphalt mixtures have been undertaken under four stress levels in the laboratory while the discrete element model (DEM) has been used to simulate the laboratory tests. A modified Burger’s model has been used to represent the time-dependent behaviour of an asphalt mixture by adding time-dependent moment and torsional resistance at contacts. Parameters were chosen to give the correct stress-strain response for constant strain rate tests in Cai et al. (2013). The stress-strain response for the laboratory creep tests and the simulations were recorded. The DEM results show reasonable agreement with the experiments. The creep simulation results proved to be dependent on both bond strength variability and positions of the particles. Bond breakage was recorded during the simulations and used to investigate the micro-mechanical deformation behaviour of the asphalt mixtures. An approach based on dimensional analysis is also presented in this paper to reduce the computational time during the creep simulation, and this analysis is also a new contribution.

In-situ high-energy synchrotron X-ray diffraction study of micromechanical behavior of multiple phases in Ni47Ti44Nb9 shape memory alloy

High-energy synchrotron X-ray diffraction technique was used to in-situ characterize microstructure, lattice strain, and phase transition behavior of a Ni47Ti44Nb9 shape memory alloy. Phase transformation kinetics and deformation mechanisms were studied under a uniaxial tension at three testing temperatures, i.e., −70°C, 25°C, and 150°C. At a testing temperature of −70°C, a complicated phase transformation with four stages of micromechanical deformation was identified which is associated with changes of martensite substructures. At room temperature of 25°C, there was no stress-induced selection process of variants of B19′ phases observed. Whereas at a testing temperature of 150°C, there was no any phase transformation observed. It is verified that β-Nb phase, an effective stabilizer for the austenite, delays the process of martensitic transformation and relaxes the strain energy without strengthening the matrix. This new finding is important to understand the relationship between the micromechanical deformation behavior and phase transformations in the Ni47Ti44Nb9 SMA.