Microstructure Strengthening Mechanisms Research Articles

Microstructure evolution of Ti17 titanium with ultrahigh strength and moderate plasticity fabricated by laser powder bed fusion

This study utilized laser powder bed fusion (LPBF) to fabricate Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) specimens, exploring their ultrahigh strength microstructural strengthening mechanisms and examining heat treatment effects on α lamellae and related mechanical property trends. In the as-deposited LPBF-Ti17, α lamellae features extremely fine dimensions, with an average lamellar thickness of 45 nm, and comprises approximately 52.2% of total volume. This finely dispersed α lamellar microstructure provides numerous α/β phase interfaces, enabling LPBF-Ti17 mechanical properties to reach ultra-high strengths close to 1500 MPa. During heat treatment, the Ostwald ripening mechanism primarily drives growth in α-phase lamellar thickness. The rate of α lamellar thickness growth reaches its peak between 650 °C and 700 °C. The main dislocation distribution of LPBF-Ti17 lies at α lamellae edges, adjacent to β phase, and dislocation density diminishes with rising heat treatment temperature. Concurrently, the Termination migration mechanism causes α lamellae ends to become progressively rounded from sharp features with increasing heat treatment temperature. The increase in α lamellar thickness and reduction in dislocation density collectively contribute to the effective enhancement of Ti17 plasticity.

Study on the strengthening mechanism of AlCoCrFeNi2.1 eutectic high-entropy alloy

Eutectic high-entropy alloys (EHEAs) possess unique microstructures and excellent properties. In this study, it was found that the microstructure of as-cast AlFeCoCrNi2.1 can be classified into lamellar regular structure, irregular structure, and grain boundaries. The solidification process of AlFeCoCrNi2.1 was theoretically speculated. Compression tests revealed that the yield strength of AlFeCoCrNi2.1 reached 724.39 MPa, the ultimate fracture toughness reached 3556.4 MPa, and the as-cast average hardness reached 349.9 HV0.3. Furthermore, this study investigated the microstructural strengthening mechanisms of AlFeCoCrNi2.1 EHEA. The results indicated that different compression stages had varying effects on orientation distribution and dislocation slip during the compression process. Furthermore, it was found that the irregular structure and grain boundaries played a major role in the alloy strengthening.

A physically-based constitutive model for a novel heat resistant martensitic steel under different cyclic loading modes: Microstructural strengthening mechanisms

Cyclic responses of a novel heat resistant martensitic steel, 9Cr3Co3W1CuVNbB steel, under different loading modes were studied to reveal its complex strengthening mechanisms at high temperature. Based on the experimental observations, dislocation strengthening, precipitation strengthening by M23C6 phase, MX phase, and Cu-rich phase, and subgrain boundary strengthening were the main mechanisms for its excellent fatigue and creep-fatigue properties. In particular, the dynamic process of interaction between phase and dislocation were studied with the help of molecular dynamics method, and the different contributions of hard and soft phases in the studied steel were determined in fatigue and creep-fatigue loading. Based on these phenomena, a physically-based constitutive model was proposed for both fatigue and creep-fatigue (dwell fatigue at elevated temperature) tests considering various micromechanical mechanisms. Three ways for dislocation annihilation were proposed to simulate the dislocation evolution under different loadings. In addition, the effect of Cu-rich phase was modeled by critical breaking angle and dislocation line tension. The capability of the proposed model under different loading modes was verified by comparing cyclic responses, hysteresis loops, stress relaxation, and dislocation density evolution. The proposed model provides an alternative perspective on understanding fatigue and creep-fatigue behaviors of heat resistant martensitic steels owning the similar strengthening mechanisms.

Microstructural strengthening and toughening mechanisms in Fe-containing Ti-6Al-4V: A comparison between homogenization and aging treated states

Microalloying of Ti–6Al–4V alloy by Fe addition has attracted interest as a promising way to improve castability and comprehensive mechanical performance. The mission of this work is twofold by employing the experimental examination and the phenomenological analysis, (1) to investigate the effect of Fe addition on the microstructure features and mechanical properties of the Fe-containing Ti–6Al–4V (TC4-xF) alloys subjected to casting and homogenization treatment, and (2) to unveil the critical microstructure features in homogenization, hot-worked and aging treated alloys, respectively, that benefit the yield strength and the fracture toughness. Experimental observations evidence that the addition of 0.5 wt.% Fe is most effective in enhancing the tensile ductility and the mode I fracture toughness. Further Fe addition up to 0.7–0.9 wt.% results in plateau values of yield and ultimate strengths with some fluctuations. Phenomenological analyses screen out the microstructural strengthening and toughening determinants which exhibit distinct sensitivities on Fe content under different processing conditions. The solid solution strengthening is confirmed as the primary effect that governs the yield strength of the homogenization treated TC4-xF alloys, followed by the refined size of colony and α lamella, so does it for the hot-worked and the aging-treated alloys. The strengthening effect of Fe could be further promoted by hot-working but impaired by a prolonged annealing time or a lowered cooling rate. The type of crack propagation path and the α morphology are discerned to play their own leading roles in different cases to influence the performance of fracture toughness. A long crack propagation distance that traverses broad α/β lamellae embraces a high crack propagation resistance and gives rise to enhanced fracture toughness. The experimental results enrich the dataset of microstructure features and mechanical properties of Ti–6Al–4V relevant alloys. While upon the phenomenological analysis, the discovered microstructural strengthening and toughening factors provide deeper mechanism insights into the mechanical behaviors of Fe-modified Ti-6Al-4 V alloys and are of the technical importance to future machine-learning of microstructure-property relationship.

Microstructure Refinement, Strengthening and Ductilization Mechanisms in Al-Mg-Mn-Er-Zr Alloy by Friction Stir Processing

Microstructure Refinement, Strengthening and Ductilization Mechanisms in Al-Mg-Mn-Er-Zr Alloy by Friction Stir Processing

Fe-reinforced TTCP biocermet prepared via laser melting: Microstructure, mechanical properties and bioactivity

Tetracalcium phosphate (TTCP) is considered as an ideal potential implant biomaterial due to its good biodegradability, biocompatibility and bioactivity, while the insufficient mechanical properties limit its use in loading-bearing implants. In this work, TTCP-matrix biocermet scaffolds reinforced with Fe particles contents of 15, 30, 45 and 60 vol% were fabricated by selective laser melting (SLM) technology. Experimental results indicated that the introduction of Fe significantly enhances the mechanical properties of TTCP scaffold. Among them, the 45Fe/TTCP biocermet scaffold exhibited the maximum compressive strength (354.12 ± 5.2 MPa), which was about 29.8 times higher than that of pure TTCP scaffold (11.87 ± 0.64 MPa). Meanwhile, its fracture toughness and tensile strength reached 1.88 ± 0.12 MPa m1/2 and 28 ± 2 MPa, respectively, which were higher than the 1.12 ± 0.1 MPa m1/2 and 17 ± 2.1 MPa of pure TTCP scaffold. Herein, the improvement of strength and fracture toughness under compression mode was probably accomplished by the microstructural strengthening and toughening mechanisms such as the crack deflection, crack bridging, and crack tip closure. The reinforcing mechanism under tension mode was probably due to the ductile microvoid coalescence fracture, which commonly observed in the fractured surface of pure Fe under tensile loading. Besides, the biocermet scaffold also has a good ability to induce the formation of bone-like apatite. In short, the porous Fe/TTCP biocermet scaffold with significantly enhanced mechanical properties and good bioactivity is expected to be a potential candidate for load-bearing bone implants.

Comparison of Microstructure and Mechanical Properties of Scalmalloy® Produced by Selective Laser Melting and Laser Metal Deposition.

The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (µ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength.

General methodology to estimate the dislocation density from microhardness measurements

A general methodology to estimate dislocation density in cubic metals using microhardness measurements has been established. The proposed methodology is based on the Indention Size Effect (ISE) and microstructural strengthening mechanisms. The methodology was validated using published experimental data of a pure Nickel (FCC) and Tungsten (BCC), as well as our own data on dual phase (BCC and FCC) lean duplex stainless steel 2101(LDSS 2101). The estimations of dislocation densities for LDSS 2101 phases were confirmed via X-ray diffraction measurements. Our results collectively validated the proposed approach as a general method to estimate dislocation density with acceptable accuracy.

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermo-mechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic γ–γ′–δ Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four γ–γ′–δ Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080 °C and 1160 °C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.

XRD study of variation of strengthening effects in carbonitrided 20Cr3MoVW steel by heat treatment

This paper discloses the findings of an XRD investigation of microstructural characteristics and calculations of their contribution to hardening of carbonitrided 20Cr3MoVW steel subjected to three heat treatment conditions. A method for evaluation of the total yield strength using X-ray diffraction data is proposed herein. It is based on a model of the steel strengthening being linearly affected by several microstructure strengthening mechanisms. Analysis of changes in depth profiles of the estimated yield strength with heat treatment is given together with comparison against microhardness distribution in the hardened case.

Residual Constitutive Behavior of Aluminum Alloys after Fire Exposure

Aluminum alloys are increasingly being used in lightweight transportation applications such as naval vessels and light passenger rail. The aluminum alloys considered include 5xxx-series (Al-Mg) and 6xxxseries (Al-Mg-Si) alloys due to their mechanical strength, corrosion resistance, and weldability. A major concern in the use of aluminum alloys in lightweight structural applications is fire exposure. Aluminum mechanical properties are significantly reduced at 300°C. After fire exposure, structural damage will vary due to the local thermal history which is governed by the fire size and proximity and other environmental factors. It is paramount to understand the nature of this structural damage in terms of residual (post-fire) constitutive behavior so as to allow for informed damage assessment. AA5083-H116 and AA6061-T651 residual constitutive behavior was characterized using quasi-static tension tests. The alloys were exposed to elevated temperatures at controlled heating rates using an induction heater to simulate the varying conditions in a fire environment. The thermal history dependence of residual constitutive behavior was elucidated in terms of the microstructural strengthening mechanisms. The primary strengthening mechanisms of AA5083 and AA6061 evolve at elevated temperatures due to recrystallization and precipitate coarsening, respectively. Strengthening mechanism evolution has been shown to be kinetically dependent which explains the thermal history dependence of residual constitutive behavior. Structural damage from a wide range of fire scenarios may be understood using these underlying strengthening mechanisms which govern residual constitutive behavior.

Hot deformation characteristics of a polycrystalline γ–γ′–δ ternary eutectic Ni-base superalloy

Nickel-base superalloys possess an unparalleled combination of mechanical and physical properties for elevated temperature applications and are likely to remain the first choice for structural components in turbine engine applications in the future. In order to keep pace with increasingly demanding design requirements associated with advanced gas turbine technologies, the properties of Ni-base superalloys can be greatly enhanced via thermo-mechanical and/or compositional changes. Recent studies have revealed that ternary eutectic γ–γ′–δ Ni-base superalloys exhibit promising high temperature mechanical properties that may potentially be superior to state-of-the-art polycrystalline Ni-base superalloys that are currently used in advanced gas turbines. As the properties of this novel class of alloys are largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters need to be optimized concurrently. Deformation at temperatures and strain rates between 1040–1140°C and 0.1–0.001/s resulted in a transition of dominant deformation mechanisms from dislocation based plasticity to grain boundary rotation and sliding. The governing deformation mechanism was found to influence the alignment of δ phase precipitates and the degree of anisotropy that occurs during hot deformation of γ–γ′–δ Ni-base superalloys. Coupling high deformation temperatures with low strain rates promotes grain boundary sliding and rotation leading to less internal cavitation damage and a more isotropic microstructure.

Microstructure strengthening mechanisms in an Al–Mg–Si–Sc–Zr equal channel angular pressed aluminium alloy

Microstructure dislocation strengthening mechanisms in severely deformed aluminium strongly depend on the different boundary evolutions. Thereafter, models of proof stress determination should take into account the different nature of the boundaries that form during severe plastic deformation. In the last few decades, Hall–Petch modified relationship and other proof stress modelling were extensively discussed. This paper deals with further insights into the Hansen's and other authors approach to the modelling of aluminium poof stress after equal channel angular pressing. The present model is based on a detailed transmission electron microscopy microstructure characterization of the different strengthening contributions in an age-hardened Al–Mg–Si–Sc–Zr alloy.

Microstructure strengthening mechanisms in different equal channel angular pressed aluminum alloys

Microstructure characterization and identification of the different nature of boundary formation during severe plastic deformation is the basis for a quantitative analysis of material flow stress. In equal channel angular pressing, the microstructure evolves to form low-angle dislocation boundaries and high-angle boundaries whose origin is different. For this reason, boundary misorientation, size and fraction evolve differently with strain. The Hall–Petch relationship in severe plastic deformed aluminum and aluminum alloys has been extensively discussed by Niels Hansen and others in several published works. It appeared that in such cases, the dislocation boundary strengthening contribution is to be taken into account. This paper deals with further insights into the Hansen's and other authors approach to the Hall–Petch relationship. Present approach is based on a detailed microstructure characterization of the different strengthening contributions in severe plastic deformed aluminum alloys. AA1200, AA3103, AA6000 series, and AA2091 were quantitatively characterized by TEM inspections. The calculated alloys yield strengths were compared to measured tensile yield stresses obtaining a quite satisfactory matching. This, ultimately, confirmed the proposed approach and models. Finally, an experimental value of the hardness-to-yield stress, H/σy, was found for all the studied alloys and discussed accordingly.

X-ray and optical crystallographic parameters investigations of high frequency induction melted Al-(α-Al_{2}O_{3}) alloys

This article deals with the microstructural strengthening mechanisms of aluminium by means of hard alpha-Al(2)O(3) alumina fine particles. A broad of understanding views covering materials preparations, elaboration process, characterization techniques and associated microstructural characteristic parameters measurements is given. In order to investigate the microstructural characteristic parameters and the mechanical strengthening mechanisms of pure aluminium by hard fine particles, a set of Al-(alpha-Al(2)O(3)) alloys samples were made under vacuum by high fusion temperature melting, the high frequency (HF) process, and rapidly solidified under ambient temperature from a mixture of cold-compacted high-pure fine Al and alpha-Al(2)O(3) powders. The as-solidified Al-(alpha-Al(2)O(3)) alloys were characterized by means of X-ray diffraction (XRD) analyses, optical microscopy observations and Vickers microhardness tests in both brut and heat-treated states. It was found that the as-solidified HF Al-(alpha-Al(2)O(3)) alloys with compositions below 4 wt.% (alpha-Al(2)O(3)) are single-phase microstructures of the solid solution FCC Al phase and over two-phase microstructures of the solid solution FCC Al and the Rhombohedral alpha-Al(2)O(3) phases. The optical micrographs reveal the presence of a grain size refinement in these alloys. Vickers microhardness of the as-solidified Al-(alpha-Al(2)O(3)) is increased by means of pure fine alpha-Al(2)O(3) alumina particles. These combined effects of strengthening and grain size refinement observed in the as-solidified Al-(alpha-Al(2)O(3)) alloys are essentially due to a strengthening of Al by the alpha-Al(2)O(3) alumina particles insertion in the (HF) melted and rapidly solidified alloys.

Preferential Site Precipitation and Subcell Stability in AA6061 Sandwich Cores

AbstractPeriodic cellular metal (PCM) sandwich cores can be considered hybrids of the solid and gas type. These can be designed at both the architectural and microstructural levels. PCM cores with 95% open porosity have been constructed from perforated 6061 aluminium alloy (AA6061) sheets using a perforation-stretching method. This method places planar, periodically-perforated sheet metal in an alternating-pin jig. The pins apply force out-of-plane, plastically deforming the sheet metal into a truss-like array of struts (i.e. metal supports) and nodal peaks (i.e. strut intersections). The result is a non-uniform work-hardened profile exhibiting large deformation at the nodes and small deformation at the struts.For identical PCM architectures, this study looks at the interaction of microstructural strengthening mechanisms and the resultant performance of PCM truss cores. Beginning with fabrication, work-hardening induced a subcell network of dislocation tangles within the AA6061 matrix. Following this stage, a variety of microstructures were created through recovery, recrystallization and precipitation mechanisms. Microhardness measurements and electron back-scattered diffraction (EBSD) characterization were employed through truss core cross-sections in order to study the microstructural gradients of subcell size as well as interaction between subcells and precipitates in the truss cores. To determine the effect of microstructure on mechanical performance, PCM cores were compressed to study deformation and collapse mechanisms.The present data can be used to illustrate engineering at the architectural and microstructural levels to achieve a range of mechanical properties in a hybrid sandwich core.

A combined microstructure strengthening analysis of SiC p/Al metal matrix composites

A combined microstructure strengthening analysis was made to predict the yield strength of SiC p/Al metal matrix composites. The modified shear lag theory can be used to predict the yield strength of particlereinforced high strength aluminium composites, but the yield strength is underestimated for composites prepared from pure and cast aluminium matrices because the change of matrix microstructure after particle incorporation is not considered. The increased yield strength due to the change of matrix microstructure represents a large increment of the total increased yield strength of SiC particle-reinforced low strength aluminium composites. Several microstructure strengthening mechanisms are examined to evaluate their effects on the matrix yield strength. The result of the combined microstructure strengthening analysis is consistent with experimental results.

Microalloying and precipitation in Cr-V rail steels

A study of alloying effects in Cr-V rail steels has been carried out to determine the microstructural strengthening mechanisms. The laboratory steels investigated were based on the commercial 1 pct Cr rail steel chemistry (0.75C−0.8Mn−1 .25Cr−0.30Si) and represented two levels of nitrogen (nominally 0.007 and 0.015 wt pct) and two types of deoxidation practice. It is found that vanadium microalloying produces a yield stress increment of approximately 90 MPa for low-nitrogen steels and 140 M Pa for highnitrogen steels. This strengthening effect appears to saturate at or below 0.15 wt pct vanadium, with no further strength increase for higher vanadium concentration. The hardness and UTS exhibit the same variation with vanadium content as the yield stress. There is no consistent effect of aluminum content (deoxidation practice). Several distinct precipitate morphologies, a wide range of precipitate sizes, and a nonhomogeneous distribution of precipitates are observed. There is also significant partitioning of V, Cr, and Mn to cementite. It is suggested that an increase in the amount of coarse, austenitenucleated V(C,N) and possibly other coarse precipitate modes is responsible for the saturation of the vanadium strengthening effect.