Dislocation-precipitate Interactions Research Articles

Strengthening mechanisms for microstructures containing unimodal and bimodal γ′ precipitates in ATI 718Plus

The influence of γ′ precipitate size distribution on the deformation mechanisms under tensile loading in ATI 718Plus was studied. A set of aging treatments within the temperature range of 720 °C–900 °C was performed on solution-treated samples to obtain various γ′ precipitate size distributions. Unimodal and bimodal γ′ precipitate size distributions were achieved through single-step and two-step aging sequences, respectively, and such microstructures were tensile tested to failure to assess their yield strength, ultimate tensile strength, and elongation-to-failure. Some of the tensile samples were interrupted after achieving 3–4 % plastic strain, and the deformed microstructures were examined using transmission electron microscopy to investigate the γ′ precipitate-dislocation interactions. For the unimodal γ′ precipitate size distribution samples with the smaller γ′ precipitates (radius ∼ 7 nm), dislocations sheared through the precipitates. Both dislocation loops and paired dislocations were observed for the microstructures containing larger γ′ precipitates (radius ∼ 24 nm). The microstructure containing a bimodal γ′ precipitate size distribution, which included average γ′ precipitate radii of ∼6 nm and ∼28 nm, exhibited shearing as the dominant deformation mechanism, and this microstructure exhibited the highest strength values. The experimental observations were rationalized based on the theoretically-calculated critical resolved shear stress values for shearing and looping and a modified model for predicting the yield strength for bimodal microstructures was introduced.

Tensile and creep resistance improvement of GH4975 alloy for turbine discs via electron beam smelting layer solidification technology

A difficult-to-deform superalloy for turbine disks, which can be used at 850 °C for a long time, was prepared by electron beam melting layer solidification (EBSL) technology with high purity and fine structure. Subsequently, the ingot was processed by hot extrusion technology instead of conventional hot forging, and then the tensile and creep tests were performed after standard heat treatment. The results show that the EBSL alloy exhibits excellent high-temperature strength and creep rupture life compared to the conventional preparation method. Among them, the yield strengths at 20 °C and 850 °C are 8.1 % and 5.2 % higher than those in the literature, and the creep rupture life at 850 °C and 412 MPa is 1.53 times higher than that in the literature. The dislocation-precipitate interaction reveals that dislocation shearing γ′ phase is the major mechanism at 20 °C and 850 °C tensile, and Orowan looping and stacking fault shearing are the decisive mechanism during creep at 850 °C and 412 MPa. MC carbides remaining during solidification are the main source of cracks during deformation by fracture analysis. The reasons for refining the microstructure and improving the purity by electron beam melting are analyzed and explained. We believe this work can provide new ideas and methods for preparing superalloys.

A novel creep model with synergetic Orowan bypassing and climbing mechanisms in nickel-base superalloys

A novel creep model was proposed to accurately and conveniently predict the steady-state creep rate of precipitation-strengthening nickel-base superalloys. Based on the space distribution of precipitates, the synergetic mode of Orowan bypassing and dislocation climbing mechanisms in dislocation–precipitate interaction was coupled into the new creep model according to probability–correlative method. The results show that the new model can accurately describe the steady-state creep rate compared with the experimental data, which is far better than the classical model. Moreover, the present model no longer relies on the adjustable parameters. The quantitative relationship between threshold stress and temperature is also determined. This work sheds new light on the dislocation–precipitate interaction during creep deformation, and provides an effective model for designing precipitation-strengthening alloys with improved creep properties.

Coupling strengthening mechanism of precipitate and solute atoms in edge dislocation motion of Al-Mg-Si alloy

To investigate the coupling strengthening mechanism of precipitate and Cu solute atoms in Al-Mg-Si alloy, the molecular dynamics model of edge dislocation motion interacted with precipitate and Cu solute atoms was established. The dislocation length is increased in the dislocation-precipitate interaction due to the bending and bowing of the edge dislocation around the precipitate. No coupling between precipitate and solute can be found in material strengthening in cases with lower solute concentrations. The exponential effect of the coupling effects between precipitate and solute on material strengthening is found when the solute concentration is increased above 0.3.at% at room temperature. The change of the edge dislocation caused by solute atoms around the precipitate is the key reason for the strengthening coupling effect. Furthermore, the effects of temperature variations on the coupling effect are more pronounced than the effects of shear strain rate variations. When the temperature is increased from 200K to 500K, the critical ratio of solute and precipitate strengthening is decreased from 1.5 to 0.4.

An investigation on damage tolerant design parameter of additively manufactured Al 2024: Fatigue crack growth rate and fracture toughness

Additive manufacturing has emerged as a key forefront of the fourth industrial revolution due to its exceptional design freedom and ability to produce lightweight components for different structural applications. However, the unique thermomechanical characteristics of these additive processes introduce several challenges to the structural integrity and the corresponding life of these components. The structural components, specifically in the aerospace industry, are usually designed according to the damage tolerance approach. The Al 2024 alloy is one of the most widely used alloy in structural applications in the aerospace industry due to its high strength-to-weight ratio and excellent fatigue properties. However, studies on the FCGR behaviour of additively manufactured Al 2024 are scarce in the literature; hence, the current study investigates the FCGR behaviour under stress-controlled loading using a stress ratio of 0.2 and 0.3. The aged specimen showed superior mechanical properties compared to the as-printed specimen, and the properties of the T6 aged specimen are comparable to their wrought counterparts. The different precipitate–dislocation interaction is investigated and correlated with corresponding mechanical behaviour. The primary strengthening mechanism in Al 2024 alloy was observed to be the Orowan bypass mechanism in θ(Al2Cu), and S(Al2CuMg) precipitate.

Implementation of a modified Graham-Walles viscosity function within a Chaboche viscoplastic constitutive model

This work provides a numerical framework for the accurate prediction of operational life of metallic components exhibiting a non-classical creep behavior under constant loadings and very high temperature. A modified Graham-Walles type analytical viscoplastic function is implemented into a Chaboche unified viscoplastic constitutive model. The numerical model is integrated into the finite element software Lagamine following a fully-implicit two-step radial return mapping algorithm. The non-linear system of equations is solved using a robust Newton-Raphson method. The computational efficiency of the model is enhanced by implementing a sub-step routine, thereby decreasing the average number of iterations of the finite element software. The validation of the model is performed using experimental data available in the literature on the non-classical creep behavior of Incoloy 800H, a Ni-superalloy exhibiting a two-step creep strain rate minima attributed to multiple complex dislocation-precipitate interactions.

Insights into flow stress and work hardening behaviors of a precipitation hardening AlMgScZr alloy: Experiments and modeling

Flow stress and work hardening behaviors, as two important aspects of mechanical behaviors, have been studied extensively and their interpretation for the case of pure metals via dislocation theory is well established. The introduction of precipitates inevitably affects the flow stress and work hardening rate, since the precipitates can evidently change the dislocation behaviors, both their gliding facilities and their storage mechanisms. Thus, different precipitate-dislocation interaction modes (i.e. shearing and bypassing mechanisms) would lead to different dislocation behaviors and resultant different flow stress and work hardening behaviors. In this study, we investigate the influence of shearable, non-shearable and mixed shearable/non-shearable precipitates on flow stress and work hardening behaviors in the case of AlMgScZr alloys, where precipitates and solid solution can be decoupled, based on experiments and a modified dislocation-based model. We show that, the introduction of shearable precipitates and shearable/non-shearable transition have important effects on flow stress and work hardening behaviors. By quantitatively characterizing different precipitate-dislocation interactions and the evolution of dislocation density during the deformation, the intrinsic influencing mechanisms of precipitates on flow stress and work hardening behaviors are demonstrated.

Texture and damage evolution in additively manufactured 18 %Ni-M350 maraging steel under dynamic impact loading: Processing parameters and heat treatment effects

The mechanical and microstructural responses of 18 % Ni M350 maraging steel produced by additive manufacturing (AM) using the direct energy deposition (DED) laser process under impact loading are investigated. The material is manufactured using low and high energy area density (EAD) to identify the influence of AM processing parameters on the impact strength, microstructural, and texture evolution during deformation. In addition to the as-built (AB) state, maraging steel samples subjected to solutionizing and aging heat treatment (HT) are also investigated. Electron backscatter diffraction (EBSD) is used to characterize texture evolution under impact loading. The test results reveal that the low-EAD material exhibits the greatest impact strength for the applied strain rates that ranged from 200 to 3300 s−1 in its as-built state. This finding is attributed to the high local misorientation that is closely correlated with the dense dislocation network generated as a result of the rapid cooling process. The activation of a new slip systems {112}〈111¯〉 and {123}〈111¯〉 is observed after deformation at high strain rates in the as-built material. A higher degree of new slip system activation in low-EAD samples suggests a higher susceptibility to the initiation of an adiabatic shear band (ASB). The effects of processing parameters on impact strength, strain, and propensity to defects are eliminated by heat treatment. The improvement in impact strength of the steel materials after heat treatment can be attributed to the dominance of precipitate-dislocation interactions as the primary strengthening mechanism. Furthermore, heat-treated materials exhibit the highest susceptibility to deformation-induced defects, such as adiabatic shear bands and fracture.

Fe-based high-entropy alloy with excellent mechanical properties enabled by nanosized precipitates and heterogeneous grain distribution

High-entropy alloys (HEAs) consisting of CoCrFeNiAlTi systems, with a face-centered cubic (FCC) matrix reinforced by ordered L12 precipitates, have demonstrated exceptional strength-ductility combinations. However, the current compositional design of HEAs heavily relies on high Ni and Co contents, compromising the balance between properties and cost. Thus, it is crucial to optimize the cost-performance trade-off by fine-tuning the range of Fe, Co, and Ni, while maintaining excellent strength-ductility combination. In this study, we propose a novel Fe-based HEA with nanosized precipitates and a heterogeneous grain distribution, achieving a strength-ductility combination comparable to state-of-the-art Ni- or Co-based HEAs. The alloy benefits from both precipitation hardening and hetero-deformation-induced strengthening attributed to the heterogeneous grain distribution, resulting in excellent yield strength of 1433 MPa, tensile strength of 1599 MPa, and ductility of 22%. The microstructural evolution and its influence on mechanical properties are unraveled with respect to the observation of precipitate-dislocation interaction and hetero-deformation-induced stress (HDI stress) evaluation. This study suggests that the challenge of balancing properties and cost can be addressed through optimized compositional and microstructural design.

Enhanced strength-ductility synergy in pure Al by assembling grain and chemical gradients

Titanium (Ti) is generally known for its limited solid solubility and used as grain refiner in Al-based alloys, accelerating heterogeneous α-Al nucleation during solidification. In this study, a newly developed surface modification of pure Al followed by sputtering Ti film (∼1 μm) and subsequent laser remelting has been proposed. Gradient structural features in both grain size and chemical composition have been accomplished in the top surface layer (<10 μm), showing three typical domains with different grain morphology and Al3Ti nanoprecipitates. The simultaneous increase in strength and ductility is attributed to the strong dislocation-precipitate interaction inside gradient nano-grained top-surface layer, which has been confirmed by the partially dissolved Al3Ti nanoprecipitates induced by deformation. The findings in this work provide a new routine for upgrading mechanical properties through the manipulation of both grain- and chemical-gradients.

A Molecular Dynamics Study on the Dislocation-Precipitate Interaction in a Nickel Based Superalloy during the Tensile Deformation.

In the present paper, the dislocation-precipitate interaction in the Inconel 718 superalloy is studied by means of molecular dynamics simulation. The atomistic model composed of the ellipsoidal Ni3Nb precipitate (γ″ phase) and the Ni matrix is constructed, and tensile tests on the composite Ni3Nb@Ni system along different loading directions are simulated. The dislocation propagation behaviors in the precipitate interior and at the surface of the precipitate are characterized. The results indicate that the dislocation shearing and bypassing simultaneously occur during plastic deformation. The contact position of the dislocation on the surface of the precipitate could affect the penetration depth of the dislocation. The maximum obstacle size, allowing for the dislocation shearing on the slip planes, is found to be close to 20 nm. The investigation of anisotropic plastic deformation behavior shows that the composite system under the loading direction along the major axis of the precipitate experiences stronger shear strain localizations than that with the loading direction along the minor axis of the precipitate. The precipitate size effect is quantified, indicating that the larger the precipitate, the lower the elastic limit of the flow stress of the composite system. The dislocation accumulations in the precipitate are also examined with the dislocation densities given on specific slip systems. These findings provide atomistic insights into the mechanical behavior of nickel-based superalloys with nano-precipitates.

Detailed investigation on microstructure and strengthening contribution of Al-xCu-1.3Li-X alloy sheets

The microstructure-mechanical properties connection and strengthening contribution of Al-xCu-1.3Li-X (x = 3.8, 4.2, 4.5 wt%) alloy sheets subjected to various heat treatments were systematically investigated. After solid-solution treatment, the decreased schmid factor value was the main factor for higher strength of Alloy-3 (4.5 wt% Cu). The number density and diameter of T1 precipitates simultaneously increased with rising the Cu content during T8 aging, leading to the highest strength (655.4 MPa) of T8-aged Alloy-3. However, the high Cu content level resulted in the preferential precipitation of Guinier-Preston (G.P) zones during T6 aging at 165 °C. The θ' precipitates were therefore significantly increased and the nucleation of T1 precipitates was suppressed in T6-aged Alloy-3, causing the reduced strength compared to that of T6-aged Alloy-2 (4.2 wt% Cu). Furthermore, the experimental yield strength was successfully predicted by invoking the modified precipitate strengthening model, in which the θ' precipitate-dislocation interaction was by-passing mechanism, whereas the shearable and by-passed T1 precipitates coexisted for all alloys. The strengthening transition from by-passing to shearing occurred in T1 precipitates with a thickness <1.9 nm or diameter <65 nm, and was particularly associated with the decrease in thickness of T1 precipitates.

Twin-boundary and precipitate interaction in Mg–Al alloy: an MD study

Strengthening of Mg-alloys by precipitation is much less efficient than in other metallic alloys (e.g. Al) as the Mg17Al12 precipitates grow as thin plate or lozenge shaped or long rod shape parallel to the basal plane. Recently atomistic simulations reveal that the dislocation-precipitate interaction is very week to claim for the precipitation hardening mechanism. However, the interaction of twin-boundary with the Mg17Al12 precipitate remains unexplored using atomistic simulation. In the present study we focus on the twin-boundary/precipitate interaction at different temperatures, precipitate sizes and varied applied loads, carried out using classical molecular dynamics methodology. In particular, the activation energies necessary to overcome various precipitates are determined as a function of the temperature, precipitate size and applied load. The velocity profile of the twin is calibrated with these different external conditions. An attractive nature of interaction has been observed while the twin-boundary comes closer to the precipitate and a network of dislocations are observed when the twin-boundary bypass the precipitate, as manifested through our atomistic microstructures. These results provide valuable information about the precipitate hardening mechanisms and suggested new avenues to improve the mechanical properties of Mg–Al alloys.

Introducing both chemical and structural gradients in AISI 321 stainless steel via aluminizing and laser shock peening for superior properties

Aluminizing is a diffusion based chemical surface treatment on materials to provide excellent protection against high temperature oxidation and corrosion, but is likely to be accompanied by a degradation in mechanical properties. Laser shock peening (LSP) is a superior laser based surface strengthening technique for enhanced mechanical properties. In this study, we focus on combining these two methods on the AISI 321 stainless steel to introduce both chemical and structural gradients for superior properties. Comparative investigation on both the microstructure and mechanical properties of the as received 321 steel, aluminized 321 steel and the aluminizing + LSP processed steel indicated that: (1) A three-layer protective coating was introduced on the surface of the aluminized steel and the concentration of Al decreased gradually from the outermost layer to a certain depth, forming B2-FeAl(Cr) solid solution and Al2O3 in Layer-I, B2-FeAl and DO3-Fe3Al phases in Layer-II and BCC-Fe(Cr,Al) solid solution phase in Layer-III, with B2-NiAl precipitates distributed in Layer-II and Layer-III. (2) After LSP treatment, a dislocation density gradient from the surface to the matrix, dislocation-precipitate interactions and a significant compressive stress were introduced. (3) Though the aluminized steel exhibited a decrease in the high temperature tensile properties compared to that of the as received 321 steel, the aluminizing + LSP processed steel achieved a striking enhancement. The formation of three-layer coating, dislocation density gradient, dislocation-precipitate interactions and the residual stress contribute jointly to gain the yield strength, ultimate strength and fracture elongation up to 265 MPa, 430 MPa and 37.5 % at 620 °C, respectively.

Discrepancy in ductility improvement in repeated stress relaxation of AA7075

The stress relaxation test is a commonly used transient test to determine the deformation kinetics during plastic deformation. This study reports the time-dependent deformation behavior of AA7075 alloy under different temper conditions. The deformation parameters, such as the apparent and true activation volume were estimated using single and repeated stress relaxation tests respectively. The results indicate that dislocation–dislocation interaction plays a critical role in the deformation behavior of the solution-treated condition, whereas dislocation–precipitate interaction dominates in the aged specimens. Additionally, the study found that single stress relaxation enhances the ductility of the deforming specimen, regardless of the temper condition. The ductility during repeated relaxation however had negative effects in aged samples. The possible mechanisms responsible for this discrepancy are discussed, providing a comprehensive overview of the time-dependent deformation behavior of AA7075 alloy.

Planar slip triggered by successive dislocation-precipitate interaction in titanium alloys

The planar slip associated with the interaction between dislocations and α2 precipitates in titanium alloys was investigated using atomistic simulation and transmission electron microscopy. The simulation reveals that <a>-dislocations experience different critical resolved shear stress (CRSS) during their passing through the α2-Ti3Al particle. The CRSS of the first pass increases linearly with the particle diameter both in the prismatic and basal planes, resulting in the formation of the anti-phase boundary within particle, which decreases the CRSS of the second pass. This triggers a slip planarity that ends up of dislocation pairs. The present results provide a deeper understanding on the plasticity and fatigue of titanium alloys containing α2 precipitates.

Beyond Orowan hardening: Mapping the four distinct mechanisms associated with dislocation-precipitate interaction

The conventional role played by precipitates in crystalline solids is in blocking the motion of dislocations and for consequentially hardening, a mechanism attributed to Orowan's finding. Recent experiments and theoretical analysis demonstrated that a few nanometre-sized precipitates, when dispersed in advanced metals at fine spacing, can further boost their strength at no sacrifice in ductility. In this paper, we construct the deformation map of four distinct mechanisms associated with dislocation-precipitate interaction: at low-to-intermediate stress level, dislocations may loop around a precipitate or cut-through it. In both scenarios the precipitates harden the materials and there is no net gaining of dislocations. At high stress level, nanoscale precipitates may in contrast act as dislocation sources and generate dislocations from the matrix-precipitate interface — an interface-nucleation process; or emit dislocations when highly stressed dislocations transverse them — a radiation-emission process. While the interface-nucleation mechanism could supply sustainable dislocation multiplication, the radiation-emission leads to the multiplication of two additional dislocations. Based on large-scale simulations and theoretical analysis, we construct a deformation map on dislocation-precipitate interaction in terms of stress and precipitate size. The revealed mechanisms and the dislocation-precipitate interaction map pave the way for strength–ductility optimization in materials through precipitation engineering.

Uncovering dislocation-precipitate interactions during tensile loading of wire arc additive manufactured nickel-aluminum-bronze

Uncovering dislocation-precipitate interactions during tensile loading of wire arc additive manufactured nickel-aluminum-bronze

Influence of heat treatment, microstructure evolution, and damage mechanism on high cycle fatigue behaviour of additively manufactured Ti-modified Al 2024 alloy

The high cycle fatigue behaviour of titanium modified Al 2024 alloy produced by laser powder bed fusion (LPBF) is investigated to elucidate the effect of the microstructural features, residual stress, and defects (such as porosity and lack of fusion) on dynamic behaviour. The titanium acts as a grain refining agent; it can reduce the solidification cracking and produce the ultrafine-grained (UFG) microstructure in aluminium alloys. The 2% titanium addition resulted in the UFG microstructure with an average size of 0.52 μm in as-built samples. The microstructural features were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron back-scattered diffraction (EBSD) analysis. The influence of the microstructural features, such as grain size, dislocation density, precipitate-dislocation interaction, etc., on the fatigue life of as built and T6 heat treated samples was analysed in detail. The effect of varying stress amplitude at a stress ratio of 0.1 and a frequency of 30 Hz was investigated for the as-built and heat-treated sample. The T6 heat-treated sample showed 21.9% higher fatigue strength compared to the as-built samples (87.8 MPa in the heat-treated sample compared to 72 MPa in the as-built sample). The corresponding damage mechanisms are thoroughly explored through scanning electron fractography, TEM, and EBSD. The defects like porosity and lack of fusion defects in the surface or sub-surface level were observed to be critical regions as crack initiation sites.

Quantitative assessment of the influence of the Portevin-Le Chatelier effect on the flow stress in precipitation hardening AlMgScZr alloys

Portevin-Le Chatelier (PLC) effect often occurs in many alloy systems and leads to flow stress variations. When precipitation occurs, the PLC behavior changes and its resultant influence on the flow stress becomes more complicated, which has not yet been clarified. The well-known interaction mechanisms between precipitates and dislocations are shearing (shearable precipitates) and bypassing (non-shearable precipitates). This study systematically investigates the influence of the PLC effect on the flow stress in three cases, namely, without precipitates, with shearable precipitates, and with non-shearable precipitates. This study is performed on a AlMgZrSc alloy, where the precipitation does not change the concentration of solute species that is responsible for PLC. A modified constitutive relationship considering different dislocation-precipitate interactions is proposed, which can quantify the contribution of the PLC effect to the flow stress in the above three cases. The modeling results agree well with those of experiments performed on AlMg and AlMgScZr alloys exhibiting the PLC effect. It is theoretically demonstrated that PLC-induced strengthening can account for as much as 14.5% of the total flow stress in AlMg alloys. When shearable and non-shearable precipitates appear, this percentage decreases to approximately 4.5% and 9.5%, respectively, indicating that the precipitates weaken PLC-induced strengthening. Moreover, shearable precipitates can shorten the strain rate range of PLC, which is more effective in suppressing the PLC effect than non-shearable precipitates. Finally, the intrinsic mechanism responsible for the PLC-induced strengthening and roles of different precipitate-dislocation interactions are discussed.