Typical Microstructural Features Research Articles

The mechanical coupling effect of α and β phases in Ti6Al4V alloy undergoing laser shock peening: A molecular dynamics study

In the dual- or multi-phase metallic materials, the influence of different microstructural constituents on the plastic strain/stress distribution is still an area of great controversy. Herein, the heterogeneous deformation behavior of α and β phases in Ti6Al4V duplex titanium alloy undergoing laser shock peening (LSP) has been investigated by molecular dynamics simulations, and the corresponding typical microstructural features are characterized at varied strains. Also, the relationship between the theoretical microstructural morphologies and the micro stress is revealed. It evidently indicates that, the grain refinement process can be summarized as: (i) dislocation motions1 interacting with deformation twins in α phases, (ii) a combination of dislocation propagations and the deformation-induced phase transition from β phase to α phase. In addition, unlike the α single-phase titanium alloy, the representative stress – strain curve of α + β titanium alloy presents a gentle fluctuation trend with smaller amplitude value, suggesting an occurrence of the coupling deformation behavior between α and β phases. The initial interfacial dislocation nucleation prefers to site in the α/α interface rather than the α/β interface, then emitting to the interior of α grains. Features of dislocation slip, twin growth, stacking fault, and phase transformation dominate the release of LSP-generated micro stress, while grain and interphase boundaries, as well as dislocation locks, contribute to the increment of the LSP-generated micro stress.

The Constituent Phases and Micromechanical Properties of Steel Corrosion Layers Generated by Hyperbaric-Oxygen Accelerated Corrosion Test.

Hyperbaric oxygen-accelerated corrosion testing (HOACT) is a newly developed method to study in the labor the corrosion behavior of steel bars in concrete. This work aimed to intensively investigate the mechanical properties and microstructures of HOACT-generated corrosion products by means of nano-indentation tests, Raman micro-spectrometry, and scanning electron microscopy. The local elastic modulus and nanohardness varied over wide ranges of 6.8-75.2 GPa and 0.38-4.44 GPa, respectively. Goethite, lepidocrocite, maghemite, magnetite, and akageneite phases were identified in the corrosion products. Most regions of the rust layer were composed of a complex and heterogeneous mix of different phases, while some regions were composed of maghemite or akageneite only. The relationship between the micromechanical properties and typical microstructural features is finally discussed at the micro-scale level. It was found that the porosity of corrosion products can significantly influence their micromechanical properties.

Characterization of microstructures and micromechanical properties of naturally generated rust layer in 60-year old reinforced concrete

The mechanical properties of steel corrosion products reported in the literature present a great dispersion. The present study aims to explore the mechanical behavior of naturally generated corrosion products at the microscale level in relation with typical microstructural features. Microstructural characterization was performed on corrosion products by nano-indentation, Raman micro-spectrometry, scanning electron microscopy and electron backscatter diffraction. The Young’s modulus and nanohardness range between 66.5 and 158.1 GPa and 2.97–11.71 GPa, respectively. The various morphologies and corresponding mechanical properties of goethite, maghemite and magnetite have finally been discussed, clarifying the influences of porosity, crystallinity and grain size on the mechanical properties.

Nanosecond pulsed laser-assisted modified copper surface structure: Enhanced surface microhardness and microbial corrosion resistance

Microbiologically influenced corrosion (MIC) is an unavoidable problem in several industries. Copper (Cu) and its alloys are widely used engineering materials. However, MIC of Cu remains a persistent challenge to their performance and functional lifetime under aggressive environments. This study investigated nanosecond pulsed laser processing (LP), which may enhance the corrosion resistance of Cu. The microstructural evolution and corrosion behavior of LP-Cu in the presence of sulfate-reducing bacteria (SRB) were evaluated. Typical deformation-induced microstructural features of high-density dislocations were analyzed on the top surface of LP-Cu coupon. Electrochemical measurements suggested that LP-Cu coupons exhibited better corrosion resistance in SRB-inoculated solution compared with their original counterpart. The enhanced corrosion resistance by LP primarily resulted from the combined influences of compressive residual stress and work hardening in the surface. However, overlap percentage played a key role in improving corrosion resistance. LP produced optimal corrosion resistance at 50% overlap. Therefore, this study introduces a unique and an option for anticorrosion control in manufacturing processes and potentially implements it onto other materials to improve its microbial corrosion resistance through LP.

Comparison of Aqueous and Gelled 3.5% NaCl Electrolytes for Assessing the Corrosion Resistance of Thermal Spray Stainless-Steel Coatings in Electrochemical Corrosion Tests

Corrosion testing with gel electrolytes gained attention in the past decade due to the advantage of almost non-destructive and in situ electrochemical measurements of bulk materials. Regarding thermal spray coatings, gel electrolytes offered the opportunity to prevent the infiltration of the typical microstructural features such as pores and microcracks. Using the example of stainless-steel AISI 316L coatings deposited by high velocity air fuel (HVAF) spraying on mild and stainless-steel substrates, the electrochemical corrosion behavior was analyzed in 3.5% NaCl electrolytes in an aqueous and gelled state. In this context, potentiodynamic polarization tests were carried out in a three-electrode corrosion cell, which was adapted for gel electrolyte testing. Gelling was realized with a technical gelatin. The characteristic corrosion values, such as open circuit potential, corrosion potential, and corrosion current density, revealed for the gelled state that the influence of the substrate material used could be eliminated and thus, the coatings itself could be characterized. In contrast, the coating specific microstructure and substrate material significantly affected the potentiodynamic polarization curve in the 3.5% NaCl aqueous electrolyte. Optical microscopy of the coating surfaces and cross-sections proved that the corrosion attack caused by aqueous electrolytes could be mimicked with the gel electrolyte.

High-speed-rate direct energy deposition of Fe-based stainless steel: Process optimization, microstructural features, corrosion and wear resistance

Different types of the Fe-based stainless steel coatings were prepared by a new laser additive manufacturing technology, i.e. high-speed-rate direct energy deposition (HDED), at different deposited speeds. Subsequently, typical microstructural features, corrosion and wear behaviors were investigated. Results indicated that all the coatings were dominantly composed of ultra-fined structures with the grain size less than 2 μm. The HDEDed coating manufactured at the deposited speed of 60 m/min exhibited superior corrosive resistance with the ranking Rt (about 463.185 kΩ·cm2), which is about four time that of coating manufactured at the deposited speed of 30 m/min (about 123.910 kΩ·cm2). In addition, the lowest repassivation current density (0.16 μA/cm2) also indicates the superior secondary passivation capability. The micro-hardness of the coatings manufactured at higher deposited speeds (50 and 60 m/min) was 20% higher than that of the coatings manufactured at lower deposited speed (30 and 40 m/min). However, the wear resistance of the coatings would reach a threshold value when the deposited speed attained a certain level with the other process parameters remaining constant.

Role of the local stress systems on microstructural inhomogeneity during semisolid injection

High pressure metal die casting is an extremely dynamic process with widely ranging cooling rates and intensifying pressures, resulting in a wide range of solid fractions and deformation rates simultaneously existing in the same casting. These process parameters and their complex interplay dictate the formation of microstructural solidification defects. In this study, fast synchrotron X-ray imaging experiments simulating high pressure die casting of aluminium alloys were conducted to investigate the effect of solid fraction, loading conditions and semisolid flow on local microstructural inhomogeneity. While most of the existing literature in this field reports speeds up to 10 µm/s for in situ deformation, the present work captures much faster filling and solidification, at speeds closer to 100 µm/s and at different solid fractions. Semisolid deformation of low solid fractions reveals two typical microstructural features: (i) coarser grains in the middle and finer ones near the walls, and (ii) remelting near the solid-liquid interface due to Cu enrichment in the liquid by the flow. Ex situ scans and digital image correlation analysis of the higher solid fraction samples reveal a porosity formation mechanism based on the local state of stresses, microstructure and feeding. Four different characteristics were identified: (i) plug flow, (ii) dead zone (densified mush), (iii) shear and (iv) bulk zones. These insights will be used to develop zone-specific strategies for the numerical modelling of defect formation during die casting.

Microstructural investigation of plasma sprayed ceramic coatings focusing on the effect of the splat boundary for SOFC sealing applications using peridynamics

The present study employs a continuum mechanics approach called peridynamics to investigate the deformation as well as the damage behaviour of a 2D microstructure obtained from a plasma sprayed ceramic coating used in solid oxide fuel cell (SOFC) sealing systems. Due to the production process, plasma sprayed ceramic materials possess typical microstructural features, such as globular pores, interlamellar pores as well as microcracks, which are orientated along the splat boundaries parallel or perpendicular to the spraying direction. Several studies postulated a splat interface or boundary, which influences the effective mechanical behaviour of such a microstructure without increasing the observable porosity. By means of an image analysis technique a splat interface was identified and considered as a separate phase in the simulation models. Based on this procedure, a microstructural model of a plasma sprayed ceramic coating of a SOFC sealing system is investigated. The simulation results are compared to experimental and analytical results. These micromechanical simulations show that structural defects influence the crack initiation as well as the crack propagation while interconnecting these defects. Typical crack mechanisms, such as crack deflection, crack shielding or multiple cracking, are observed. Additionally, an anisotropy of the effective mechanical properties is observed in this heterogeneous material, which is generally expected for plasma sprayed materials. As a result, considering the splat boundary in the simulations increases the anisotropy of the mechanical behaviour, decreases stiffness as well as the strength and influences the crack pattern strongly.

Processing Q&P steels by hot-metal gas forming: Influence of local cooling rates on the properties and microstructure of a 3rd generation AHSS

Tube hydroforming is a well-established process in industry for producing complex shaped parts with closed cross section geometry and high geometrical accuracy. Hot-metal gas forming (HMGF) extends the conventional tube hydroforming process by a complex thermo-mechanical approach and thus potentially enables the complex processing of tubes from the relatively new class of quenching & partitioning-steels (Q&P steels). To prove the feasibility of an integrated Q&P treatment in the HMGF process, a simple demonstrator geometry is considered in the present study. The applied forming pressure is limited to 70 MPa, resulting in incomplete forming of the parts in the corner areas. The resulting, locally varying contact situation between workpiece and die allows the investigation of different cooling rates and their influence on local microstructural changes and on the corresponding mechanical properties. A numerical finite element simulation is used to estimate local cooling rates in three different areas of the tube (maximum cooling rates between 60 and 280 K/s), which are then correlated with hardness measurements and typical microstructural features. The hardness distribution is inhomogeneous over the cross section of the part (varies about 150 HV), with a minimum in the areas without die contact during quenching. Ferritic areas are observed in these regions due to the significantly lower cooling rates. Tensile tests show that the stress-strain behavior after the shortest partitioning time of 10 min is not only better from an energy efficiency point of view, but also provides both high strength (Rm =2050 MPa) and high ultimate strain (19.7 %), while longer partitioning times result in inferior properties. Thus, the present study shows that the Q&P treatment can be integrated into the HMGF process, but the local cooling rates must be taken into account as they strongly influence the final mechanical properties of the workpiece.

Initial investigation of B4C–TiB2 composites as neutron absorption material for nuclear reactors

In this study, a specifically designed B4C–TiB2 composite with the typical microstructural feature of a TiB2 network (cages) that encapsulates a B4C matrix was fabricated by the molten-salt and spark plasma sintering (SPS) method. The finite-element (FE) calculation results show that the connected TiB2 cages constitute a thermally conductive network, which effectively improves the overall thermal conductivity of the composite; these results agree well with the experimental results. Moreover, the Vickers indentation results reveal that the TiB2 network (cages) can effectively impinge/block the propagation of cracks, which increases the composite toughness. The composite was subjected to helium (He) ion irradiation to simulate the situation in which the B4C–TiB2 composites serve as neutron absorption material, and for which case a high quantity of He atoms is produced by the B10 (n, α) Li7 nuclear reaction. According to the transmission electron microscopy (TEM) results, the interfaces between TiB2 and B4C act as effective sinks for He atoms, and are preferential nucleation sites for He bubbles. The theoretical and experimental results show that when the B4C–TiB2 composites serve as neutron absorption pellets in nuclear reactors, they exhibit a better resistance to their disintegration than pure B4C pellets. Consequently, the performance of the control rods of nuclear reactors can be improved.

High-performance integrated additive manufacturing with laser shock peening –induced microstructural evolution and improvement in mechanical properties of Ti6Al4V alloy components

High-performance integrated additive manufacturing with laser shock peening (LSP), is an innovative selective laser melting (SLM) method to improve mechanical properties, and refine microstructure in the surface layer of metallic components. Phase, residual stress distribution, surface micro-hardness, tensile properties and microstructural evolution of SLMed and SLM-LSPed specimens in horizontal and vertical directions were examined. In particular, typical microstructural features in the surface layer were characterized by transmission electron microscopy (TEM) observations. Results indicated that surface micro-hardness subjected to massive LSP treatment had significantly improved, tensile residual stress was transformed into compressive residual stress by LSP-induced plastic deformation, and both SLMed specimens in two directions exhibited a good combination of the ultimate tensile strength (UTS) and ductility. Meanwhile, high-density dislocations and a large number of mechanical twins were generated in the coarse α′ martensites by laser shock wave (LSW), and gradually evolved into refined α′ martensites. Furthermore, according to the included angle between LSW and the deposited plane, two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented, and the influence mechanisms of the included angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens were revealed. The hybrid additive manufacturing technology combined SLM with LSP realizes the high-efficiency and high-quality integrated manufacturing of the formed metallic components for practical applications.

Effect of bimodal microstructure on the tensile properties of selective laser melt Al-Mg-Sc-Zr alloy

Selective laser melting (SLM) has become a promising manufacturing route for the aluminum alloy models and parts in automotive and aircraft industries. In this work, an Al-4.0Mg-0.7Sc-0.4Zr-0.5Mn alloy was fabricated by SLM and near-full densification (relative density 99.7%) was obtained. A fine equiaxed grain regions and columnar grain regions alternately distributed characteristic is observed in the as-fabricated sample. The results of microstructural analyses show that the rapid solidification process and the precipitation of Al3Mg2, Al6(Fe, Mn), and Al3(Sc, Zr) particles lead to the fine grain structure, which provides a combination of high yield strength and excellent ductility. The mechanical properties of the as-fabricated sample were analyzed based on the typical microstructural feature of a combination of coarse columnar grain regions and fine grain regions. The strengthening effect is mainly attributed to the grain boundaries and the effect of grain boundaries on yield stress is according with the classical Hall-Petch relationship.

Dependence of fretting wear resistance on microstructural features of alloyed steels

Abstract Systematic experimental observations concerning the response of microstructural features on fretting wear of steel grades were conducted. Five steel grades with different typical microstructural features were selected. The effect of various microstructures on fretting wear behaviour and the resulting wear mechanism were evaluated. Results show that microstructural features play a significant effect on fretting wear resistance depending on the applied loads. The DP and Q&P steel having low hardness show comparable or even better fretting wear resistance than the FM steel of the highest hardness regardless of loads, which are all better than TWIP and IF steel. The result suggests that the combination of soft ferrite and hard martensite with/without retained austenite having low hardness displays superior fretting wear resistance.

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.

Response of copper’s microstructure to laser shock peening

The response of copper’s microstructure to laser shock peening (LSP) was investigated in this study. LSP experiment was conducted with a Spitlight neodymium (Nd)-doped yttrium aluminum garnet laser. Then microstructural observations in the affected zone and substrate were conducted by means of cross-sectional optical microscopy and transmission electron microscopy. The experimental results indicated that the distribution of grain size becomes more uniform near the top surface after LSP treatment, and the average grain size in the top surface is about 5 μm. There are four typical microstructural features along the depth direction due to the ultrahigh plastic strain in the affected zone, and the microstructure changes with the number of LSP impacts. The movement of and interaction between dislocations plays an important role in the evolution of the dislocation structure. Additionally, the grain refinement mechanism of copper along the depth direction, induced by the plastic deformation during multiple LSP impacts, was proposed based on the microstructural observations.

Slurry erosion behaviour of thermomechanically treated 16Cr5Ni stainless steel

Abstract 16Cr5Ni martensitic stainless steels are used for making underwater parts of hydro turbines, which suffer severe attack from sand particles during service. The effect of thermomechanical processing that can potentially improve the mechanical properties of steels on the slurry erosion behaviour is studied. Hot compression tests were performed at strain rates of 0.001 s−1 and 10 s−1 at 950 °C and 1050 °C for a true strain of 0.7. The erosion test is conducted in a slurry pot erosion tester for 24 h at room temperature. Thermomechanically processed specimens exhibited enhanced slurry erosion that is attributed to its lower toughness arising from typical microstructural features. The erosion mechanism is correlated with the microstructure, hardness and tensile properties.

A Numerical Analysis of Deformation Processes in Oxide Dispersion-Strengthened Materials - Influence of Dislocation-Particle Interactions

A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

異質核凝固の原理を応用した高造形性・高強度を実現する3Dプリンタ用金属粉末

Additive manufacturing, also known as 3D printing, can be used to fabricate 3D objects according to 3D CAD data. Among some metallic additive manufacturing technologies, powder bed fusion method is most suitable for the precise fabrication of complex shaped products. However, columnar grains along the build direction are a typical microstructural feature of the samples fabricated by the powder bed fusion method, since this method involves leaving built products at sufficiently high-temperatures for long time during. Meanwhile, Al alloys are grain refined during solidification in order to promote the formation of an equiaxed grain structure in the casting and to eliminate columnar grain formation. This is commonly achieved by adding a small quantity of Al-Ti-B master alloy with Al3Ti and TiB2 heterogeneous nucleation sites to the melt just prior to casting. In this study, therefore, heterogeneous nucleation sites, TiC particles, are added into Ti-6Al-4V powder for 3 D printer, to achieve better microstructure and better mechanical properties.

In vitro investigation of NiTiW shape memory alloy as potential biomaterial with enhanced radiopacity

In the present study, a novel kind of NiTiW shape memory alloy with chemical composition of Ni43.5Ti45.5W11 (at.%) has been successfully developed with excellent X-ray radiopacity by the introduction of pure W precipitates into the NiTi matrix phase. Its microstructure, X-ray radiopacity, mechanical properties, corrosion resistance in simulated body fluid, hemocompatibility and in vitro cytocompatibility were systematically investigated. The typical microstructural feature of NiTiW alloy at room temperature was tiny pure W particles randomly distributing in the NiTi matrix phase. The presence of W precipitates was found to result in enhanced radiopacity and microhardness of NiTiW alloy in comparison to that of NiTi binary alloy. NiTiW alloy exhibits excellent shape memory effect, and a maximum shape recovery ratio of about 30% was obtained with a total prestrain of 8% for the NiTiW alloy sample. In the electrochemical test, NiTiW alloy presented an excellent corrosion resistance in simulated body fluid, comparable to that of NiTi alloy. Hemocompatibility tests indicated that the NiTiW alloy has quite low hemolysis (lower than 0.5%) and the adherent platelet showed round shape without pseudopod. Besides, in vitro cell viability tests demonstrated that the cell viability is all above 90%, and the cells spread well on the NiTiW alloy, having polygon or spindle healthy morphology. The hemocompatibility tests, in vitro cell viability tests and morphology observation indicated that the NiTiW shape memory alloys have excellent biocompatibility. The excellent X-ray radiopacity makes the NiTiW alloys show obvious advantages in orthopedic, stomatological, neurological and cardiovascular domains where radiopacity is quite important factor in order to guarantee successful implantation.

ChemInform Abstract: Application Prospects and Microstructural Features in Laser‐Induced Rapidly Solidified High‐Entropy Alloys

AbstractReview: 69 refs.