Dislocation-precipitate Interactions Research Articles

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.

Design of ultra-strong but ductile iron-based alloys with low supersaturations

High-performance but low-cost structural materials with nanoscale precipitations are vitally important for engineering applications in advanced industry systems. Thus far, the classical nucleation mechanism assumes a critical nucleus of a homogeneous composition up to a sharp boundary with a large energy barrier of nucleation. A fine dispersion of nanoscale precipitates often requires strong solute supersaturations with expensive alloying additions or otherwise deteriorated strength-ductility, weldability, and fabricability. The simultaneous satisfaction of all these mutually exclusive requirements is the longstanding issue for the design of advanced materials for structural applications. In this alloy design, we have adopted a revolutionary approach of ultra-strong iron-based alloys that have resolved all these key issues through non-classical nanoscale precipitations and multi-elemental partitioning. Such an alloy design strategy offers a unique approach to control nanoscale precipitates with low solute supersaturations. Fine precipitations so designed lead to the strong enhancement of both the precipitate-dislocation interactions and work-hardening capacity, resulting with a sharp increase of the yield strength (YS) to as high as 1500 MPa and tensile ductility up to 15%. Furthermore, our design assures outstanding fabricability together with superior post-weld properties through dense nanoscale reprecipitations (∼1024m−3) in a well economical manner for production. The non-classical actions unfold brand new pathways for precipitation of various intermediate structures and chemistries with low energy barriers, especially useful for the design of sustainable and economical modern alloy systems.

Microstructure evolution and shearing behaviour of δꞌ/θꞌ/δꞌ precipitates in an aged Al-Cu-Li-Mg aluminium alloy

We report here on the microstructure evolution and the strengthening behaviour of the δꞌ/θꞌ/δꞌ precipitates in the aged Al-Cu-Li-Mg aluminium alloy by atomic high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and in-situ TEM compression straining. On the chosen zone axis, atomic HAADF-STEM images reveal the in-situ microstructure evolution of the δꞌ/θꞌ/δꞌ precipitates. A monolayer GP(θꞌ) zone, grown on the (001)Al plane, evolves from the Cu-rich region. After the transformation of the GP(θꞌ) zone to θꞌ phase, the boundaries of the θꞌ phase are coated with the δꞌ phases, forming a matured δꞌ/θꞌ/δꞌ precipitate. Additionally, the growth of δꞌ phases on the two sides of θꞌ is not affected by the inclined boundaries of θꞌ and the hard impingement of the thin T1 precipitate. During the in-situ TEM compression straining on the peak-aged nanopillar, the deviated slip band structures and the large displacement bursts are presumably associated with the dislocation-precipitate interaction of the T1 and δꞌ/θꞌ/δꞌ precipitates. Furthermore, evidence from atomic HAADF-STEM images shows that dislocations can cut the δꞌ/θꞌ/δꞌ precipitate, producing the stacking fault layers within the θꞌ phase of the sheared δꞌ/θꞌ/δꞌ precipitate.

Strengthening effect of NiAl and Ni3Ti precipitates in Co-free maraging steels

One class of maraging steels are strengthened by a combination of β-NiAl and η-Ni3Ti intermetallic phases which are precipitated during an aging heat treatment. To establish a meaningful structure-property relationship the precipitation strengthening effect from each phase must be isolated from the other strengthening mechanisms. To achieve this, a series of model alloys based on the Fe-12Ni alloy system were aged and characterized to determine the precipitation strengthening effect. In the course of this study, using a new approach, atom probe tomography was used to determine the spacing between precipitates and to calculate the individual strength contribution of the β and the η phase using a model describing the precipitate-dislocation interactions. It was found that the precipitation strengthening of the combined β and η phases is close to 1000 MPa and that the relative strengthening effect of each phase is sensitive to the Ti and Al concentration.

Dislocation dynamics analysis influence of precipitate shape in dislocation-precipitate interactions

Dislocation dynamics analysis influence of precipitate shape in dislocation-precipitate interactions

Bayesian optimization of discrete dislocation plasticity of two-dimensional precipitation-hardened crystals

Discovering relationships between materials' microstructures and mechanical properties is a key goal of materials science. Here, we outline a strategy exploiting Bayesian optimization to efficiently search the multidimensional space of microstructures, defined here by the size distribution of precipitates (fixed impurities or inclusions acting as obstacles for dislocation motion) within a simple two-dimensional discrete dislocation dynamics model. The aim is to design a microstructure optimizing a given mechanical property, e.g., maximizing the expected value of shear stress for a given strain. The problem of finding the optimal discretized shape for a distribution involves a norm constraint, and we find that sampling the space of possible solutions should be done in a specific way in order to avoid convergence problems. To this end, we propose a general mathematical approach that can be used to generate trial solutions uniformly at random while enforcing an Euclidean norm constraint. Both equality and inequality constraints are considered. A simple technique can then be used to convert between Euclidean and other Lebesgue $p$-norm (the 1-norm in particular) constrained representations. Considering different dislocation-precipitate interaction potentials, we demonstrate the convergence of the algorithm to the optimal solution and discuss its possible extensions to the more complex and realistic case of three-dimensional dislocation systems where also the optimization of precipitate shapes could be considered.

Effect of hydrogen accumulation on θ' precipitates on the shear strength of Al-Cu alloys

We investigate the effect of accumulation of H atoms on θ' strengthening precipitates in Al-Cu alloy. A relationship is established between the H accumulation and decrease in shear strength of alloy depending on the H content in precipitates of second phase. We use the previously proposed multiscale approach combining molecular dynamics (MD), continuum modeling and discrete dislocation dynamics (DDD). H atoms are located inside precipitate and at interphase boundaries. Energetically preferable places of H accumulation near and inside θ' phase in Al-Cu alloy are revealed. It is shown that the H accumulation on θ' leads to a decrease in precipitate shear resistance and causes the change in the dislocation-precipitate interaction mechanism from the Orowan loop formation to the cutting of the inclusion. The H accumulation on precipitates causes an increase in the threshold stress for the dislocation nucleation on the inclusion. The mechanism of softening of inclusions associated with a change from looping to cutting is transferred to the DDD level. The DDD results demonstrate that H charging of Al-Cu alloy can substantially decrease its shear strength. For high strength alloys with small precipitate size and/or inter-precipitate distance and with the yield stress above about 460 MPa, the reduction of strength can reach 10–25%.

Dislocation-obstacle interaction evolution in rate dependent plasticity of AlSi10Mg as-built microstructure by laser powder bed fusion

Novel microstructural features obtained from laser powder bed fusion methods challenge our understanding of the rate controlling mechanisms for plasticity which are important for structural applications. The strain rate dependent plasticity of as-built AlSi10Mg laser powder bed fusion samples was systematically studied as a function of stress state (tension and compression) and strain rate at 293 and 78 K. Utilizing constant strain-rate and rate-change testing methodologies, a strong dependence of flow stress on strain rate was revealed for both stress states. The compressive yield stresses have a potent strain rate sensitivity in both quasi-static (10−4-10−1 s−1) and medium-dynamic (1-5 s−1) strain rate regimes. A positive strain rate sensitivity was observed in all strain rates except for a shift to negative values in the 10−2-10−1 s−1 range for both tensile and compression states. This negative strain rate sensitivity was correlated with the deformation-induced microstructural evolution, i.e., manifested as a gradual transition in the dominant dislocation-precipitate interaction mechanism from shearing to looping, along with the disappearance of dislocation-solute interaction in the form of dynamic strain aging (DSA). The precise measurement of activation volume evolution with flow stress further confirmed the transition from shearing to looping in the 10−2-10−1 s−1 range.

Superposed hardening from precipitates and dislocations enhances strength-ductility balance in Al-Cu alloy

The age hardening response and tensile properties in an Al-Cu alloy pre-deformed by 20-80% and post-aged at 120-165°C have been examined to study the effect of mixed dislocations/precipitates on the strength-ductility balance. As compared to the T8 temper, both yield strength and uniform elongation are significantly improved by a pre-deformation of 20-40%, followed by ageing at 120°C. As compared to the T6 condition, the yield strength is higher (by about 50%), while the uniform strain is similar. The dislocation self-organization evolves from forests to tangles and cell boundaries with the pre-deformation level increasing to 60-80% and keeps preserved after post-ageing. The heterogeneous formation of θ' phases at dislocations and the homogeneous matrix precipitation of θ"/GP zones constitute the hardening particles. The quantitative analysis suggests dislocations and θ' precipitates are the major strength contributors. Off-alignment between the deformed precipitate and the un-deformed one occurs for both θ' and θ" during tensile straining. The dislocations tend to accumulate around large θ', eventually leading to the θ' rotation by 2-5° and severe matrix distortion near the precipitate. The fine and relatively sparse θ' phases in the 20-40% pre-deformed alloy post-aged at 120°C firstly block and then transmit dislocations, as is evidenced by the shearing at multiple locations, resembling the θ" phases. The correlation between the precipitate-dislocation interaction and the obtained properties supports that localized dislocation pile-up at hard barriers, e.g. large θ' and cell boundaries, could initiate void and deteriorate the plasticity. Suppressing the dense formation of non-shearable precipitates delivers exceptional strength-ductility synergy in the pre-deformed and post-aged alloy.

Correlating in-plane strength anisotropy with its microstructural counterpart for a hot rolled line pipe steel

The present study focuses on the microstructural entities, emphasizing the role of precipitates on the in-plane directional tensile responses, including the strain hardening behaviour of a hot rolled X-65 Line pipe graded steel. Tensile samples have been prepared from four different directions lying on the RD-TD (rolling direction-transverse direction) plane at the varying angle of 0°, 30°, 60°, and 90° with respect to the RD of the hot-rolled plate. Superior mechanical properties have been evidenced in the 90° sample compared to other orientations in terms of maintaining yield continuity and better strength-ductility combination. The yielding behaviour and tensile response have been explained from the viewpoint of grain/grain boundary characteristics, dislocation-precipitation interaction, and elasto-plastic incompatibility across grain boundaries. The role of interaction of dislocations with fine precipitates along with the Elastic Modulus (EM) and Schmid Factor (SF) differences across high angle grain boundaries (HAGBs) have been found to be significant in determining the superiority of the mechanical properties of 90° sample.

Solidification microstructure, aging evolution and creep resistance of laser powder-bed fused Al-7Ce-8Mg (wt%)

The solidification microstructure and the aging- and creep-resistances of a laser powder-bed fused (LPBF) hypoeutectic Al-7.3Ce-7.7Mg (wt%) alloy were investigated. Optimized process parameters allowed to produce specimens with > 99.7% relative density and high hardness (~1650 MPa), without solidification cracking despite a relatively coarse grain size (~ 30 µm). Two distinct solidification microstructures are observed within the solidified melt pools: (i) eutectic α-Al(Mg) and Al 13 CeMg 6 (a metastable phase) at the bottom pool region, and (ii) eutectic α-Al(Mg) and Al 11 Ce 3 (a stable phase) precipitates at the top pool region. The high hardness is predominantly attributed to Orowan strengthening and load-transfer effects from the Ce-rich precipitates, but solid-solution hardening from Mg is also quite pronounced, as most Mg resides in α-Al. Aging at 325 or 400 °C causes a continuous, slow hardness drop, but 85–90% of the initial hardness is retained after over 1000 h aging, consistent with the observation that the Ce-rich precipitates within the grains show very sluggish coarsening, making the alloy highly resistant to aging. The alloy exhibits two creep regimes at 300 ºC. At stresses below 18–20 MPa, diffusional creep is operating with signs of grain-boundary sliding inhibition due to Ce-rich precipitates at grain boundaries. At higher stresses, dislocation climb with either stress-invariant or stress-dependent subgrain structure is dominant, with additional load-transfer contributions and possibly a threshold stress due to dislocation-precipitate interactions. The alloy also exhibits much lower steady-state strain rates (e.g., two orders of magnitude lower at 20 MPa) as compared to a compositionally-similar, but conventionally-cast Al-Ce-Mg alloy with much coarser microstructure, demonstrating that LPBF Al-7.3Ce-7.7Mg alloy – because of its fine precipitates and despite its fine grain size - is well suited for long-term uses under stress at 300 ºC.

Nanoscale Cu particle evolution and its impact on the mechanical properties and strengthening mechanism in precipitation-hardening stainless steel

Copper-rich precipitation can effectively strengthen stainless steel; however, the underlying mechanism is still unclear due to an insufficient understanding of the transformation of Cu-rich precipitates. Herein, using a high-resolution transmission electron microscope, we characterized the evolution of nanoscale Cu-rich precipitates during the aging of precipitation-hardening stainless steel and determined their effects on mechanical properties. Results indicate that the structure of the Cu-rich nanoprecipitates consecutively evolved from B2 (ordered Fe-Cu) to BCC (body-centered cubic) to 9R (the stacking structure of Cu-atom layers is ABC/BCA/CAB/A) and eventually to FCC (face-centered cubic) with aging. Consequently, the dislocation-precipitation interaction changes from a cutting to a looping mechanism. The BCC-structured Cu-rich nanoprecipitates formed at 480 °C after 1 h of aging exhibit excellent strength and ductility combination properties of 412 MPa and 18%, respectively.

Improved strength with good conductivity in Cu–Cr–Zr alloys: Determinant effect of under-aging treatment before rolling and aging

To achieve high strength and good electrical conductivity, thermo-mechanical treatments are usually applied for precipitation strengthened Cu–Cr–Zr alloys. However, the dislocation-precipitate interaction is still unclear for these alloys during deformation and post-aging, which hinders a further improvement in their mechanical-electrical properties. In the present study, to investigate the effect of precipitates’ size and their orientation relationship with the Cu matrix on the dislocation accumulation during rolling and post-aging, a thermo-mechanical treatment was fulfilled by adjusting the pre-aging state of the alloys. Samples with four precipitate states (solid solute, under-aged, peak-aged, and over-aged) were subjected to rolling and post-aging, respectively. The results showed that, the under-aged sample with less precipitates accumulated the similar deformed structure as the peak-aged one during rolling and relieved the negative effect caused by aging recovery during post-aging, retaining more deformed structure. Therefore, the ultimate tensile strength reached to 588 MPa, ∼30 MPa higher than those with other pre-aging treatments. Additionally, the re-dissolution and refinement of precipitates caused the abnormal decrease of electrical conductivity during rolling, and the post-aging treatment restored the electrical conductivity to the normal level (81% IACS). The results indicate that a proper under-aging pre-treatment before rolling helps to obtain better mechanical properties, which is feasible for large-scale industrial production and can be extended to other thermo-mechanical treatment forms.

The martensitic transformation behavior and shape memory effect of laser powder bed fusion NiTi alloys influenced by rare earth addition

Additively manufactured (AM) has attracted extensive research due to its ability to precisely form NiTi alloys with complex structures compared to conventional fabrication processes. However, the strong-plastic mismatch, poor shape memory effect (SME) and functional properties severely limit the wide application of AM-NiTi alloys. Rare earth oxides are common additives to improve the mechanical properties of alloys. This paper is the first to report the effect of different additions of CeO2 (0.03% CeO2, 0.3% CeO2 and 3% CeO2) on the properties of NiTi alloys prepared by laser powder bed fusion (LPBF) technology. The results showed that different amounts of CeO2 seriously affect the transition temperatures (TTs), SME and mechanical strength of LPBF-NiTi alloys. The change of TTs is mainly related to the precipitates formed after the addition of CeO2. The formation of NiCe reduces the Ni content in the matrix and thus increases the TTs, while the generation of CeTi2O6 reduces the content of Ti in the matrix, resulting in a decrease in TTs. The improvement of mechanical properties is mainly attributed to the grain refinement strengthening, precipitate-dislocation interaction and solid solution strengthening after rare earth addition. More significantly, the addition of an appropriate amount of CeO2 (0.03% CeO2) promoted the grain order of LPBF-NiTi alloys and improved the yield strength and SME.

Dislocation-θ′ (Al2Cu) interactions during creep deformation of an Al-Cu alloy

While precipitate-dislocation interactions are well-understood for Al-Cu alloys in tension, creep behavior has seen far less study. New, thermally-stabilized Al-Cu alloys have θ′ (Al2Cu) as strengthening precipitates that remain stable up to 300 °C (∼60% of the melting temperature) and higher, where creep becomes essential to the mechanical behavior. This investigation identifies the precipitate-dislocation interactions in such an Al-Cu alloy using in-situ neutron diffraction and scanning transmission electron microscopy. Significant load transfer to the θ′ precipitates occurs, which can be attributed to dislocation loops on the interfaces of θ′ and the Al matrix. Thus, Orowan looping is identified to be the primary activity for precipitate-dislocation interactions. As Orowan looping and load transfer are associated with significant strain hardening, these results explain the excellent creep resistance seen in this alloy, and provide insights into the design of precipitation strengthened alloys with superior creep performance.

Prediction of the strength of aged Al-Cu alloys with non-hybrid and hybrid {1 0 0}Al plates

The effect of precipitate hybridization on macroscopic strengthening in aluminum alloys is investigated on the example of Al-Cu alloy using multiscale approach combining molecular dynamics (MD), continuum modeling and discrete dislocation dynamics (DDD). Non-hybrid and hybrid {100}Al plates are considered to involve θ′-phase and θ′-phase in the core and Guinier-Preston zone (GP-like) structure along the broad interfaces, respectively. MD simulations evidence a complex dislocation-precipitate interaction mechanism involving bypassing of both hybrid and non-hybrid {100}Al plates by dislocations at early deformation stages and their shearing by the following dislocations. MD results are used to calibrate a continuum model of dislocation-precipitate interactions in 2D DDD. The shear strength of alloy with hybrid precipitates is found to be 20% higher than that for non-hybrid plates at the same Cu content exceeding 2 wt%.

An investigation into the mechanistic origin of thermal stability in thermal-microstructural-engineered additively manufactured Inconel 718

An obstacle hindering the applicability of surface modification techniques such as laser peening (LP) in high temperature systems stems from thermally-driven degradation of desirable, strain-induced material modifications. Illustrated in this work is a novel LP scheme termed laser peening plus thermal microstructure engineering (LP + TME) comprised of cyclic LP and the addition of intermittent 600 °C (0.55Tm) heat treatments designed to impart thermally-stable microstructural modifications in additively manufactured (AM) Inconel 718 (In718). Instrumented microindentation uncovered significant surface and sub-surface hardness enhancements exceeding 600 HV following LP + TME, a 20% increase over the as-built material. High magnitude compressive residual stresses exceeding −310 MPa were also measured following a 350-h 600 °C thermal exposure; a 25% increase compared to the material subjected to only a single laser shot. Thermal stabilization and overall material enhancement were determined to be the result of the formation of thermally-stable subgrains, subgrain and grain growth regulation through pinning effects, and dislocation-precipitate interactions.