Effect Of Microstructure Heterogeneity Research Articles

Effect of microstructural heterogeneity and grain morphology on the annealing behavior of additively manufactured metastable β titanium alloy

Abstract Titanium alloys fabricated by additive manufacturing usually exhibit significant heterogeneity in grain size and morphology. Recrystallization treatment is an effective approach to alter the microstructure and achieve a more homogeneous structure. In this study, recrystallization and grain growth behaviours of a metastable β titanium alloy fabricated by laser powder bed fusion (LPBF) and direct energy deposition (DED) were investigated. The LPBF sample developed a chessboard-like microstructure, with considerable variations in grain size and morphology across molten pools. Recrystallization started from the boundary of these pools, where grains were finer and the stored energy was higher. The DED sample had a heterogeneous structure with alternating layers of equiaxed grains and columnar grains oriented with <100> along the building direction. Results obtained from in-situ EBSD demonstrated that grain growth occurred preferentially in regions with fine equiaxed grains, and regions with columnar grains showed limited grain growth due to the relatively lower driving force and mobility.

Effect of microstructure heterogeneity on the cryogenic fracture toughness of the dissimilar joint between SA645 and 304L made by laser welding

The cryogenic fracture toughness of the SA645/304L dissimilar weld, produced using continuous wave IPG fiber laser welding with a 0.3 mm beam offset towards the 304L side, and the microstructural effects on the fracture behavior were systematically investigated. The weld metal consists of ∼89% columnar dendritic martensite and ∼11% retained austenite (RA), where the ununiform distribution of constituent phases as well as the distinctions between martensite and RA in terms of morphology and mechanical properties lead to the microstructure heterogeneity of weld metal. Pop-in phenomenon appears on the Force-Displacement (F-V) curve during the crack tip opening displacement (CTOD) test for the weld. The CTOD value for the weld is ∼0.057 mm, about 8 times less than that when pop-in effect is ignored (∼0.563 mm). Rapid propagation of the pre-fatigue crack tip along the center of the weld leads to the pop-in phenomenon. Fracture surface in pre-fatigue crack tip (PCT) region shows quasi-cleavage features, while fracture surface in stable crack propagation (SCP) region presents a ductile-fractured surface with dimples. The crack propagation path in SCP region is frequently deflected. Inhibition effect of grain boundaries on crack propagation and TRIP effect of retained austenite (RA) result in the improvement in fracture toughness of the SCP region.

Effect of microstructural heterogeneities on variability in low-temperature impact toughness in multi-pass weld metal of 420 MPa offshore engineering steel

Effect of microstructural heterogeneities on variability in low-temperature impact toughness in multi-pass weld metal of 420 MPa offshore engineering steel

3D printed Inconel mechanical response related to volumetric energy density

This work aims to provide a methodology for producing Inconel 718 specimens ISO 527–2-5B with a tailored mechanical response, by Selective Laser Melting (SLM). Since the macroscopic mechanical behavior of 3D printed metals is dictated by their microstructure, the effect of the printing parameters such as the laser power, the scan speed, the hatching distance and the layer thickness was considered. To evaluate the maximum dimensional accuracy and optimum Volumetric Energy Density (VED) value, the specimens were printed with a constant layer height, hatching distance and spot size and with a varying combination of laser power with scan speed. The 3D printed specimens were subjected to uniaxial tension. The experiments have shown an increased reproducibility of the specimens’ microstructure that led to a more consistent mechanical behavior, when lower scan speed and power were used. Specimens printed with higher speed and power seem to have a more heterogeneous microstructure leading to a more varying mechanical response. While the differences in the mechanical response between specimens of different batches are attributed to different laser power and scan speed, different response between specimens of the same batch is attributed to the inherent inhomogeneity of their microstructure. A simple elasticity model was used to describe their elastic behavior, while a 1D gradient plasticity model was utilized to describe their post-yield tensile response, in which the effect of microstructural heterogeneity on macroscopic behavior was taken into account through a respective internal length. The combined elasto-plastic model was used to capture mechanical response until failure, in good agreement with the experiments. The predicted mechanical characteristics were related to the specimens’ VED to come up with simple relations providing optimum 3D printing parameters for producing specimens with a certain mechanical response. For a complete characterization, the specimens’ roughness was studied and its VED dependence was performed.

Effect of microstructural heterogeneity on fatigue strength predicted by reinforcement machine learning

AbstractThe posterior statistical distributions of fatigue strength are determined using Bayesian inferential statistics and the Metropolis Monte Carlo method. This study explores how structural heterogeneity affects ultrahigh cycle fatigue strength in additive manufacturing. Monte Carlo methods and procedures may assist estimate fatigue strength posteriors and scatter. The acceptable probability in Metropolis Monte Carlo relies on the Markov chain's random microstructure state. In addition to commonly studied variables, the proportion of chemical composition was demonstrated to substantially impact fatigue strength if fatigue lifetime in crack propagation did not prevail due to high threshold internal notches. The study utilizes an algorithm typically used for quantum mechanics to solve the complicated multifactorial fatigue problem. The inputs and outputs are modified by fitting the microstructural heterogeneities into the Metropolis Monte Carlo algorithm. The main advantage here is applying a general‐purpose nonphenomenological model that can be applied to multiple influencing factors without high numerical penalty.

Effect of microstructural heterogeneity on corrosion fatigue crack growth behavior of 60-mm thick-plate TC4 titanium alloy NG-GTAW welded joint

The microstructure, mechanical properties and corrosion fatigue crack growth behavior of 60-mm thick-plate TC4 titanium alloy narrow gap gas-tungsten arc welding (NG-GTAW) welded joints applied to the shell of the Jiaolong deep submersible were studied, and the effect of microstructural heterogeneity on the corrosion fatigue crack growth behavior was discussed. The results show that significant microstructure evolution occurred in the welded joint with acicular α′ martensite coarsening and a random orientation distribution. The hardness and tensile strength distribution of the welded joints correspond to the difference in the microstructure, with a smaller size and more randomly oriented α′ martensite providing superior strengthening effect. The acicular α′ martensite of the weld metal (WM) has excellent resistance to crack growth compared to the α and β phases of the base metal (BM). The degree of crack serration depends on the size and orientation distribution of the α grains and acicular α′ martensite. As the stress increases, the fatigue resistance of BM may have a more serious deterioration trend compared with WM.

Effects of microstructural heterogeneity and structural defects on the mechanical behaviour of wire + arc additively manufactured Inconel 718 components

This study investigates the extent to which the build orientation and heat treatment schedule affect the microstructure and mechanical properties for thin-walled and additively manufactured IN718 components produced with the cold metal transfer process (CMT-WAAM). Uniaxial tensile tests using digital image correlation (DIC), microhardness analysis and fractography were used to characterise the mechanical behaviour, both in the as-deposited condition and after heat treatments used in the aerospace and oil and gas industries. Wrought material was also tested to benchmark the measured properties. The solution treatment of 1040 °C for 1 h in the oilfield specification reduced the area fraction of Laves phases significantly (∼80%) and promoted higher homogenisation of ageing constituents. However, grain growth near interlayer boundaries resulted in localised low hardness (∼50 HV0.2 below the average) after age hardening. In the as-deposited condition, the yield strength was ∼10% lower along the build direction and changed to being 8–13% higher along the same direction in heat-treated samples, while the elastic modulus relative to deposition orientation was unaffected. Furthermore, solidification defects, such as porosity and hot cracking, caused strain localisation during tensile testing and substantial scatter in macroscopic strain. The ductility was improved with oil and gas heat treatment, but it was substantially lower along the build direction due to the longer axis of defects being perpendicular to loading direction. This study highlights the importance of optimising process parameters to minimise defects and tailoring heat treatments to achieve a higher ductility in IN718 processed by WAAM.

Effect of microstructural heterogeneity on the balanced-biaxial and tensile behavior of a Zn alloy sheet

A 0.65 mm thick Zn sheet was processed at room temperature by equal channel angular sheet extrusion (ECASE), which allowed for the fabrication of a heterogeneous material. The processed sheet's heterogeneity manifested as larger dislocation clusters and smaller grain sizes on the surface than in the sheet's center, generating as well a 35% increase in yield strength while preserving a fracture elongation of 26%. ECASE processing resulted in a slightly higher deformation limit than that of the as-received material with a weight reduction of 9%. The improvement in the processed sheet's mechanical properties was associated with the formation of twins on the surface, resulting in a smaller grain size and heterogeneity between the twins-free and twinned grains. These two microstructures produced a combination of hard and soft zones. ECASE processing formed twins in particular grains that in turn created a high density of geometrically necessary dislocation (GND) arrays in twin-free neighboring grains. It should be noted that the twinned grains had a low density of geometrically necessary dislocations (GND). In addition to the heterogeneity between twinned and twin free grains, a heterogeneous GND distribution was formed between the surface and the processed sheet's center, with the basal slip system dominating.

Effect of microstructure heterogeneity on the mechanical properties of friction stir welded reduced activation ferritic/martensitic steel

The microhardness distribution in the different zones of a friction stir welded reduced activation ferritic/martensitic steel has been investigated and correlated to the hierarchical martensitic microstructure in the respective zones, characterized by electron backscatter diffraction orientation analysis. It is found that the variation of prior austenite grain size, packet size, and block width in different subzones is influenced by the peak temperature and effective strain rate during the friction stir welding process. The distribution of the microhardness correlates directly with the geometrically necessary dislocation density observed in the different zones.

Correlation between microstructural heterogeneity and mechanical properties of WC-Co composite additively manufactured by selective laser melting

A unique heterogeneous microstructure was generated in the tungsten carbide-cobalt (WC-Co) composite additively-manufactured by selective laser melting (SLM). In this work, the effect of microstructural heterogeneity on mechanical properties and crack propagation in SLM-processed WC-Co is investigated. The strength and strain calculation model is constructed to analyze mechanical properties in heterogeneous WC-Co. Especially, this heterogeneity contributes to further deformation by interrupting crack propagation. The SLM process can be a candidate for manufacturing process to tailor microstructural heterogeneity of WC-Co composite.

Effect of microstructure heterogeneity on the damage resistance of nacre-like alumina: Insights from image-based discrete simulations

Recently, several types of nacre-like materials have been developed using various methods. However, the resulting materials exhibit a significant randomness and defects compared to natural nacre. Therefore, evaluating the influence of these microstructural variations on the material’s mechanical behavior is of high interest for the development of nacre-like materials and their industrial scale-up. In this work, we present an original method that allows the modeling of complex morphology of brick and mortar materials, by combining Discrete Element Method and Electron Backscatter Diffraction images. Using different realistic microstructures we studied the effect of the microstructural variations on crack propagation and damage resistance. The obtained results were compared to the regular microstructure of nacre and showed a non-negligible decrease of mechanical properties.

Strengthened flow instability in hot deformation of titanium alloy with colony structure: On the effect of microstructure heterogeneity

The effect of microstructure heterogeneity on flow instability is investigated during isothermal compression. The mechanism of plastic flow instability is studied from the perspective of the strain mode and geometrical orientation dependence of microstructure. It is found that the instability criteria underestimate the range of unsafe processing region of lamellar microstructure due to its stronger trend to flow instability. The key factors are the geometrical orientation and colony size of lamellar microstructure. In the colonies with hard geometrical orientation, deformation resistance is big since only the pyramidal slip system can be operated. The difference of the deformation resistance among colonies can reach 49% in a primary β grain. The incompatibility of flow among various colonies promotes the generation of micro shear bands in colonies with hard geometrical orientation, which facilitates the slip transmission across α/β interfaces and weakens the Hall-Petch strengthening effect. The large colony size facilitates the rapid expansion of micro shear bands leading to the continuous softening of colonies, which strengthens the strain localization and accelerates the occurrence of flow instability. The critical value of Semiatin's model is modified to 4 for lamellar microstructure. Based on this research, a method to confirm more precise unsafe processing regions of titanium alloys of different microstructures is proposed.

Study of the effect of microstructural variation on the low cycle fatigue behavior of laser welded DP600 steel: Simulation and experimental validation

Non-isothermal heat treatment during welding results in a wide variety of microstructures which strongly affect the fatigue properties of the weldment. To improve the durability of weldments, the effect of microstructural heterogeneity on the fatigue behavior of welded joints should be thoroughly understood. In this paper, the role of microstructure features on the low cycle fatigue behavior of laser welded dual-phase 600 steel was studied by both experimental analysis and simulation. In-situ cyclic tests combined with scanning electron microscopy (SEM) imaging and electron backscattered diffraction (EBSD) were performed to identify microstructural deformations in different laser welded regions during cyclic loading. Microstructural characterization revealed that the microcracks caused by fatigue mostly occurred in the ferrite phase and close to the interface boundaries of ferrite/martensite. In the numerical part, the hierarchical micromechanical modeling using a representative volume element (RVE) was employed to study the regional mechanical properties of welded joints. 2D RVE calculations for different parts of the welded joints were based on the metallographic morphologies. The cyclic behavior of constituent phases (ferrite, martensite) in the microstructure of different regions was quantified based on the dislocation-density-based model and Ramberg-Osgood model. The laser weld joint model containing the RVEs of different weld regions could accurately predict the fatigue weak area of the laser welded joint. The hysteresis stress-strain loop of the welded joint with a heterogeneous multiphase microstructure was simulated and was in line with the experimental data.

Effect of microstructure heterogeneity on microscopic stress triaxiality of a C-Mn weld metal

Strain incompatibility between soft grain boundary ferrite and hard acicular ferrite in C-Mn weld metal during tensile gives rise to stress triaxiality accumulation in microscale. Effect of microstructure heterogeneity on microscopic stress triaxiality were investigated in the study by conducting microstructure-based finite element analysis. It is found that stress triaxiality buildup in microscale is attributed to the microstructure heterogeneity of weld metal (the microstructure contrast and the local microstructure distribution); the stress triaxiality in grain boundary ferrite approaches to the plane strain state in longitudinal specimens, whereas it spreads from pure shear to plane strain states in transverse specimens; the local constraint of grain boundary ferrite has pronounced effects on the stress triaxiality of acicular ferrite. At final, the tension process is divided into three characteristic stages: initial stage, transition stage and stable stage according to the evolution of the stress triaxiality, which is consistent with the macroscopic observation. As a results, the triaxiality can be used as an indicator to measure the heterogeneity of the microstructure, correlating well with the macroscale deformation.

The effect of microstructural heterogeneities on the High Cycle Fatigue scatter of cast aluminium alloys: from an elementary volume to the structure

Cast Al-Si alloys have been widely used in automobile applications thanks to their low density and excellent thermal conductivity. A lot of components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, the well know size effect in fatigue, whereby the fatigue strength is reduced in proportion to an increase in size, can be important. This is caused by a higher probability of initiating a crack in larger specimens (i.e. statistical size effect). This paper analyses the role of casting defects on the statistical size effect. For that, a uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. Different specimens (smooth and notched) with different stressed volumes have been investigated. The first part of this article is dedicated to the experimental characterization of the statistical size effect in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size via diagram of Kitagawa-Takahashi. The results show that the presence of statistical size effect is strongly linked to the characteristics of the pore population present in the alloy. A numerical approach, linking the observed pore distribution to the volume of loaded material, is proposed and discussed.

The effect of microstructural heterogeneity on pore size distribution and permeability in Opalinus Clay (Mont Terri, Switzerland): insights from an integrated study of laboratory fluid flow and pore morphology from BIB-SEM images

Abstract Opalinus Clay (OPA) is considered as a potential host rock for the deep geological disposal of radioactive waste. One key parameter in long-term storage prediction is permeability. In this study we investigated microstructural controls on permeability for the different facies of OPA. Permeability and porosity were determined under controlled pressure conditions. In addition, the pore space was investigated by SEM, using high-quality surfaces prepared by broad ion beam (BIB) milling. Water permeability coefficients range from 1.6×10 −21 to 5.6×10 −20 m 2 ; He-pycnometer porosities range between approximately 21 and 12%. The sample with the highest He porosity (shaly facies) is characterized by the lowest permeability, and vice versa (carbonate-rich sandy facies). This inverse behaviour deviates from the generally reported trend of increasing permeability with increasing porosity, indicating that parameters other than porosity affect permeability. Visible porosities from SEM images revealed that 67–95% of the total porosity resides within pores smaller than the SEM detection limit. Pore sizes follow a power-law distribution, with characteristic power-law exponents ( D ) differing greatly between the facies. The carbonate-rich sandy facies contains a network of much larger pores ( D (shaly) ≈2.4; D (carbonate-rich) c. 2.0), because of the presence of load-supporting sand grains that locally prevent clay compaction, and are responsible for a higher permeability.

The effect of microstructure heterogeneity on the microscale deformation of ultrafine-grained aluminum

Abstract

Effect of microstructural heterogeneities on scatter of toughness in multi-pass weld metal of C–Mn steels

Often, scatter in mechanical properties of multi-pass steel weldments is qualitatively attributed to the underlying heterogeneous microstructure brought about by spatial variations of multiple thermal cycles. In this research, a method for quantitative heterogeneity calculation based on measured variations of microstructure and hardness throughout the multi-pass weld metals including various reheated regions was explored. Published data from multi-pass welds with controlled titanium additions (7–32 wt ppm) were correlated to comprehensive microstructure characterization. The scatter in 7 wt ppm Ti steel welds was larger than that of 32 wt ppm Ti steel welds. This change in scatter is correlated to spatial distributions of microstructural heterogeneity parameter, rather than the average value of heterogeneity parameter for the whole weldment.

The effect of crystallographic orientation on the micromechanical deformation and failure behaviors of DP980 steel during uniaxial tension

The effect of crystallographic orientation on the deformation and failure behaviors of DP980 steel was investigated using the crystal plasticity finite element method (CPFEM). A phase identification method that was based on the image quality of EBSD data and a filtering process provided the individual crystallographic orientations for ferrite and martensite phases in DP980 steel. By using a technique for the direct mapping of filtered microstructure into finite element meshes, CPFEM can capture the heterogeneity of strain–stress partitioning and the effect of microstructure heterogeneity on the hot spots for void formation in DP980 steel during uniaxial tension. The failure mechanisms were studied through scanning electron microscope (SEM) observations of the polished sections of a failed tensile specimen. An isotropic elasto–plastic FEM was used to simulate the heterogeneity of strain–stress partitioning and the failure behaviors of DP980 steel without considering the crystallographic orientation of the constituent phases. The simulation results demonstrated that the initial crystallographic orientation of the constituent phases significantly affects the heterogeneity of strain–stress partitioning and the hot spots for void formation in DP980 steel during uniaxial tension.

Structural Aspects of the Behavior of Lead-Free Solder in the Corrosive Solution

In oxidizing environments, most tin-based lead (Pb)-free alloys form a tin oxide that is easily eroded or mechanically damaged, affecting corrosion resistance and thus reliability of the soldered joints. In this study, the effect of microstructure heterogeneity on corrosion behavior of Pb-free solder candidate systems has been investigated on the example of as-cast and heat-treated alloys. The research was focused on a comparison between the corrosion resistance of binary Sn-Zn and ternary Sn-Zn-Cu alloys. Accelerated corrosion tests were performed by means of electrochemical methods in the sodium sulfate solution (VI), Na2SO4, of about 0.5 M concentration, pH adjusted to 2 by means of concentrated H2SO4 acid. In these tests, the corrosion potentials as well as polarization curves were determined for the selected alloys in as-cast state and after their heat treatment using different combinations of processing parameters. The measurements of basic electrochemical characteristics were made, i.e., the corrosion current (i corr μA/cm2) and Tafel coefficients, both cathodic (b c V/dec) and anodic (b a V/dec) ones. Detailed structural characterization of as-cast and heat-treated alloys before and after accelerated corrosion tests has been made under a wide range of magnifications using light microscopy and scanning electron microscopy observations. The results showed that structural heterogeneity of the examined alloys, attributed to the presence of secondary phases, and affected by their size and distribution, significantly influences the behavior of the examined Pb-free Sn-Zn-based alloys in the corrosive environment.